Trem Problem with AB763 Deluxe Reverb-Low Volume

[Sasquach]

First real post.
I built a DR from a Weber kit a year or so ago but I didn't notice this problem until recently. I included the mod for trem and reverb on both channels from robinette's website. (0.047uF cap and 220k resistor to isolate each channel)
This Weber kit includes a built in switch on the trem pot to disengage the trem circuit output for higher volume. I'm not sure if I have ever turned the switch off before now. I understand that the trem pot attenuates the signal because of the 50k pot to ground, hence the switch. The problem is that when the trem is off (switch in the off position) and the amp is turned on, the output is very low. If I engage the trem the volume is restored and turning off the switch increases the volume even more. Why would the volume be low when first turning on the amp with the trem switch off?

Order of events to try to explain this better:
Trem switch off---->Amp powered----> Result: Very Low Volume ---->Trem switch on----> Result: increase in volume----> trem switch off---> Result: higher increase in volume

I also have a homebuilt VVR for fixed bias installed but it is bypassed for troubleshooting purposes.

My plan is to use my scope to trace the signal from the input until I see where the signal drops. I may also use an external switch but the pot switch seems to be working fine. I could keep the switch on all of the time but that defeats the purpose of the switch.

Suggestions?
Thanks

[pompeiisneaks]

Do you have the schematic of the mod? It would help greatly.

It's possible that with the switch off, on startup, that tube isn't conducting, and therefore somewhat 'cold'/'off' and that causes some impact to the tone, once 'enabled' it starts conducting and bootstraps the tube to be more 'operational' and 'shutting it back off' doens't put it back in that state. This type of behavior sometimes can be caused by a missing ground reference somewhere else...

Also photos of the amp would greatly help. especially around the mod.

~Phil

[Sasquach]

The mod can be found here:

https://robrobinette.com/AB763_Modifica ... Reverb_Mod

What tube are you referring to that isn't conducting? The preamp tube?

This picture was taken before the amp was finished.

Mod pic:

[pompeiisneaks]

I didn't know 'what' tube was doing anything since I didn't have a schematic or any details.

I don't specifically see what could be the problem, and I'm confused about the switch since I don't know if the original had one, other than on the footswitch, so I don't know how that's wired either.

in theory any of the tubes that are 'on that switch' could be causing this type of thing, but I don't know for sure, and it could be something else. Maybe others will have a good idea where to look next.

If you have a link to the original schematic pre mod as well, with the panel switch you have mentioned that's from the weber kit, that may help as well to compare the circuit you built originally to the modded version.

~Phil

[sluckey]

The Weber 6A20 schematic shows the switch. It's just a simple "boost" mod that can be found all over the net. Works on any AB763 with roach tremolo.If wired according to the Weber schematic even with robrob's mod I don't see how you could have the symptoms you describe.

[Stevem]

If you uncouple either channel one at a time does the issue still manifest itself?

[Sasquach]

Mod schematic- There is another 220k between C15 and the same node

Switch closeup

Weber schematic
EDIT: Sorry, had the wrong schematic

[Sasquach]

If you uncouple either channel one at a time does the issue still manifest itself?

That's what I plan on doing later today. Thanks

[Sasquach]

I isolated the channels one at a time and the problem is still present in both channels. I isolated each channel at the junction of the mod.

Something else I noticed. I switched the trem on and then off again to get the boost. After about 30 seconds, the output dropped back out. This hasn't happened before. Once it dropped out I switched the trem on and then off again and got normal volume and it dropped out again after about 30secs. This was consistent each time I tried it. Seems like it's a heat issue. I'm going to check all of the grounds but I'm not sure how they can be causing this. I'm also going to swap some tubes around and see if something changes. Any other ideas?

[pdf64]

That Weber switched pot is rubbish, at least mine was, the switch failed after a few uses.
It did me a favour really, the trem off tone / boost was too much.
I was much happier with a 100k lin trem intensity pot. That provided a couple of dB boost compared to a 50k pot, but without being excessive.

[Sasquach]

That Weber switched pot is rubbish, at least mine was, the switch failed after a few uses.
It did me a favour really, the trem off tone / boost was too much.
I was much happier with a 100k lin trem intensity pot. That provided a couple of dB boost compared to a 50k pot, but without being excessive.

Thanks pdf64. You nailed it. I disconnected the switch and everything worked fine. I also replaced the 50k with a 100k with no switch.

I re enabled the VVR and everything is working like it's supposed to.

Thanks everyone for the help.

[Stevem]

Good news to hear!
When I do the Trem kill mod in silver face Fenders with a master volume I disconnect the gain boost on that master volume and use that switch section for the Trem mod.
I then use that shelled cable removed to replace the non shielded feed from both channels to the 220k mixers.

[Sasquach]

Good news to hear!
When I do the Trem kill mod in silver face Fenders with a master volume I disconnect the gain boost on that master volume and use that switch section for the Trem mod.
I then use that shelled cable removed to replace the non shielded feed from both channels to the 220k mixers.

I may use one of the "bright" switches to do this. I'm using one of them to switch feedback resistors between 820 and 1.5k so I can use the other one as a boost switch. Thanks