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Tim Gagan
Page 1 of 3
([email protected])
2/11/2002 6:25 AM
Further adventures in oscillation
Hello,
Now it's in the reverb circuit in this amp I'm building. (The reverb is essentially Fender Blackface,
but for the schematic of the amp, see
http://gaganbros.com/images/ultimategregall.gif )
It's a very high pitched squeal that comes in when the reverb and the master volume are set fairly
high. If the reverb tank is disconnected the oscillation is much louder and lower in pitch.
It seems to be originating somewhere around where the signal comes back from the reverb tank.
I've tried moving the output transformer, the reverb transformer, the reverb send and return jacks
(which are close to the ot), shielding the tubes, etc etc, but the squeal remains. Also, with a .002
cap to ground from pin 2 of the recovery stage(parallel with the 220K resistor, like some Silverface
Fenders), the oscillation is the same as when the tank is disconnected, much louder and lower in
pitch.
Any thoughts on how to remedy this?
Thanks,
Tim
http://www.gaganbros.com
The World's Only Double Doubleneck Lefthand Band
From:
Date:
Subject:
Jan
([email protected])
2/11/2002 9:46 AM
Re: Further adventures in oscillation
Tim.
Just guessing here, but this worked for me:
Try adding a grid stopper to the reverb recovery stage. A 33k soldered on the tube socket cured a
similar symptom in my amp.
Good luck.
Jan
From:
Date:
Subject:
MBSetzer
([email protected])
2/11/2002 7:09 PM
Re: Further adventures in oscillation
Hi Tim,
These things can get frustrating, I have no real answers, but maybe a few things to try.
Not so sure how the reverb recovery triode might be getting so much worse with an open input
when you have a suppressing cap attached, as an experiment you might be able to replace the
220K with a 250K linear pot (wired as a variable resistor) to see how close to ground you need to
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Page 2 of 3
get to kill oscillation now. Then further experiments might be able to have their success gauged by
how much closer to 250K you can crank it. You would want the pot case grounded to keep the
resistive element from picking up its own interference, and shield the wire running from the grid to
the pot, and from the reverb recovery jack to the grid as well. If the reverb return cable in
particular is not perfect, especially the ground connection, it can be a problem. If you are relying
on chassis ground for the reverb jacks, you might need to run a wire from the return jack shield to
the same point where the cathode of the recovery triode is grounded.
Maybe there is insufficient power supply decoupling, I would try adding ohms between points D &
E, also probably E & F might also be audible if you increase from 1K to about 2K or 3K at least
between these nodes. The diodes should be no problen, but if I was suspicious I would disconnect
them temporarily.
Also, the plates of the reverb send triode (I assume 12AT7) are connected to the screens of the
EL84's, if it werent for the 470ohm stoppers and the small reverb transformer primary they would
be directly coupled. I would probably prefer the plate supply for the reverb driver to come from
point C.
Could be unrelated but the vibrato tube now also has a plate in common with these, this should be
OK by itself to an extent, especially if you move the reverb transformer to point C, but later you
may want to listen to the difference when you add another dedicated RC filtering stage between
the screen supply and the tremolo plate supply :)
Also, just checking, but on the reverb transformer have you got the blue wire to the plates and the
red one to the power supply? Then also you should have the black output wire grounded and the
green one is the hot output. Even so it might be worth a try just to reverse either the primary or
secondary temporarily for testing.
This Twin Reverb II I've been working on has a slightly evolved reverb circuit but it does seem hard
to tell any difference from the BF as far as reverb goes, look at that schem in the Tube Amp Book
or the '83 Concert (last page in Volume 3) or I could email it to you, it has a 10K 2W CC between
the power supply and the red lead of the reverb transformer, this definitely cuts down on the stress
for the 12AT7 and still sounds killer. If you are using much lower B+ 10K may be too much, but
after this I now like the idea of having something there. The only change I did here was to replace
the 680ohm cathode resistor with a 2200 for improved tube performance, this one does not have a
bypass cap on the reverb driver, I would try removing that also, you really do not need the
maximum gain from these parallel triodes. Then they have a 560pf ceramic 3KV cap from the
parallel plates to the parallel cathodes, this seems counter-intuitive but it has been OK since the
amp was built and so far I have not removed it to listen for any differences.
Really make sure the ground connection on the 220K at the grid of the recovery triode is grounded
excellent and in the right place in the star grounding scheme of things.
Looks like there is an auxiliary output or input into the mixing triode after the recovery triode, even
if this is shielded, as an open input it can work as an antenna picking up higher level signals from
other places in the chassis, depends on how much gain there is whether there will be a problem.
Also, the conventional wiring of the reverb recovery and the following triode look familiar, and it
would also be conventional to footswitch off reverb by grounding the grid of the recovery triode, on
the bypassing arrangement you have, any interference from open inputs at open switch terminals
will be bypassed too, but even when reverb is on, these might still be sensitive if they are not
shielded on their way to & from the switch, plus the two leads running from the switch to the
reverb area of the schematic could couple especially well if they were parallel or not short & direct
enough.
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In the power amp, the triode switch and plate voltage jacks can be an insidious problem. When its
in triode mode, point B still has its screen supply wire running to the switch, but is an open lead
and such close proximity to the large power tube fields can input trash noise to the power supply
as it acts like an antenna, OTOH, in pentode mode, the signal fluctuation on this screen supply lead
(with added length to reach the switch) can be naturally large and the longer unshielded wire can
then act as a broadcast antenna sending interference in phase with an earlier stage more
effectively. Same with those plate bias jacks, one time I wanted the equivalent for plate voltage
monitoring, but plate leads are the strongest broadcasters in the amp and it was instant oscillation
when I added them to a previously stable amp. I added 100K resistors to the plates to isolate the
leads which ran to the panel jacks, it stopped the oscillation and I could still read accurate plate
voltages through them but this would not be feasible for shunt bias readings. Actually it only
stopped the oscillation until I added another triode gain stage later, then that put it over the top
and now I want minimal length OPT leads twisted on their way to the plates. I wouldn't even trust
the jack on the primary center-tap now with some of the gain levels I am working with.
Maybe this will give you some ideas, but it may just be one simple thing . . .
Mike
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