no sound from an AB763 clone

chris shaw · 11-20-2009, 09:10 AM

hi
am completely new here so please excuse my ignorance
I have built a single input ab763 clone without the trem circuit , hasmmond transformers JJ valvesand a hammond reverb tray
I have about 5.9 volts dc to ground to heaters, 330v ac to pins 4 and 6 of rectifier tube (GZ34)
all voltages from here seem a little high (but i have a cheap meter)
as follows

first preamp tube (nearest to input) 2.3 v to cathode, (sschematic says 1.3) 187 tio plate1 (schematic says 170 ) similar to pin 6 , 1.3 v to pin 8
reverb tube is 450v to pin1 (schematic 410)and only 4.2 v to pin 8(schematic says 8.7)

the BIG DIFFERENCE in second preaamp trube - reading 316 to pin 1 (schematic 170), 5.2 v to pin 3 (schematic dsays 1.3)and 330 to pin 6 - schematic says 180 v

last 12ax7 readings before 6v6 are 190 to pin 1 and 6 and 80v to pin 3 and8 so are ok I think
450 v dc to pins 3 of each 6v6 (schematic suggests 415)
200v to pin one of last 12AT7 (schematic says 170)
I AM GETTING NOTHING BUT A SLIGHT BUZZ from speaker and when guitar goes thro its just a faint distorted noise BUT I CAN HEAR REVERB TRAY VIBRATING A TREAT WITH VOL UP, nothing to tray with vol down

please help It is driving me to drink
Chris (sheffield uk)

no
about 190 to pin 6 of same tube and
82 volts dc to pin 3 (schematic says 77v)

I have po

bluesfreak · 11-20-2009, 11:05 AM

Any pics and which schematic did u use? w/o that its difficult to do an analysis

MWJB · 11-20-2009, 12:30 PM

"the BIG DIFFERENCE in second preamp tube - reading 316 to pin 1 (schematic 170), 5.2 v to pin 3 (schematic dsays 1.3)and 330 to pin 6 - schematic says 180 v" Doublecheck you have the right values for plate & cathode resistors, check that the cathode resistor & cap have a solid ground connection.

Your preamp voltages are low, a BF deluxe is usually around the 220vdc mark. A little off because your B+ is high, What plate current do you have?

With the power on & all volumes/tone controls up, probe power tube plates, should hear a faint "ffsst!" in the speaker, now move on to PI tube, and so forth...by the time you get to the input tube you should need to turn the volumes down as probing should be VERY noisy! We're trying to see where the break in the signal is.

Amp Kat · 11-20-2009, 02:24 PM

I have built the same circuit with a Deluxe Reverb AA763 Tremelo stage only except in a tweed chassis. I had to use 20k dropping resistors because the 10k were just yielding to high of a plate voltage on the preamp tubes. This got all of the voltages almost exact which refers to what MWK was saying. What I found and even with 15+ years of amp experience was some wiring issues that I had overlooked and I would bet that is part of you're problem also. Check all of the voltage nodes on the schematic (A,B,C,D,E,FZ,X ) I mean everything and make sure they are there at idle with no signal (quiescent point) . Check your pot connections real well and make sure the pots are all wired right. The best test you can probably do after that is insert a tone with a generator to the input and lift the wire from the treble pot right after the 250pf tone stack cap and attach a wire or test clip to it and send it to connection of the .001uf / 220k right at the junction of the PI. Any sound ? If not you either have problems before that first tube or after the PI and isolation troubleshooting should cut you're work in half but if you can't get sound there you won't get it anywhere else so that's a first. Let me know as mine sounds great but still has some hum in the reverb I have to find.

chris shaw · 11-20-2009, 06:19 PM

gentlemen(sexist assumption?)
I have neither done a build nor done a forum before. I am thrilled that experts across the pond and in europe are helping me
!!
bluesfreak - I'm too embarrased re soldering and cable neatness to post photo here!!
but circuit is
https://taweber.powweb.com/store/6a20_layout.jpg except only 1 channel and no vibrato
will try over weekend but its daughters birthday so busy! will post early next week

you are all very kind
cheers
Chris

chris shaw · 11-20-2009, 07:30 PM

https://taweber.powweb.com/store/6a20_layout.jpg

above is schematic
as you can see the dropping resistors (the ones before the plates?) are all 100k so I dont know how that affects things, really grateful for help , will try the bypassing tip tomorrow

bluesfreak · 11-20-2009, 09:34 PM

Chris, have you tried to unplug Reverb Pan and touch the cable feeding back to the Recovery Tube? You should have decent Hum if not there is a generic failure in the circuit.
However, since this is your first build doublecheck all resistors/caps for correct values, Pinout of the tube sockets in clockwise from the bottom of the socket!
YOur Voltages on V1 seem ok to me but confuision starts for me on V2 as you stated that you have built this circuit without the Vibrato Channel but the Schematic refers to it. So how did you modify the circuit? Did you leave the Vibrato Input Branch with V2/V5 out of the circuit (need to modify bias supply then!) or what di you exactly? I suspect that you made a mistake around the cutoff of the vibrato input..

chris shaw · 11-20-2009, 11:23 PM

bluesfreak
i just removed the bit vibrato circuit , from the cathode cap and resistor of pin 3 , through to the cathode cap and rsistor of pin 8, I therefore only had one wire going to the bias circuoit ads the one from pin 1 of the vbrato bit of the circuit didnt exist. I then just had a 'Y' connection going into the PI , as hte 'x element of the circuit was not from the reverb/ vibrato channel (I didnt include it in the build

I suspect you are saying 'no, no, no'

please ell me what i have done that I cant do !!

