Red Glow in Front Right EL34 of ST-70?

Hi All.

Might Bob or anyone please be able to suggest what I should look at, when I get a big red glow in one of the EL34s in my VTA ST-70?

I purchased my VTA ST-70 kit (240v power transformer, both cap upgrades, with valves) through Bob in September 2012 and finally got it built after nine years!  It is a gorgeous kit and it was a joy to build.  

I double-checked my work and switched on and set the bias.  Maybe the front right was a bit hard to get right because it seemed to change a bit - increasing over time.  Anyway, for a couple of hours, the amp seemed to be running just fine and played very sweetly with my trusty Apple iPod as music source - through my Focal Chorus 714S 8 ohm speakers.  

But I was still having a bit of an issue with what I regarded as a slightly unstable bias on the front right EL34, starting quite low when cool (perhaps 0.265v) and climbing up to and over (>0.400v) in a minute or so, sometimes seeming to continue to climb. So I was trying to set the bias at around 0.385v to be on the safe side. And that seemed to work OK.

Then once, after a couple of hours of total on-time, and a bit less than an hour of playing in this session (and when I thought the bias was fairly stable), I heard a small "plop" sound in the speaker along with the music, and the front right EL34 quickly started glowing bright red (very different from the small red filament glow).  I switched off within maybe 10 - 15 seconds.  I did try again, but noticed the same slightly unstable bias and after maybe a half hour this time, got the same "plop" and big red glow and switched off promptly.  So I re-checked the wiring and the soldering on and around both right hand EL34 sockets, V6 and V7, and it all looked good.  Tried again and got the big glow a third time - quite quickly this time - after maybe a few minutes of playing Beethoven at mid volume.  So switched off in a few seconds.  There is a perceptible darkening of the glass on the tube, but it is too hard to photograph.  The glow seems to be happening quicker now, but I don't want to keep pushing it.

Something is NQR!

I would much appreciate suggestions for what to look for and/or check.  I have a spare set of new JJ E34Ls but am loathe to put these in unless I've reduced the risk of the problem recurring with them too.

May I include a picture of my build.  The cheap Chinese "VU" meters are illuminated via a diode and 470 ohm resistor from the heater circuit tapped off the front left valve socket V3, and take a signal feed from the 8 ohm speaker terminals via a diode, a 1.8k ohm resistor and a 10k ohm trim pot, using a circuit like this one - https://www.giangrandi.org/electronics/vu-meter/vu-meter.shtml.  They're fun, and I can switch them off with the 3-gang switch on the left.  I don't think these are contributing to my issue but I'm happy to hear other theories.

I apologize for my lack of tube knowledge, but I'm hoping that the above description is enough for folks to work with.  If I can provide more, please let me know.  Thank you for any assistance you can offer.

kind regards,
Simonism

Simon Murray
Melbourne, Australia

If you change place between the two tubes at right, will the redplating follows the tube ? If so the
tube is defective. If not you have an issue with the g1 circuit ( coupling cap, resistors, solderings)

Thank you Peterh.  Good suggestion.

I swapped the two right hand EL34s and switched on, with no music, monitoring the bias on all 4 tubes.

The old front right tube, now at front rear, continued to show bias instability in its new position.   I also noticed that if I tapped this tube with a screwdriver, it produced massive crackles in the right speaker.  And sure enough, after a few minutes, there were some pops in the right speaker, some hum, and a big red glow in the rear right tube (previously front right).  I promptly shut down the amp.  OK, that tube is defective/unstable.

OK, so the next thing is to install my set of JJ EL34s, then re-bias and test.  Thanks again for the guidance.  I'll report back in due course.

Simon

OK, thanks again - the new set of JJs have biased easily and seem stable. Now in retrospect, I can see that I had issues with one of the original Valve Art EL34s right from first install.
Simon

Simonism wrote:OK, thanks again - the new set of JJs have biased easily and seem stable.  Now in retrospect, I can see that I had issues with one of the original Valve Art EL34s right from first install.
Simon

Yes, JJ works fine with me.
The imprtant thing is to buy them from someone that take responsiblility i.e. gives a decent warranty, not what brand it is.

Thanks Bob, good advice.

Those output tubes were in their boxes, in the carton that you shipped them in, for 8 1/2 years, in the cupboard.  I just finished building the amp two weeks ago, and the tubes got plugged in for the first time.  I believe one of them was unstable from first use and anyway, they only had a couple of hours fired up.  I'm not complaining at all, just pointing out that they did not have any significant play time.  I'm really happy with the entire kit and my purchase, and when you're back to shipping internationally again, I may have to buy another kit, my experience was that good.   Thanks.

Simon

Unfortunately, "red plating" is far too common with new modern tubes and buying them from someone with a good return policy is a must. I NEVER saw a US made 6L6 or EL34 in a guitar amp red plate out of the box but only after too many hours of heavy abuse. I have had 2 instances in the last several years with Chinese/Russian tubes red plating after only a short warm up time.

Glad you found the problem and that amp sure looks good!

Hi, I have not changed a tube in my St-70 since 1989. I use Seimens EL=34's. My amp was modded to a Purist Audio St-70 which instructions came out in 1989.The mod uses the 12AX7 tube instead of the 7199. My bias is set at .75 which is half the stock ST-70's voltage 1.56.The top of the amps cage is only warm to the touch even when asked for some power. Maybe if Hafler had that advantage in his design way back you guys might be saving some money on tubes....Mark Korda