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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

Showing 2 responses by jax2

jax2
3,031 posts
 
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 whoop...it 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.

Marco
jax2
3,031 posts
 
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.
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