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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Sat May 07, 2011 8:32 pm
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keep us updated for sure, i had the best of luck with them in several DRRI's

I have a pair for back up for my 5E3 Clone, The glue is still holding on the center pin of the "other pair" in my amp now :wink:

The Tung-Sol reissues will hopefully hold up and become a trusted source of present day 6V6's.

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Sun May 08, 2011 12:44 am
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I'm hopeful that BMW2002Ti's experience was an isolated incident.

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Sun May 08, 2011 4:17 pm
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Me too! Both pairs (bought from two different suppliers).... went red plate at about 20-30 hours of pretty heavy use (volume well above "5" in two different Princeton Reverbs).

There was a post here & I've seen it on other boards where the poles holding and aligning the plates and grids MAYBE a bit thin. And subject to thermal distortion, if the amp is really pushed. All just conjecture.

The dude I sent my bad tubes to, is off to see Mother this weekend. He should be reporting what his lab results, including dissecting the bottles--- next week.


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Sun May 08, 2011 9:12 pm
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I wonder if both pairs were from the same production run?

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Mon May 09, 2011 5:46 am
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Like I said somewhere before,talking to a guy I buy NOS tubes from a week ago,he touched on the metallurgy,(sp?) of the new tubes and that in his opinion there's not the science involved like we had in this country when our tube companies were producing...that's not to say the Russians and ChiComs don't research their products,but from what I've read in the past,when US tube companies stopped production,several people like Aspen Pittman and the guy from Eurotubes,can't remember his name,went overseas and talked the foreign companies into producing tubes for guitar amps.
Relabeling foreign products with past well known US brands like Tung-Sol and Jensen speakers,and UK brands like Mullard even down to the brand logos and boxing will sell those products because of the familiarity and trust associated with the brands of the past...but it does not ensure the same quality.


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Mon May 09, 2011 8:55 am
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Results from a Tube Tester/Seller I know: (Jim McShane)

I'm sorry this took a bit - with the Mother's Day festivities here my weekend was pretty much shot. And I lost a power trafo on one of my HP712B bench supplies first thing Friday AM - off to Gary Brown from TRS in Maine to rewind it and the other power trafo in that unit. But that set me back 1/2 a day too.

So EARLY (only for you would I get up at 5:30 to test tubes Steve!!) today I put the Tung-Sol reissue 6V6GTs that Steve sent me through their paces. And as a control, I took a brand new one out of my stock and checked it the same way.

My observations...

1. The tubes came from Antique Electronic Supply (or their wholesale division, CE Distribution). I have never really gotten a clear answer from them regarding how much run in the tubes get before they match them. The most recent explanation I got was they run them until they are fully warmed and measure to match at that point. The tubes were well matched for total cathode current, but they were also a pretty "hot" pair - meaning at a given set of voltages these tubes passed more cathode current than I usually see by about 33%. Honestly, I would not have sent out this pair for guitar amp service.

Examining the tubes I saw no evidence of any damage from overheating or any other physical abnormalities. There was no rainbowing around the edges of the getters nor any loose material inside the tubes indicating the tubes had badly overheated or run away.

2. Preliminary testing on my modified TV-7B Hickok/military tester was done, and both tubes passed both shorts and "gas" (grid current) testing with flying colors. The tubes went in the TV-7 while they were cold and were allowed to warm until stable test readings could be obtained. The tubes had a measured Gm of about 3500 to 3600 umhos.

3. The tubes were placed in the "McShane" match rig and set up to run at 330 volts on the plate and screen and -25 volts on the control grid. The grid resistor value I use is 100K Ohms.

The tubes were run for 10-15 minutes to stabilize, and then I began making observations:

a. The cathode current (plate current + screen current) on both tubes was around 41 ma. That means that at that point the tubes were dissipating 330 volts times .041 amps, or 13.5 total watts. This is just under the rated total dissipation of a 6V6GT or GTA. Plate dissipation was 12.9 watts which is .9 watts above the Tung-Sol data sheet design-center limit of 12 watts.

b. Both tubes ran around 1.6 - 1.7 ma of screen current, so screen dissipation was just over 1/2 watt, well under the Tung-Sol 2 watt spec.

c. Under these operational conditions a bit of red-orange could be seen in a plate crease on one side of the plate structure in a room lit only by daylight through one window (moderately dim). Shielding the light from the tube made a red-orange stripe in that crease clearly visible. This was the case on both tubes, although one showed more color than the other at this test point.

The tubes' electrical performance was stable and no signs of runaway, etc. could be found.

d. The bias voltage was changed to reduce cathode current to 35 ma. This reduced the total dissipation from 13.5 watts to 12 watts. The tubes were allowed to stabilize and I repeated the observations as detailed above.

