Reverb pop when the amp is on, and long starting vibrato

[Strat76]

Hi everybody.
I built several Blackface amps and all of them have the same behavior. The last one I built is a Deluxe Reverb AB763.
First, I hear a light but audible reverb pop when I put the amp on. There is no pop of course when the reverb pot is on 0. No pop as well when the reverb is disengaged. It's not annoying but I wonder what can be the reason.

Second : The vibrato is very long to come for the first time once the amp in on. About 15 seconds. It's more annoying, especially when I am gigging.

As all my amps do the same thing, I can say without being pretentious that this is not a failure or a tube problem. The layout is very close to the Fender layout and I followed the schematic without any change. The only difference are the filter caps (22uf). Except that, the amps work very fine with no annoying hum nor oscillation.

As I already learnt a lot from you guys, I wonder if someone could give me some explanation.
Any idea ?

[Strat76]

Just want to clarify. The reverb pop comes just one time, when the standby switch is engaged. Thanks

[Enzo]

Have you ever turned on the light switch in your room and heard a little pop come from your radio or TV or stereo? Or maybe the furnace or airconditioner or water pump started in your home and it made a sound in some piece of audio gear? That is all that is happening. The small spark in your standby switch is amplified by the amp - picked up by radiation of the signal, coupling into your amp. I would consider that perfectly normal. Not every amp may do it, but many many will.


I don't knoiw why your trem is slow to start. It is an oscillator, it depends on low frequency, but it also depends upon feedback around that triode tube. If all your built amps do this, one thing comes to mind might be your selection of the three feedback caps. And also consider the cathode bypass cap on the oscillator.

[Strat76]

Hi Enzo, many thanks for your quick answer. I appreciate a lot.

The feedback caps are Orange drops but I remember that I used before Jupiter caps and I had the same problem. Maybe I have to use ceramic caps just like the original ones. The cathode bypass caps are Sprague 25uf/50V. Very common ones.

[Strat76]

No more ideas ?

[Enzo]

Not really. See if your circuit voltages make sense, try different caps. How about rem,oving the cathode cap just to see. etc.

[g1]

It seems very odd that 3 different builds all have the same slow starting vibrato. Seems like too much of a coincidence. Just to verify, this happens the first time the vibrato is engaged, even if you have been playing the amp for several minutes already? On all three amps?
Are you using the same footswitch for all and have you tried another? I would be looking for a part that is the incorrect value in all three amps or a wiring error that is the same in all three amps, surely it must be the same problem for all 3?

[Enzo]

I wouldn't even use a footswitch, other than to verify it wasn;t the problem. I would just use a clip wire inside the amp while under service.

I agree that three amps with the exact same symptom is too much for coincidence.

[Strat76]

Hi,
This happened with different footswitches. This doesn't happen with the Princeton builds. Only with AB763 models.
And as you know this is not the same vibrato circuit.

But I agree with you. I made several tests and I saw that when the footswitch is in a different area or if the wire is less close to the speaker, the start up is almost immediate. So I will investigate if the slow start up is relative to the ground quality of the footswitch wire. For me it makes sense.
I already rewired the footswitch and remade the grounding inside. So for me it is not depending of the footswitch itself.

But It is also interesting to see that a lot of people have the same problem on their amps. And I believe that nobody has solved this issue even after changing almost all the components of the vibrato circuit.
So the poor quality of the ground could be an answer.