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Distorting reverb tube
(09-03-2020, 06:32 PM)K O\Connor Wrote: The PA has a closed-loop gain of 55, so a 20V output at the speaker requires just 365mV at the PA input. This is where the 3M3||10pF meets the 470k grid-leak and the 470k from the reverb recovery stage. The attenuation through the dry path is about a factor of seven, so only 2v5 is needed from the preamp for the 20V at the speaker.

The AA1164 schematic I'm looking at looks a bit different - there's no grid leak on the PA input. I do see it in the AB763 circuit for example, though. I will add it then. 

(09-03-2020, 06:48 PM)K O\Connor Wrote: Is there a signal threshold where the reverb oscillation starts?

Yes, around 20Vpp. 

(09-03-2020, 06:48 PM)K O\Connor Wrote: V1A,B are cascaded stages and even Fender did not tie their grounds together. They did however tie the reverb driver cathode to the V1B ground, which is not problematic for their intended use of the amp and "reasonable" signal levels.

Good point, I will split those. 

(09-03-2020, 06:48 PM)K O\Connor Wrote: V3B trem should ground to Vs cap along with bias since the amp uses bias-modulation.

It does. 

(09-03-2020, 06:48 PM)K O\Connor Wrote: The concertina really should have its own filter since it provides a supply-referenced signal plus a ground-referenced one.

I think it does too, there's an additional 10K resistor after the 4th filter cap down the chain, and V3 has its own filter cap. 

(09-03-2020, 06:48 PM)K O\Connor Wrote: One other thing: Since the closed-loop gain of the PA is so high, at least diagnostically it might be worth shorting out the 47R to see if stability returns? In any case, you can also hear how that sounds. bass might be a bit looser.

I'll certainly try that. I also noticed that the signal at the V3B plate is a bit wiggly when the NFB loop is closed. 

I've been tinkering with the amp tonight and I still don't have a clue how to fix it. Looks like there's some bad interaction between V1 and V2 that starts above a certain preamp volume level. I'm pretty sure that V1B is misbehaving as well as the sine wave shape changes a bit, but it's not clipping. I guess the driver tube is the culprit. Disconnecting the reverb tank input immediately cleans up all signals. With no load on the reverb transformer secondary I can get a clean sine wave at the V2 plate. I even tried unscrewing one side of the reverb transformer and moving it around a bit, but it doesn't seem to have any effect on the reverb driver distortion.

The amplitude at the driver input is now actually much smaller - I previously operated the amp with the EQ lifted and I forgot to tie it back to ground.

I guess I'll start over and try to re-bias the V2 again, increase the value of the grid stopper and see if adding a small capacitor between the plate and the cathode improves the tube stability.
Looks like the problem is gone and the solution was pretty straightforward. I decided to try the 12AU7 as the reverb driver which was a move in good direction, but the tube would still start oscillating if the volume was too high. I then disconnected the cathode bypass capacitor and that finally stopped the oscillation and lowered the maximum signal swing to around 260Vpp with the stock Rk value of 2K2. I changed it to 1K6 which yields 300Vpp at the plate and just about enough reverb. The tube seems to still have some headroom as it wouldn't oscillate even with values below 1K, but I think I'll leave it as it is right now.
Hi Tomislaw

You should measure Vk and calculate the dissipation for the tube to make sure it is within its limits. 12AU7 is 5.5W total for both plate, which happens to be the sum of the individual plate ratings Smile ; 12AT7 is listed simply as 2.5W per plate.

Maybe you should try reducing the dry path signal levela bit, so the driver can be either tube and be stable while providing the reverb depth you want?

Have fun
Hi Kevin,

I made sure the tube is fine - it's drawing 8,5mA at 285V, dissipating 2,42W.

I did think about lowering the gain on two first gain stages or running the signal through a voltage divider, but not in this particular build. Apparently my friend for whom I'm building it enjoys the amount of distortion he's getting by lifting the EQ. The unloaded signal still upsets the reverb driver tube and the oscillation even increases the preamp distortion.

The amount of reverb is just about right with the current V2 bias, but in my first build it's way too much at 10. I will probably be rearranging my own "AA1164" because I built it more for the clean tone and I definitely don't need that much reverb it now has. Actually I didn't suspect the 12AT7 driver tube was oscillating in it because the reverb sounds totally fine and there's no distortion in the wet signal. I can easily sacrifice the over the top effect for the sake of tube stability. I will probably incorporate some simple solution to cut down the signal on the V2 grid to a reasonable level when the EQ lift switch is engaged. The grid leak resistor could probably become switchable to a lower value and form an appropriate voltage divider with the grid stopper. I'm sure TUT will come in handy in this matter Wink

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