chris shaw · 11-21-2009, 07:53 AM

hi
have just read your last post in cold light of day and realised i havent answered your point
no i didnt remove the v2 - v5 part of the circuit, i removed the v1 part i.e. the channel with neither vibrato nor reverb , and then removed the v5 part of the remainder, leaving just v2 , v3 and v4 before the PI
i suspect i have a bias problem as a result?
also what do you mean by 'pin out of the tube sockets clockwise from below '
I've wired them all pin out clockwise from above - I e , as I'm looking down from above , pin 1 is the first clockwise from the space on the socket - that seems to make sense and tie up with socket numbers and photos of other builds have I even got that wrong?!!

chris shaw · 11-21-2009, 07:55 AM

hi ampkat
make sure they are there at idle with no signal (quiescent point)

what does the above mean please?"

bluesfreak · 11-21-2009, 09:00 AM

I should have been more specific: Pinout of the sockets is clockwise if you look on it from the solder (bottom) side!! and CCW if you look from the top (Tube Insert) so the Weber Layout shows it correctly if you look from the solder side. Ensure that you didn't make a mistake there as you will most likely then have blew tubes already.
Looking at the Schematic its getting a bit clearer to me now so you went from V2B Pin8 to GND via R26/C9 ? This is a shared cathode R so when you use it with only one tube I would recommend to double its value to 1.5k. What about the mixing Rs R35/R36, did you get rid of them and the addtl coupling cap C14? The bug might be sittin here...

chris shaw · 11-21-2009, 10:26 AM

tapping plates yielded noise from 6v6's and pin 6 of the second preamp valve (interestingly less noise from pin 1?)
nothing earier in circuit
re the r26 /c9 - I dont really know the terms but I reckon you must mean the 820 resistor which takes cathode of both normal and vibrato pin 8 cathodes - I need to double it?-- will do
also re r35/36 I dont know what you mean but I assume you mean the point where X and Y signals from normal and vibrato channles join?
answer is no I just didnt put anything on x -- ( as I have no normal channel!)
should I have removed the two 220 k resistors asnd the 0.001mf capacitor?)
I did a similar thing re bias connection from vibrato bit of circuit which I left out - is that why I got -60 instead of -35v on bias input?!

re cutting bits out to test circuit
I tried connecting output from plate of first preamp tube pin 1 direct to point Y immediately prior to 220k resistor at start of PI-- there was nothing -- but i could hear lots of input into reverb tray ?
re plate current
I dont know how to measure it-- I put a probe on the plate, one to chassis and dialed up current on meter, but it faded to 0 fairly quickly from every point - am I doing something wrong or is ithat meaningful?
in summary it sounds like power amp stage is ok
One question from the readings- the drop in voltage from the 'D' supply in the schematic going to the first preamp plates across the two 100k resistors drops to 180v and yet the same power supply 'D' feeding the second preamp stage only drops to 310 - how does that work or shouldnt I ask ??!!
back to you chaps !!

pdf64 · 11-21-2009, 03:21 PM

Dropper resistors are the resistors in the power supply section, 'in the doghouse' compartment of a regular fender. They connect the B, C, and D nodes, dropping some Vdc and, in conjunction with the power supply 22uF high voltage caps, decoupling Vac on the power supply.
The 100k resistors connected to pins 1 and 6 of the pre-amp tubes are 'plate load' resistors, they turn the current flowing through the tube into voltage.

're plate current
I dont know how to measure it-- I put a probe on the plate, one to chassis and dialed up current on meter, but it faded to 0 fairly quickly from every point - am I doing something wrong or is ithat meaningful? '

Don't try to measure current like that, it's a sure way to damage your meter. Voltage is measured in parallel with the circuit under test, across 2 points, whereas current is measured in series, you have to break into the circuit and put the meter so that it bridges the break. A current meter is effectively a short circuit.
Hopefully you didn't plug your test leads into the current sockets on your meter, or maybe you blew a fuse in your meter.

chris shaw · 11-21-2009, 03:45 PM

hi pdf 64
thanks, I said I was a beginner, will try meter tomorrow, feel stupid but am learning all the time, I really appreciate your help
thanks for your time and knowledge
Chris

Enzo · 11-21-2009, 06:31 PM

Actually if you want to measure current through the 100k resistor, or any other resistor, leave your meter on volts. measure the voltage across the resistor. Divide that voltage by the resistance. The result is current.

That is classic Ohm's Law, one of the most fundamental relationships in electronics. I keep a little calculator next to me at all times for exactly this reason.

Volts - V
Resistance in ohms - R
Current in amperes. - I

V = I x R = Ohm's Law

SO then I = V/R

For example, if the B+ supply is 300v and the plate of the tube sits at 150v, and that plate load resistor is 100k, then we have 150v dropped across that 100k resistor. So we know that Ohm's Law says the current through it is 150/100,000, which is 0.0015 amps. .0015 amp is the same thing as 1.5 milliamp.

SO we know there is 1.5ma current in my example, without having to break into the circuit. Of course we could disconnect one end of the plate load and insert out current meter in series, but that would be clumsy and dangerous. Current meters are very useful, but in other situations. I wouldn;t have a meter without it.

chris shaw · 11-21-2009, 07:15 PM

thanks enzo
I did physics before university and i knew i'd use it one day V+ IR brilliants, I was worried about having to disconnect everything to measure stuff!

