Strange coincidence, possible connection?

Just got back from a couple days of skiing. Boy was it cold, -22c at the summit, -30c with windchill!

So today I swapped the leads over on my bucker transformer. Now with 122v in seeing a cozy 115v out to the M125's. Hoping this increases operating margins sufficiently to be able to safely use Triode mode. I also replaced one tube pair where the red plating was primarily noticeable, but not the other pair. Because I think I should be able to get much better then 2000 hrs as long as they haven't been baked. GZ37 soldiers on, amazingly robust tube.

Will monitor the situation closely.

Peter W. wrote:...incoherent mumblings....

Once more - a power pentode is NOT a triode. Operating it such that it mimics a triode (will never be more than an emasculated tetrode in actual service) will do all sorts of things - not necessarily fatal, but certainly contrary to the proper function and actual design of the tube. Want another analogy - going off-label with a drug. Might be benefits and there will definitely be differences - and possible consequences.

If that's your opinion you are certainly entitled to it. However to make the blanket statement that triode connected pentodes or tetrodes makes it a poor tube is simply not the case.

Triode connected pentodes have been in use fior a long time and in many (most) cases they deliver excellent performance. There are even cases where pentodes have been triode connected inside the tube when it was made to create a "new" tube. The most widely known example of that is probably what Sylvania/Philips did with the 6AV5GA. By triode connecting it internally it performed just like a 6B4G - these were sold to the US military as JAN 6B4G tubes. Triode connected EF86s make a very nice tube as well.

I have personally run KT-90 and KT-88 tubes in triode on many occasions and I have been VERY happy with the result. My own amp is running KT90s in triode and it is really a terrific amp according to people who have heard it.

So if you don't like triode connected pentodes that's one thing. To say they are not able to function properly is a bit of a reach - don't you think?

gktamps wrote:VTA M125 "emasculation" occurs through connecting the suppressor grid to the plate through a 100R resistor as a grid stopper to eliminate oscillation, just as you indicated earlier.

Actually no - the plate is connected to the screen grid not the suppressor grid. In most applications connecting the plate to the suppressor will give you lots of sparks and smoke since often the suppressor is internally tied to the tube cathode.

Jim McShane wrote:

So if you don't like triode connected pentodes that's one thing. To say they are not able to function properly is a bit of a reach - don't you think?

And a few of the Cadillac 4/6/8 engines ran very, very well. I do not dispute that it is certainly possible to run a pentode as if it were a triode. De gustibus et coloribus non est disputandum.

I do, however, strongly question why one would choose to do so. I think in this case, specifically, a series of coincidences put the OP's speakers at risk from parasitic oscillation. And I think that one of the contributing (and necessary) factors was the fact that the amp was in 'triode mode' at the time.

Oops, thanks for the correction, Jim. I intended to state screen grid but misstated. The schematic shows it clearly.

Zero issues so far, even after 10 hrs run time. Wonder if dropping the supply voltage did the trick?

So with new tubes absolutely zero issues over the past 3 months. But is there anyway I can protect my speakers from seeing a similar massive sonic overload in the future when a tube starts to go? The SPL level was incredible, still surprised my speaker didn't get cooked.

The lack of responses suggest there is not much I can do outside of adopting a PM routine where I replace tubes on a pre-set schedule. Which based on my prior experience of tube failures suggests each 6 months. My speakers cost enough that I cannot risk another sonic overload episode. So in another 3 months it will be time to replace the power tubes again. But I'm now wondering if maybe I should invest in a tube tester? Every 6 months is going to add up! 8 x Gold Lion KT88's into Canada after exchange and the rest is around $800, ouch.

