Seeking recommendations for replacing Power Tubes (KT88, 6550) - Partial Quads

[Guest davidness]

I have and use two power amps; a Bob Latino ST-120 (uses 4x KT-88/6550's), and a VRD ST that also uses the same power tubes. Over the years I have bought several matched quads (Gold Lion KT-88's, Valve Art KT-88's, Tung Sol 6550's), and occassionally a power tube fails leaving me with a partial set of tubes.

In a perfect world, I guess if one tube of the quad failed, you'd replace all the tubes with a new matched quad, but in reality that's expensive and wasteful. What does everyone else do?

I recently found myself in this situation with my STEREO VRD, and I was forced to use a pair of Valve Art KT-88's on one channel, and a pair of Gold Lion Reissues on the other, until I can order more tubes. But do I order just a few extra Valve Arts and/or a few Gold Lions to round out my partial quads, and if I do this, do I need to try to order tubes that "match" my existing used tubes? (This would be nearly impossible as I don't have any way to test tubes and no way to come up with accurate measurements to tell a supplier to try to match to.)

I guess in the old days, if a KT-88 failed, a person would buy any brand of KT-88 and replace it. If it was the same brand, all the better, but I suspect it would have been nearly impossible to "match" a new tube's "measurements" with their old tubes.

Anyways, you get the gist of my question here... what does everyone do to replace power tubes that were part of matched quads?

[kevinmi]

I have the St-120 also, and it's my understanding that you don't have to worry about tube matching since each tube is individually biased. I've replaced a single KT-88 tube before and had great results. -Kevin

[Guest " "]

What needs to happen is all these tubes need to be tested for Ip (bias current) and transconductance. Requires a tube tester and a bias probe. Then graded. That will let you know if your mixed match quads can be used with each other. There are a few inherit problems. If you have individual tube bias, you can compensate for differences in tube Bias and make them draw the approx same amount of current. BUT, that does not mean that they will amplifiy in concert. If you have tubes and they score Ip 33/Gm 12000, Ip35/Gm 9000, Ip28/Gm6000, Ip31/Gm11000. The tubes are not a matched quad. If you lump the 12000 and the 11000 on one channel and the 6000 and 9000 on the other, your stereo imaging will go down the toilet. If you lump the 12000 and the 6000 on one channel and the 11000 and 9000 on the other, you will probally tank you noise floor. Over time tubes increase in ther ibas current while at the same time gas accumalates while the getters ability to absorb the gas deteriates. The one day, you flip the power switch, and get a tube flash. The tube flash could blow a tranny if you do not have a b+ fuse or sacrifical resisters designed to blow to save your tranny.

[Guest " "]

I have the St-120 also, and it's my understanding that you don't have to worry about tube matching since each tube is individually biased. I've replaced a single KT-88 tube before and had great results. -Kevin

Moreso with new tubes. But as tubes age, their amplification power decreases. So we are talking about adding new tubes with new amplification power to sets of old tubes with deteriating amplifcation power. Individual bias adjust will only adjust the tube current and will not compensate for differences in amplification power of mixed matched tubes.

There are a few inherit problems. If you have individual tube bias, you
can compensate for differences in tube Bias and make them draw the
approx same amount of current. BUT, that does not mean that they will
amplifiy in concert. If you have tubes and they score Ip 33/Gm 12000,
Ip35/Gm 9000, Ip28/Gm6000, Ip31/Gm11000. The tubes are not a matched
quad. If you lump the 12000 and the 11000 on one channel and the 6000
and 9000 on the other, your stereo imaging will go down the toilet. If
you lump the 12000 and the 6000 on one channel and the 11000 and 9000 on
the other, you will probally tank you noise floor.

Heres a group of about 32 tubes. After testing, they produced two matched quads and 10 matched pairs . 4 tubes couldnt be matched to anything, of the 10 matched pairs, there wasnt anything close enought to create another matched quad.

[Audible Nectar]

When I got into KT88 amps (VRDs in particular) and decided they were "lifers", I went about he process of finding proper tubes for those amps. I tried just about every recent production tube out there, settling on two: The old Valve Art KT100, and the Penta Labs KT88. I have 20 VA KT100, all within a few percent, and 30ish Penta KT88, again, all within a few percent and in matched quads. Then a couple dozen Mullard and Philips Holland vintage rectifiers.

I load a set of a given tube, and merrily use those until say, a tube goes south. Usually, those other three are still good tubes, so I set them all aside, IDing the one bad and three good, and store them for later. Then load a new quad. Repeat, then rematch and use the used good ones when reasonable to do so. I then waste less tubes and can use them with no fears.

I do the same with 14 matched GEC KT66 for one set of my McIntosh MC30s. Use them with no fear of losing one. I'm prepared if this happens. I do this with much of my gears that I plan to keep - lots of Tele 12AX7 for my vintage Mac gears, Amperex and Siemens 6DJ8 family stuff....i just got batches of this stuff once I figured out what I liked and stocked up when the opportunities presented themselves.

So that's how I deal with that issue.

In your case, you might order another quad of the same tube, and from the same vendor with same new in box readings if you can. Use that quad, when the first one fails (hopefully many listening hours down the line) rematch your used ones and squeeze the useful life out of those (best four out of the remaining six) and so forth.

[russ69]

I one of the tubes dies early, I'll just pop in one new one, as closely matched as possible. If the quad, or sixtet is getting old and near the end of life then I'll do the full set.

[Marvel]

It's a little easier for me. I only have one 2A3 per side ...[]

Bruce

[Don Richard]

Another reason to go SS. That's what I would recommend.

[garymd]

For my VRDs, I'm just fine with the reasonably priced Valve Arts. I currently use the slightly higher priced Penta Labs KT88s that Craig recommended. They have just a touch more bass but not a very noticable difference.

From my experience, I haven't found VRDs as output tube sensitive as some other KT88 amps like my old Macs so I spend my money on vinyl instead. I know of others who swear by their 6550s and have spent a small fortune on them. I'll stick to the theory that what I don't hear can't hurt me (financially that is).

Edit: As to your initial question, I've also had excellent results replacing single tubes. I understand the potential problems stated above but have never encountered one myself. I think it's worth trying to do it on the cheap before spending money on a new quad. Use one of your spares. You might be pleasantly surprised.