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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
stevecham
3,298 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.
newbee
4,093 posts
 
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.
mario_b OP
147 posts
 
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.
mario_b OP
147 posts
 
For the record, pencil tapping did not work with this case. Just a bad tempermental tube. Thanks all. Case closed.
- Mario
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