chris shaw · 11-21-2009, 09:11 PM

to first pre amp tube
voltage drop across first pre amp tube plate resistors was 357- 220 on pin 1 and 357-228 on pin 6 (should they be different?- could be measure error)so current to each is 1.5 ma

to reverb tube
-- i dont know how to measure without disconnecting so havent done that , voltage to red wire of rev transformer is 450v, similar to blue wire returning to pins 1 and 6 of tube

to second pre amp stage
voltage drop across 100k resistors to next preamp stage is 357 - 327 = 30 divided by 100,000, so plate current =0.3 ma
to PI tube
voltage drop across 100k resistor to pin 6--390 -215= 175 divided by 100000 = 1.75 ma , drop across 82 k resistor was 390- 217= 173 divided by 82,000 = 2.1 ma
do they sound right?
all tubes in by the way

chris shaw · 11-22-2009, 09:53 AM

plate currents

--------------------------------------------------------------------------------

to first pre amp tube
voltage drop across first pre amp tube plate resistors was 357- 220 on pin 1 and 357-228 on pin 6 (should they be different?- could be measure error)so current to each is 1.5 ma

to reverb tube
-- i dont know how to measure without disconnecting so havent done that , voltage to red wire of rev transformer is 450v, similar to blue wire returning to pins 1 and 6 of tube

to second pre amp stage
voltage drop across 100k resistors to next preamp stage is 357 - 327 = 30 divided by 100,000, so plate current =0.3 ma
to PI tube
voltage drop across 100k resistor to pin 6--390 -215= 175 divided by 100000 = 1.75 ma , drop across 82 k resistor was 390- 217= 173 divided by 82,000 = 2.1 ma
do they sound right?
all tubes in by the way

pdf64 · 11-22-2009, 11:20 AM

'to second pre amp stage
voltage drop across 100k resistors to next preamp stage is 357 - 327 = 30 divided by 100,000, so plate current =0.3 ma
to PI tube'

There's a problem there. Do you have any voltage on the grids of that tube, pins 2 and 7?
Did you double check that the cathode resistors and bypass caps on pins 3 and 8 are correct, see post #3?

chris shaw · 11-22-2009, 12:00 PM

thanks PDF
Will check grid voltage and also cathode res and cap connections
what I do have, as stated in previous post, is a click/ thud when probing pin 6 of the second preamp tube , although the noise from pin 1 is substantially less -- just a little click, ----nothing when probing first tube and reverb tube, but with reverb pot turned up I can hear the springs vibrating quite a bit in the reverb pan sat on the bench.
I am getting a lot from your interventions , thanks chaps
will report back
ps what should grid voltages be on the second preamp tube and also what should the plate current be through the same tube ?

chris shaw · 11-22-2009, 12:10 PM

just to clarify
the words
'to PI'
quoted in my 'plate current' post were the heading relating to the numbers which followed it, so the current to the second pre amp tubbe were the 0.3 ma, whereas
current to PI was 1.75ma. sorry if my failing to put a line between second preamp current details and PI heading and subsequent current details

thrust this explains but will assume everyone understood anyway

pdf64 · 11-22-2009, 02:07 PM

Grid voltages should be 0.
All those 12AX7 stages should have very similar operating conditions, so the voltages ( and therefore currents) should be within 10 or 20%.

chris shaw · 11-22-2009, 04:40 PM

Gentlemen
Of course you were right , I checked all the connections and I had connected the 2nd preamp cathode cap and resistor serving both sides of the tube to the grid instead
Having resolved this I now have (330-210) /100,000 =1.2 ma on the 2nd pre amp plate---
at least I now have a buzz through the speakers, which, right now , feels like a success!!!
HOWEVER it is only a buzz, which with reverb turned full on becomes a fairly large buzz. - guitar is heard v distorted and faint , slightly more with reverb full on
question1 - will my bad connection have damaged anything before it was resolved?
question 2
how do I start finding source of buzz nd get it to make a guitar noise?!!!
question 3
I am using 1 m copper single strand cable as preamp ground across baclk of all pots and for preamp cathodes and preamp power cap -- can it be too big?
other observations
bias varies from -61 to -29 across pot range so have turned it to -35.
i have 0v on all grids EXCEPT I have 24v on grid of PI , which, when grounded gets rid of buzz/ hum totally
the amp doesnt seem very loud and poking plates of v1 and reverb valve creates some but not a massive noise
any more advice would be much appreciated
I couldnt have done this so far without you chaps

pdf64 · 11-22-2009, 06:23 PM

Check out the Jack Darr link on this site
Links to amp building forums and resources
see his section on tracing a signal
Plenty of other good links there also, Aiken especially.
Rather than set the bias level by measuring the voltage from the pot, you ned to monitor the current going through the power tubes with no signal. This is normally done by putting a 1 ohm resistor between each power tube cathode and ground, then measure the mV across that resistor. See Aiken for more detail.
Have you got something you can use as a signal generator, such as a keyboard with a jack output (ie headphone).
Feed a constant signal into the amp input and measure the resultant Vac in the signal path. That should enable the problem area to be identified.

chris shaw · 11-22-2009, 06:46 PM

hi PDF
I guess biasing is a way away judging by the buzzing, but i'll do it this way when time comes
dont have keyboard, cn get one. How does the measuring thing you suggested work?
sorry for basic questions

bluesfreak · 11-22-2009, 07:03 PM

YOu could also pull V1 first and then check if the buzz goes away, if not you got the problem more deeper in the circuit so pull the next tube. If its gone you found the part causing the buzz. For biasing the poweramp it would be more advisable to measure current as pdf already mentioned, just dialing in a voltage will not tell you if the tubes do run correct. However, you didn't tell us if you had the tubes hooked up correct, "I've wired them all pin out clockwise from above " is not correct! Also, I recommended some changes according to this schematic: https://taweber.powweb.com/store/6a20hp_schem.jpg , have they been done or not? We need to get a common base on information, questions and answers, otherwise the discussion will end without a working amp...

chris shaw · 11-22-2009, 07:54 PM

pdf
will check the links as described, review thoroughly and come back but will be next week
thanks
chris

chris shaw · 11-22-2009, 08:22 PM

bluesfreak
yes all sockets are correct, and thanks for schematic link, I understand doubling the cathode resistor valuefrom 820 to 1500 at c2b r26
and removing the mixing resistor r35, r36 and mixing cap c14 of v6 PI tube,

will do and then do tube removal process.
will report back later this week
thank you

chris shaw · 11-26-2009, 09:29 PM

Gentlemen
the connection from power caps to power tubes pin 6 was, ahem, missing
it is in now and the mods around the removal of r35, 36 and c14 plus doubling or r26 from 820 to 1500 all done.

