Deluxe Reverb strangeness

[Chuck H]

Ok. Any modifications. Did it get "blacfaced" or have the small power tube grid caps been cut out, etc? The DR didn't have the same problems as other models, but they still added those grid to ground caps to the design. Probably to mitigate oscillation in shipped amps because the older wiring wasn't being duplicated.?. But I don't really know.

I'm surprised the cap job didn't solve it. I still think it's an oscillation though. If the amp has been modified it's possible some grounds have been moved?

I'll have to give this another read through tomorrow.

[Randall]

Checking the filter cap grounds now, they all look good, less than 1 ohm to chassis. It is a 1973 DR.

[Chuck H]

Were all the filter cap ground lead terminations checked and/or redone? What year Deluxe Reverb is this?

[Randall]

Well, darn. I replaced the filter caps, and the oscillation is still there in channel 2. It hasn't gone in the weak snarly condition yet, but I expect it will.

Now what?

[Enzo]

That^^^

If I had a bunch of values, I'd put the largest one first. But you are talking 16 versus 22, and that is such a tiny difference as to not matter.

[Chuck H]

I got the go ahead to replace the filter caps. Anyone care to chime in on where to put the 22uF? I'm thinking screen node.

Sure. Why not.?. Though I don't think it matters. Back in the day those caps had a -20%/+80% value spec!!! Whatever modern part you're putting in will be spec with the original design regardless of where the 22uf goes. As to where it might benefit the circuit most? I dunno. I'd probably put the youngest cap in your collection in the D position in the hope that it will be the freshest and most viable. Not sure how long you store your electrolytics before using them.?.

[Randall]

I got the go ahead to replace the filter caps. Anyone care to chime in on where to put the 22uF? I'm thinking screen node.

[Chuck H]

Notice that the vibrato channel has like phase gain stages sharing a filter node. The normal channel doesn't.

I'm not sure what you are saying here. It looks to me that both channels as well as the reverb tube recovery are served by the last filter cap D.

The reverb doesn't generally ascribe to the same phase considerations as the main (dry) signal path because there are multiple phase angles in the reverberated signal and it's rendered quite dissimilar from the main signal.

Both channels are not used by the same instrument simultaneously in this design. We can ignore unused gain stages because there's no signal on them. If you follow the phase inversions in each channel you'll see that the normal channel only has one inversion before the phase inverter (which is supplied by the C node). The vibrato channel has two inversions of the dry signal, so there are like phase gain stages filtered at the same node. If that node isn't filtering there can be like phase interaction and that can cause oscillation. In fact like phase interaction is how oscillator circuits are made.

[Randall]

The bandwidth of my scope is 20MHz.

And I stand corrected, the Sprague Atoms in the can are marked 9-99, so they are over 20 years old. I will lobby for replacing them. I already did the cathode caps, the bias cap is an F&T, I'm not too worried about it.

I hope this gets it because this condition only happens sporadically, maybe once every 5 times it is powered up cold, so it is a time glutton, and is clogging up my bench. Not that I have all that much work right now anyway.

I have only four 16mF/475v caps on hand, if you were to replace one of the Spragues with a 22uF, which one would you pick? Or does it really matter?

"Notice that the vibrato channel has like phase gain stages sharing a filter node. The normal channel doesn't. "

I'm not sure what you are saying here. It looks to me that both channels as well as the reverb tube recovery are served by the last filter cap D.

[Chuck H]

I'm going to second Helmholtz line of reasoning here. I was actually thinking it earlier, but wasn't quite sure enough to post.

Notice that the vibrato channel has like phase gain stages sharing a filter node. The normal channel doesn't. This would explain why you're only experiencing the problem with the vibrato channel and support the probability of bad filter caps.

You can try paralleling a good cap with the last preamp filter cap. I'll bet it stops the problem. And...

I know it's not your MO to change caps that aren't detected as bad. But let's try to rationalize this. If it turns out that replacing the preamp filter solves the problem I think you should just perform a full cap job on the amp. Unless there's some weirdness in there like THAT particular cap is the only original cap.?. If the caps are all of the same make it's entirely probable the other caps are not sound. They can test fine on a tester and still not be snuff in the higher voltage environment of that amp. The amp may even perform ok even if the other caps are failing or close to failing since the other nodes share antiphase circuits.

It just seems counter productive to me to have an amp open with ONE of the filter caps is exhibiting failure and not replace them all. Including the bias supply caps since they're part of a critical circuit that prevents catastrophic failures!!! They ARE age sensitive parts. But it happens all the time. Someone finds 'a' bad cap and replaces 'the' bad cap. Amp "works" and away it goes. Chances are good the amp could "work" a lot better and more reliably if a full cap job were performed.

JM2C

[Helmholtz]

If it is filter caps, why does it only effect the vibrato channel, but not the normal channel?

Oscillation often involves more than one amp stage and I think that the reverb circuit is involved.
So especially check the screen filter cap and its ground connection. You could temporarily wire a known good cap in parallel.

When I turned it on just now went into failure, and the reverb was also effected, scratchy and barely there. Also, in failure mode the 45K wave almost disappears into about 0.1v of hash. I probed the reverb send tube plates and it made no change, but it snapped back to life when I probed the B+ at the power supply side, and the 45K wave returned.

One possibility is that the amp toggles between 2 different oscillation frequencies, the higher one being above the bandwidth limit of the OT.
What is the bandwidth of your scope?

[Randall]

Scope settings are 5 uSec/division and 0.1 v/div.

I have more data. When I turned it on just now went into failure, and the reverb was also effected, scratchy and barely there. Also, in failure mode the 45K wave almost disappears into about 0.1v of hash. I probed the reverb send tube plates and it made no change, but it snapped back to life when I probed the B+ at the power supply side, and the 45K wave returned. I also swapped the preamp tubes before powering up to rule them out.

If it is filter caps, why does it only effect the vibrato channel, but not the normal channel? And anyway they have been done fairly recently.

[Helmholtz]

Please always post scope settings with scope pics.

A 45kHz signal at the output is clear evidence of oscillation. Prime suspects are typically filter caps.

[Randall]

I'm not sure about the reverb, I missed it last time around. But have done the whack and probe test, no change.

However, following Hemholtz lead, I do find what I think is a 45K wave at the speaker jack, plugged into the vibrato channel (the one in question), and channel vol turned up to 4. If my calculations are correct, I measure around 22 uSec for a complete wave. 1/0.000022 = 45, 454 Hz. Correct? No such wave appears with the normal channel plugged in and dialed up. Keep in mind, this is after the condition had reset, and the amp sounding normal. I will try to get the same measurements next time it fails. My problem here is when I touch a probe to some things, it resets, but not others.

The wave disappears if the channel volume is turned full off, if the guitar is turned full off, or if either grid on V2 is grounded. So far moving leads has made no change.

Am I barking up the wrong tree here, or is Hemholtz onto something?

[Helmholtz]

I may be wrong but I suspect that the amp is at the verge of instability and that the observed effects are caused by HF oscillation.