New Prima Luna Amp In Distress / Me Too

Prologue II 40WPC Integrated – (4) KT-88s (2) 12AX7s (2) 12AU7s. The amp arrived new six weeks ago and has maybe a bit over 200 hours operation. Today, sequences of double popping began coming over the speakers. About a second between each pop then after a minute or so another double pop. There was no input signal at the time.
I switched the input selector from phono over to CD and the popping continued.
I then played a CD and noticed a gain and fidelity increase in the middle of the pops (the short sequence). I then noticed that the blue glow from one of the KT88s increased within these short bursts of fidelity/gain that were bracketed by the pops as well.
I shut off the amp and let it cool before swapping tubes to see if the problem followed the tube or was dedicated to that socket. When the amp was powered back up, the popping was gone but both middle output tubes (the ones that were swapped) did not come up to full blue glow and a good part of the bass section has fallen through a trap door in the soundstage – and remains that way. It is now off line. I’m ready to box it up and send it back for warranty work. One note – this amp has a proprietary on-the-fly auto biasing circuit.
Anyone who can explain what’s going on will have praises sung by me until my dying day. I’m heartbroken!
Contact Kevin at Upscale Audio. He'll help you.
I'd agree, check with Kevin. It sounds like one, or more of your output tubes
may have bit the dust. If so, no big happens...especially on
modern-made Chinese and Russian tubes. I have had brand new tubes go
bad before. The (regular) popping sound is a tell-tale sign of tube failure. I'd
still check with Kevin though...I'm sure at the very least he'd replace the set of
tubes for you if it is only two months old. I would suggest trying that before I
packed up the whole unit and sent it back. Welcome to the wonderful world
of tubes! For the sonic advantages, the occasional minor hassle like you are
experiencing is worth it IMO. It's not a regular occurance, but it does happen
on occasion. Don't let it ruin your weekend.

Just FYI:KT-88s for the Prologue 2 run $35 each.Anybody throw any different 88s in??
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.
Have you replace the rectifier tubes ?
I beleve that the Prima Luna has solidstate rectification.
Just got my PL2 on wednesday and rolled in 4 jj E34L and voila what a difference! With auto biasing on this amp you can try many combinations. So far the hype is for real.
hi,,I currently am intersted in the prima luna amp..I saw your thread on audioggon,,do you sttill have it and are you still happy with it,,this week i heard the prima 2 and one i found the one to be warmer,,maybe it was because of the kt88 tubes,,so was it warmer when switching the output tubes,,i could use the little more power that the 2 has over the one ...
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
Why would you be running 3 KT88s and 1 6550?
Because that's (the 6550) the only tube I could lay my hands on while my replacement KT-88 ships.
It is always a good idea to tap on the sides of new tubes with a pencil, only when they're cold, of course, in order to shake loose and settle any internal metallic "dust" that results from manufacture and that may have resettled on contact/conducitng surfaces on the elements during shipping.

This was recommended to me by a very knowledgeable engineer. What may be happening is that there could be small conductive particles in the tubes that will create micro shorts and give rise to the popping heard. After doing this the first time, there should be no need to repeat, unless the amp or tubes ar shipped.

Hope this helps.
This little integrated replaced $24K worth of pre/power equipment.Many friends have come by to listen ,without any of them feeling the system has gone backwards.
I hope that the new trade agrements with China don't up the price of this gem,it would be a shame.
Vm8444, FWIW I have a Prologue One and a Prologue 5. I think your description of the One as being warmer is correct. I think the KT88 models are a bit more incisive, but I'm not sure how much more powerful they really are. The spec on the 5 is 36wt pc w/KT88's which is pretty much the same as the 1 w/EL34's. Its probably just a matter of personal choice, but with my speakers I find I prefer using EI 12AX7E's and SED power tubes. Upper mids and highs are a tad warmer/smoother.
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.
For the record, pencil tapping did not work with this case. Just a bad tempermental tube. Thanks all. Case closed.
- Mario
thanks for the reponse,,I ended up with a 3 month old prima one ,and I like it very much over my previous 25 watt tube intergarted ,,but I was wondering about tubes,,I am sure this thing can sound better then eh 12 ax7 in the front,,I had telefunken 12 ax7 and when i plopped that tube into the mix i found a more musical sound,that is in my previous amp,,,so I spoke to kevin,,he said give it a while with stock tubes but he likes ge 12ax7 long plate tubes with mullards el34's,,anybody out there let me know what you tried,,
Vm8444, I've already mentioned the tubes I prefer in the One...nuff said. However, if you do some checking I think you will discover that the SED EL34's are a Mullard copy and the EI's are a Tele copy. These are inexpensive new production tubes and I recommend them highly for what they are, without regard to the Prologue One. But, if you've got the money to spare, that NOS recommended to you might float your boat as well or better.
thanks newbee,,loving my prima one,,currently still using stock tubes ,,but I am getting the itch to roll,,maybe the cv4004 mullards,,and maybe slvania 12ax7 ,,then we'll see about output tubes later ,,thanks bro.
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.
Tell me more about the blue glow? I know this is an old thread.
I want to see it.
In summary blue gaseous glow in power tubes is good, and pretty. Lack of it is neither unusual nor indicative of anthing bad to come.

For a detailed explanation of the reasons go to Audio Asylum and look at their FAQ's. I'm too lazy to explain.
In summary blue gaseous glow in power tubes is good, and pretty. Lack of it is neither unusual nor indicative of anthing bad to come.

OTOH I recently had a blue gaseous glow on start-up in the lower part of a new NOS 5AR4 rectifier tube. I should have known something was wrong as it had just glanced the needle ever so slightly upon testing for emissions on my tester. It was so slight I didn't think much of it until I saw the glow, and then heard the occasional popping sound in that channel once the amp was up and running. Swapped out the tube and no popping sound. Turned out it had a hairline crack at the base. I don't know if the blue glow had anything at all to do with it, but I'd never before noted any blue glow at the base of a rectifier tube before this. You tell me. The (ebay) seller of the tube refunded my purchase.
Never saw any blue gas glowing in a rectifier or small tube either. Only power tubes. I guess we should just keep a bottle of Beano around. Too much gas around here. :-)
Front end tubes for PL1/2.Shuguang Triple-mica 12ax7:Telefunken based copy.Mazda/Radio Clifte French military 12au7s.These tubes will run about $100/4 and greatly improve on tonal body,immediacy and sound-staging.The "originals" are a little cool sounding.
I had this same exact problem it really bothered me. I talked to Kevin and everything. I finally swapped the kt88s and haven't had any issues. I'm hoping it was just tube "sediment" but I guess I will never know. Hopefully it didn't affect the amp or anything, I would be heartbroken.
Had a similiar problem when a tube was failing in the pre section of the Prima Luna Premium Integrated, selector knob would pop with any source change, erratic tics during operation. Looking at the bad tube it was apparent that arcing was occurring inside.