Earth bassman 4x12 Low volume/dirty tone issue

[AnnMar Studios]

Hey guys,

My first post here, but have been reading for a long time.

I have a Earth SR bassman 4x12 and the sound coming out of it is really low in volume and very distorted.

Could someone give me a list of what to diagnose first? I know and understand the saftey issues, I know how to read schematics enough and I understand how to use a dm. I just need some help in how to start diagnosing this issue.

[Enzo]

First, is the speaker OK? Play some other amp through the speaker. And don't ignore the speaker wiring.

Make sure the speaker is plugged into the main speaker out jack, not the extension speaker jack - it matters.

ANd what part of the amp? Does this have the slave out jacks or recording out jacks? COnnect them to some other amp. How does that signal sound? That is the preamp out.

And if it really is the amp itself...


I don't much care what circuit we look at, if it doesn;t have good power supply, it won't work right. SO start by looking at power. Is your main B+ something like 480v? Give or take 20? And flip the meter to AC volts to measure the ripple. Is ripple at most a few volts?

Is there B+ at pin 3 of every power tube? is ther B+ at every pin 4 of the power tubes?

Pin 5 of the power tubes is the bias. You got somethinglike -45v on each grid?

And the small tubes, is there some reasonable B+ on each plate - pins 1 and 6? About 220-250VDC I'd guess.

[AnnMar Studios]

hey enzo,

thanks for responding. I will check everything you have said and get back to you. the amp only has 2 tubes, 6L6's its one of those hybrid solidstate/pwr tube amps.

I appreciate the help, this one has stumped me. I know my way around an amp pretty good for repair but these solidstate/tube deals are not my strong point.

i will post my results later today.

[Gtr_tech]

This amp is called the "Revival". Sounds like there's a problem in the front end somewhere. Possibly a bad output transformer if its been run real hard. Here's a page with the schematic:
Prowess Amplifiers - Misc - Schematics - Revival.jpg - earth revival 212 combo

The repair tech you'll prob'ly end up taking it to will find it helpful to have that info.

[AnnMar Studios]

Hey ,

I actually have the schematic already and its the 4x12 version.

It looks like the caps are bad. I put it on the ocisilloscope and it does appear that the caps are bad.

I will be digging into this and finding more out about it.