no sound from an AB763 clone

[chris shaw]

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]

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

[chris shaw]

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]

'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]

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 ?

[chris shaw]

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]

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]

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

[pdf64]

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]

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

[bluesfreak]

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]

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

[chris shaw]

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

[chris shaw]

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

[MWJB]

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)?