Reverb Noise

[jmaf]

jmaf,
OK, I did it again. The resistor is 10 ohms, not 1K. The tank input impedance is 8 ohms so it should be close. No distortion with the resistor. I'm using the "standard" 500pf cap to couple to the driver stage.
I realize this thread has gotten long and wordy, so to recap... I tried a different reverb transformer, two sets of different type cables and three reverb tanks all with the same rsults. D'oh! I appreciate your help and thinking out loud. This is one of the weirdest amp problems I've come across. Thanks!
Dave

Dave, sorry, I suggested something you had already tested(the resistor and you even tested with a speaker), I was rereading now and just saw it.

So, continuing the out loud speculation:

1) With a 8 OHM speaker you went full volume and it did not distort at all, worked perfectly.

2) With a resistor it did not distort either. If our reverb tank was either of those, the problem was solved.

Our problem is now between that transformer secondary and the reverb tank.

You have more than one matching transformer there, right? The other one is known to be good. So, if you unsolder the current transformer secondary from the leads to the reverb jack, and then hook the other transformer's secondary as a load to this transformer instead of the speaker, does the distortion still happen on that matching 8 OHM connection? What about on the primary("the other side which would be our test transformer's primary under normal circumstances")? Can you run this test?

[Daver]

jmaf,
OK, I hooked up the extra reverb transformer's secondary as the 8 ohm load. I left the original soldered in place and connected the spare with jumpers at the tank end of the cable. No distortion. So, I hooked up the scope to the "load" transformer's primary (now being used as the secondary) and that also showed no distortion. It's almost like there is some weird interaction between the reverb transformer and the tank. Now, so far, I've tried two reverb transformers and three tanks. That "should" rule those components out, you'd think. I know the ground for the send cable is the sheild and ground "return" for the tank transducer. Without it you get no reverb. Every other amp I've built or seen that uses this type of reverb has that grounded right at the jack where it's mounted to the chassis.
Thanks for the insight.
Dave

[jmaf]

jmaf,
OK, I hooked up the extra reverb transformer's secondary as the 8 ohm load. I left the original soldered in place and connected the spare with jumpers at the tank end of the cable. No distortion. So, I hooked up the scope to the "load" transformer's primary (now being used as the secondary) and that also showed no distortion. It's almost like there is some weird interaction between the reverb transformer and the tank. Now, so far, I've tried two reverb transformers and three tanks. That "should" rule those components out, you'd think. I know the ground for the send cable is the sheild and ground "return" for the tank transducer. Without it you get no reverb. Every other amp I've built or seen that uses this type of reverb has that grounded right at the jack where it's mounted to the chassis.
Thanks for the insight.
Dave

Dave, I thought you'd get the distortion when an inductive load was present. You didn't. I think you've tried most of what I'd try....I'm really out of ideas. If you do solve this case please share the result with us. (Edit: not IF, WHEN you solve it)

Last thing I'd suggest is hooking up the tank improvisedly directly to the transformer, jumping the jacks and cable, see if the distortion remains. If it does, then you've definitely isolated the problem to an interaction between those two inductors.

Good luck.

[vikingz]

Troubleshooting.

It seems to me that you have built it wrong.Are you sure that you have separated the signal paths correctly,i´m thinking of the ground returns.-Even Fender had problems with this it seems.It is not easily copyable rightaway.-You seem a bit thick to me,if you ask me,a simple beginner.-Check your ground returns fo god sake..

[Daver]

jmaf,
I just tried using unsheilded jumpers to connect the tank bypassing the jack and cables. No return cable was attached. Same problem. This is not a ground hum problem. It produces a spiky leading edge on the waveform that eventually forms a second peak if the signal is high enough. This results in an odd distortion when you play the guitar loud or hard enough. The madness goes merrily on.
Dave

[Enzo]

OK, it works into a resistor. It works with clip wires instead of the cable.

Without trying to overthink it, my gut tells me there is a wiring problem. it tells me your ground and hot are maybe reversed. The input jack on the pan is insulated from the pan walls, right? CLip wires to the two wires from the reverb transformer secondary to the end of a cable running to the pan drive end. DOn;t ground anything there. does it matter which wire is grounded? I don't think it should, but does it? if it does, then there is a clue. If it doesn;t matter, then take another clip wire and try grounding one of the transformer wires, and then the other one instead. Make any difference which?

[Daver]

Enzo,
To clarify, when using the jumpers instead of the cable I still got distortion. No change. The tanks input is isolated from its' frame, correct.
I disconnected the reverb transformer from the amp connections. I used clip leads to the tank and no return cable. So there is no ground connection to the tank at all. I still get the distortion, but it's on the trailing edge of the waveform. ??? So, I reversed the leads and got exactly the same thing; distortion but on the trailing edge. I then connected the return cable so the tank was grounded to the amp chassis and that changed nothing. I still have the distortion.
I then did as you suggested and swapped the black and green wires on the reverb transformer. I used the black as the hot and the green as the ground. I still get distortion on the trailing edge. Does any of this make sense? Thanks for all the help.
Dave

[Daver]

Any more ideas? Anything else I can test? I'm starting to redo things I've already tried. I just don't understand why this thing distorts the reverb only when a pan is attached. I even tried paralleling the input to the tank with a 220K resistor for giggles. I'm out of ideas of what the cause could be or what to try. Thanks.
Dave

[trem]

Can you leave it alone for a few days? Then, with a fresh pair of eyes, check every component/solder joint from input jack to output jack. Forget about the reverb, just check for continuity and proper component value and proper schematic placement.
You may just have a simple anomaly somewhere.
Good Luck

[tubeswell]

Hi Daver

can you post a schematic?

(I still wonder if you need more padding - like a voltage divider and/or smaller (.002uF) coupling cap in front of the reverb driver, or maybe a local NFB loop there etc).

[Enzo]

Um, how large is the signal hitting the grid of the drive tube? You said somehting earlier about distortion appearing there too?

[Daver]

Thanks for the help, guys. I have tried padding down the signal by removing the cathode bypass cap on the second stage, paralleling the Dwell pot with a 1M resistor and removing the cathode bypass on the reverb driver stage. The input cap is already 500pf. I've included a quick schematic of the preamp. When the distortion appears at the reverb transformer secondary is when I can see some distortion on the grid. Again, this is not regular old square wave type of clipping. There is sheilded cable running to and from the Dwell pot. Maybe posting a picture of the waveform would help?
Dave

[Enzo]

You didn;t answer my question. How large is the signal at the drive tube grid when the distortion occurs?

[Daver]

Enzo,
When I start to see the leading edge of the waveform produce a spike at the input to the pan, the voltage at the driver grid is 18V peak to peak.
I hooked up my scope to my Super Reverb today and got a similar, but not as severe looking waveform when the volume was above 3/4. Maybe this is more "normal" than I thought.
Dave

[Enzo]

Look at the DC voltage on the cathode of the driver tube. Assuming the grid sits at zero DC like it should, your peaks are 9v less the cathode. Might be enough to get the grid conducting on those peaks and that would explain the distortion.