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Post subject: '72 Champ Voltage & Bias questions
Posted: Sat May 06, 2017 8:49 pm
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Hi!

A couple months ago I dug my '72 Champ out of the closet where it's been sitting for a lot of years. I bought it new way back in the day. Been enjoying it, but it really isn't up to snuff.

Back around '79 or '80 it died, and I had to replace a burnt resistor, a bad cap and the speaker was actually open as well. I had forgotten about that until I pulled the amp apart tonight...

Anyway, I got some NOS tubes, and figured it was time to recap the amp and just check everything out before I slammed expensive new glass in it.

It's been a couple or three decades or so since I worked on any hollow state electronics, so while I've been reading a lot of stuff on the 'ol innerwebs, I still got some questions.

I figured I'd measure / benchmark stuff to know where I was before I started swapping out components.

First thing I noticed, the B+ voltage seems pretty high, but the gist of what I read seems to indicate SF champs had higher voltage then the schematics indicate. There's 400v on the plate of the 6V6.

I've got 378v plate to cathode, and after measuring cathode resistor value and voltage drop, that works out to 17.39 watts plate dissipation. That seems pretty high to me; that's almost 25% higher then the tube's rating of 14w max plate dissipation.

Nominal line voltage here is ~123v, so I would expect a _little_ higher voltage...

I guess it's been like that all it's life... But before I go mucking around with cathode resistors, I was wondering if anyone had some real world comparisons on their own amps?


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sat May 06, 2017 11:55 pm
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Your measurements seem prototypical given the 123 VAC wall voltage. The 400 VDC at the 6V6GTA's plate is a skosh high compared to the spec shown on the schematic but Leo was known to push the envelope when it came to tubes. And the cathode resistor might be tired which could account for the high plate dissipation. But so long as the bottle is not red-plating and the amp's tone is pleasing I don't think I'd worry about it.

Arjay

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 1:15 am
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Leo ran those 6V6GT in his single-ended amps, very hard. And that is a big part of the great tone. You need tough tubes. US made NOS ones are the best, IMHO. IMHO, 6V6GTY and 6V6GTA are the best.

:)


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 2:02 am
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Change plate resistor to have lower plate dissipation and listen the amp, if you like, live it like that.


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 2:03 am
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As far as modern made tubes, EH 6V6 are a perfect match for Champs and Vibro Champ amps. :idea:

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 3:43 am
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shimmilou wrote:
As far as modern made tubes, EH 6V6 are a perfect match for Champs and Vibro Champ amps. :idea:


+1

They're a pretty good fit for Princeton Reverbs as well.

Arjay

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 5:53 am
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I've tried increasing the cathode resistor (470) in a few Champs in small increments. To me and my ears it just keeps leading it further, in small increments, away from the Champ's tone.

UN


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 7:55 am
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As I'm sitting here, I'm thinking that the notes on the AA764 schematics say:

Voltages read to ground with electronic voltmeter. Values shown + or - 20%.

400v on the plate IS within the 20% range... Also the 470 ohm 1w resistor is a 10% tolerance, so at 462 ohms is well within spec. So really, nothing much to see here.

So I went ahead and installed my new tubes and checked the plate voltage; it's now 375v! 24.5v drop across cathode resistor, and 352v between plate & cathode, which nudges up plate dissipation by about another watt. Allowing for 5% of the cathode current to be screen current (something I did not do in my initial assessment), then it's 17.6 w. No sign of red plating and things seem happy.

I'll replace the three 25mfd Mallory caps, one of which has begun leaking, and probably leave well enough alone. Although that cap can is what, 45 years old? I see Weber has some cap cans that are supposed to be decent, maybe I should think about changing that out...

Thanks for the responses!


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 8:11 am
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7chord wrote:
...I'll replace the three 25mfd Mallory caps, one of which has begun leaking...maybe I should think about changing that out...


ASAP! :!:

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 10:19 am
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shimmilou wrote:
7chord wrote:
...I'll replace the three 25mfd Mallory caps, one of which has begun leaking...maybe I should think about changing that out...


ASAP! :!:


+1000!

Hate to see a vintage Champ's power tranny bite the dust due to procrastination.

And there are a number of sources for good drop-in replacement cap cans. Weber's seems to be a bit pricey......

Arjay

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 1:24 pm
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Retroverbial wrote:
shimmilou wrote:
7chord wrote:
...I'll replace the three 25mfd Mallory caps, one of which has begun leaking...maybe I should think about changing that out...


ASAP! :!:


+1000!

Hate to see a vintage Champ's power tranny bite the dust due to procrastination.

And there are a number of sources for good drop-in replacement cap cans. Weber's seems to be a bit pricey......

Arjay



I agree with it being pricey - Was just poking around and I see that CE is remaking the old Mallory cans, but I haven't found that really any cheaper - https://www.tubesandmore.com/products/c ... v-402020uf for an exact replacement with a higher max voltage rating.

If that's the going rate, then I'll just get it and be done with it. But if you have any better sources / suggestions, I'd appreciate hearing them!


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 4:23 pm
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The CE cap cans from tubesandmore.com are what I use.

Be sure to check all the resistors on the power rail when you replace the filter can -- heave any that measure out of spec and install fresh ones (metal oxide are perfectly acceptable substitutes for the carbon-comps).

Arjay

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 8:00 pm
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Retroverbial wrote:
The CE cap cans from tubesandmore.com are what I use.

Be sure to check all the resistors on the power rail when you replace the filter can -- heave any that measure out of spec and install fresh ones (metal oxide are perfectly acceptable substitutes for the carbon-comps).

Arjay


I ordered the cap can from tubes and more. I always used to put cement style resistors in power rails. That's what's I put in there decades ago when the smoke escaped from the 1k resistor. I'll spot check it and the 10k as well.


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Sun May 07, 2017 10:37 pm
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It's probably still hokay but check it nonetheless. Even concrete crumbles with the combined effects of time and heat.

Arjay

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Mon May 08, 2017 7:04 am
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Metal oxide or wire-wound "bricks" will be fine in this amp's PSU. Both will last nearly forever. Provided ratings and resistances are correct. :)

Image


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