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Old 28th April 2014, 04:04 AM   #1
VMUNIX is offline VMUNIX  Argentina
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Default pushpull 6aq5 distortion

Opened This one some time ago but most of the problems you guys helped me to solv were either missprints or mistakes from myself.

Now I get power, no hum, no oscilations, it just distorts the sound in a way that high frequencies are somewhat modulated by a lower frequency. (gets worse at high volume with high bass compensation) I made some modifications to the feedback and the way it connects adding the 100ohm resistor, replaced the 4k7 from the feedback for a pot to adjust it, and also tried bypassing the 6c4 by connecting the grid of the 6c4 to the first half of the 12ax7 to bypass the first stage and the tone control , no help.

Click the image to open in full size.

Two things
- The cathode of first 12ax7 is not bypassed by a capacitor.
- And the transformers, were purpose built for this (unknown reputation) but I calculated the ratio by connecting 100v AC to the primary and measure the secondary, if the secondary is properly wound and has 8 ohms the secondary has 5k7 plate to plate, could this contribute to the distortion so bad?
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Old 28th April 2014, 05:23 AM   #2
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Hi
I am only a beginner with valve amps but, Pin 7 of each of the 6C4 valves looks to me a more appropriate place to apply the feedback to rather than the 12AX7

Cheers / Chris
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Old 28th April 2014, 10:29 AM   #3
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Quote:
Originally Posted by Chris Daly View Post
Hi
I am only a beginner with valve amps but, Pin 7 of each of the 6C4 valves looks to me a more appropriate place to apply the feedback to rather than the 12AX7

Cheers / Chris
No. Three reasons:
1. Phase. Good if you want to create an oscillator. You would then need to swap OPT primary connections.
2. Tone controls in the feedback path, BIG no-no.
3. Increased chance of instability.

Reason number two precludes the others.

Last edited by cogsncogs; 28th April 2014 at 10:33 AM.
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Old 28th April 2014, 01:13 PM   #4
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I wonder if it could be local oscillation in the output stages, in which case grid stoppers
and screen stoppers on the output tetrodes could help. How about trying 4.7k grid stoppers and 1k screen stoppers?
I can't see where connection 'A' goes to? Does it replace the ground at the centre tap
of the filament winding to give some 'heater elevation'?
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Old 28th April 2014, 01:58 PM   #5
Ketje is offline Ketje  Belgium
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Some ideas , other side idem.
Mona
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Old 28th April 2014, 02:17 PM   #6
VMUNIX is offline VMUNIX  Argentina
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Quote:
Originally Posted by Malcolm Irving View Post
I wonder if it could be local oscillation in the output stages, in which case grid stoppers
and screen stoppers on the output tetrodes could help. How about trying 4.7k grid stoppers and 1k screen stoppers?
I can't see where connection 'A' goes to? Does it replace the ground at the centre tap
of the filament winding to give some 'heater elevation'?
I will try the stoppers, I see that in all "modern" designs, the "A" used to be connected to the heater winding, the original design didn't include a center tap, I grounded the center tap and left "A" alone.
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Old 28th April 2014, 05:00 PM   #7
kevinkr is offline kevinkr  United States
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How about some measurements first and a few pix of the amp before we all run off and try to fix a problem that we do not understand?

Is this a scratch build, or is it a rebuild of an existing amp? What speakers are used with this amplifier? Are you asking it for more performance (power, bass extension) than it was designed to provide?
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Old 28th April 2014, 08:24 PM   #8
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remove the feedback resistor and cap or short 100 ohm resistor to ground
to eliminate any NFB.
test
then check and record the 12ax7 concertina voltages again
the concertina showing 60 ish volts accross it may be starved for voltage
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Old 28th April 2014, 08:48 PM   #9
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According to the measured voltages shown, the bias of the concertina triodes seem to be (76-83)=-7 volts and (82.1-89)-6.9 volts. This is too negative and would put a 12AX7 well into cut-off at quiescent conditions. This would cause severe distortion
for anything more than a small signal.
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Old 28th April 2014, 09:33 PM   #10
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Malcolm Irving View Post
According to the measured voltages shown, the bias of the concertina triodes seem to be (76-83)=-7 volts and (82.1-89)-6.9 volts. This is too negative and would put a 12AX7 well into cut-off at quiescent conditions. This would cause severe distortion
for anything more than a small signal.
Oh shure,that's why the anode voltage is that low
Mona
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