Glowing plates - is this OK?

Randy

Member
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3,911
I just tried a set of 6V6's in a cathode biased Two Rock. The tubes sounded great but were running pretty hot. I noticed that on one of them the plates were glowing red for a little while, and then it seemed to go away. I wasn't running the amp very loud. The tubes were Russian made EH labeled.

Is this normal? I've heard that most modern 6V6's are not hearty enough to handle some modern amps. What will happen to the amp if I fry a tube? I don't want to damage anything.

Thanks
 

Fuchsaudio

Member
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7,721
The EH-6V6's can be marginal when run above 400 volts. They suffer from inconsistant screen grids (from tube to tube), which results in some tubes running okay and some turning red from electron bombardment of the plate. This is because the screen is not holding back enough electrons from the plate. It's not really an amp fault, but more a tube fault.

I'd see about buying a few spares and swapping them out until you have a set that's not "red-plating", or buy a set of GE,Mazda, or RCA NOS blackplates. The older tubes could take the abuse a little better. Svetlana will have a new 6V6 out by summers end that sounds encouraging......
 

Randy

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3,911
Thanks for the info Andy. I've got some NOS Phillips that I was saving but I guess I'd better break 'em out.
 

jzucker

Supporting Member
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20,720
Originally posted by Fuchsaudio
The EH-6V6's can be marginal when run above 400 volts.
Lord Valve tested them at 515 volts and they seemed to work fine biased for 12 watts and setting the bias using this formula:

12W / 515 = 23mA maximum bias current
70% of 23mA = 16 mA (safe level)
 

Fuchsaudio

Member
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7,721
Originally posted by jzucker
Lord Valve tested them at 515 volts and they seemed to work fine biased for 12 watts and setting the bias using this formula:

12W / 515 = 23mA maximum bias current
70% of 23mA = 16 mA (safe level)
I never said they wouldn't work, but I worry that they're not gonna stay happy too long above 400V. I use them at 400V in my ODS-30's with few problems. I see them in Deluxes running 430 to 450 V and sometimes they have red-plates , even when biased correctly.
 

jzucker

Supporting Member
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20,720
Originally posted by Fuchsaudio
I never said they wouldn't work, but I worry that they're not gonna stay happy too long above 400V. I use them at 400V in my ODS-30's with few problems. I see them in Deluxes running 430 to 450 V and sometimes they have red-plates , even when biased correctly.
I'll have to ask LV about that.

Here's what he said about them:
SOVTEK 6V6EH
Well, those Sovtek fellers have finally gotten off their
fat Red asses and made a *GOOD* 6V6. Frankly, I never had
much hope that I'd ever see a good current-production 6V6
again and I was pretty sure that if it *did* happen, it'd
come from damn near anywhere besides Sovtek. Surprise,
surprise! This tube sounds *great* in Deluxe Reverbs, and
it handles the high plate voltage in these amps with ease.
(Mike Matthews told me they designed it with 500 volts in
mind, just to be sure that it would survive at the 440V
typically found on Deluxe Reverb plates.) Field failure
rates on these are *very* low. I do, however, recommend
that you change your screen resistors to 1K/2W (or you
can use the more common 5W "sandbox" style) for added
reliability...with *any* 6V6, not just the EH.
and...

The new Sovtek 6V6EH can easily run at 450 volts and over. (I burned four of 'em in at 515 volts, 15 watts static to see if they'd die and they just laughed at it.) Just change the screen resistors to 1K 2 (or 5) watt and tweak the bias component values to get the supply into the right range. All of these changes are easily reversible, and the whole conversion is going to cost less than $25 INCLUDING the tubes.
Give it a shot and let us know how it sounds.
 

Fuchsaudio

Member
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7,721
Jack:

Perhaps Sovtek has improved them recently. The current batch I have on-hand in the shop has been reliable and sound good, although I haven't torture-tested them.

I did have some problems in the past with higher voltage amps like the Deluxes, and esepcially Kelleys, but perhaps they've been addressed with current production. The original batch I got from Sovtek, included a data-sheet, and they showed some scary (amazing) currents and voltages on the curves which simply did not work without those spooky "red-plates" showing up. Their expectations (or production standards) did not meet the actual tubes at all. It's nice to know their striving to improve quality.

I'm also looking forward to the new Svetlana 6V6's due out in the fall. I think GT would be wise to consider an American 6V6 in the future.
 

jzucker

Supporting Member
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20,720
I think LV's tests were on the original batches. Maybe the quality has gone down since then. That wouldn't surprise me knowing Sovtek...

BTW, Who's making the best EL-34 these days?
 

Blue Strat

Member
Messages
30,066
Check the Fender Forum for reports from dozens of techs across the country who've had EH 6V6s fail in Princeton and Deluxe Reverbs. From a small sampling, it looks as though 10% of them fail early. This is a horrible record for any electronic component.

BTW, is that amp supposed to use 6V6s? If not, it's probably not a good idea to use ANY 6V6 in it.
 




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