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Post subject: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 03, 2011 12:03 am
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The night before I noticed odd harmonics and early distortion on my PR. I checked the tubes and the TSRI were RED HOT! Almost flaming... The CBS rectifier was perfectly fine. When I replaced them with used 1950 Hytron 6V6GTY, same settings, no problems. So today....

1.) I replaced all the parts of the bias power supply and the intensity pot (as it controls the vibrato via the negative voltage off the bias supply). I used a Fairchild UF4004 rectifier. I even dropped the negative voltage a bit (to about -36-38VDC--- OEM recommends -33VDC).
2.) I changed out the new cap can (in case it was defective).

3.) I changed all power supply resistors.

4.) I replaced the 1k-1watt droppping resistor with a 1k-ohm, 2 watt Caddock metal film resistor (this one drops the voltage to the screen grids). I happen to have a few laying around.

5.) Replaced all resistors more than 5% out of tolerance. Checked all coupling caps for leaks and replaced one.

Unfortunately, when I put those TungSol RI 6V6GT back into the amp, There were problems. Not the flaming red of the other day, but the center of both sides of the plate had a distinct orange-red glow. Even even the volume at around "2."

If I cranked up the amp, the glow became brighter and covered more of the the tube's anode plate. And the amp distorted at a rather low volume (around "4-5"). After a few minutes I pulled both tubes. They were extremely hot.

At the same settings, I tried the Hytron, RCA 6V6GT, GTA's, and Sylvanias. No problems.

I believe that I bought these tubes from Tube Depot or Musiciansfriend. So, I'll try other sources.

BTW... the PR runs about 425 VDC onto the plates and 415 VDC onto the screens. Class AB1, push-pull. Fixed bias.

Sorry for the bad news. Maybe just a bad pair of tubes.


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 03, 2011 12:45 am
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Hook-shot those turds into a convenient dumpster lest they inadvertantly find their way into another amp.

Better to eat a pair of tubes than to lose a vintage tranny -- or worse.

:(

Arjay

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 03, 2011 5:26 am
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Yesterday afternoon after reading your earlier post about this,I replaced the T-S 6V6GTs in my '65 Deluxe Reverb,I get skittish when it comes to these things...mine seemed to be doing fine though,but I still want to try and darken the amp up some,it is really bright,I was going to install a NOS pair of RCA 6V6GT blackplates,but saw a NOS pair of Brimar 6V6GTY tubes I'll try for awhile.
I like trying different tubes,but I'm going to watch some reports of the new Tung-Sols for awhile.


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 03, 2011 5:48 am
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:? You had bad output tubes.....so, you rebuilt the power supply in the amp? Mind if I ask what you were thinking? :?:

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 03, 2011 9:36 am
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Shimmy,

The 1-k-ohm,1-watt resistor off the "square" designated cap drops the voltage to the screen grids (usually around 10 VDC). I changed the cap can, because I have gotten bad NEW cans (and we ain't just talking females :shock: ) before.

I checked all tolerances to rule out any source of parasitic or other oscillations. Including soldering, wiring, wire layout, and groudning points.

I changed all components in the bias power supply to insure steady negative voltage. I even checked and tightened all sockets tangs for the heck of it.

Coupling caps were double checked for DC leak.

Basically, I wanted to rule-out as many other issues that may have caused the tubes to glow red (and this happened about 20-30 hours into use... it did not happen initially).

The reason why is I've heard such good reports on the new TSRI 6V6GT. But, you know... I had a similar problem. About 8 years ago I bought some Russian made 6N6C mil spec tubes. They worked fine, until aout 30 hours of use. Then, they red plated so bright you could see them from in fron of the amp. I'm wondering about these "new" TS....

I will try another tested/matched pair... just to be sure.

HTH! Have a good one.


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 03, 2011 10:22 am
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What Plate current are you running for each type of tubes? I would think that after trying four other sets of tubes with no problems, that would be a good indication that the amp isn't the problem, only the tubes, or actually the idle current setting for the tubes. :wink:

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 03, 2011 2:22 pm
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Shimmy,

You are correct. But the TSRI 6V6GT has gotten such good ratings... and BOTH tubes went equally ballistic. I wanted to rule-out any amp problems. Mostly, oscillation issues, as that is what a lot of folks would ask before deciding I happened to buy two bad tubes.

And these tubes didn't just go bad, right from the get-go. They had about 30 hours on them, before going VERY red & sounding terrible. That would sometimes indicate a parasitic problem. A problem that maybe the older NOS 6V6GT can handle, but the newer TSRI is more sensitive to.

