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Post subject: 1965 Deluxe Non-Reverb Bias problem
Posted: Sun Jul 24, 2011 4:52 pm
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I have a 1965 Deluxe Non-Reverb. I have put the original Utah speaker away for safe keeping and have been trying different speakers. I also replaced the tubes with JJs and biased the amp to 24ma. Its been sounding great with a Weber 12F150 and I've played a couple gigs with it with no problem. Tonight I decided to try and Emminence Texas Heat 8ohm speaker and after installing the amp had an unpleasing growl. I plugged it into my 8ohm Marshall cab as I thought it might be the new speaker......same noise. I then shut it down and checked the bias as it also sounded really choked and strained. The bias reads 130-160 ma and jumps around!!! Holy Crap. What's up??? I can't adjust it enough to drop below 130ma and I am scared to death to leave it on to troubleshoot any further. Everything looks right. Tubes are in good and solid and the amp has been working great. Could something have come loose inside while I was changing the speaker? What would cause this. Any ideas are very welcome.


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Post subject: Re: 1965 Deluxe Non-Reverb Bias problem
Posted: Sun Jul 24, 2011 6:21 pm
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Do you have access to a tube tester? Might want to see if one or both of those new JJ are shorted. Unfortunately, JJ power tubes and rectifiers have taken a nose-dive in QA over the last couple of years.

If no tester, try replacing the old power tubes and checking bias. If OK... then it's prolly one of the new tubes. If not ok, further inspection of sockets, bias power supply, and bias pot, etc may be needed. Unless you are familiar with this part of the circuit and how to remedy the issue(s)--- best to find a good amp tech.

Good luck!


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Post subject: Re: 1965 Deluxe Non-Reverb Bias problem
Posted: Sun Jul 24, 2011 6:34 pm
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I find it highly unlikely that both power tubes shorted simultaneously. I'd look for a catastrophic failure within the biasing circuit itself -- either the 25mfd cap has taken a dump or the pot itself has flown south. There's also a 10K shunt resistor and a diode which should be inspected as well.

JMO

Arjay

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Post subject: Re: 1965 Deluxe Non-Reverb Bias problem
Posted: Sun Jul 24, 2011 7:11 pm
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Thanks for the quick replies. Yes I have a tester and I had the original RCAs. I had already hoped it was tube failure but pretty much knew that wasn't it but tested with the RCAs and readings are the same. About 3 weeks ago I had a similar situation (not as bad). When I tested, my readings were in the low 30s and like I said, 24 is where I had set it. I set it back and have been playing quite a bit since with no problems until tonight. Its going to an amp tech tomorrow. This is too big for me to want to deal with.

Thanks again!


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Post subject: Re: 1965 Deluxe Non-Reverb Bias problem
Posted: Sun Jul 24, 2011 8:05 pm
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That's the best thing to do gam727,I learn from these guys also and have a friend who is a great amp tech,but I can't figure out everything...I can fix most anything when it is pointed out to me,but sometimes this stuff gets very complicated...best of luck!


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Post subject: Re: 1965 Deluxe Non-Reverb Bias problem
Posted: Sun Jul 24, 2011 8:23 pm
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Arjay, a couple of times I've had one power tube go ballistic and draw mega current. Then, it drew scary current through the other "good" tube (though prolly not as much current). Last time it happened, one BP RCA 6L6GC in a BF Twin Reverb went bad. The amp started going into that terrrible howl. By the time I hit the switch, the push-pull paired 6L6GC had started going red plate.

Prolly not the situation with the OT. But, more of a solder or failing part issue with the bias power supply. Or loosey-goosey tube sockets.

Good luck with the amp!


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Post subject: Re: 1965 Deluxe Non-Reverb Bias problem
Posted: Sun Jul 24, 2011 9:22 pm
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BMW2002Ti wrote:
Arjay, a couple of times I've had one power tube go ballistic and draw mega current. Then, it drew scary current through the other "good" tube (though prolly not as much current). Last time it happened, one BP RCA 6L6GC in a BF Twin Reverb went bad. The amp started going into that terrrible howl. By the time I hit the switch, the push-pull paired 6L6GC had started going red plate.

Prolly not the situation with the OT. But, more of a solder or failing part issue with the bias power supply. Or loosey-goosey tube sockets.

Good luck with the amp!


Ouch......know that must've hurt losing not one but a pair of holy grail blackplates!

Not saying it isn't possible. But of course, I'm "old-school" like you......I would err on the side of caution and check everything. Whatever the cause, it's surely a grave situation that threatens the amp's health.

Rawk on!

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: 1965 Deluxe Non-Reverb Bias problem
Posted: Mon Jul 25, 2011 7:59 pm
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gam727 wrote:
Thanks for the quick replies. Yes I have a tester and I had the original RCAs. I had already hoped it was tube failure but pretty much knew that wasn't it but tested with the RCAs and readings are the same. About 3 weeks ago I had a similar situation (not as bad). When I tested, my readings were in the low 30s and like I said, 24 is where I had set it. I set it back and have been playing quite a bit since with no problems until tonight. Its going to an amp tech tomorrow. This is too big for me to want to deal with.

Thanks again!


Good luck, let us know how it turns out. fingers crossed for you.

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Post subject: Re: 1965 Deluxe Non-Reverb Bias problem
Posted: Tue Jul 26, 2011 6:57 am
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I've actually seen a couple of Ampeg SVT's with multiple push-pull paired Tungsol 6550's blown. The first pair goes ballistic and start drawing all kinds of excess current. Then, if the screen resistors don't blow--- the next pair blow out, etc.

The user sometimes don't even realize the damage as the amp doesn't really drop in apparent loudness. And he is not attentative to odd overtones or smelling burning Tungsols. :(

The signal keeps going down the parallel output stages and being amplified, so all seems ok. Until there is no music. Or a fire. Anyhow, there's plenty on the Net about stabilizing these big monsters. And adding matched (1% tolerance) grid stoppers and screen resistors to each stage. Even fuses for the output.

I did that to my BR Twin Reverb. Sylvania 7581A are hard to come by, these days. Maybe not the $$$ of a vintage BP TS 6550. But close.


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Post subject: Re: 1965 Deluxe Non-Reverb Bias problem
Posted: Tue Jul 26, 2011 7:23 am
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Well I took it to the amp tech that has worked on it in the past (before I purchased it). He said he replaced a grid resistor before and should have replaced the other. At that point he was speculating but from reading some of the replies here, I'm hopeful that is the problem. He said he would have it fixed tomorrow.


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Post subject: Re: 1965 Deluxe Non-Reverb Bias problem
Posted: Tue Jul 26, 2011 9:47 am
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+1

There's nothing to be gained by taking any chances with a nice vintage piece such as yours.

Best of luck

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: 1965 Deluxe Non-Reverb Bias problem
Posted: Tue Jul 26, 2011 5:09 pm
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What kinda good amp tech just replaces one screen resistor on the power tubes.They where put in there at the same time and more then likely they are both gonna fail at the same time.Would you just replace on power tube in an amp if it failed no you replace the set.


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