2. amps-preamps
  3. 1290787781

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
rar1's avatar
rar1
1,784 posts
 
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.

Rich
newbee
4,767 posts
 
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).
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
4,767 posts
 
Fear not. Go forth and slay that beast! It really shouldn't make a difference on the longevity issue. :-)
More to discover