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Post subject: 64' Twin Reverb bias problem
Posted: Sun Oct 27, 2013 7:26 am
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Aspiring Musician
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Hello,

New to this forum, so I hope I'm doing this correctly.

I've got a 1964 Twin Reverb that was in rough shape. I replaced all the needed caps and resistors, (everything was original and ruptured), and it fires up and plays. I put new JJ 6L6GC's in, along with some new Fender AX7's and AT7's I had, and when I tried to bias it I couldn't get it over 29. I switched out the resistor on the bias pot and was able to get higher settings, but at 35 my plate voltage dropped to below 400v. I thought about doing the 1 ohm resister on the sockets for measuring, but I don't really think that's going to matter. I've heard JJ's draw more? Has anyone had this issue with JJ's?

Any help would be appreciated.


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Post subject: Re: 64' Twin Reverb bias problem
Posted: Sun Oct 27, 2013 7:43 am
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Joined: Thu Jan 10, 2013 7:02 pm
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Location: Manchester
When you say "all the caps", do you mean just the filter and blocking caps? With an amp that old you should look at replacing the caps in the bias supply, chances are they're leaking badly and stopping you getting enough bias voltage. As the amp heats up they will leak even more, your bias drops closer to zero Volts and you can end up in thermal runaway as the tubes conduct more and heat up more. Turning the adjuster to lower resistance is a bad idea when the caps are suspect. It'll fix the bias voltage but as soon as the caps start leaking badly then there's more chance it'll go into runaway.

I've got to do this exact same repair on two Marshalls next week!

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Post subject: Re: 64' Twin Reverb bias problem
Posted: Sun Oct 27, 2013 7:51 am
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Joined: Thu Jan 10, 2013 7:02 pm
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The 1 ohm resistor is always a good idea. Without it you're just guessing the total anode current and dissipation.

Can't say why the plate voltage would drop with increased -ve bias, stick the resistors in and get a clearer idea of what's happening in the amp.

Also, did you use 105 degree rated caps? Cheap electrolytics are rated 85 degrees, those aren't ideal when you're running a hot tube amp for hours on end.

/my 2c

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Post subject: Re: 64' Twin Reverb bias problem
Posted: Sun Oct 27, 2013 8:49 am
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if you post this over on the vintage or modern amp section you'll get instant replies from about 3 guys who know all about this at a very deep level. retroverbial and bluesky, for 2.


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Post subject: Re: 64' Twin Reverb bias problem
Posted: Sun Oct 27, 2013 7:57 pm
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Aspiring Musician
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Just so everyone knows, I moved this post to the vintage amp section and the problem is resolved I believe, which was a senior moment error on my part, even though I'm only in my 50's!


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Post subject: Re: 64' Twin Reverb bias problem
Posted: Sun Oct 27, 2013 8:36 pm
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upnorth wrote:
Just so everyone knows, problem is resolved I believe, which was a senior moment error on my part, even though I'm only in my 50's!


Welcome to the Forum uporth :D

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