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Post subject: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 3:23 am
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The Scene: at last night's gig, one of the Tung Sol 6V6 tubes in my DRRI surged into the red plate zone (the tube nearest the GZ4 rectifier) = I was alerted to this by loss of power with crackling/popping/farting sound.

I switched off and after a minute, it was all OK, but it soon returned to its glowing state making the amp unusable. (also noted some 'purple haze' happening on that tube only whilst it was red)

So, immediately dropped in a couple of replacement tubes (also Tung Sol) and carried on OK with the rest of the set.

But curiously, I then then experienced the same issue again with the second pair of tubes - same position again, adjacent to the GZ4. Switched off, back on again and this time they held out OK for the rest of the night.

Now I know there has been some concern on this forum about the reliability of Tung Sol 6V6s, but I love their adorable thick blues tone without the need to resort to expensive and for me probably unobtainable NOS tubes.

I should say I'd biased the TS at 25mV and amp has hitherto proven to be very reliable as fitted with TS tubes.

Just keen to understand if this appears likely to be a tube issue with Tung Sol 6V6, OR perhaps something to do with the associated circuitry? (grid resistors?).

Any views to help pin this down issue with a little more clarity would certainly be appreciated as I try to locate a trustworthy tech. in my local area.

Thanks
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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 6:59 am
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On schematic , this amp have close to 400 volts on 6V6 plate. 25 mv is too hot for one 6V6, IMO

I assume you mesure this 25 mv at the cathode of the 6V6 tube with a 1 ohms resistor ?


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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 7:28 am
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The DRRI has 2 x 6Vs output tubes?

25mV does sit within within the recommended tolerances I've picked up on and seems reasonable?

I measure it using a propriety test valve base (with flying test leads) into which I then re-insert the 6V6.

I would not expect 25mA to cook it?

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 8:19 am
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One of our forum members "BMW2002ti" has done extensive bench testing of these Tung-Sol re-issue 6V6GTA's and experienced similar results in Fender class AB amps (his primary test vehicle was the Princeton Reverb platform). Several test pairs of new T-S tubes exhibited severe red-plating under normal operating conditions to the point that the test amp was imperiled. IMO there is something seriously amiss with these tubes and I would not recommend their use, at least in a Fender amp. You might try the Electro-Harmonix or JJ 6V6GTA for a reliable, cost-effective alternative.

HTH

Arjay

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 8:20 am
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6V6 GT are good for 12 watts . If you bias it at 25 mv ( this mean 25 ma ) your tube work at 10 watts , no signal, very high and way too high IMO
If you biased it at 21 it work at 8.1 watts, no signal

If you play loud you have more power dissipated. That 's probably why you have red plating at 25 ma.

You just have to test it with lower bias like 21 ma and you'll see. Tubes won't red plating if is not too late..........


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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 8:25 am
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stratele52 has a good point

What is the max Plate voltage rating, and the max Plate watt rating of the Tung-Sol that you use? The chart that I have, shows a max of 315 Plate volts and max 12 watts. At 400 Plate volts, 21 mA Plate current will give 70% of the tubes rating.

http://www.dougstubes.com/6v6gt-tungsol.pdf

If the chart is right, both your Plate voltage and bias setting are too high for this tube.

I suspect that this particular tube just can't be run that hot (25 mA) at that high of a Plate voltage. If the tube can handle 400 Plate volts, then in this amp it has to be at the limit, pushing it IMO.

Contact the manufacturer, or reseller for a current chart for the Tung-Sol 6V6 to see if the values are still the same. :idea:

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 8:31 am
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I am also thinking they don't like 25mA at high volume (I was pushing them nicely).

I'll try lowering the bias to 21mA, but 25mA was the 'sweet spot' for my ears.

Worth noting my other post today on the Telecaster section regarding damage sustained to my Telecaster fret-board also last night - it was not a good gig by any measure.
:(

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 8:34 am
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Feedbackboy wrote:
Worth noting my other post today on the Telecaster section regarding damage sustained to my Telecaster fret-board also last night - it was not a good gig by any measure.


"Some days you eat the bear, some days the bear eats you."

$hit happens.

Arjay

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 8:41 am
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Retroverbial wrote:
...You might try the Electro-Harmonix or JJ 6V6GTA for a reliable, cost-effective alternative.

HTH

Arjay


+1 on the EH

I have a pair of EH 6V6 idling at 14 watts ( 38 mA at 368 Plate volts), in an old amp similar to a 5E3 Deluxe, for about 5 months now, with not so much as a hiccup. Beautiful sound, and the Plates get no where near red! This is very extreme, so under normal conditions, this tube should do very well. Just got another set to install in my HRDlx, with 430 Plate volts. The EH 6V6 are rated for 450 Plate volts and 14 watts.

