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Post subject: 65 Twin Reverb Reissue Issue
Posted: Wed Jun 14, 2017 1:29 pm
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A friend has one of the 65 Twin Reverb reissues, I think it's 1997 vintage. He was tapping on the tubes testing for microphonics. When he tapped on V9 it must have shorted internally cause he said he saw sparks in there and the fuse blew. He got a quad of ElectroHarmonix 6L6's and retubed the amp, replaced the fuse and it was playing again.

He mentioned this to me & I said we better check it out, check bias, etc. So he brought it over this morning. I plugged in my BiasMaster and immediately knew there was a problem. V9 showed 60ma while the other 3 were around 38ma. Switching tube positions made no difference, the problem stayed at the same socket.

Pulled the chassis, and found both v10 & v9 had toasty 1.5k control grid resistors. Replaced those, and found v9's 470 ohm screen grid resistor was outta spec. Pulled it out and found it was cracked around the middle, so replaced that too.

Checked all the other resistors in the immediate circuit, didn't find any other issues. Powered up the amp and the situation was the same - V9 showing 60ma, other 3 6L6's looked fine @ 38ma. I spot checked a few voltages, but nothing stood out to me. Also eyeballed the filters and all looked good. I also swapped out the 12AT7 that sits in front of the outputs, made no change.

He's a working musician / teacher, and couldn't afford the down time, so he insisted on buttoning it back up at that point as he had to hit the road. We discussed what might / might not happen if he used the amp like that, but he didn't have any other options at the moment.

I'd appreciate any ideas what else to look for once I can get it back on the bench!


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Post subject: Re: 65 Twin Reverb Reissue Issue
Posted: Wed Jun 14, 2017 3:06 pm
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Look negative bias voltage don't go to V9
Check how many negative volt you got both side of V9 1.5 K resistor
You may have close to minus -56 DC volts

A short in V9 tube toast 1,5 k and may cut the copper printed circuit from de bias voltage supply .


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Post subject: Re: 65 Twin Reverb Reissue Issue
Posted: Wed Jun 14, 2017 3:35 pm
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stratele52 wrote:
Look negative bias voltage don't go to V9
Check how many negative volt you got both side of V9 1.5 K resistor
You may have close to minus -56 DC volts

A short in V9 tube toast 1,5 k and may cut the copper printed circuit from de bias voltage supply .


I had the same negative voltage on pin 5 of all four 6L6's.


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Post subject: Re: 65 Twin Reverb Reissue Issue
Posted: Wed Jun 14, 2017 4:02 pm
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Joined: Fri Feb 05, 2010 4:31 am
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Location: Province de Québec, Canada
And with another tube in V9 socket you got 60 ma too ?

It is socket contact 5 is broken or no contact to V9 tube's pin 5, you have to thigten the contact in the socket and clean it with good electronic contact cleaner, not one at Radio Shack
Or replace the contact or the socket .The socket contact could be burn and can't be cleaned

If tube is good, it must be the bias negative voltage can't go to tube


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