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Post subject: red plating issue
Posted: Wed Sep 30, 2015 8:05 pm
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Started to smell a musty smell cooking from my 76' Champ, took off back and the 6v6gt is redplating, what would be the common causes? [img[/imgImage


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Post subject: Re: red plating issue
Posted: Wed Sep 30, 2015 10:26 pm
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Two probable causes:

1.) Tube is shorting out. Did you try another 6V6GT?

2.) Cathode-tied resistor is defective. And/or bypass cap is shorted out.


It could also be too much voltage on the 6V6GT plate. You running a 5Y3GT type rectifier? Rule-out numbers 1 & 2, first.


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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 1:08 am
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BMW2002Ti wrote:
Two probable causes:

1.) Tube is shorting out. Did you try another 6V6GT?

2.) Cathode-tied resistor is defective. And/or bypass cap is shorted out.


It could also be too much voltage on the 6V6GT plate. You running a 5Y3GT type rectifier? Rule-out numbers 1 & 2, first.


+1

Are there any audible symptoms?

Arjay

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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 3:38 am
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Thanks guy's, first off, amp sounds fine, noticed the musty cooking smell, I had been using a military grey glass RCA 6v6gt when it started, so I put in this RCA 6v6 in the pic. unfortunately with the grey glass you can't see whats going on, this one is clear to see.Yes, that is a 5Y3GT rectifiyer in there.I will try another 6v6 after work, also I had issues with the 6v6 socket when I picked up this amp, a unused pin fell out, and even after having tech tighten socket, tubes still feel kind of loose, could this old socket be an issue?


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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 6:06 am
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Socket's Pins may move in the socket but being tight on tube pins, a qualifid tech see that easely.
See a qualifeid one .

First answer tell all.
Check tube cathode contact to bias resistor,

With a new tube we must always check bias


Last edited by stratele52 on Thu Oct 01, 2015 8:49 am, edited 1 time in total.

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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 6:53 am
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ok, isn't the 76' Champ a fixed bias? before I left this morning I put a brand new JJ 6V6 in and it red plated in just a min or two of playing, so I think it's safe to say it's not a tube problem.


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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 7:19 am
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Your Champ has a cathode-biased output tube - single-ended. No separate bias circuit, like in the push-pull Fender amps (fixed bias).

Ok. If the JJ and the RCA red plate --- you have an issue with either the cathode tied 470-ohm resistor and/or its 25mfd/25VDC bypass cap.

If the 220k-ohm grid-to-ground resistor on the 6V6GT control grid is way off, like more than 500k-ohms, this can cause the 6V6GT to idle too hot.

One other source of a problem maybe the 0.22mfd coupling cap from the 12AX7's anode plate to the 6V6GT. If it leaks more than a few hundred mV of DC... this can cause the 6V6GT to become unstable.

Also, measure the plate voltage on the 6V6GT. If the measurement comes out way over 350VDC (like in the 400 volt range) --- there is something wrong with the power supply.

If after you've checked all the above & everything seems ok --- it could be a shorted output tranny. Rule-out the above potential issues first.

Here's the schematic to help your diagnosis:

http://ampwares.com/schematics/champ_aa764.pdf

Good luck! Keep us posted.


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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 7:36 am
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Thanks, just dropped it at my amp tech.he basically listed all the same scenarios you listed(he's a good tech so I'm sure he will get it sorted out. At this point this is beyond my ability, I hope it's just a resistor and not a more serious power supply issue. I will report back with diagnosis.


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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 8:54 am
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my56strat wrote:
Thanks, just dropped it at my amp tech.he basically listed all the same scenarios you listed(he's a good tech so I'm sure he will get it sorted out. At this point this is beyond my ability, I hope it's just a resistor and not a more serious power supply issue. I will report back with diagnosis.


I see just 2 things :

1- cathode resistor for tube 's bias

or

2- cathode pin contacts


Both inexpensive repair


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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 3:01 pm
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Stratele, I don't think poor cathode pin contact will cause red plating. Intermittent conduction and noise --- yes.

One other remote possibility, is the main power supply filter cap. If this multi-section can is dried out --- you maybe getting poor filtering and instability in the DC on the anode plate. Has the amp played ok, in the past? No mods done to it?

Anyway, we'd be interested in finding out what the problem is. These simple amps are usually very reliable.


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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 3:12 pm
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BMW2002Ti wrote:
One other remote possibility, is the main power supply filter cap. If this multi-section can is dried out --- you maybe getting poor filtering and instability in the DC on the anode plate.


That's the reason I asked if there were any audible symptoms. If the filter stage were shot (or heading south) the amp might manifest some type of noise or hum. Still, it could be a marginal cap can.

Arjay

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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 3:53 pm
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Retroverbial wrote:
That's the reason I asked if there were any audible symptoms. If the filter stage were shot (or heading south) the amp might manifest some type of noise or hum. Still, it could be a marginal cap can.

Arjay


Yeah. I have a feeling this maybe a variety of fix needs. Prolly new cathode resistor/bypass cap. New cap can. New 220k-ohm grid-to-ground resistor. And check the OPT for shorts.

One other thing {always one other thing... 8^) --- the fuse}. Have the tech make sure the correct fuse is in the amp. A current sucking 6V6GT or 5Y3GT should pop the fuse, before frying the power transformer or worse, causing a fire. Should be a 1-amp slow-blo.

Also if not already done, have the tech wire in a three-prong outlet cord to the amp. And remove the "death cap."


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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 4:58 pm
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The amp sounds great, no hum or noise, when I first got the Champ I brought it in to him, it did have the wrong fuse, he changed it to correct size then, in 76' the Champ already came with the 3 prong, red end cord. Would it have a death cap on a 76? His shop is 10 mins. from my office, so after trying the new JJ 6V6 this morning I took it to work and dropped it to him, he said he may get to it this weekend, so hopefully soon.Thanks for all the info!


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Post subject: Re: red plating issue
Posted: Thu Oct 01, 2015 5:06 pm
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Yes, it seems to still have the "death cap." The 0.047mfd between the fuse and ground. I have no idea why that is still installed in these three-prong wired amps.

http://bmamps.com/Schematics/fender/champ_cbs.pdf

In this CBS era schematic (sorry for the quality) --- I noticed a 330 pico-farad disc cap running between the 6V6GT cathode and the grid. Prolly there for stability issues (high frequency filter). This cap maybe shorted out. Prolly don't need it, as long as the voltages on the plate are ok. And you are using a good 6V6GT.


Good luck with the fix! Keep us informed.


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Post subject: Re: red plating issue
Posted: Fri Oct 02, 2015 7:13 am
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BMW2002Ti wrote:
Stratele, I don't think poor cathode pin contact will cause red plating. Intermittent conduction and noise --- yes.

.



you are right a open cathode circuit do not allow tube to conduct.o


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