The tube that showed more color now just showed no color in the ambient room light. Shielding the tube from the light made it possible to observe a barely visible hint of color in the crease. At this point both tubes showed the tiniest hint of color that was only visible under near darkness conditions.

As before the tubes' electrical performance was stable and no signs of runaway, etc. could be found. In normal room lighting no color was visible on the tubes.

e. Finally the bias was adjusted again to reduce the cathode current to 30 ma, making total dissipation just under 10 watts.

At 10 watts no visible color was observed even in very low light conditions, and tube operation was 100% normal.

f. While still blazing hot from the tester the tubes were re-checked for shorts and gas on the TV-7B. These tubes both illuminated the "shorts" indicator when first placed on the tester. This is normal, as the shorts test calibration is based on the tubes being run in the TV-7, not at the dissipation limits. MANY power tubes do this, whether old stock or current production. These particular tubes however did illuminate the shorts indicator more brightly than most, indicating more electrode leakage than "normal". Once the tubes ran in the tester for a few minutes the shorts indication completely disappeared. Again, this is consistent with typical behavior - when the tube cools to the temperature the tester normally runs them at there is no visible indication at all.

g. The "control" tube removed from my stock showed just the tiniest bit of color at 13.5 watts, and no color at 12 watts. 10 watt testing was unnecessary. When placed in the TV-7 this tube showed very little shorts indication and what it did show went away in a few seconds.

Conclusion - as engineers sometimes say, this seemed to be an unfavorable combination of tolerances coming together to cause trouble.

1. I believe Steve's pair of tubes had not been properly run in and checked. I also believe these particular tubes biased hotter than some amps would like to see.

2. The presence of red in just that one particular spot indicated a bit of screen misalignment may be present in the tubes. In service in an audio amp (or in many guitar amps) this would have been completely unnoticed. The tubes would have given fine service under most conditions.

3. Steve's observations of color on the plates at well below 10 watts indicates something else is affecting the tube performance besides the tubes themselves. It is possible the plates simply didn't have adequate time to cool after Steve reduced the dissipation and therefore showed color for a while which he saw as a continued problem.

4. The grid resistor values are too high for use with fixed bias. The Princeton Reverb uses 220K grid resistors - Tung-Sol's data sheet shows 100K maximum for use in fixed bias, 220K in cathode bias.

I believe that the combination of:

- hot biasing tubes causing more interelectrode leakage than "normal" and...

- tubes with SLIGHT screen misalignment, and...

- excessive grid circuit resistance

was the cause of his red plate difficulty. The combination of those particular tubes and that particular amp was simply not "made in heaven".

I would replace the grid resistor 220K with 100K parts to bring it onto spec. In the future I would suggest that Steve purchase tubes that bias cooler, and that have been run in for 24 hours or more to ensure stable
operation. If it were me I would install 1 Ohm or 10 Ohm cathode resistors to make bias measurement easier down the road.

So Steve - change the grid resistors and plug in the new pair of tubes I sent you!!

More info:
Yes, switching the resistors will have some tone impact I suspect. Remember the grid resistors are in parallel with the plate load resistors of the prior stage. Dropping the resistor values reduces the stage gain and that can change tone (as I'm SURE you know)

You could try using a resistor that's not so far out of spec - try a 180K or 200K and if that doesn't work drop down until you get stable happy tubes. Maybe with a better tube pair you'll be able to cheat a bit!

You could also make a small increase to the plate load resistor on the last 12AX7 section. Stock it appears to be 56K. You could go to 68K or maybe more - just be sure you change the one in the cathode to the same value since this section is the PI. If the plate voltage drops too much you could raise the supply to that stage by reducing the value of the 18K dropping resistor in the power supply just ahead of the 20/450 filter cap section. Current through that resistor is about 5.2 ma - so each drop of 1K Ohms will raise the voltage about 5 volts. Maybe 15K would be about right, you'll have to experiment some if you decide to do it. Don't go too low as the first stage voltage will go up some as well. One last thing - that resistor dissipates almost 1/2 watt - I'd use a 2 watter in there as opposed to 1 watt for durability and reliability.

Also, you might want to increase the value of the .1 coupling cap - again keep in mind that the cap forms an R-C network with the grid resistor. With .1 uf and 220K you get a -3db point around 7 Hz, with 100K it's around 15.5 Hz. If you change to .22 and 100K you'll have a -3db point identical to stock. But if you use a 180K or such and it works well then you'll be close enough at 8.8 Hz. Below 180K you pretty much need to raise the value of the cap. At a grid resistor value of 150K a .047 in parallel with the .1 would be excellent.

If the 220Ks in there were original they may have drifted up well above 220K - FYI I just replaced some 1 watt carbon comps in my bench power supply that were banded as 200K - but they measured 271K!! S**t happens after 40-50 years of use!!

