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Post subject: Twin Reverb '65 Reissue
Posted: Wed Jun 14, 2017 1:07 pm
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***EDIT*** SORRY - I REPOSTED IN MODERN AMP FORUM****

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!


Last edited by 7chord on Wed Jun 14, 2017 1:30 pm, edited 2 times in total.

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Post subject: Re: Twin Reverb '65 Reissue
Posted: Wed Jun 14, 2017 1:13 pm
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Did you perform any dynamic voltage checks -- plate, cathode, screen & control grids, etc? How about a close visual inspection of the tube sockets and relevant wiring?

BTW, this question should be posted in the "modern amp" section.

Arjay

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Post subject: Re: Twin Reverb '65 Reissue
Posted: Wed Jun 14, 2017 1:19 pm
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Doh, yes, I guess it should be. Should I repost there, or can it be moved?


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Post subject: Re: Twin Reverb '65 Reissue
Posted: Wed Jun 14, 2017 1:27 pm
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The deed is done. I'd leave it here.

Arjay

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Post subject: Re: Twin Reverb '65 Reissue
Posted: Wed Jun 14, 2017 1:31 pm
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Retroverbial wrote:
The deed is done. I'd leave it here.

Arjay


Nah, I already made a bigger mess of it - It's what I do! ;-)


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Post subject: Re: Twin Reverb '65 Reissue
Posted: Wed Jun 14, 2017 1:34 pm
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7chord wrote:
Nah, I already made a bigger mess of it - It's what I do!


Not nearly as big as the new "relic" posted in the Strat section.

:lol:

Arjay

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Post subject: Re: Twin Reverb '65 Reissue
Posted: Wed Jun 14, 2017 1:39 pm
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I compared voltages between the other output tubes, and didn't see anything that looked out of the ordinary. Bias / plate voltages/grids all looked decent. I didn't have a lot of time to spend with it, but hopefully can get it back on the bench before he burns it up. I *thought* I eye-balled everything pretty good, but how many times have you heard that one...


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Post subject: Re: Twin Reverb '65 Reissue
Posted: Wed Jun 14, 2017 1:43 pm
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If the dynamic *idle* voltages all check good, I'm thinking that the V9 socket is arcing under load. Look closely for any sign of cracked porcelain there.

Arjay

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Post subject: Re: Twin Reverb '65 Reissue
Posted: Wed Jun 14, 2017 4:23 pm
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With all 6L6GC tubes pulled, check to see if you are getting around -50VDC on pin 5 (grid). This is with the bias pot at midpoint of its range. If V9 is not showing -50VDC, start checking the wiring continuity from the board to the socket.

Also, check to be sure that all socket tangs are tight and secure onto the tube. You may need to tighten them.

Good luck! :)

http://ampwares.com/schematics/65_twin_ ... manual.pdf


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Post subject: Re: Twin Reverb '65 Reissue
Posted: Wed Jun 14, 2017 5:17 pm
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BMW2002Ti wrote:
With all 6L6GC tubes pulled, check to see if you are getting around -50VDC on pin 5 (grid). This is with the bias pot at midpoint of its range. If V9 is not showing -50VDC, start checking the wiring continuity from the board to the socket.

Also, check to be sure that all socket tangs are tight and secure onto the tube. You may need to tighten them.

Good luck! :)

http://ampwares.com/schematics/65_twin_ ... manual.pdf


As I recall, there was ~ -56v on pin5 of all four 6L6's. However, whether that's making it through the socket to actually get to the tube, I can't say.

Inasmuch as I plugged and unplugged both the BiasMaster and the tubes multiple times, I'd have to say that socket would have to be seriously damaged because the BiasMaster plug pins are a really tight fit. I'll definitely check that out though. Never say never, right? Maybe compare the voltage drop across R46 - R49...


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Post subject: Re: Twin Reverb '65 Reissue
Posted: Wed Jun 14, 2017 6:30 pm
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7chord wrote:
Maybe compare the voltage drop across R46 - R49...


The malady may reveal itself there......

Arjay

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Post subject: Re: Twin Reverb '65 Reissue
Posted: Thu Jun 15, 2017 5:57 pm
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R46 through R49 are the grid stoppers. If the screen resistors blew (470-ohm), there is a very good chance a grid stopper blew. Check for physical damage and resistance across them. Should be 1500-ohm (+/- 10%).

I'm not sure about the TRRI, but in most earlier blackface and silverface amps the grid stopper resistor is soldered across pin 1 and pin 5 of each 6L6GC.


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Post subject: Re: Twin Reverb '65 Reissue
Posted: Thu Jun 15, 2017 6:51 pm
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I got the amp back from him today, so it's back on the bench.

As I mentioned in my original post, the first thing I noticed was the crispy grid stoppers on v10 & v9. The screen resistor for v10 was fine, but v9's was reading high, upon removal I found a crack around the middle, so it was replaced. I also replaced the screen resistor for v10 as a test. I mentioned previously I recalled the bias voltage was -56v but that was incorrect, the actual bias voltage is -44.3.

Tonight I closely inspected the sockets, even tightened the pins. No change. I took a piece of wire and twisted a small loop around pin 5 of the 6L6 and plugged it back into the v9 position. I measured the negative bias voltage was present on the wire, so clearly the tube is seeing it. I even jumpered that wire from pin 5 of the tube to the bias voltage connection on the base of the tube socket. As expected, no change. I don't think it's a socket issue.

v10 - 45ma
v9 - 68ma
v8 - 34ma
v7 - 36ma

I've moved the tubes around, the issue stays at v9 / v10. I've checked all resistors in the output circuit, and the 12AT7 stage preceding it.

What the heck am I missing?? I'm not seeing it...

Oh, and yes the grid stopper is soldered across the socket in the TRRI.


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Post subject: Re: Twin Reverb '65 Reissue
Posted: Thu Jun 15, 2017 7:47 pm
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How about the 220k dropping resistors that tie the bias-supply voltage to the grid stoppers? There's one for each pair of output bottles (V7 &V8, V9 & V10). Have you checked them for damage or marginal solder joints?

Arjay

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Post subject: Re: Twin Reverb '65 Reissue
Posted: Thu Jun 15, 2017 9:03 pm
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Retroverbial wrote:
How about the 220k dropping resistors that tie the bias-supply voltage to the grid stoppers? There's one for each pair of output bottles (V7 &V8, V9 & V10). Have you checked them for damage or marginal solder joints?

Arjay


Sure did, and they are fine; well at least from the top side of the board. I checked every single resistor in the output circuit and the previous stage, marking them with a sharpie as I go thru them. I'm probably going to have to pull the board to get to the underside & solder joints, and will also isolate and test caps while I have it there.

Voltage on all four output tubes are within 2 or 3 volts respectively. The only marked difference is that v10 & v9's screen resistors are showing a greater voltage drop, around 2v each compared to .75v for v7 & v8.

Heck, I should pull the output tubes and look at the voltages that way too. (Wanders back down to shop...)


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