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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Fri Aug 03, 2012 10:44 pm
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Glad to hear you know your way around electronics, Pneuma. That really helps! If you've gone through all of the circuits, by means of continuity, voltage, and current tests, I could only suspect carbon paths on the power tube sockets. We haven't talked about the choke or the OPT. Can you test these guys? It's just a matter of elimanating possible suspects. Like Shimmy said, the Pt seems like he's trying to do his job. The PT is ususally one of the last suspects, unless heat or abuse has compromised his performance. Hate to say it, but this ain't a simple one! There is no static, abnormal hum, or intermittent power interruptions, right? All voltages are seemingly low but steady, right? Art

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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Sat Aug 04, 2012 4:19 am
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" I put one of the caps for the bias circuit in backwards. "

I have one suggestion, post what you wrote --- be SURE you are getting a constant, steady -48VDC at the read point off that bias power supply. Putting that cap in backwards may have caused damage to other components in the circuit (like the single diode).

Bias supplies are a bit confusing. As the diode and cap(s) appear to be put in backwards. They are, to get negative DC voltage. It must be negative and near the nominal value, in order for you to properly adjust the idle dissipation of your 6L6GC's.

Good luck. You seem to be progressing.


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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Sat Aug 04, 2012 4:32 am
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Thanks for the advice. I think I got lucky and didn't damage anything else. There is definitely a proper, steady negative bias voltage now. I tuned the hum balance pot until I had equal current through both power tubes - about 38 mA. With a bias voltage around -50vdc. And yeah, I know the caps should be put in "backwards", just made a lame brain mistake. I went through the amp several times after I did the cap job looking for something I did wrong, and looked at that cap several times thinking "that one looks good". Funny how you can look at your own work and everything looks like it is perfect, but if you go away and look again you can find mistakes...


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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Sat Aug 04, 2012 6:46 am
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Awesome! I am glad that everything is in order now. Don't be too hard on yourself, I am certain that everyone here, including myself, has had similar "WTF?" moments. You were clear, and thorough, I just offered a way to test the PT. It is almost always something simple. I find that what you said about going away for awhile, then coming back with a fresh start, is very effective. Good job sir! 8)

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I said "hard on"....huh-huh...huh-huh... :lol:

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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Sat Aug 04, 2012 6:49 am
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That's why a Variac or Light Bulb Limiter trick is a Godsend. I've caught a few self-inflicted boo-boo's before Hell cut loose. :D


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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Sat Aug 04, 2012 7:40 am
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pneumatica wrote:
Boy, don't I feel like a dufus now! Especially after that "I've been an electronic technician for 20 some-odd years" rant. I put one of the caps for the bias circuit in backwards.


We're sending you back to Jacksonville to repeat "A" school.

Rust-pickers!!!

:mrgreen:

Glad you finally located the problem -- sometimes a little break will give you a perspective you didn't initially have.

Rawk on!

8)

Arjay

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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Sat Aug 04, 2012 5:11 pm
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Excellent!!! Art

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