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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?

Thanks
jwglista
jwglista OP
206 posts
 
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.
newbee
4,794 posts
 
Fear not. Go forth and slay that beast! It really shouldn't make a difference on the longevity issue. :-)
hifihvn
943 posts
 
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.
coltrane1's avatar
coltrane1
828 posts
 
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...
maxmad
182 posts
 
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.
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