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Post subject: New tubes. TOOO HOT!!
Posted: Fri Aug 13, 2010 4:28 pm
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Recently I bought 4 shuguang coke bottle tubes for my vintage 76 fender twin reverb. I put em in and they sound great, but moments later i noticed they were way too hot. I'm fearing for the integrity of the amp they're so hot. I read up on biasing and was wondering if this is the classic case and, if so, how do i fix it?


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Posted: Fri Aug 13, 2010 4:33 pm
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A '76 Twin Reverb does not have an adjustable bias.

The chassis is equipped with the infamous "balance" pot only.

I'll leave it in the hands of others to comment upon the relative merits of chicom-made glass.

Arjay

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Posted: Fri Aug 13, 2010 5:39 pm
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I would have bought some NOS 6L6 tubes. RCA, GE, Sylvania. I wouldn't put chicom-made bottles in any of my amps. If I don't go NOS I stick with JJ/Tesla.

I always let a good tech take care of my 76 Twin. Are the tubes red plating?

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Posted: Fri Aug 13, 2010 6:02 pm
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I wouldn't a got em if they weren't cheap as hell. the old tubes were busted and i needed something, but whatever, ill fix that later. They are red plating, at least all but the far right. Where should i go for this and how much do you think itll cost me?


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Posted: Fri Aug 13, 2010 6:35 pm
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For decent current-production glass I'd go with JJ's, EH's, or Tung-Sols. Expect to pay a C-note (at minimum) for a matched quartet.

Best of luck, HTH

Arjay

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Posted: Fri Aug 13, 2010 7:02 pm
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Eurotubes has a matched quad of 6L6's for 68 bucks.

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Posted: Fri Aug 13, 2010 10:59 pm
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Just thought it would be a good idea to test the sockets before putting another quad in your amp. I would swap the tube to the right with it's neighbor. If the one cool running tube reacts the same way(cool) in the different socket, then it seems the quad is not matched. It would probably be a good idea to clean the tube sockets before getting the new tubes. Oh yeah, what effect did the balance control have on those tubes ? It should generally be set at the mid point of rotation, when installing new tubes. Then adjust for the balance. Art

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Posted: Sat Aug 14, 2010 3:34 am
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You must check the bias when put news tubes. This amp should be modified for change the balance pot in bias adjustment pot.
Easy to do by any good tech.

But it's a must

Red plating tube : don't use the amp util a proper bias done. You will destroy this tube.


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Posted: Sat Aug 14, 2010 8:31 am
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After cleaning the sockets and pins of the power tubes & the amp still seems to run hot....

Have you used a DVM & are comfortable probing around areas with 400+ VDC? (I know it sounds silly, but it's what us amp nuts do all the time). :roll:

Anyhow, check the layout scheme (bottom portion of link) and you'll see an eyelet where there is "-37 VDC." [Yes, it MUST be minus volts]. It's on the small breadboard in the upper lefthand corner of the chassis layout, next to the pilot light (looking at the amp upsides-down). With amp plugged into speakers, zero volume, no plugged in guitar... this point should read -37 VDC, [+/- 2 VDC] (red DVM lead on eyelet, black clipped to chassis, DVM set to VDC). If not, you prolly will need to check the electrolytic cap, diode, and resistors in the BIAS power supply (those on the small breadboard).

If this sounds too ambitious or crazy (high voltage and all). Seak a good amp tech. Good luck with your amp!

http://www.ampwares.com/schematics/twin ... _aa769.pdf


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Posted: Sat Aug 14, 2010 9:31 am
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BMW said: .".......speakers, zero volume, no plugged in guitar... this point should read -37 VDC, [+/- 2 VDC] (red D....."

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I don,t agree with that.
This voltage is good only with the originals tubes when Fender built the amp.

You have to do a new bias with your new tubes


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Posted: Sat Aug 14, 2010 10:46 am
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No, I'm not saying he should not fine-tune the tube's bias to a tonal end-point, But, most Fender amps are designed to putout a constant negative voltage on the grid (hence, fixed bias). I suspect that the rectifying diode in the bias power supply is bad, a resistor has drifted way out spec, or the bias supply filter cap is bad. I wouldn't be surprised if his negative voltage reads near zero. No amount of adjustment with an added pot will bring the tubes into spec.

Oh BTW... you can measure this BIAS power supply voltage with the power tubes out of the amp (in fixed bias amps). In fact, ALL of the tubes out. Therefore, protect $$$ investment in tubes.

Just my 2¢ worth. But, I have seen this is older amps with original parts.


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Posted: Sat Aug 14, 2010 11:10 am
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BMW2002Ti wrote:
I suspect that the rectifying diode in the bias power supply is bad, a resistor has drifted way out spec, or the bias supply filter cap is bad.


That'd be my diagnostic starting point as well.

Arjay

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Posted: Sat Aug 14, 2010 12:02 pm
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okay, so how much would this run me to have a tech do it? I don't have a dvm.


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Posted: Sat Aug 14, 2010 12:12 pm
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oooooo! I swapped the working tube with one of the ones that was red plating and they didn't change! the working one still worked fine and the red plating still red-plated! also, one of the jj's it came with (it came with 2, but one broke, hence the new tubes) works fine in any socket, as did the other one before it broke (a leak i think). Does this mean I just need new tubes? i found a quartet of NOS russian 6l6's on ebay for 30$, good buy?


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Posted: Sat Aug 14, 2010 12:44 pm
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Prolly doesn't mean you need new tubes. If one set works "fine" and another doesn't, it may mean the "red plate" set may not be a true 6L6GC-type tubes and the current idle bias cathode setting of your amp is too hot for these tubes.

May wish to have the idle bias current checked wih a tech. And have him rewire the balance pot to adjust the bias. Just as was stated before by another poster.

Good luck with your amp!


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