Going to try two new TSRI in the same amp and get back to the board on the results.

BTW... I ran 18mA initially on the tube. Then after mods, around 16mA per tube.

Thanks, for the comments.


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 03, 2011 2:49 pm
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Let us know the results after trying a different set of TSRI's -- I'm damned curious as to what caused them to prematurely fail.

Arjay

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 03, 2011 2:55 pm
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Cool, I think that I understand, thanks for the answers. I thought that maybe you had a fixed bias amp, and that the TS were simply pulling more current at the same bias setting than the other tubes, but apparently not. :)

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 03, 2011 10:27 pm
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Sorry,

I used two TSRI 6V6GT bought from another seller. These had 10-02 date of manufacturing. The first set had 10-12 dates and OTK 2 stampings.

The 10-02 went into the same amp (1972 silverface Princeton Reverb, AB1164). And within about a minute, one plate started glowing red. The other tube started glowing about 5 minutes later. It glowed less.

As you know, one thing to bad about nearly all Fender amps, is you CANNOT see the tubes unless you remove the top back panel. So it's easy not to see overheating, in a normal setup amp.

I warmed the amp up about 10 minutes and played a few chords and solo stuff for about 15 minutes. At about 10 minutes, the overdrive distortion became kinda fuzzed out (not a 6V6GT characteristic) around volume "4." I am using no pedals.

Replaced the TSRI with the Hytrons and the amp returned to a more normal tone. Creamy, slow onset of overdrive past "6."

I will pull out my AA764 (1964) PR and try the TSRI on them. This amp runs a GZ34 rectifier. Rather than the 5U4GB most PR run. For whatever difference that makes.

I tried to take photo, but couldn't get the glow to properly show with my cheapy Canon A720IS.

Will report on tubes in the 1964 PR...


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Tue May 03, 2011 11:26 pm
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Sad news # 3 & 4:

Ran the TSRI 6V6GT's in my AB764 Princeton Reverb (with GZ34 rectifier)---and it developed red plates even faster. Prolly due to rectifier(?). It took over 30 miuntes for the tube's base to cool down enough to touch by hand, in an AC room, after I pulled them. I always use Mechanix gloves to pull power tubes and rectifiers.

I even tried a couple in my SE-Vibro Champ with same results. There's a possibility that the tubes are now damaged and will always glow red. I will attempt to buy at least another pair to try out.

Note: the red glow is the length of the plates. And when I put one into my Champ, it almost smelled like the base (or glue on the base) was starting to melt. This was maybe after 5 minutes in the amp.

I had high hope for this tube. Maybe just a bad lot or something.


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Wed May 04, 2011 12:08 am
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If they weren't toast before, they certainly are now.

It wouldn't be the first time a tube (or pair of tubes) failed right out of the gate.

Bummer!

:(

Arjay

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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Wed May 04, 2011 6:19 am
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Retroverbial wrote:
If they weren't toast before, they certainly are now.

It wouldn't be the first time a tube (or pair of tubes) failed right out of the gate.

Bummer!

:(

Arjay

Oh yeah...even NOS tubes.
Years ago I bought a pair of RCA 6V6GTs from a trusted seller...and after awhile checking the bias settings in a '66 Deluxe Reverb,I noticed the readings on one tube getting hotter and hotter till it cherry plated,the other tube staying stable.
After shutting it down,I let it cool and tried the bias adjustment again,the tube would stay the same for several minutes and then start running away again...I contacted the seller who questioned my bias procedure and said he normally didn't take back tubes since he put them through testing before selling them...since he had sold me tubes before several times,he agreed to take it back to see for himself....I told him to just watch it for a few minutes before he dismissed my claims....it did the same thing in his test equipment,and he sent me another NOS RCA tube.
Many of the so called NOS tubes can be rejects from years ago,you have to watch them also.


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Wed May 04, 2011 8:58 am
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while the TS RI 6V6s are a good sounding tube they are known for their small post which hold the grids. These can warp due to heat, whcih is normal for a tube, but then the grids can short. 6V6s made in the russian plant, EH, TS, and others are of the same basic design. JJ's have the most beefy build but kinda stray from the true 6V6 tone.

I have a pair of TS RI 5881 in my BFSR....I hope they are built a bit better...


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Post subject: Re: BAd news about a pair of TungSol 6V6GT reissued tubes.
Posted: Wed May 04, 2011 1:07 pm
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I sent the "bad" TSRI 6V6GT back to a guy that has more gizmos to test power tubes, than Dr Frankenstein.

Will post results.


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