PrimaLuna ProLogue Two - mixing tubes?

I have a newer quad set of SED 6550Cs that I've been using as a replacement for the stock KT88 tubes. I decided to take advantage of the auto biasing feature of this amp and run one 6550 and one KT88 per channel, only because one of the stock KT88s is bad. I'd like to use up the other two good KT88s. I've read that this is acceptable with this amp, but I'm not sure if this will have negative impact on the 6550s. I plan to rotate the 6550s so that I still have a somewhat matched quad set of them. Has anyone done this before, and are there any pitfalls I should take note of?

So, thinking aloud ...

the two 6550's in use will have more wear on them. I had absolutely no luck with SED Winged Cs when it came to tube life and my PL2 (a few hundred hours), so wear & tear may enter this.

if you use the 6550s in an amp which requires biasing, there may be some differences in the bias values.

if this is a short term thing, I would not worry about it.

According to Kevin at Upscale and the PL ads you can use most any tube in combo and the auto bias circuit will make the necessary bias adjustments. BUT he doesn't say it will sound good! Just that it can work.

I don't have a PL 2. Just a PL1`, PL3 and PL5. Each of the amps sounded best with the tube type that they were originally designed to use. I ran the PL1 and the PL5 with all of the standard power tubes mentioned in the ads, in both amps. Ultimately the EL34 sounded best in the PL1, and the KT88 in the PL5. Although the 6l6 in the PL1 and the 6550 in the PL5 were quite good. More linear, not as warm.

BTW, I have run a lot of SED 6550's, KT88's, EL34's, as well as 6L6GC's and I've never had a premature failure problem. Rich's problem may have some relation to the pre-set bias points in his amp (which btw can be altered and should be if for example you decide to try 6l6's or EL34 in the PL2 - at least that was my experience).
Thanks for the input guys. Running one 6550 and one KT88 per channel certainly sounds different than running all 4 tubes as the same type. As I would expect, it sounds like a cross between the 6550 and KT88. Running all 6550s, the sound is nice, but I would describe it as very warm, and not very authoritative in the bass. The bass quantity is there, but it's a bit flabby. Mixing the KT88 with the 6550 brought about tighter bass response, and more extension in the treble.

My only fear is that running one of each will ultimately reduce the longevity of both tubes. On the other hand, I may be able to get more use out of 6 tubes (2 KT88s, rotating the 4 6550s) than if I were to just run all 4 6550s until they burned out.
Fear not. Go forth and slay that beast! It really shouldn't make a difference on the longevity issue. :-)
Also,if you swap the two tubes on the same channel,one of them may/will(depend on amps&tubes)over dominate the other sonically.IT depends which one is in the push,and pull sockets.So,if you like the sound of one tube type more than the other,rotate the two within the channel,to see if you get a blend of the two you like.

For amps that could use a 6550/KT88 type,or a 6CA7/EL34 type,I do not think a combination of a KT88,and an EL34 would be good in the same amp.The tubes out of its design types are to far apart.Ask the manufacturer first, although I wouldn't myself.
So if I understand the question correctly, you're risking doing damage to an item with a used value around $1k rather than just buying a new set of tubes? Do the math and weigh the prospect of having to pay for a repair bill vs. buying a set of $100 tubes. The amp isn't designed to operate with different tube types, no amp is, though it technically could. The answer seems obvious to me. Buy a pair of new $100 tubes and avoid the risk of doing damage to your $1k amplifier. I can't believe I'm even responding to this...
When I had PL Two I mailed with the owner of the company and he wrote me that you can use any tubes together without any problems, cos their auto-bias system is the only one that can be called the real thing, each tube gets own bias, which allow you to use this combinations without problems. I used KT88 and 6550 all together, no probs what so ever. If you want less warmth and better extention both ways, go for NOS TUNG SOLS 6550 Black Smooth plates, with no holes, thats why they call them smooth. Or, even better, NOS GEC KT88, period, you just stop rolling after that, thats what I did and never looked back.
The PL amps are designed to safely accomodate this. However, Kevin at Upscale can probably give you the upside/downside of operating the amp in this fashion. I think that I've used every outpube tube out there in my PL 7s and there's a fair bit of character shift when you change the output tubes. I've pretty much settled on KT 66s for most apps.

Good Luck

@Coltrane1, While PrimaLuna amps may not be directly designed to use several power tube types at the same time, the ability to do so is a pleasant side effect of our Adaptive AutoBias circuity. We've actually run 4 different power tubes at the same time in our own ProLogue Five, without any problems.

It will not damage the amp in any way, nor will it effect the longevity of the tubes... it's just not a practice we recommend, as it's not optimal in terms of sound quality.

We find this feature to be a useful tool for someone who's maybe throwing a party and has a tube failure... but the only spare power tube lying around is an EL34... that customer can put the EL34 into the circuit with KT88s (or whatever) and still have music going until a proper replacement can be had.
Hi Upscaleaudio, if different tubes can mix well and sound nice, why do we need tubes of the same kind in the amp? I'm also a prologue 2 user. thanks
I never said it will sound nice. While using different tube types at the same time CAN be done in a PrimaLuna... that's not the same thing as sying it SHOULD be done. While you may blend the color of each tube, the slightly different operating parameters may make the sound less coherent and resolving.

I always recommend using only one type of power tube at a time. You can always change the 12AX7 or 12AU7 tubes to further fine-tune the color of the amp to your taste.