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 9:40 am
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I gave up on Tung-Sol tubes after experiencing my own problems with them... then, after reading what BMW2002ti had to say about 'em, it confirmed that I wasn't alone.

I've been extremely happy with the TAD 6V6 in my DRRI. About 4 months ago we had a discussion here about DRRI bias... so I decided to run mine hot at 23 mA. Well this lasted 4 months. Monday night at the jam I had the amp kinda loud, it was Halloween :D we did 5 or 6 songs and it sounded wonderful as usual. Then I handed the guitar to another musician and after a couple songs he mentioned that the amp was acting up and making funny noises, I didn't really hear it... it wasn't until yesterday that I got to hear it while playing at home, looks like one of the tubes was dying! Not to risk any damage, I removed them and put the RCAs back in, all back to normal now. I'll drop the bias and put the TADs back in to test them.

Maybe we're pushing these tubes too much when we run 'em that hot and unlike NOS they can't take the abuse... I ran the RCAs at 23mA for a while and never had an issue!


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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 9:53 am
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I don't know about the DRRI,but vintage DRs push 6v6s past the specs...if you look at the tube manual and then the voltages you would think they would fry,but then American made tubes were built better in those days....nowadays 6v6s are "iffy" for lack of a better word,you have to experiment some to find what works,and hope your amp doesn't get screwed up in the process.


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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 10:26 am
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Sorry, I didn't see this thread until now. I've had a lot of problems with the Reissued TS 6V6GT. They sound so good, that I tried a lot of things to keep them happy.

1.) Changed the grid-to-ground resistors to precise 150K-phm resistors (from the OEM 220K-ohm ones).

2.) Put is new 1.5 k-ohm grid stoppers.

3.) Put in 470-ohm precise screen resistors.

4.) Re-do the Bias Power supply. Caps, diode and resistor to get nominal negative voltage off the supply.

5.) Bias the tubes a bit colder. Around 6.6 watts of dissipation per tube (16mA at 415 plate volts--not taking into account any drop across the cathode to grid).


I still had thermal runaway, with two out of 6 tubes.

My conclusion (plus some similar experiences with those on this & other boards): This tube can have issues being run at high plate voltages. It may have been designed for 300-350 VDC range, like all of the original TS and RCA data describe. I say that, because I have not had issues with thermal runaway in single-ended Champs that run the tubes around 350 VDC.

http://www.mif.pg.gda.pl/homepages/fran ... /6V6GT.pdf

I do not use this tube in any of my amps. And haven't really persued the plate voltage issue rigorously.

Currently running TAD 6V6GT-STR in one of my PR. Same amp that had issues with the TSRI 6V6GT. So far so good.


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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 10:40 am
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Pleased you note how delicious these tubes sound though.... :P

But thanks for a really useful insight BMW - do you think I am wasting my time lowering bias current?

If so, I'll revert to the Fender GTs that it shipped while I search out an alternative UK source equivalent.

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Last edited by Feedbackboy on Sun Nov 06, 2011 10:41 am, edited 1 time in total.

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 10:41 am
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BMW2002Ti wrote:
My conclusion (plus some similar experiences with those on this & other boards): This tube can have issues being run at high plate voltages. It may have been designed for 300-350 VDC range, like all of the original TS and RCA data describe. I say that, because I have not had issues with thermal runaway in single-ended Champs that run the tubes around 350 VDC.


+1

The DRRI pushes 391VDC to the plates -- the vintage AB763's, even higher (around 415VDC). Using the T-S's with anything above +/- 350VDC is a crapshoot.

And losing that "bet" might proved to be catastrophic.

JMOOC

Arjay

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 11:00 am
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"Do you think I am wasting my time lowering bias current?"

Well, that's a personal descision, I guess. The one thing that kinda bothered me was I bought a couple of matched quads from a very reputable dealer that tests tubes at amplifier currents and voltage range, not tube tester ranges.

One set worked well for a good 20-30 hours--- then one tube went ballistic. I'm not sure if the TSRI tube is more sensitive to amp instabilities or certain current or signal frequencies--- but I had the same issue with another set. Both went red plate after a few hours of use. Not right away.

The amp that was use has had NOS RCA and Raytheon 6V6GT and 6V6GTY's in it for years, with no problems. So, even though I redid the entire power section to keep the TSRI happy, the components were not old and within specs.

If you really like the TSRI's, you might want to try dropping the plate voltage down to 350 VDC range. Rebias for about 6 watts idle bias per tube. I haven't tried this, nor anyone I know of, that has had issues with this tube. Just a suggestion.


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