I get a rude buzz with vol at 0 but this dissappears as I turn up vol to about 2 - and is replaced by good tone.
HOWEVER if i remove jack plug with vol turned up a little the signal is replaced by a deeper tone- sounding almost moog like . If I replace the plug the signal comes back and moog tone dissappears
If I use jack plug with no signal the moog tone remains - i.e. a jack plug with a signal seems to be good at removing the moog tone. However I also have the bnuzz when the vol is turned down, which only disaapears as I turn up the vol as stated
ps when I turn treble right up it gets all warbly and feedback- ish
any ideas gentlemen?. I used a cd as the signal, when working it sounds great!!!

voltages are as follows
v1 - remember there is no v1 in this version, nor is there an v55 as there is no vibrato)
v2 pin 1 188v plate current 2.5ma, pin 2- 0v, pin 3 - 1.7v , pin 6 -203v, (plate current 1.4ma) pin 7-0v pin 8 -1.59v
v3( reverb tube) pin 1 -404v, pin 2-0v, pin 3-7.63v, pin 6-410v pin 7-0v, pin 8-7.6v.
v4 (make up tube)- pin 1-207v(plate current 1.7ma), pin 2-0v, pin 3-1.6v, pin 6-207v (current 1.7 ma) pin7- ov, pin8--1.5v
v5 not fitted
v6 pin1- 234v, pin2-32.5v, pin3-57v, pin6-2.229v, pin7-37v , pin8-57v
supply voltages- A-405v, B 402v, C 338v, D 300v
power tubes- both identical
pin 1 -minus 35v, pin 3 -397v, pin 4- 406v, pin 5 -minus35v, pin 6 -407v

MWJB · 11-27-2009, 09:11 AM

V2 plate current 2.5mA? Confirm cathode & plate resistor values & tube type...this is not right for a 12AX7 with 1.5K/100K. Voltageat pin 1 should be similar to pin 6.

Is your preamp filter cap grounded to the input jack along with the preamp cathodes? It should not be grounded with the main & screen caps. PI filter cap should either be grounded halfway along the control panel, or also to the input jack...wherever eliminates hum.

You say you have -35vdc at power tubes pin 5, what plate current do you have?

What method did you use to ground the amp (brass plate, buss wire, grounds soldered to chassis or bolted)?

chris shaw · 11-28-2009, 10:09 PM

correction re v1 pin 1 plate current-- the voltage drop across the 100k dropper is about 120v so current is 1.2mA as expected
in answer to grounding questions - preamp filter cap is grounded at input ground, as are pots, and preamp cathodes, tried grounding PI at PT, half way along controls and input, big hum remains
DONT KNOW PLATE CURRENT TO POWER TUBES-- DONT KNOW HOW TO MEASURE IT!
most important/ relevant info - when I lift the output from tube 1 at the 250 pf tone stack cap and connected to the inverter input - the hum remained,, -- I then completely disconnected the input at the PI and the hum remained-- so its in the PI or the Power section!!!
its buzzing but if i turn it up to 1 or 2 it turns into a moog growl, which dissappears when I put a signal through

heeeelp please!

MWJB · 11-30-2009, 11:56 AM

"DONT KNOW PLATE CURRENT TO POWER TUBES-- DONT KNOW HOW TO MEASURE IT!"

Add 1x 1ohm 2W resistor between each power tube pin 8 & ground, make sure resistors are exactly 1 ohm, make sure you have a good ground connection. Read the voltage at pin 8 in mV <200mV range, reading converts directly to mA. Should be less than 30mA per tube (around 20mA per tube if using Russian Reflector/EH/Tung Sol 6V6s).

...or buy some bias probes, they'll save you time & money in the long run.

MWJB · 11-30-2009, 11:57 AM

Did you use 2x100ohm resistors as a virtual centre tap for the heaters, or did you use a heater winding centre tap on the PT?

chris shaw · 11-30-2009, 12:01 PM

hi MWJB
used red/ yellow wire from hammond transformer as centre tap
Thanks for answer re resistors for current, will look to find bias probes for sale in UK

bluesfreak · 11-30-2009, 12:24 PM

Chris, you can get a probe kit at Tube-Town - Hot Stuff cool sounds. However, installing a 1 Ohm Resistor between Pin8 and GND on the power tubes is basically the same and it will tell you (based on ohms law) the exact current in mV (e.g. 48mV equals 48mA). There is also a real good bias FAQ in the Info/DIY section of tube-town...

11-20-2009, 09:10 AM	#1
chris shaw Member Join Date: Sep 2009 Posts: 51	no sound from an AB763 clone hi am completely new here so please excuse my ignorance I have built a single input ab763 clone without the trem circuit , hasmmond transformers JJ valvesand a hammond reverb tray I have about 5.9 volts dc to ground to heaters, 330v ac to pins 4 and 6 of rectifier tube (GZ34) all voltages from here seem a little high (but i have a cheap meter) as follows first preamp tube (nearest to input) 2.3 v to cathode, (sschematic says 1.3) 187 tio plate1 (schematic says 170 ) similar to pin 6 , 1.3 v to pin 8 reverb tube is 450v to pin1 (schematic 410)and only 4.2 v to pin 8(schematic says 8.7) the BIG DIFFERENCE in second preaamp trube - reading 316 to pin 1 (schematic 170), 5.2 v to pin 3 (schematic dsays 1.3)and 330 to pin 6 - schematic says 180 v last 12ax7 readings before 6v6 are 190 to pin 1 and 6 and 80v to pin 3 and8 so are ok I think 450 v dc to pins 3 of each 6v6 (schematic suggests 415) 200v to pin one of last 12AT7 (schematic says 170) I AM GETTING NOTHING BUT A SLIGHT BUZZ from speaker and when guitar goes thro its just a faint distorted noise BUT I CAN HEAR REVERB TRAY VIBRATING A TREAT WITH VOL UP, nothing to tray with vol down please help It is driving me to drink Chris (sheffield uk) no about 190 to pin 6 of same tube and 82 volts dc to pin 3 (schematic says 77v) I have po

11-20-2009, 11:05 AM	#2
bluesfreak Member Join Date: Jun 2009 Location: Germany Posts: 96	Any pics and which schematic did u use? w/o that its difficult to do an analysis __________________ I can fix everything, where is the duct tape?

