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Post subject: Red knob "The Twin" is always distorted
Posted: Mon Jan 30, 2017 12:55 pm
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Location: SF, CA
I have a red-knob "The Twin" that is always distorted on both channels. Neither channel is ever clean unless it's played really quietly, like less than 1. Above that both channels are distorted in an unnatural and unpleasant way. It sounds horrible.

It's not the speakers - when I plug another head into the Twin's speakers they sound great. When I plug the amp into another set of speakers I get the same harsh distortion.

I had it serviced but it didn't solve the problem, the servicer really had no clue what the problem is. The service invoice says he "repaired the bias section, replaced grid and screen resistors on V10-11, replaced 6l6s, tested all tubes, cleaned all pots, jacks and tube sockets, replaced switching input jacks."

(I'd wanted to use another local servicer who's amazing but he won't even touch red-knob amps - he hates them because of the printed circuit board.)

The amp has sat around for years like this (ahem..2008/2009) since I have other amps to play but I'd really like to use it. I've got the time now so I'd like to dig into it.

Any ideas on how I can make this amp sing again?


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Post subject: Re: Red knob "The Twin" is always distorted
Posted: Mon Feb 06, 2017 3:15 am
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Location: Province de Québec, Canada
Amp need to see a qualified tech, not a handy man :(
Ask a refund because job is not done and the guy you see replace some good parts like input jacks :lol:

Be sure to test the amp with no effect plug in , guitar only and guitar in good working order.
If not, tests are wrong.

Check for bad tubes.
Phase inverter resistors.
Power supply voltage.
Output transformer


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Post subject: Re: Red knob "The Twin" is always distorted
Posted: Mon Feb 06, 2017 1:18 pm
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Hi stratele52, thanks for your reply. The input jacks he replaced did need to be replaced, we talked about that before he performed the work. On this model they tend to get smashed because they protrude from the face of the amp. I was just listing everything he did per the invoice. The guy is a legit guitar tech, he was referred by the other tech I've used who won't work on Red Knob Twins.

"Check for bad tubes.
Phase inverter resistors.
Power supply voltage.
Output transformer"

Tubes were replaced, and I've tried a few different pairs - JJ's, Svetlana's, even some cheap Chinese tubes I can't remember the name of (Ruby's maybe).

I'll look into this stuff. Thank you.

I was wondering if anyone had had a similar experience that caused such a malfunction.


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Post subject: Re: Red knob "The Twin" is always distorted
Posted: Mon Feb 06, 2017 1:39 pm
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Were all tubes replaced or just the 6L6GC tubes? What you are describing sounds more like input & gain stage issues. Usually these are driven with 12AX7 tubes.


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Post subject: Re: Red knob "The Twin" is always distorted
Posted: Mon Feb 06, 2017 1:57 pm
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I only replaced the 6L6GC tubes. He said he checked all the tubes - but who knows what that means.

Thanks for the suggestion! Yes. it has five 12AX7A 7025 tubes, and two 12AT7 tubes. I'll check those out.

I don't know schematics but it looks like it's time to learn about them. I'll research the input and gain stage. Thanks for your help!


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Post subject: Re: Red knob "The Twin" is always distorted
Posted: Mon Feb 06, 2017 3:35 pm
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ufront wrote:
The guy is a legit guitar tech, he was referred by the other tech I've used who won't work on Red Knob Twins.
.


I understand you bring amp to fix distortion and the guy don't fix it !
I can't tell is a qualified tech.


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Post subject: Re: Red knob "The Twin" is always distorted
Posted: Mon Feb 06, 2017 4:38 pm
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BMW2002Ti wrote:
Were all tubes replaced or just the 6L6GC tubes? What you are describing sounds more like input & gain stage issues. Usually these are driven with 12AX7 tubes.


With both channels manifesting the same symptom, I'd look to the phase inverter first as the source of the issue. It's common to both channels.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Red knob "The Twin" is always distorted
Posted: Thu Feb 09, 2017 9:57 pm
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These are rockin loud amps if you get em runnin good.
You can start to break it down into sections to track it down. Start by plugging into the power amp input. The sound will be soft, but. is it distorted?

Is this the schematic of the unit you have?

http://www.blueguitar.org/new/schem/fender/the_twin.gif


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