How to tell when a tube's going bad?

[JBryan]

If a tube glows red - its suspect and I wouldn't put it back in unless you're absolutely sure that its not the culprit. Use another tube and if it acts the same, chances are that the problem is with the amp. Even then, I'd be very wary as the circuit could have damaged the tube - you don't want to get into that cycle. This is where a tube tester comes in very handy as its capable of handling a tube in almost any condition.

But you have the best option available in your case - call Mark.

[fini]

Well, I pulled out my tube tester, a Hickok 600A (I haven't had it calibrated) to test a few tubes. I figure it should at least give me tube measurements relative to one another. The questionable tube failed miserably the mutual conductance and gas tests. Yep, bought on eBay as NOS NIB (but no guarantee). Live and learn.

With the Pcats, each tube is individually biassed, making tube matching unnecessary. Say I have two tubes with a Gm of around 2500, and two with Gm readings around 6000. Would using one of each per monoblock make biassing more difficult than if I were to match similar tubes per monoblock?

[fini]

Thanks for chiming in, Mark! I didn't want to bug you until I'd understood the problem a little better (good learning experience, too). I will go back to the KT-88s to see how that works.

BTW, Zeno, I checked the GE 6550s you lent me. They read mucho bueno!

[Guest " "]

If a tube glows red - its suspect and I wouldn't put it back in unless you're absolutely sure that its not the culprit. Use another tube and if it acts the same, chances are that the problem is with the amp. Even then, I'd be very wary as the circuit could have damaged the tube - you don't want to get into that cycle. This is where a tube tester comes in very handy as its capable of handling a tube in almost any condition.

But you have the best option available in your case - call Mark.

I've seen driver tubes cause power tubes to glow red. Power tube and amp would be fine. If not believed, send me a PM and I will send you two 12ax7/12at7 driver tubes that will cause a power tube to glow red.

[JBryan]

That would be a new one on me but then, I guess I can see the connection and shouldn't be surprised. Thanks for the offer but you can hold onto your tubes - that's one of those experiences I'd rather know about 2nd hand.

[NOSValves]

If a tube glows red - its suspect and I wouldn't put it back in unless you're absolutely sure that its not the culprit. Use another tube and if it acts the same, chances are that the problem is with the amp. Even then, I'd be very wary as the circuit could have damaged the tube - you don't want to get into that cycle. This is where a tube tester comes in very handy as its capable of handling a tube in almost any condition.

But you have the best option available in your case - call Mark.

I've seen driver tubes cause power tubes to glow red. Power tube and amp would be fine. If not believed, send me a PM and I will send you two 12ax7/12at7 driver tubes that will cause a power tube to glow red.

In a McIntosh amp I bet.........

[fini]

Well, it was definitely the bad tube. The original KT-88 tubes biassed just fine, as did used (tested good on my 600A) Tung-sol 6550s. Thanks, everyone!

[jt1stcav]

Aaaahhh, another happy ending and no harm done to your pCAT.[]

Now to give that eBay seller some negative feedback for a false auction description and selling you a dud...[8o|]

Just kiddin'...[:S]

[Clipped and Shorn]

Well, it was definitely the bad tube. The original KT-88 tubes biassed just fine, as did used (tested good on my 600A) Tung-sol 6550s. Thanks, everyone!

Fini---

Great! Happy to hear everything is AOK there. Drop me a line and let me know how you like the 6550A in the 'Cat. I like the Tung-Sols in UL amps, but not as much in triode. Like to hear what you think.

Mark,

Is your preference for the KT88 over the 6550 in PP triode based on a comparison between newly available tubes or with NOS. I think fini found the Old 6550s smoother than his newish EH KT88s that came with the units. I have a feeling your comparison was based on EH 6550 vs. EHKT88. One might be inclined to conclude that the best tube for the pCat might be some NOS or old stock KT88.

My smoky Genelex KT-66 sound pretty nice in my protoPcats. That's triode too. Now I am not recalling how much if any improvement I heard over the "new" 6l6GC that were in there when I got them. I am thinking I better liked the old KT-66 in there, but perhaps I was just anxious to use those vintage tubes.

I think vintage type KT-88s might be a bit pricey these days. What was considered desirable brands of vintage KT-88s. Forget the WEs, they are probably off the chart.

——LJÍ

[fini]

Mark,

I'll let you know as soon as I figure it out! I've been in hunting mode the last month, acquiring a variety of vintage used (to wildly varying degrees) 6550s. I have a couple pairs of GE 6550As, and six Tung Sols (rebranded as RCA, mostly). Since I switched to the older 6550s (a couple borrowed pair from Zeno), I haven't gone back and tried the KT-88EHs. I haven't been home alone much over the past several weeks, so critical listening has been restricted (hard to listen when the TV is blaring over there, and the washing machine and dryer are chugging away over that away...).

I did like what I heard at that initial change-out, though. Less of that Klipsch Heritage upper-mid harshness and "aural confusion." I've been seriously considering upgrading the Khorns, either with new crossovers (Josh has some to lend me...good to have friends with cool stuff) or with new horns/drivers, etc.