Working on a friend's 1972 Deluxe Reverb - vibrato question

[mlp-mx6]

Hello Friends,

I'm working on a 1972 DR and the vibrato circuit is doing something strange. At startup - once the standby is on, the amp 'thumps' 2-3 times per second for about 15-20 seconds, then settles down and works normally. This thumping coincides with the neon lamp in the vibrato bug flashing brightly. By the way, it isn't "ticking" - the amp was doing that also when I got it, but reflowing 3 bad solder joints (previous work by someone) fixed that.

Has anyone seen anything like this before? Any suggestions?

Thanks,
Michael

[Firestorm]

Try replacing all the caps in the LDR phase circuit with 3KV ceramics.

[Randall]

Hey Firestorm, can you explain why?

[Randall]

Another double post. Something is messing with me....

[Firestorm]

Hey Firestorm, can you explain why?

If the caps leak, they do all kinds of strange things. Disconnecting the footswitch is supposed to shut the tube off, but maybe physically disconnecting the intensity pot will help localize the issue.

[Structo]

Randall, do you have your mouse set to double click when you press the left mouse button?

[mlp-mx6]

I replaced the 3 series caps as suggested, no change. Exact same behavior - thumps for 15-30 seconds, then settles down and works just fine.

Any other ideas?

Thanks again.

[Milkmansound]

what about replacing the LDR bug? or the .1 coupling cap on the plate of V4A?

you could also pull the tremolo output from the junction of that .1 and the 220k resistor and see if the thump is still there

[mlp-mx6]

Yep, the LDR bug was next on my list, but I'll have to order one of those. Don't have a spare.

What's strange is how it settles down and works normally after about 20-30 seconds. That makes no sense to me. Tube changes for the vib changed nothing performance-wise.

[Firestorm]

Disconnect the wire from the wiper of the intensity pot and see if it still does it. This takes the whole trem circuit out of the signal path.

[mlp-mx6]

Yep, still thumps exactly with the neon flashes. It seems when thumping it isn't passing any signal to the output section, because the last thump goes through the reverb and sounds bigger - like the 'final thump of doom' or something. Also the tempo of the thump is not changed by the pot settings on the vib circuit. In fact, the tempo of the thump is not constant.

Let me also say 'Thanks' to Firestorm for the suggestions and help.

[JazzGuitarGimp]

I wonder if the power supply filter cap on the trem oscillator B+ node is bad.

[mlp-mx6]

I'll check it again this evening, but the caps under the doghouse appear to have been replaced not too long ago. Thanks.

[Firestorm]

Yep, still thumps exactly with the neon flashes. It seems when thumping it isn't passing any signal to the output section, because the last thump goes through the reverb and sounds bigger - like the 'final thump of doom' or something. Also the tempo of the thump is not changed by the pot settings on the vib circuit. In fact, the tempo of the thump is not constant.

Let me also say 'Thanks' to Firestorm for the suggestions and help.

So if it thumps with intensity pot wiper disconnected, the "signal" isn't coming through the PI but through the screen supply node where the neon side of the trem circuit. By all means check the screen filter, but it may be fine. Because of the way the standby switch is wired, closing the standby makes the choke ring. This should settle down after only one or two thumps, but I wonder if it couldn't set up a more persistent oscillation involving the neon bulb. I know nothing about neon bulb failure modes, but maybe replacing the bug is worth a try. Also make sure the 6V6 screen resistors are still in place. These serve to quiet the choke ringing (among other things).

[mlp-mx6]

OK, further exploration revealed that if I remove the 12AX7 for the trem the thumping still happens, but of course no flashing neon. This removes the trem osc. from the cause of the problem. Swapping power tubes didn't do anything either. Screen resistors still in place. This silverface also has the grid caps to ground, FYI.

Any suggestions?