Hiya all,
I am helping a friend to build a Marshall 2204 clone and while everything has gone relatively smoothly thus far, yesterday we ran into trouble.
The build is essentially finished and we already tried it out successfully couple of weks ago: everything worked fine and the sound was great.
Now my friend wanted to add some led lighting for the front panel, so we built a simple diode rectifer based on this article, drawing power from the tube heater. Unfortunately, there was a short circuit (legs of two capacitors touched) and a bit of smoke emanated from the rectifier. But we acted pretty swiftly, the amp was on only for few seconds and we turned it off immediately when we saw the smoke.
However, after the incident the amp has started to oscillate. Depending on the speaker (my friend has a couple of drivers at hand) the noise either a high-pitched squeal or a low rumble. Normal guitar signal is coming through and the oscillation is not affected by the master volume.
The oscillation stops, when we drop the bias to about 15mA (the correct value at 60% dissipation is about 35mA), but enything above that and it starts again.
There is one more piece of info: we have implemented a half-power switch (we have been using this Ceriatone layout for our project) and the oscillation only appears with full power mode. When the switch is set at half power, everything is working fine.
So all this leads me thinking that the probem lies with the screen grids of the power tubes. Could one or both tubes have been damaged during the rectifier debacle? Would new pair of tubes fix this issue or is there something else we should check out?
I am helping a friend to build a Marshall 2204 clone and while everything has gone relatively smoothly thus far, yesterday we ran into trouble.
The build is essentially finished and we already tried it out successfully couple of weks ago: everything worked fine and the sound was great.
Now my friend wanted to add some led lighting for the front panel, so we built a simple diode rectifer based on this article, drawing power from the tube heater. Unfortunately, there was a short circuit (legs of two capacitors touched) and a bit of smoke emanated from the rectifier. But we acted pretty swiftly, the amp was on only for few seconds and we turned it off immediately when we saw the smoke.
However, after the incident the amp has started to oscillate. Depending on the speaker (my friend has a couple of drivers at hand) the noise either a high-pitched squeal or a low rumble. Normal guitar signal is coming through and the oscillation is not affected by the master volume.
The oscillation stops, when we drop the bias to about 15mA (the correct value at 60% dissipation is about 35mA), but enything above that and it starts again.
There is one more piece of info: we have implemented a half-power switch (we have been using this Ceriatone layout for our project) and the oscillation only appears with full power mode. When the switch is set at half power, everything is working fine.
So all this leads me thinking that the probem lies with the screen grids of the power tubes. Could one or both tubes have been damaged during the rectifier debacle? Would new pair of tubes fix this issue or is there something else we should check out?
i made 5 or 6 time ..this tube guitar amp .this is classic el34 push pull.if oslition happen fist remove feed back connection.when remove feedback if stop oscillations....change tow input opt frome plate of el34s in pin no 3 output socket . qesition: oscillation start by power on switch ?or by playing guitar ?
I am sorry, I do not quite understand... anyway, this is supposed to be a faithful clone of the original 2204, we have been using this schematic
and the aforementioned Ceriatone layout. I am a bit hesitant to make major changes to it, since we do not want to change the character of the amp.
Oscillation is constant, it starts when the standby is flipped on. It makes no difference whether there is a guitar plugged or if the master volume is off.
What hooman was suggesting is to reverse the plate leads from the primary of the output transformer. You may have faithfully wired a clone, but your transformer may be wired out of phase.
Disconnecting the negative feedback wire is easier for test though. If disconnecting the NFB makes the oscillation stop, then your primary wires are backwards.
meanwhile, you removed the failed power supply experiment, but afterwards had the new oscillation problem. SO are we SURE that the smoke ONLY came from that power supply circuit? or is it possible some part of the original circuit was damaged - for example a ground connection to something burnt off?
Disconnecting the negative feedback wire is easier for test though. If disconnecting the NFB makes the oscillation stop, then your primary wires are backwards.
meanwhile, you removed the failed power supply experiment, but afterwards had the new oscillation problem. SO are we SURE that the smoke ONLY came from that power supply circuit? or is it possible some part of the original circuit was damaged - for example a ground connection to something burnt off?
Ah okay, thanks. I´ll try that next time I can get my hands on the amp. The problem is that my friend is a busy pro musician and it will probably be a week or two before we can both find time to continue the build. It has already taken 4 months... But that´s why I´ll try to gather all the information I can in between, so I´ll have it handy when we continue.
That is a good point too, thanks. We did do a cursory check-up after the smoke show, but of course it never hurts to go through everything once again.
Success! I wasn´t able to do it myself, but I relayed this info to my friend and he disconnected the FB wire like suggested and the oscillation was gone. He then connected it back and switched the OT primaries and now the problem is solved. Massive thanks to both Hooman and Enzo 
However, he did say that even after the fix, he could still get the amp to oscillate by turning the master volume all the way up, so there is still something awry. Also, he said there is a small but noticeable 50hZ hum, but that might be a lead dress issue. It is a small chassis so things are a bit tight in there.
He´s leaving for a 2 week tour next tuesday, but we´ll try to squeeze in a couple of hours of work in monday. Any ideas about the aforementioned issues? Could the hum have something to do with the oscillation?
However, he did say that even after the fix, he could still get the amp to oscillate by turning the master volume all the way up, so there is still something awry. Also, he said there is a small but noticeable 50hZ hum, but that might be a lead dress issue. It is a small chassis so things are a bit tight in there.
He´s leaving for a 2 week tour next tuesday, but we´ll try to squeeze in a couple of hours of work in monday. Any ideas about the aforementioned issues? Could the hum have something to do with the oscillation?
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before finish your amp and fix all problem its better remove the feedback line.
hum is main symptom in tube guitar amp projects .so need some advise to solve hum .
- turn on amps ..no guitar connect to input .... master volume in normal condition . can here hum in speaker .... please short the legs no 2 of master volume to ground .....if hum stop ....means problem in preamp ...if hum cuntinio ....means power amp problem ! this is first step .
before finish your amp and fix all problem its better remove the feedback line.
hum is main symptom in tube guitar amp projects .so need some advise to solve hum .
Thanks,
I am not so worried about the hum, as I said it´s probably a lead dress issue or perhaps grounding was compromised somewhere during LED power debacle, as Enzo suggested. I´ll be wiser when I see and hear the thing myself.
But I am curious about the oscillating at full volume. I´ve been reading a bit about the feedback network and I am wondering if adjusting the feedback resistor could remedy the issue? Our OT is Hammond 1750N, if that helps.
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You are right that feedback is probably causing the oscillation, but it is probably not the "official" feedback network that's to blame.
When you cram an amp with high gain and high input impedances into a fairly small space, it's quite easy for some tiny fraction of the output voltage to couple back into the input, either through the air (wires a few inches apart pick up radiated electric fields), or through imperfect layout (ex. power ground shared with signal ground), or because the power supply decoupling caps aren't doing their job quite well enough.
This unwanted (and unplanned) sort of feedback is usually what causes the sort of oscillation you're describing.
If the oscillation is only at full volume, and you are okay with having slightly less gain in the amp, the simplest fix is to simply insert a fixed resistor in series with the volume or gain pot, sized to drop the gain of the whole amp just enough to stop the oscillation.
Beyond that, if not already done, I would start by using shielded wire for the input jack wiring, and also wiring to the volume and tone pots early in the signal chain. This drastically reduces the amount of unwanted signal picked up by these wires from stray fields, and may be all you need to solve the problem.
-Gnobuddy
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