Twin Reverb Reissue with Reverb on Normal Channel

[sonicmojo]

Hello,

I've got this issue with an apparently STOCK Twin Reverb Reissue from the 2000s probably where the reverb has a hum when turned up and there is about 3/4 reverb effect when using the normal channel. Has anyone had this issue and know of an obvious place to look for where the fault may be? I've tried different tubes to no avail for the hum and I think the reverb hum is probably related to whatever is causing the reverb bleed.

Thanks!

[martin manning]

Can you learn anything by pulling tubes, V2, V3, V4?

[sluckey]

The stock reissue only has reverb and vibrato on the Vibrato channel. Look closely. Someone has been messing with that amp.

Here's the schematic...

https://el34world.com/charts/Schematics ... manual.pdf

[sonicmojo]

Thanks for replying and I wish someone had messed with it. Apparently it is the original owner and bought it new. I don't see any mods or see any solder joints affected on the upside down preamp control board. Attached is a pic if you see anything I don't.

If I pull V2 or disconnect the jumper for the Vibrato channel, still reverb. If I pull V3 (reverb send), barely any reverb on either channel but something very slightly there in the background, but I think that is normal sympathetic vibration happening. If I pull V4, no reverb as expected. I've tried different tubes in all those sockets. I think I even tried the PI and V5 just to see if anything helped tube wise. I don't think it is tubes.

If I turn volume down on Vibrato channel, no difference.

I'll be measuring voltages next. If I can find the fault that is causing the hum in the reverb, maybe that will point me better. Somehow, someway, some signal is getting there pre reverb send. I've been going through the schematic and the 220K mixing resistors check out R34/R34. Not seeing anything obvious. I'm wondering if something under that control board is amiss. I guess I will probably have to pull it and look.

As for the schematic, I don't get why TP15 says 4.2V. I get 200+ something as I would expect.

Thanks!

TRRI.jpg

[martin manning]

How about C11? If that is bad, the channels will talk to each other.

[sonicmojo]

How about C11? If that is bad, the channels will talk to each other.

Wouldn't have pulling the V2 tube fully disconnect any possible crosstalk there?

[martin manning]

Yes, you're right.

[sonicmojo]

Seems odd to me that R34 and C18 are in the order they are in. I don't think that is the problem but seems off.

[sonicmojo]

So the original reported problem with this amp was that it would not turn on. The fuse retaining clip appears to have been installed backwards and created bad connections and load and blew that fuse so I bent out the tabs, installed a fuse and it works again. Took me a few minutes to figure out why the fuse was good but not connecting. I can't imagine that has anything to do with the current reverb problem but thought I should mention. Maybe there is something in the control board connected that should not be or a bad ground somewhere up in there that somehow connects the normal channel to V4 and is also causing the hum. Could be this issue has always been there but the customer never noticed or cared about the hum. Who knows...

fuseretainer.jpg

[martin manning]

Nevermind.

[sonicmojo]

Could a leaky/bad filter cap create crosstalk like this?

[martin manning]

Are the filters original? Worth a try. The reverb driver is a couple of RC's away from the rest of the preamp, though.

[sonicmojo]

Yes, caps look untouched. I'll try that first, especially the 22uF caps that connect to the "X", "Y", and "Z" connections.

[sluckey]

As for the schematic, I don't get why TP15 says 4.2V. I get 200+ something as I would expect.

All those voltages inside an oval are signal voltage, not dc voltage. They are based on the 37mV @1KHz test signal applied to the grid of the first preamp ***AND*** having the controls set to a default setting. These test points are used to trace the test signal through the amp for troubleshooting.

[sonicmojo]

As for the schematic, I don't get why TP15 says 4.2V. I get 200+ something as I would expect.

All those voltages inside an oval are signal voltage, not dc voltage. They are based on the 37mV @1KHz test signal applied to the grid of the first preamp ***AND*** having the controls set to a default setting. These test points are used to trace the test signal through the amp for troubleshooting.

Thanks! I may have to do some signal insertion and tracing but will try a couple of filter caps first for the low hanging fruit.