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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Mon May 08, 2017 10:36 am
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I got started doing bench work at a TV repair shop in the late 70's. Worked on a _lot_ of tube equipment. And changed a lot of burnt resistors, flyback transformers, burnt circuit traces and bad capacitors...


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Mon May 15, 2017 9:57 pm
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Finally had some time to work on my Champ! Replaced the power supply filter can, all the electrolytics, a couple resistors, a broken input jack, and the tone caps. Unfortunately I didn't have the proper replacement for the plate #1 capacitor off the 12AX7. So I'll have to change that out later. For now I just want to get the amp back together so I can use it!

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Mon May 15, 2017 10:54 pm
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Very clean, very orderly.

How's it sound now?

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: '72 Champ Voltage & Bias questions
Posted: Tue May 16, 2017 6:45 am
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Didn't have a chance to try it out yet, got too late. For sure I will this evening!

I've seen some pictures of chassis that were wired very neatly, but mine was a little haphazard. I spent a few extra minutes rerouting wires and cleaning up the layout. (I know, but I can't help it sometimes...).


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Tue May 16, 2017 8:53 am
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7chord wrote:
I've seen some pictures of chassis that were wired very neatly, but mine was a little haphazard. I spent a few extra minutes rerouting wires and cleaning up the layout. (I know, but I can't help it sometimes...).


That's the nature of silverfaces built after 1968 -- lead dress got sloppy. Use caution when re-routing wires though. You don't want to inadvertently induce any hum into the circuit.

Arjay

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Wed May 17, 2017 6:57 am
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I did lift the grounded heater line to one VC, a long time ago. Ran twisted paired heater lines to the lamp and sockets. That does reduce the hum, esp low volume type. A bit of work in the tight spaces of a Champ or Vibro-Champ.


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Wed May 17, 2017 9:49 am
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BMW2002Ti wrote:
I did lift the grounded heater line to one VC, a long time ago. Ran twisted paired heater lines to the lamp and sockets. That does reduce the hum, esp low volume type. A bit of work in the tight spaces of a Champ or Vibro-Champ.


Was it quiet enough to justify the work?


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Wed May 17, 2017 4:26 pm
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IMHO, if you play the amp at less than "3-4" most of the time, the reduced hum is nice. esp when recording. But, if you play the amp near full blast --- the difference is not that great.

Single-ended amps tend to be more noisy (versus push-pull). There's only so much hum one can eliminate.

Also, if you use a very efficient speaker or plug into a larger external cab --- the twin-twisted heater line will help eliminate the more noticeable hum ---- due to the speaker's efficiency.


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Thu May 18, 2017 9:36 am
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Guess I've just been lucky with this Champ, it's always been pretty quiet. Although I still haven't had a chance to test it since I did the work. I got a couple Sound Motion units in that I needed to repair and turn around quickly. And my actual mortgage paying gig has been hectic lately. Hopefully this weekend...


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Fri May 19, 2017 9:16 pm
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The Champ sounds fantastic now - that was totally worth the effort! Every single cap has been replaced except the 250pf ceramic, a couple crusty resistors and all new NOS tubes. I thought about lifting the ground on the 6.3v filament string and doing the virtual center tap mod, but really there's more of a hum issue in the garbage power supply for my pedal train. I'll solve a lot of ills by addressing that. Now, as far as the Champ, I've got some other ideas for improvement...


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Fri May 19, 2017 11:31 pm
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7chord wrote:
I've got some other ideas for improvement...


Just don't "fix" something that isn't broke.

Arjay

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Fri May 19, 2017 11:34 pm
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I like silver mica in place of the ceramic disc caps, in the tone section.


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sat May 20, 2017 6:20 am
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Retroverbial wrote:
7chord wrote:
I've got some other ideas for improvement...


Just don't "fix" something that isn't broke.

Arjay


When I mod things, I do it in as non-destructive a way as possible, in order to leave a path back to originality. You've all seen poorly conceived or executed mods; things like holes drilled in face plates or worse. :shock: I'll follow up when I get this idea fleshed out. No amps will be harmed in this exercise.


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 21, 2017 12:48 am
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Be sure to document each and every alteration with notes and (if possible) photos.

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: '72 Champ Voltage & Bias questions
Posted: Sun May 21, 2017 10:52 pm
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I think perhaps that it's biased a little too hot. After playing it for a while this evening, I stuck my head behind the amp and could see the plate of the output tube blushing a little. Not a full on red plating, but certainly beyond where I think it ought to be.

Guess I'll get back in there and recheck the cathode resistor. Wonder if my brand new cathode cap is leaky...

I still think the plate voltage is rather high, recall that I measured 400v...


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