11-20-2009, 02:24 PM	#4
Amp Kat Senior Member Join Date: Jul 2006 Location: Baton Rouge,LA Posts: 1,062	I have built the same circuit with a Deluxe Reverb AA763 Tremelo stage only except in a tweed chassis. I had to use 20k dropping resistors because the 10k were just yielding to high of a plate voltage on the preamp tubes. This got all of the voltages almost exact which refers to what MWK was saying. What I found and even with 15+ years of amp experience was some wiring issues that I had overlooked and I would bet that is part of you're problem also. Check all of the voltage nodes on the schematic (A,B,C,D,E,FZ,X ) I mean everything and make sure they are there at idle with no signal (quiescent point) . Check your pot connections real well and make sure the pots are all wired right. The best test you can probably do after that is insert a tone with a generator to the input and lift the wire from the treble pot right after the 250pf tone stack cap and attach a wire or test clip to it and send it to connection of the .001uf / 220k right at the junction of the PI. Any sound ? If not you either have problems before that first tube or after the PI and isolation troubleshooting should cut you're work in half but if you can't get sound there you won't get it anywhere else so that's a first. Let me know as mine sounds great but still has some hum in the reverb I have to find. __________________ KB

11-20-2009, 06:19 PM	#5
chris shaw Member Join Date: Sep 2009 Posts: 51	indebtedness gentlemen(sexist assumption?) I have neither done a build nor done a forum before. I am thrilled that experts across the pond and in europe are helping me !! bluesfreak - I'm too embarrased re soldering and cable neatness to post photo here!! but circuit is https://taweber.powweb.com/store/6a20_layout.jpg except only 1 channel and no vibrato will try over weekend but its daughters birthday so busy! will post early next week you are all very kind cheers Chris

11-20-2009, 09:34 PM	#7
bluesfreak Member Join Date: Jun 2009 Location: Germany Posts: 96	Chris, have you tried to unplug Reverb Pan and touch the cable feeding back to the Recovery Tube? You should have decent Hum if not there is a generic failure in the circuit. However, since this is your first build doublecheck all resistors/caps for correct values, Pinout of the tube sockets in clockwise from the bottom of the socket! YOur Voltages on V1 seem ok to me but confuision starts for me on V2 as you stated that you have built this circuit without the Vibrato Channel but the Schematic refers to it. So how did you modify the circuit? Did you leave the Vibrato Input Branch with V2/V5 out of the circuit (need to modify bias supply then!) or what di you exactly? I suspect that you made a mistake around the cutoff of the vibrato input.. __________________ I can fix everything, where is the duct tape?

11-20-2009, 12:30 PM	#3
MWJB Old Timer Join Date: Jun 2006 Posts: 1,926	"the BIG DIFFERENCE in second preamp tube - reading 316 to pin 1 (schematic 170), 5.2 v to pin 3 (schematic dsays 1.3)and 330 to pin 6 - schematic says 180 v" Doublecheck you have the right values for plate & cathode resistors, check that the cathode resistor & cap have a solid ground connection. Your preamp voltages are low, a BF deluxe is usually around the 220vdc mark. A little off because your B+ is high, What plate current do you have? With the power on & all volumes/tone controls up, probe power tube plates, should hear a faint "ffsst!" in the speaker, now move on to PI tube, and so forth...by the time you get to the input tube you should need to turn the volumes down as probing should be VERY noisy! We're trying to see where the break in the signal is. Last edited by MWJB; 11-20-2009 at 02:29 PM.

11-20-2009, 07:30 PM	#6
chris shaw Member Join Date: Sep 2009 Posts: 51	https://taweber.powweb.com/store/6a20_layout.jpg above is schematic as you can see the dropping resistors (the ones before the plates?) are all 100k so I dont know how that affects things, really grateful for help , will try the bypassing tip tomorrow

11-20-2009, 11:23 PM	#8
chris shaw Member Join Date: Sep 2009 Posts: 51	bluesfreak i just removed the bit vibrato circuit , from the cathode cap and resistor of pin 3 , through to the cathode cap and rsistor of pin 8, I therefore only had one wire going to the bias circuoit ads the one from pin 1 of the vbrato bit of the circuit didnt exist. I then just had a 'Y' connection going into the PI , as hte 'x element of the circuit was not from the reverb/ vibrato channel (I didnt include it in the build I suspect you are saying 'no, no, no' please ell me what i have done that I cant do !!

11-21-2009, 07:53 AM	#9
chris shaw Member Join Date: Sep 2009 Posts: 51	hi have just read your last post in cold light of day and realised i havent answered your point no i didnt remove the v2 - v5 part of the circuit, i removed the v1 part i.e. the channel with neither vibrato nor reverb , and then removed the v5 part of the remainder, leaving just v2 , v3 and v4 before the PI i suspect i have a bias problem as a result? also what do you mean by 'pin out of the tube sockets clockwise from below ' I've wired them all pin out clockwise from above - I e , as I'm looking down from above , pin 1 is the first clockwise from the space on the socket - that seems to make sense and tie up with socket numbers and photos of other builds have I even got that wrong?!!

