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Post subject: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Tue May 14, 2013 6:24 pm
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Hi Guys,

New set of issues with my Super Reverb. Starting gradually popping fuses (after about 2 minutes) and started to notice very "hot" smell prior to fuse pop. So started to investigate. Getting 502VDC at pins 3/4 of each power tube socket (with rectifier in and no power or pre tubes) in place. But getting 0VDC at bias point (middle contact of bias pot).

Any ideas? Recently I have done a disconnect of two speakers (so now running only two for reduced output) which seemed OK - no fuse pops, untoward burning smells etc. Then did a grill cloth replace and was surprised when I put it all back together that I now encountered this issue - very strange.

Any ideas?

Chris


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Post subject: Re: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Tue May 14, 2013 6:38 pm
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Your plate voltage is way too high -- you should be seeing something close to 437 VDC. And with 0 volts at your bias pot, I'd hazard that something in the power supply has taken a three-roll dump. Pull the tubes and start tracing voltages, beginning with all the secondary taps on your power tranny. If the iron checks good, carefully inspect your bias supply (especially the pot and the electrolytic cap).

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: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Tue May 14, 2013 7:40 pm
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Thanks Arjay,

I checked the bias resistor, pot and cap and no obvious issues there as they measure close to spec and cap is not shorted - same for diode - seems OK closed one way, open the other.

So will check trannies tomorrow. Nothing visibly wrong in the dog house either.

Regards, Chris


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Post subject: Re: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Wed May 15, 2013 3:49 am
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taskerc,

I'd check the DC voltage from the pot back to the diode. Then, the VAC from the diode back to the PT. You may have a broken line someone in the bias supply. Or worse... a bad winding in the PT ( I hope not). You should hit a point where you get voltage (either VDC ---after the diode to the pot. Or VAC --- before the diode to the PT ).

If you have a shorted electrolytic cap in the bias supply, it maybe dumping all your voltage to ground. This won't pop the fuse --by itself --- as the bias supply only pulls a few hundred mA, max.

You gotta get proper & STEADY negative voltage on the 6L6GC grids. Or you'll toast output tubes. And keep popping fuses.

After solving that problem.. move onto the anode plate voltage.

Good luck with all that. Hope you find the culprit.


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Post subject: Re: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Wed May 15, 2013 4:52 am
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Thanks BMW :-)

The Bias cap is for sure not shorted - will trace back as you suggest.

If I don't get it sorted looks like I have a nice toaster oven though :wink:

Those tubes were sure glowing bright and hot and are almost certainly dead by now.

Chris


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Post subject: Re: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Wed May 15, 2013 12:04 pm
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Retroverbial wrote:
Your plate voltage is way too high -- you should be seeing something close to 437 VDC.



Yes WITH power tubes IN . It is not the case.

No tubes = no load = higher voltage


Last edited by stratele52 on Wed May 15, 2013 12:09 pm, edited 1 time in total.

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Post subject: Re: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Wed May 15, 2013 12:09 pm
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taskerc wrote:
Thanks Arjay,

I checked the bias resistor, pot and cap and no obvious issues there as they measure close to spec and cap is not shorted - same for diode - seems OK closed one way, open the other.

So will check trannies tomorrow. Nothing visibly wrong in the dog house either.

Regards, Chris


Don't check components , read voltages first .

AC volt at power transformer at bias tap , close to 40 AC if good check DC after the diode.

If no AC voltage , check solder , be sure you put your voltmeter probe ON the Power Transformer wire.


Last edited by stratele52 on Wed May 15, 2013 1:44 pm, edited 1 time in total.

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Post subject: Re: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Wed May 15, 2013 1:15 pm
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stratele52 wrote:
Don't check components , read voltages first .


+1!

You need dynamic measurements, not static data.

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: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Wed May 15, 2013 1:49 pm
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Thanks all - will check voltages tonight as suggested.

I really appreciate the support (again).

Chris


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Post subject: Re: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Wed May 15, 2013 7:41 pm
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Hi Louis,

Did measurements with Rectifier tube in place only - Red Xfmr wire to Bias is 52VAC and 0VDC after Diode.

Diode dead?

Regards, Chris

stratele52 wrote:
taskerc wrote:
Thanks Arjay,

I checked the bias resistor, pot and cap and no obvious issues there as they measure close to spec and cap is not shorted - same for diode - seems OK closed one way, open the other.

So will check trannies tomorrow. Nothing visibly wrong in the dog house either.

Regards, Chris


Don't check components , read voltages first .

AC volt at power transformer at bias tap , close to 40 AC if good check DC after the diode.

If no AC voltage , check solder , be sure you put your voltmeter probe ON the Power Transformer wire.


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Post subject: Re: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Thu May 16, 2013 1:23 am
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taskerc wrote:
Hi Louis,

Did measurements with Rectifier tube in place only - Red Xfmr wire to Bias is 52VAC and 0VDC after Diode.

Diode dead?

Regards, Chris




Bias AC voltage at 52 is good = Power Tansformer is good .

No Volt DC after diode ; diode is defect or filter cap is bad or you have a short somewhere in bias circuit. Diode is a 1N4007 or "bigger" , cost $0.75

1- Check diode with ohmeter . You must disconnect one end. Disconnect the side where is the bias filter caps

If the diode is good , be sure to know how to test it . Read DC voltage at the end you desoldered . You must read DC .

2-Connect this side of the diode to his filter cap . Before , remove this end of filter cap from the board and connect it to the end of your diode ,

Now you have now ONLY the bias diode WITH his filter cap ; read DC Volt .If no good , filter cap is defect.
If good , look other part of the bais circuit

For ALL tests : Yes tube rectifier in place but NO OUTPUT POWER TUBE until you fix bias


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Post subject: Re: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Thu May 16, 2013 3:12 am
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stratele52 wrote:
Bias AC voltage at 52 is good = Power Tansformer is good .

No Volt DC after diode ; diode is defect or filter cap is bad or you have a short somewhere in bias circuit. Diode is a 1N4007 or "bigger" , cost $0.75

1- Check diode with ohmeter . You must disconnect one end. Disconnect the side where is the bias filter caps

If the diode is good , be sure to know how to test it . Read DC voltage at the end you desoldered . You must read DC .

2-Connect this side of the diode to his filter cap . Before , remove this end of filter cap from the board and connect it to the end of your diode ,

Now you have now ONLY the bias diode WITH his filter cap ; read DC Volt .If no good , filter cap is defect.
If good , look other part of the bais circuit

For ALL tests : Yes tube rectifier in place but NO OUTPUT POWER TUBE until you fix bias


+1

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: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Thu May 16, 2013 3:14 am
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Thanks - I will continue measurements tonight.

Chris


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Post subject: Re: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Thu May 16, 2013 6:28 pm
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Hi Louis,

Following your instructions, Diode seems to check out. So I then connected Filter cap end to diode end and get -71.2VDC.

So it sounds like the diode and cap are fine and something along the path is suspect - yes?

Chris


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Post subject: Re: Super Reverb - Redplating like crazy, no -52vDC bias
Posted: Thu May 16, 2013 7:43 pm
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Also checked bias pot again and it seems fine as does the 27K resistor to ground.

Now putting it all back right I am getting -47VDC at junction of 220K resistors :-( Confused.

Tonight I am now tired so I will take another look tomorrow to confirm and maybe test again with power tubes.

Chris


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