When you finish I'll bet it'll sound spectacular!!



Thanks for giving me the opportunity to investigate this for you.


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


Last edited by BMW2002Ti on Mon May 09, 2011 12:18 pm, edited 1 time in total.

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Mon May 09, 2011 10:25 am
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Looks like you should save those tubes that red-plated for use individually in a SE application or restrict their paired use to a Princeton Reverb.

Arjay

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Mon May 09, 2011 4:47 pm
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Later this week, gonna rip open that PR and double check those 220K-ohm (5%) resistors hanging off the grids. Waiting for 180k-ohm and 200-kohm resistors to come in from Mouser.

Will try 200-Kohm first with the new set of matched tubes (lower current draw mates on got from Jim). Check new bias, then put on a few hours and re-check bias. Tell you the results.

If the plates start to go red again, will drop down to 180k-ohms. I'd rather not have to go to 100-kohm and change the coupling caps. But, if that will make it stable...

Not too sure how this all will affect NOS tube tone? So I'll swap out the TSRI with NOS RCA's and tell you the tonal response.

Hope all this helped. Wanted to thank Jim, again for all the data & the great posting summary. Much work went into both. He is one of the few I know that matches and tests power tubes and rectifiers at real amplifier voltages and currents.

http://www.ampwares.com/schematics/prin ... aa1164.pdf


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Mon May 09, 2011 7:24 pm
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Wow,never seen so much investigation go into a pair of tubes that cherry plated.... :shock:


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Mon May 09, 2011 8:15 pm
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Rebel, There's a hidden meaning (maybe not so hidden) to all of this. This is prolly one of the best new 6V6GT available. However, the Russian tube is (and it's my WAG) built to original data supplied by TungSol, in the 1950's.

TS and most all those old companies were noted for overbuilding all of their tubes (as most mil spec suppliers were in those days). And Leo prolly knew that "paper tolerances" and real life tolerances were two different things. Thus the 220k-ohm grid resistors, which are important in final idle bias of the tube.

These New Sensor TS branded 6V6GT maybe made using Reflektor rigs when they were making 6P6S (6N6C) tubes in 1970-80's (another WAG). And similar issues of red plating did occur with those tubes, too. Built to "paper tolerances" so to speak.

Bottom line: may need to change certain parameters around the power tube stage, to keep this new tube happy and long living. As original parameters were built for old time NOS tubes. Why I still love to use old NOS mil spec 6V6GT and GTY's. They are tougher than ANY 6L6GC made. Or I should say, can take hours of abuse much better.


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 10, 2011 4:44 am
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I would say, not so hidden. :wink: These TS tubes it seems, simply can't handle too hot of a bias (way hot). I still say that it is a simple matter of the TS tubes drawing more idle current than other tubes at the same setting. This seems to be a very likely conclusion that is substantiated by all of the testing and data that you have given, particularly the fact that it was both output tubes red plating, and happened with two different sets of TS, while the other tubes were fine. A bias pot and a reasonable bias level (ie 70% or less), it would seem, would solve all problems. I am still not clear, is there no bias adjust pot on your amp? :idea:

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 10, 2011 5:02 am
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Red glowing tube is ALWAYS a wrong bias : too hot.


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 10, 2011 5:21 am
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Hi stratele52,

Haven't seen your posts for a while, have you been away? I have seen some very extreme differences in idle current draw between different types of output tubes with the same bias setting (same negative volts on grid). The most extreme difference that I have seen being 32 mA per tube with one set of tubes, and approximately 325 mA per tube for the new set, both readings were with the exact same voltage on the grid circuit. Luckily I had my meter connected when I powered up and the red plating had just started, about one second, so I was able to shut down before any damage was done. A quick adjustment of the bias pot, and all was well. In fact, the old tubes would not draw more than 32 mA each, even with the bias pot at 100%, while the new tubes would draw much more than their rating at the same setting. :idea:

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 10, 2011 7:20 am
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I'm little bit tired of amps and guitars forums. Good to know you are there shimmiliou.

About bias, you are right about grid negative voltage. I know that this voltage mesurement is not very usefull for a good bias. I mesure Kathode current.

With new tubes ,I adjust the neg volt potentiometer to max neg grid voltage, after that I check kathode current and turn the neg volt pot for the good bias ( kathode current) .

No adjustment pot : I install one each time I fix amp


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 10, 2011 9:49 am
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In the future, I'd buy tubes from Mike Kropotkin - have yet to go wrong with him in 9 years, and he's helped me tube up my Princeton, Princeton Reverb, and Bandmaster.

His 6V6 tubes that he carries:

http://www.kcanostubes.com/catalog/21

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