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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!
mario_b
mario_b OP
147 posts
 
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
1,064 posts
 
Why would you be running 3 KT88s and 1 6550?
mario_b OP
147 posts
 
Narrod,
Because that's (the 6550) the only tube I could lay my hands on while my replacement KT-88 ships.
-Mario
stevecham
3,340 posts
 
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.
tpsonic
1,092 posts
 
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.
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