Out of curiosity, what are you biasing your tubes at? I run mine around 0.80 per pair. The higher the idle current, the less life you'll get out of these modern tubes.

peterh wrote:

LeGrace wrote:The lack of responses suggest there is not much I can do outside of adopting a PM routine where I replace tubes on a pre-set schedule. Which based on my prior experience of tube failures suggests each 6 months. My speakers cost enough that I cannot risk another sonic overload episode. So in another 3 months it will be time to replace the power tubes again. But I'm now wondering if maybe I should invest in a tube tester? Every 6 months is going to add up! 8 x Gold Lion KT88's into Canada after exchange and the rest is around $800, ouch.

You could obtain and connect a hour-meter :
( https://www.ebay.com/itm/1PCS-Rectangle-Waterproof-Digital-4-5V-90V-DC-LCD-Hour-Meter-resettable/111451730205?hash=item19f30a711d:g:7hQAAOSw~AVYrCYV )
This is to know how many hours the tubes has done, at least 3000h should they make.
"knowing is gold" "guessing is iron".

That is a great suggestion. I think I'd like to start a bit more conservative, maybe 2500 hours. Folks that say they have been using the same tubes for 50 years make me roll my eyes. I have a similar issue with the expensive stylus on my turntable. Is there such a thing as a turntable hour-meter?

corndog71 wrote:Out of curiosity, what are you biasing your tubes at?  I run mine around 0.80 per pair.  The higher the idle current, the less life you'll get out of these modern tubes.

I think about 0.47 per tube (0.94/pair) is about as hot as you need to go for KT-88/6550 types, new or NOS. I bias the EL-34/6CA7s in my ST-70 at 0.4/tube, the latter being Bob L.'s recommendation.
You CAN bias a KT-120 up to 0.6 per tube but I can't, for all that extra wear and tear on the tubes, tranny, et al., hear even the slightest bit of difference.
But back on point, and at the risk of being flamed even on this polite Forum, stick some in-line fuses in your speaker leads, one per speaker on the + side. They make in-line fuse-holders so there's no need for surgery "under the hood."
Purists might well scoff, but I use them on my tube and (more importantly) silicon gear where IMHO the risk of high-amplitude transients is even greater.
So, saith the Purist, "I wouldn't deign to subject my $10,000 (per channel) speaker cables to some 5-cent fuse from Ace Hardware."
Bullshite.
John Bedini always speaker-fused his amps. I have a 200/200 (kinda like a BA-803 but older) fused with 3.5 amp slow-blows. Came from the factory that way, and the fuse-holders are part of the chassis. Take out the fuses, no sound.
Maybe you just can't bear putting some automotive tripe in your gigabuck system. There are alternatives.
Lookit these bad-boys, solid gold, at a lofty price of $49.00 PER FUSE:
http://trueaudiophile.com/isoclean-audio-grade-fuse-24k-gold/
There is a CNET discussion on the topic of fusing here:
https://www.cnet.com/news/39-gold-plated-fuses-improve-sound-quality/
So there's plenty of room for fuse snobbery if one eschews Ace Hardware. And if fuses were good enough for John Bedini, they're good enough for me.(Pretty sure his came from Ace, though.)

My Dahlquist DQ-10’s had two fuses, one at binding posts (full range) and another separate fuse for the tweeters.  I do recall blowing the main fuse more than once during college house parties waaaay back in the ‘80s and I kept a pack of spares handy.  (LOL more often than not  the cops would show up and shut us down on blown fuse nights, I think not a coincidence). (Ahh, to be young and foolish)

vtshopdog wrote:My Dahlquist DQ-10’s had two fuses, one at binding posts (full range) and another separate fuse for the tweeters.  I do recall blowing the main fuse more than once during college house parties waaaay back in the ‘80s and I kept a pack of spares handy.  (LOL more often than not  the cops would show up and shut us down on blown fuse nights, I think not a coincidence). (Ahh, to be young and foolish)

A man after my own heart! I had KLH's which were not fused and during those same sorts of parties you describe always drove my Heathkit
AR-15 into clipping, usually at Roger Daltry's scream at the end of Won't Get Fooled Again. Tweeters are expensive

*scoffs*