11-21-2009, 07:55 AM	#10
chris shaw Member Join Date: Sep 2009 Posts: 51	hi ampkat make sure they are there at idle with no signal (quiescent point) what does the above mean please?"

11-21-2009, 09:00 AM	#11
bluesfreak Member Join Date: Jun 2009 Location: Germany Posts: 96	I should have been more specific: Pinout of the sockets is clockwise if you look on it from the solder (bottom) side!! and CCW if you look from the top (Tube Insert) so the Weber Layout shows it correctly if you look from the solder side. Ensure that you didn't make a mistake there as you will most likely then have blew tubes already. Looking at the Schematic its getting a bit clearer to me now so you went from V2B Pin8 to GND via R26/C9 ? This is a shared cathode R so when you use it with only one tube I would recommend to double its value to 1.5k. What about the mixing Rs R35/R36, did you get rid of them and the addtl coupling cap C14? The bug might be sittin here... __________________ I can fix everything, where is the duct tape?

11-21-2009, 10:26 AM	#12
chris shaw Member Join Date: Sep 2009 Posts: 51	progress tapping plates yielded noise from 6v6's and pin 6 of the second preamp valve (interestingly less noise from pin 1?) nothing earier in circuit re the r26 /c9 - I dont really know the terms but I reckon you must mean the 820 resistor which takes cathode of both normal and vibrato pin 8 cathodes - I need to double it?-- will do also re r35/36 I dont know what you mean but I assume you mean the point where X and Y signals from normal and vibrato channles join? answer is no I just didnt put anything on x -- ( as I have no normal channel!) should I have removed the two 220 k resistors asnd the 0.001mf capacitor?) I did a similar thing re bias connection from vibrato bit of circuit which I left out - is that why I got -60 instead of -35v on bias input?! re cutting bits out to test circuit I tried connecting output from plate of first preamp tube pin 1 direct to point Y immediately prior to 220k resistor at start of PI-- there was nothing -- but i could hear lots of input into reverb tray ? re plate current I dont know how to measure it-- I put a probe on the plate, one to chassis and dialed up current on meter, but it faded to 0 fairly quickly from every point - am I doing something wrong or is ithat meaningful? in summary it sounds like power amp stage is ok One question from the readings- the drop in voltage from the 'D' supply in the schematic going to the first preamp plates across the two 100k resistors drops to 180v and yet the same power supply 'D' feeding the second preamp stage only drops to 310 - how does that work or shouldnt I ask ??!! back to you chaps !!

11-21-2009, 03:21 PM	#13
pdf64 Senior Member Join Date: May 2006 Posts: 610	Dropper resistors are the resistors in the power supply section, 'in the doghouse' compartment of a regular fender. They connect the B, C, and D nodes, dropping some Vdc and, in conjunction with the power supply 22uF high voltage caps, decoupling Vac on the power supply. The 100k resistors connected to pins 1 and 6 of the pre-amp tubes are 'plate load' resistors, they turn the current flowing through the tube into voltage. 're plate current I dont know how to measure it-- I put a probe on the plate, one to chassis and dialed up current on meter, but it faded to 0 fairly quickly from every point - am I doing something wrong or is ithat meaningful? ' Don't try to measure current like that, it's a sure way to damage your meter. Voltage is measured in parallel with the circuit under test, across 2 points, whereas current is measured in series, you have to break into the circuit and put the meter so that it bridges the break. A current meter is effectively a short circuit. Hopefully you didn't plug your test leads into the current sockets on your meter, or maybe you blew a fuse in your meter.

11-21-2009, 03:45 PM	#14
chris shaw Member Join Date: Sep 2009 Posts: 51	learning through embarrassment hi pdf 64 thanks, I said I was a beginner, will try meter tomorrow, feel stupid but am learning all the time, I really appreciate your help thanks for your time and knowledge Chris

11-21-2009, 06:31 PM	#15
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,366	Actually if you want to measure current through the 100k resistor, or any other resistor, leave your meter on volts. measure the voltage across the resistor. Divide that voltage by the resistance. The result is current. That is classic Ohm's Law, one of the most fundamental relationships in electronics. I keep a little calculator next to me at all times for exactly this reason. Volts - V Resistance in ohms - R Current in amperes. - I V = I x R = Ohm's Law SO then I = V/R For example, if the B+ supply is 300v and the plate of the tube sits at 150v, and that plate load resistor is 100k, then we have 150v dropped across that 100k resistor. So we know that Ohm's Law says the current through it is 150/100,000, which is 0.0015 amps. .0015 amp is the same thing as 1.5 milliamp. SO we know there is 1.5ma current in my example, without having to break into the circuit. Of course we could disconnect one end of the plate load and insert out current meter in series, but that would be clumsy and dangerous. Current meters are very useful, but in other situations. I wouldn;t have a meter without it. __________________ Education is what you're left with after you have forgotten what you have learned.

11-21-2009, 07:15 PM	#16
chris shaw Member Join Date: Sep 2009 Posts: 51	thanks enzo I did physics before university and i knew i'd use it one day V+ IR brilliants, I was worried about having to disconnect everything to measure stuff!

