AC10 SRT Clone - Tremolo thumping issue

[mwaller]

Hello folks!
I recently completed building a clone of an AC10 SRT using the following schematic:

AC10_SRT_001_zpsejn5ubuv.jpg

I made a few minor modifications. Most notably, I replaced the reverb circuit with the 1-tube reverb circuit discussed here:

https://el34world.com/Forum/index.php?topic=7957.0

Here is a picture of the completed chassis:

Chassis Complete.jpg

After a few minor tweaks, everything is now working well except the tremolo. At certain speed and depth settings, the guitar signal seems to decline and a pronounced thumping sound is heard. The thumping seems to happen when either control is around 75%. Strangely, if I turn the depth control all the way up, the thumping goes away!

How do I correct this?

Thanks!

[pompeiisneaks]

I don't know this amp circuit super well, looking over the schematic, I'm a tad confused, but thumping in a tube driven circuit often is either a bad tube, bad connections, or bad caps in the phase shifting portion. On this one, I don't see exactly which one(s) would do that, as the schematic is drawn in a really hard way to understand.

That being said, have you gotten the thumping active and tried chopsticking around to see if it impacts the sound? (Use a 100% wood chopstick so you don't electrocute yourself )

~Phil

[mwaller]

Thanks for the comments!
I agree... the schematic is terrible.
C8, C9, and C10 and the caps in the phase shifting circuit.
I hadn't thought to try a new tube in V1 - I'll give that a shot. I'll grab a chop stick while I'm at it!
Thanks!

[pompeiisneaks]

I also am not 100% sure, but it may be that the footswitch routes that grid connection to earth to disable the tremolo, and I see you've got a switching jack that seems to be shorting the switch to ground, so that may be blocking the trem from working? Have you tried inserting a non connected jack into it to see if that resolves it?

It may be better to use a non switched jack and then setup the footswitch to ground it and add a manual grounding 'defeat' switch on the panel somewhere?

If I'm right, that is. It may be that the grounding creates a grid reference for the bias and makes the tube conduct and work, so I could be way off

~Phil

[mwaller]

The tremolo definitely works, but it thumps badly at certain control settings.
I believe the 220K resistor provides a ground reference for the grid of V1b; without it, the oscillator probably won't work.
I actually don't even have a foot switch to test this function out...! But its currently wired with a shunt-tip jack that connects it to ground.
Thanks!

[pompeiisneaks]

Ok cool so my second thought that I was mistaken was correct I did hit that as a theory as well, thus why I said to possibly ignore me

I wasn't 100% sure from your original post, that the trem worked other than the thumping noise. So it works but also adds the thumping noise in certain situations. Got it.

~Phil

[mwaller]

Hey, if you're batting 500, you're doing GREAT!
I'll report back after I try a few things.
Thanks!

[mwaller]

I tried a 5751 tube in the V1 location. The thumping is still present at higher intensity settings.
Does this point to a component problem? Should I replace the caps in the oscillator circuit?
Thanks!

[pompeiisneaks]

I don't know. Are those NOS WIMA caps or current production ones? If they're NOS they're supposed to be rock solid, but I guess maybe there's still a chance one or two are bad. Did chopsticking hint at anything?

~Phil

[mwaller]

I still do need to do the chopstick thing - thanks for the reminder!
The caps are NOS WIMA TFF. I believe they are a film & foil style cap, which are inherently pretty robust. But they are old...
Thanks!

[pompeiisneaks]

Yeah I was more implying I don't know the current production line, but have heard the NOS are rock solid. I think new are still pretty well built. But now that I say that, I think the new WIMA's are rectangular blocks now not round like that.

It's still possible one of them has a fault, but often pushing on their leads a bit or on them proper will make it better or worse and indicate which one may have gone bad.

~Phil

[earl2]

Your schematic or layout would help. Hard To read the values from the photo.

[mwaller]

Here's a clearer interpretation of the tremolo circuit. Values are the same as shown in the first post of this thread.
Is there an easy way to reduce the "intensity" of the tremolo when it is set to max?

AC10 SRT Tremolo Circuit.JPG

[sluckey]

Put a 1M pot between C21 and RV5. Wire it as a variable resistor. Now set the Depth to max and adjust the other pot until you are happy. Measure the pot resistance and then replace with a 1/2w fixed resistor with that value.

[mwaller]

Cool, I will try that!

Can I also slow the tremolo down by increasing the value of the resistor that is in series with the speed pot?

Thanks~