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Distorting reverb tube

I'm having a bit of a problem troubleshooting a Fender type reverb - the 12AT7 driver tube output has a tendency to distort the signal. Originally with a bypassed 1K cathode resistor the tube was oscillating wildly. I increased the Rk value to 2,2K bypassed which stopped the oscillation but the top of the signal was still distorted. Removing the bypass cap obviously reduces the gain in the triode and I can make up for that by lowering the Rk value to 470R. At maximum V1 output volume I'm getting 154Vpp signal swing whereas with a bypassed 2,2K Rk it was around 195Vpp. At 154Vpp reverb sounds OK. Voltage drop at Rk is now 4,7V and plate voltage measured cathode to plate is 340V, so the plate dissipation is still within spec.
The signal at the grid is clean and undistorted. There's a 430K grid stopper and a 680K grid leak. Reverb transformer sees 360V.

Is there any other way of fixing the problem with the reverb driver?

Hi Tomi!

You have a lot of answers for your own question, are these things you do not want to do?

It could be that some resistiors drifted off value, having a much lower resistance than the official value.

Maybe the tube is bad, but I think you tried that already.
Changing to a 12AY7 might help, or a 12AU7.

You could try changing RA.


A 12AT7 can handle more current than a 12AX7!
Twice as much actually.
I don't know about 12AY7 or 12AU7, don't try that without checking!

Hi Strelok,

I was only curious if there was another workaround for this problem. As it turned out, the 470R cathode resistor eventually made the tube oscillate again, so I switched it back to the stock value of 1K without the bypass cap. There was still enough signal swing to make the reverb sound OK. I couldn't actually play with Ra as the tube drives a reverb transformer and both triodes are wired in parallel to allow more current through. The amp suffered from a number of other issues like random ground connections causing ground loops, and there was a really nasty one in the reverb circuit. It had a Vox AC30 4xEL84 power output section with one shared 56R Ck and 100R screen resistors. Each tube was dissipating ~15W of heat in this configuration.

Hi Guys

The 470 Rk value for the reverb driver is actually okay, but the effect on the tube gain may be problematic for given layouts In an old Fender chassis I used for trying circuits out in, the stock R was 470 even though the schematic suggested a different value.

It is important in the SE circuit to have a high-value good-quality filter cap for the related supply node. Remember, PSRR is nonexistent in such a circuit, so the supply has to be quiet on its own otherwise you get IM products into the reverb tank. In the worst case that can sound as if the reverb is wildly distorted and in the best case simply that the reverb is muddy. Obviously, a bad filter cap can cause a lot of supply-modulated bad sound.

You can scope the drive into the OT using a 100x probe (if using a DSO). Visual distortion of a sine wave represents quite a high figure of THD, but it's what most people have to go by. You can also send a signal into the return path to make sure that section is clean unto itself.

EL-34s dissipating 15W is no problem; if you meant EL-84, then it is a problem and Rk needs to be split (ideally) and/or made higher in value. Those 100R screen-stops will cause the amp to eat tubes.

have fun
Yes, of course I meant EL84's, my mistake. I split Rk/Ck and adjusted the resistor values to cool down the tubes. 168R per side took the dissipation down below 12 watts. I increased screen resistors' values to 1K and I don't think that changed the sound much.

The amp was an interesting case and I have learnt quite a lot working on it. I was only to switch the PT wiring to 230V but I noticed the amp hums a bit too much so I had to investigate that. There were many random ground connections and I had to convert everything to galactic ground. I then experienced all sorts of unpredictable behaviour when the amp would stop humming and buzzing one day and start over again the next day. Turned out the reverb driver tube was oscillating and upsetting the V1, which was quite misleading at first. I fixed that by installing a bypassed 2K2 Rk usually found in typical Fender-style reverb circuits. The tube stopped oscillating right away but the sound coming out of the reverb was noticeably harsh. I scoped the tube to see what's going on and found the distorted output. I then only had to find the optimal Rk value so there was enough signal swing to drive the reverb without distortion and the tube was not oscillating. Typical values didn't work as expected and the circuit had to be carefully tweaked to get decent performance. Three other 12AT7's behaved exactly the same way in this position so the problem was not caused by a faulty tube.

The offending tube was on the screen supply which was well filtered, but didn't have an individual filter cap as such.
Hi Tomislaw

The reverb driver needs decoupling from the screen node, or from whatever node it is powered from in any build. The SE driver seems more problematic in this regard, being sensitive to layout and where its power comes from, where the push-pull driver does not have that issue.

Have fun
Oh yes, of course, the Tx is RA.

Thank you very much all, the info in this thread is a very handy reference.
Hey Strelok

What does RA mean?

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