no sound from an AB763 clone

[chris shaw]

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]

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

[MWJB]

"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]

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]

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

[chris shaw]

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]

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]

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]

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]

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

what does the above mean please?"

[bluesfreak]

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]

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 !!

[pdf64]

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]

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

[Enzo]

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.