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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
 
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
vm8444
18 posts
 
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,,
newbee
4,767 posts
 
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.
vm8444
18 posts
 
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.
v
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