11-21-2009, 09:11 PM	#17
chris shaw Member Join Date: Sep 2009 Posts: 51	plate currents to first pre amp tube voltage drop across first pre amp tube plate resistors was 357- 220 on pin 1 and 357-228 on pin 6 (should they be different?- could be measure error)so current to each is 1.5 ma to reverb tube -- i dont know how to measure without disconnecting so havent done that , voltage to red wire of rev transformer is 450v, similar to blue wire returning to pins 1 and 6 of tube to second pre amp stage voltage drop across 100k resistors to next preamp stage is 357 - 327 = 30 divided by 100,000, so plate current =0.3 ma to PI tube voltage drop across 100k resistor to pin 6--390 -215= 175 divided by 100000 = 1.75 ma , drop across 82 k resistor was 390- 217= 173 divided by 82,000 = 2.1 ma do they sound right? all tubes in by the way

11-22-2009, 09:53 AM	#18
chris shaw Member Join Date: Sep 2009 Posts: 51	plate currents -------------------------------------------------------------------------------- to first pre amp tube voltage drop across first pre amp tube plate resistors was 357- 220 on pin 1 and 357-228 on pin 6 (should they be different?- could be measure error)so current to each is 1.5 ma to reverb tube -- i dont know how to measure without disconnecting so havent done that , voltage to red wire of rev transformer is 450v, similar to blue wire returning to pins 1 and 6 of tube to second pre amp stage voltage drop across 100k resistors to next preamp stage is 357 - 327 = 30 divided by 100,000, so plate current =0.3 ma to PI tube voltage drop across 100k resistor to pin 6--390 -215= 175 divided by 100000 = 1.75 ma , drop across 82 k resistor was 390- 217= 173 divided by 82,000 = 2.1 ma do they sound right? all tubes in by the way

11-22-2009, 11:20 AM	#19
pdf64 Senior Member Join Date: May 2006 Posts: 610	'to second pre amp stage voltage drop across 100k resistors to next preamp stage is 357 - 327 = 30 divided by 100,000, so plate current =0.3 ma to PI tube' There's a problem there. Do you have any voltage on the grids of that tube, pins 2 and 7? Did you double check that the cathode resistors and bypass caps on pins 3 and 8 are correct, see post #3?

11-22-2009, 12:00 PM	#20
chris shaw Member Join Date: Sep 2009 Posts: 51	current thro second pre amp stage thanks PDF Will check grid voltage and also cathode res and cap connections what I do have, as stated in previous post, is a click/ thud when probing pin 6 of the second preamp tube , although the noise from pin 1 is substantially less -- just a little click, ----nothing when probing first tube and reverb tube, but with reverb pot turned up I can hear the springs vibrating quite a bit in the reverb pan sat on the bench. I am getting a lot from your interventions , thanks chaps will report back ps what should grid voltages be on the second preamp tube and also what should the plate current be through the same tube ?

11-22-2009, 12:10 PM	#21
chris shaw Member Join Date: Sep 2009 Posts: 51	just to clarify the words 'to PI' quoted in my 'plate current' post were the heading relating to the numbers which followed it, so the current to the second pre amp tubbe were the 0.3 ma, whereas current to PI was 1.75ma. sorry if my failing to put a line between second preamp current details and PI heading and subsequent current details thrust this explains but will assume everyone understood anyway

11-22-2009, 02:07 PM	#22
pdf64 Senior Member Join Date: May 2006 Posts: 610	Grid voltages should be 0. All those 12AX7 stages should have very similar operating conditions, so the voltages ( and therefore currents) should be within 10 or 20%.

11-22-2009, 04:40 PM	#23
chris shaw Member Join Date: Sep 2009 Posts: 51	never did such a buzz sound so sweet Gentlemen Of course you were right , I checked all the connections and I had connected the 2nd preamp cathode cap and resistor serving both sides of the tube to the grid instead Having resolved this I now have (330-210) /100,000 =1.2 ma on the 2nd pre amp plate--- at least I now have a buzz through the speakers, which, right now , feels like a success!!! HOWEVER it is only a buzz, which with reverb turned full on becomes a fairly large buzz. - guitar is heard v distorted and faint , slightly more with reverb full on question1 - will my bad connection have damaged anything before it was resolved? question 2 how do I start finding source of buzz nd get it to make a guitar noise?!!! question 3 I am using 1 m copper single strand cable as preamp ground across baclk of all pots and for preamp cathodes and preamp power cap -- can it be too big? other observations bias varies from -61 to -29 across pot range so have turned it to -35. i have 0v on all grids EXCEPT I have 24v on grid of PI , which, when grounded gets rid of buzz/ hum totally the amp doesnt seem very loud and poking plates of v1 and reverb valve creates some but not a massive noise any more advice would be much appreciated I couldnt have done this so far without you chaps

11-22-2009, 06:23 PM	#24
pdf64 Senior Member Join Date: May 2006 Posts: 610	Check out the Jack Darr link on this site Links to amp building forums and resources see his section on tracing a signal Plenty of other good links there also, Aiken especially. Rather than set the bias level by measuring the voltage from the pot, you ned to monitor the current going through the power tubes with no signal. This is normally done by putting a 1 ohm resistor between each power tube cathode and ground, then measure the mV across that resistor. See Aiken for more detail. Have you got something you can use as a signal generator, such as a keyboard with a jack output (ie headphone). Feed a constant signal into the amp input and measure the resultant Vac in the signal path. That should enable the problem area to be identified.

11-22-2009, 06:46 PM	#25
chris shaw Member Join Date: Sep 2009 Posts: 51	using signal generator hi PDF I guess biasing is a way away judging by the buzzing, but i'll do it this way when time comes dont have keyboard, cn get one. How does the measuring thing you suggested work? sorry for basic questions

11-22-2009, 07:03 PM	#26
bluesfreak Member Join Date: Jun 2009 Location: Germany Posts: 96	YOu could also pull V1 first and then check if the buzz goes away, if not you got the problem more deeper in the circuit so pull the next tube. If its gone you found the part causing the buzz. For biasing the poweramp it would be more advisable to measure current as pdf already mentioned, just dialing in a voltage will not tell you if the tubes do run correct. However, you didn't tell us if you had the tubes hooked up correct, "I've wired them all pin out clockwise from above " is not correct! Also, I recommended some changes according to this schematic: https://taweber.powweb.com/store/6a20hp_schem.jpg , have they been done or not? We need to get a common base on information, questions and answers, otherwise the discussion will end without a working amp... __________________ I can fix everything, where is the duct tape?

