New Prima Luna Amp In Distress / Me Too

Thanks all - will wait out the weekend. Tube failure and replacements would certainly make me sigh with relief. Might even take the opportunity to roll in other offerings and use the Luna replacements (they are, in fact, covered by a 90-day warranty)as back-ups. Thanks again.

Hi Vm8444,

Yes, I still have my Luna II and am extremely happy with it. Currently it is running with 3 Kt88s and one OS 6550. Thanks to its autobiasing board you can run unmatched and even different compatible tube types and it adjusts and brings everything onto a balanced output. After a long troubleshoot, I was able to determine that one of the power tubes had an intermittant short. I resoldered the pins but the problem kept cropping up. A Hickock tester finally revealed an intermittant short to the cathode screen. A warrantied replacement tube is on the way from Upscale.
The Prima Luna II really puts out tube power. I've got medium sensitive floorstanders @ 93db in an 11'X14' room and I've never had the volume knob past the 10:00 position. If you like bass slam, the KT-88s really bring it home. And if you're into vinyl, I'd recommend getting the modestly priced phono board installed. This phono stage took a full 200 hours to burn in, but once it did, it made its very musical, full-range presence known. Hope this helps.
- Mario

Narrod,
Because that's (the 6550) the only tube I could lay my hands on while my replacement KT-88 ships.
-Mario

For the record, pencil tapping did not work with this case. Just a bad tempermental tube. Thanks all. Case closed.
- Mario

Thanks for the tip Newbee. The errant tube is still with me, so I'll try it. Kevin is out of town and they may be a little swamped at Upscale - but with the Luna up and running, I can be patient.
When I added the 6550 into the mix, it was interesting to watch the bias mv values change (I had the amp opened up as part of my troubleshoot).
Prior it was (.380) (.398) (.402) (.000) the last, obviously, was the "bad boy".
With the 6550, it was (.345) (.349) (.355) (.332). That "Adaptive Auto-Bias" board did exactly what it was billed to do - nice.
What happened to the other 4 wpc in the Luna 5? The Prologue II outputs 40wpc.

It’s been a few months since the original faulty tube has been swapped out with a replacement KT-88 sent by Upscale Audio. Things sailed along smoothly until last week when another stock KT-88 tubes began popping and demonstrating the same problems.
After checking with Kevin at U.A., he suspected it might be vashode powder. On close examination, this turned out not to be the case (at least, as far as I can tell).
At any rate, since this latest intermittent failure began in the same socket as the first one, I’ve decided to replace the entire power set to rule out any “hot seat” factor that might be emanating from the amp while it’s still under warranty. I’m going with Valve Art Kt-100s which are a rugged Chinese copy of the old Genolux design KT-88.

Whether this ends up to be just an unlucky set of stock tubes or a problem with the Luna, I’ve come around to thinking that this may be the challenge that one sometimes has shoulder in tube audio.

It should be noted that Kevin reports that, outside of a few PL Kt-88s demonstrating a similar problem with Vashode powder (none in the PL-EL-34s), the Prima Luna Amps are demonstrably good builds with few problems. In fact, I’m not in a panic about all this – learned a lot about this amp through troubleshooting – learned also that when it’s on song, it’s absolutely warm and lush and competes with the best of ‘em.