Problem with vibrato in Silverface Twin Reverb

[bobshbob1]

Well, new optocoupler and now the vibe is now working like a charm...starting 30 seconds after the standby switch is thrown. Once it gets going you can turn it on and off, even turn the amp on and off, and it keeps going, but after being left overnight it takes a bit for the vibrato to really kick on the first time. I know I've read about this issue before, but I can't seem to find a solution just from a quick search. I'm thinking caps now, but not sure which ones, cathode or the ones in the feedback loop or both...?

[überfuzz]

Ask your friend if he thinks the 30 sec warm-up is a big deal. Chances are he don't...

[patlaw]

I'd like a second opinion or two before I spend $10 on a new one.

Where did you order the optocoupler from?

[bobshbob1]

Antique electronics supply, it was actually $8.95.

[g1]

I'm thinking caps now, but not sure which ones, cathode or the ones in the feedback loop or both...?

Both cathode and loop.
And it is possible the old opto was just marginal and may work with new caps.

[bobshbob1]

No luck. All new caps in vibrato circuit, same issue. Also different tube, same thing. The upside is it has to sit overnight, like 8+ hours, before it needs to "warm up" again.

Searching around here and elsewhere I've not found a definite solution for this, just some reports of a new tube and/or caps and/or roach and/or 1-meg R's (which I also replaced because they had drifted a bit high) making it go away, as well as a handful of experienced techs saying it's just a quirk that you have to live with. So I'm going to leave alone for now. I don't think it'll be an issue.

Thanks for the advice as usual, this forum is always helpful and informative.

[g1]

You are confident about the speed pot condition?
Also, there was discussion recently about the trem "kickstarter", and it was mentioned that for this circuit, it comes from the neg.bias voltage via the 2.2M resistor. So make sure that one is ok.
And finally, if you are using the footswitch, try another, or try a jumper. Stranger things have happened.

[bobshbob1]

That's interesting; unfortunately the amp is no longer with me so I can't check the speed pot for sure, nor the 2.2-meg R. The speed pot seems to work just fine, meaning it controls the speed, but that's all I know. The 2.2-meg resistor measured good so I left it alone (the other two were about 10% high).

I actually saw that thread you mentioned and thought it was not related to this issue, but I think I get how too much resistance there could prevent or delay that initial "kick" of voltage to get the oscillator started. This amp lives with a shorting plug on the vibrato footswitch jack, though, with the intensity turned down when not in use. So the negative voltage is there all the time... I never actually tried it with a footswitch, could the lack of a "kick" from the switch closing cause the oscillator to not take off immediately...?

[keithb7]

This thread is interesting to me. I have a 71 Super Reverb that is slow to kick in vibrato also. Seems as though if the amp sits off for a while stored voltage bleeds out somewhere? Then when activated by the footswitch takes 30 seconds to kick in when the amp is first fired up.
When using the amp throughout the night, after initial power up and kick in of vibrato, it seems to work fine. Turning it off and on as needed it's instant. For a while there, I discovered that by clicking on the vibrato footswitch before powering up the amp, after it had been sitting unused for a few days, the amp would power up and the vibrato would already be pulsing. Problem is I couldn't be sure if I turned the amp off last time with the vibrato left in or not. With no indicator on the switch I screwed up many times. It's a pain when you are gigging. You have to remember to kick in your vibrato before the first set to make sure it's warmed up and ready to go for the song that requires it.

I would truly like to get to the bottom of it. Reason is I have a 64 Bandmaster with what I believe is a similar vibrato circuit that kicks in instantly as needed. Hot or cold, whenever.

I have completely rebuilt the Super's vibrato channel. 3 new caps. New resistors. New cathode cap. New RCA jacks. New tube. New roach. Re-soldered all connections. New foot switch. No improvement on kick in time as described above.

I have heard mention of the fibre board becoming conductive. Claims have been made that this is the reason for the delayed engagement. I am interested in looking further into this possibility. Maybe I will open it back up and take some voltage readings to ground on the board around the vibrato components. Also I hear of playing the amp opened up in the dark. Vibrato engaged. Look for small arcs of light.

[TimmyP1955]

I find it customary for one to test every switchable function of an amp that he's doing to use, as well as to make sure it sounds right, before the downbeat. Do that, and your problem is solved.