11-22-2009, 07:54 PM	#27
chris shaw Member Join Date: Sep 2009 Posts: 51	pdf will check the links as described, review thoroughly and come back but will be next week thanks chris

11-22-2009, 08:22 PM	#28
chris shaw Member Join Date: Sep 2009 Posts: 51	changes to circuit bluesfreak yes all sockets are correct, and thanks for schematic link, I understand doubling the cathode resistor valuefrom 820 to 1500 at c2b r26 and removing the mixing resistor r35, r36 and mixing cap c14 of v6 PI tube, will do and then do tube removal process. will report back later this week thank you

11-26-2009, 09:29 PM	#29
chris shaw Member Join Date: Sep 2009 Posts: 51	big progress but new big question - big hum Gentlemen the connection from power caps to power tubes pin 6 was, ahem, missing it is in now and the mods around the removal of r35, 36 and c14 plus doubling or r26 from 820 to 1500 all done. I get a rude buzz with vol at 0 but this dissappears as I turn up vol to about 2 - and is replaced by good tone. HOWEVER if i remove jack plug with vol turned up a little the signal is replaced by a deeper tone- sounding almost moog like . If I replace the plug the signal comes back and moog tone dissappears If I use jack plug with no signal the moog tone remains - i.e. a jack plug with a signal seems to be good at removing the moog tone. However I also have the bnuzz when the vol is turned down, which only disaapears as I turn up the vol as stated ps when I turn treble right up it gets all warbly and feedback- ish any ideas gentlemen?. I used a cd as the signal, when working it sounds great!!! voltages are as follows v1 - remember there is no v1 in this version, nor is there an v55 as there is no vibrato) v2 pin 1 188v plate current 2.5ma, pin 2- 0v, pin 3 - 1.7v , pin 6 -203v, (plate current 1.4ma) pin 7-0v pin 8 -1.59v v3( reverb tube) pin 1 -404v, pin 2-0v, pin 3-7.63v, pin 6-410v pin 7-0v, pin 8-7.6v. v4 (make up tube)- pin 1-207v(plate current 1.7ma), pin 2-0v, pin 3-1.6v, pin 6-207v (current 1.7 ma) pin7- ov, pin8--1.5v v5 not fitted v6 pin1- 234v, pin2-32.5v, pin3-57v, pin6-2.229v, pin7-37v , pin8-57v supply voltages- A-405v, B 402v, C 338v, D 300v power tubes- both identical pin 1 -minus 35v, pin 3 -397v, pin 4- 406v, pin 5 -minus35v, pin 6 -407v

11-27-2009, 09:11 AM	#30
MWJB Old Timer Join Date: Jun 2006 Posts: 1,926	V2 plate current 2.5mA? Confirm cathode & plate resistor values & tube type...this is not right for a 12AX7 with 1.5K/100K. Voltageat pin 1 should be similar to pin 6. Is your preamp filter cap grounded to the input jack along with the preamp cathodes? It should not be grounded with the main & screen caps. PI filter cap should either be grounded halfway along the control panel, or also to the input jack...wherever eliminates hum. You say you have -35vdc at power tubes pin 5, what plate current do you have? What method did you use to ground the amp (brass plate, buss wire, grounds soldered to chassis or bolted)?

11-28-2009, 10:09 PM	#31
chris shaw Member Join Date: Sep 2009 Posts: 51	progress on ab763 build big hum correction re v1 pin 1 plate current-- the voltage drop across the 100k dropper is about 120v so current is 1.2mA as expected in answer to grounding questions - preamp filter cap is grounded at input ground, as are pots, and preamp cathodes, tried grounding PI at PT, half way along controls and input, big hum remains DONT KNOW PLATE CURRENT TO POWER TUBES-- DONT KNOW HOW TO MEASURE IT! most important/ relevant info - when I lift the output from tube 1 at the 250 pf tone stack cap and connected to the inverter input - the hum remained,, -- I then completely disconnected the input at the PI and the hum remained-- so its in the PI or the Power section!!! its buzzing but if i turn it up to 1 or 2 it turns into a moog growl, which dissappears when I put a signal through heeeelp please!

11-30-2009, 11:56 AM	#32
MWJB Old Timer Join Date: Jun 2006 Posts: 1,926	"DONT KNOW PLATE CURRENT TO POWER TUBES-- DONT KNOW HOW TO MEASURE IT!" Add 1x 1ohm 2W resistor between each power tube pin 8 & ground, make sure resistors are exactly 1 ohm, make sure you have a good ground connection. Read the voltage at pin 8 in mV <200mV range, reading converts directly to mA. Should be less than 30mA per tube (around 20mA per tube if using Russian Reflector/EH/Tung Sol 6V6s). ...or buy some bias probes, they'll save you time & money in the long run.

11-30-2009, 11:57 AM	#33
MWJB Old Timer Join Date: Jun 2006 Posts: 1,926	Did you use 2x100ohm resistors as a virtual centre tap for the heaters, or did you use a heater winding centre tap on the PT?

11-30-2009, 12:01 PM	#34
chris shaw Member Join Date: Sep 2009 Posts: 51	use of heater winding centre tap hi MWJB used red/ yellow wire from hammond transformer as centre tap Thanks for answer re resistors for current, will look to find bias probes for sale in UK

11-30-2009, 12:24 PM	#35
bluesfreak Member Join Date: Jun 2009 Location: Germany Posts: 96	Chris, you can get a probe kit at Tube-Town - Hot Stuff cool sounds. However, installing a 1 Ohm Resistor between Pin8 and GND on the power tubes is basically the same and it will tell you (based on ohms law) the exact current in mV (e.g. 48mV equals 48mA). There is also a real good bias FAQ in the Info/DIY section of tube-town... __________________ I can fix everything, where is the duct tape?