Reverb problems

[docz]

Hello guys, long time! I wish you all a happy new year.

Allthough this is not a tube amp related thing, it is still an amp thing

I bought a broken down Marshall Valvestate 8080. It is sort of a hybrid amp with a ECC83 tube in it. I've managed to fix everything in it except the reverb. I had to replace all the pots and some caps in it, and now everything is working like it was new - but there is no reverb. The reverb pot only adds hum and hiss. When I unplug the RCA plugs and touch the input with my finger I get noise, so that tells me the amp get's a signal. When I plug them into the tank, I get hum and hiss.

I took out the tank and started poking around. Everything looks ok, the small wires are working, and I get 200ohms between - and + on the output, and about 32ohm on the input. All springs seem ok. I don't get any resistance between - and the chassis on the input, but I do get 1 ohm > on the output.

Any ideas on what is wrong? Should I just get another tank? Or should I investigate further?

The tank is an Accutronics 8DB2C1D

DocZ

[TUBEDUDE]

The springs might be stretched out. Some surgery can help. Can you
see if the magnets are in the magnetic loop? The coil bobbins may be loose? If so, shim with a thin chopstick wedge.

[briane]

should be able to rca jack output to input. That will give you an overdrive stage instead of reverb.

If it works and sounds ok, then the problem is a bad tank. Its often just a corroded jack or tiny broken wire.

valvstate - I dont envy your tone. Last one I saw I almost threw out the window it sounded so bad - and thats when it was working perfectly.

[alvarezh]

Briane's suggestion is excellent and a quick one to perform too.

I 've got a perfectly working (almost new) ATV50……. it's my loaner. They're always happy to bring it back!

All the best.

[docz]

I tried connecting the two RCA together. The amp screamed like a banshee, and gave a pretty harsh distortion when playing. Scream and distortion was still present - but faint - when the reverb pot was turned two 0.

About the valvestate sound, I kinda like it for a practice amp, particularly the OD1 sound.

DocZ

[alvarezh]

Doc, I only work on tube amps, so I'm not going to be of much help, nevertheless I am under the impression that the tank is probably good.

Like Briane said, this has become a powerful overdrive stage, so a little distortion is understandable. Now, the screaming should not happen. I would guess you have a feedback problem, most likely caused by the reverb's recovery circuit.

I am under the impression you don't have a scope, if so, use a signal tracer (easy to make in case you don't have one) and listen to the signal at the output to tank RCA jack, it should be clean. If it's not, your problem lies with your driving op-amps. If it's clean, then your problem is on the reverb's recovery circuit. Of course, do this without the tank nor the jump cable connected to the RCA jacks, leave them open.

BTW I am looking at this schematic here:

http://www.drtube.com/schematics/marshall/pc0689.pdf

I am highly suspicious of the C45 220n cap on the above schematic. But, I may be way off.

All the best.

All the best.

[tubelectron]

OK - I may have not seen it the above posts, but : did you tried to swap input and output cables of the reverb ? I too often service bad reverb/no reverb sounding amps which have the tank connected in the wrong sense...

A+!