RCA 6L6GB in Fender Vibro King - diyAudio
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Old 30th September 2011, 08:44 PM   #1
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Default RCA 6L6GB in Fender Vibro King

A friend of mine blew up a 6l6gb by using a pair in his Vibro King. It also took the screen resistor with it. He purchased three pairs from fleabay under the advice of the seller that they are good replacements and that they can handle the high voltages seen in a guitar amp. He was trying to get less volume and earlier breakup so someone recommended 6l6gb or 5881's. The datasheet for max ratings say 360v at plate and 270v at the screen. The VK schematic says 480v for B+ and I am sure the screen sees just under that. Well my opinion is that the plate may be able to handle the high voltages as long as the dissipation is set correct (19watts) but the screen is what worries me. He does have a pair of RCA 6l6GC black plates with the side getter and I told him to bring the amp by and I will help him put those in.

Has anyone used 6l6gb in such a high voltage circuit with positive results? If so what was the screens seeing for voltage? And how much do you think the screen can actually handle? I just want to help him out and make him feel like he didn't waste is money. Maybe up the screen resistor value to get a larger voltage drop so maybe the screen only sees around 325v?
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Old 1st October 2011, 12:08 AM   #2
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There are a few threads here about the Shuguang 6L6GC's. They seem to be pretty robust,if you get a decent batch.

6L6GC -Which Shuguang can take the Heat?
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Old 1st October 2011, 12:37 AM   #3
Enzo is offline Enzo  United States
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And chances are the Vibroking didn't blow up the tube, more likely the tube failed, and damaged the amp. ANy tube can fail at any time
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Old 2nd October 2011, 08:56 PM   #4
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Obviously the tube failed, but the screen is seeing 200 volts over the max rating according to RCA's datasheet. I know the datasheets are conservative but 200 volts over is kind of a lot.
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Old 4th October 2011, 01:35 AM   #5
Enzo is offline Enzo  United States
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Yes, and yet thousands and thousands of these amps have been running over the last 50 years without incredibly high numbers of tubes being destroyed.

Leo Fender ran 6V6 tubes at 100v over their "maximum" voltage ratings for decades.

The RCA tube manual published specs for their tubes. They were the numbers they wanted engineers to use when designing consumer products - TVs and table radios. They warraqnted that within these specs, you could expect long reliable life from their tubes. AFter all, no one wanted a radio that needed new tubes every few months. But that didn't mean that the tubes were on the verge of exploding if you exceeded those specs. I do EXPECT to replace the power tubes in my amps yearly if not more often.

Thus my point was that while we both agree the tube failed, I am doubting it was really the amp's fault.

VOltage doesn;t kill tubes, dissipation does. If screen grid current is limited, then it won;t melt and short out the tube. If plate dissipation is limited, then the tube doesn;t melt.
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Old 4th October 2011, 03:36 AM   #6
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Quote:
Originally Posted by Enzo View Post

VOltage doesn;t kill tubes, dissipation does. If screen grid current is limited, then it won;t melt and short out the tube. If plate dissipation is limited, then the tube doesn;t melt.
And given the higher rated 6L6GT no harm would have been done. But putting it's smaller sibling into an amp which is built for a larger plate and screen dissipating tube it is not surprising. Either the circuit has to be modified to keep the tube withing safe limits or the same result is bound to occur.
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Old 4th October 2011, 04:04 AM   #7
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I used to put 6L6's in my Fender Vibro-Champ and ran it for a long time until a fried the OPT by paralleling 3 speakers and driving it to hard.

The only real problems I had was some of the tubes that I had would arc from the plate pin to the one next to it and this would burn the bakelite base.
This would cause a low resistance crack and it would continue to burn when I re-inserted the tube.

Other than that it worked fine and gave me a few more watts.

This was way before I knew anything about tube bias and should be rebiased if you plan on getting the most out of it.

I believe if you just replace a 6V6 with a 6L6 the 6L6 will be flowing more current than the 6V6 will when sitting at idle.

This may have been a contributing factor to the demise of the OPT besides the low impedance that I was using,As I quite abused that amp but it sure sounded sweet for a blues type amp. he,he

jer

P.S I still have that same amp ,still, with the same fried OPT I just use it for a preamp these days.

Last edited by geraldfryjr; 4th October 2011 at 04:10 AM.
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