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Robin H
Reply with quote  #1 
My Pinnacle amps have been up and running for 5 months.  I travel a lot lately for work. So, listening for around 20-30 hours per months only.

I’m running a new set of (4) PX300B in one channel since February.
At beginning, the F2, F3 fuses (slow blow fuses) keep blowing at normal listening volume.
Break-in did help.  Now after around 40 hours on these tubes, it works fine at mid volume.
However, F2, F3 still keep blowing at higher volume.
Not ear bleeding volume. Just high volume such as the ending of Beethoven violin concerto 1st movement...

Another channel is running PSVANE HiFi tube (cheaper). It blowed fuses at the begining too but it settled and works fine after around 10-20 hours.

I run PX300B in one channel and PSVANE in another channel. The PX300B amp keeps blowing fuses while PSVANE do not.
I have tried swapping 300B tubes, swapping amps and other things in both channels. I’m sure it’s due to the PX300B tubes. 

Any one have experience with PX300B on the Pinnacle?
Any thoughts or suggestions?


Reply with quote  #2 




2 Pairs of Pinnacle = 4 mono blocks with 300B JJ.


Never blow any fuse.



Reply with quote  #3 
From the description the problem exists also for the PSVANE. And your speakers? Maybe impedance too low?
Reply with quote  #4 
Maybe it has something to do with the "max. grid circuit resistance " of the PX300B which is 50k for fixed bias (Pinnacle)? I checked some data sheets and could find that only the PX300B and the EH 300B list these parameters. Normally its the max Ip with fixed and cathode biased which is given in the data sheet. Does anybody know more about this and could explain the difference or how it translates to each other?

Don't understand what you could mean, Petr, that "the problem exists also for the PSvane" as the PSVane doesn't list these kind of limitations  and for self/cathode bias (300B DC OTL) the limit would be anyway 0,5M - even in the case of the PX300b.
Robin H
Reply with quote  #5 
I'm running two sets of speakers:
One set is 8 Ohm 8" full range (around 96 db as I record).
One set is two-way MTM with passive crossover, aroound 92db. Woofers are driven by seperate amps wih an active crossover. It is 4 Ohm. I use Zero Autoformers to make it 16 Ohm. In tuture, I might be able redesign the crossover to convert the MTM drivers from parallel to serial to make it 16 Ohm.

Same blowing fuse issue on both sets of speakers. But it happens more often on the crossover speakers. Due to its lower efficiency which require the Pinnacle to drive harder.

The PSVane HAD the same issue at begining. But it settled after around 20 hours and never happen again.
Break-in also did help the PX300b and it was getting better. Lauder volume is possible and fewer blowing fuses happened. But it still happens when driving hard.
I suspect the PX300Bs take longer time to break-in. While the Pinnalce operates the tubes pretty gentle, it takes a very long time to settle. 
I'm still keep burnning it further and see if it will totally settled.

Max grid circuit resistance is a good thought.
I have no idea how it works and how it affects the tubes...

Reply with quote  #6 
The problem are almost certainly the autoformers. They are basically short circuit at DC and this may upset the push-pull amp. I assume the amp is directly (DC) coupled - no capacitor at output. Then if the DC balance fluctuates - more probable with new tubes - the DC offset may be enough for the fuse to act.
Robin Ho
Reply with quote  #7 
I thought about this possibability.
But the PSVane have no problem with the autoformer after broken-in.
And the PX300b still blow fuse with the 8 Ohm Full renage which don't have autoformer.

Reply with quote  #8 
Then if you already swapped tubes and excluded the possibility of 1 faulty piece, maybe just try next higher fuse.
Robin H
Reply with quote  #9 
I’m not sure it would not damage other thing by increasing the fuse rating. So, dare not try it yet.
Reply with quote  #10 
The fuses in PP OTLs protect the speakers and the output tubes. I wouldn't change them to a higher value.
How high is your DC offset? Is it fluctuating very much?This could be an indicator for a bad imbalance of your output tubes. In this case swapping tubes from positive to negative side and between each other could solve the problem. One at a time and adjusting  for lowest DC offset. Blown fuses at higher power demands could be another indicator of  an extreme imbalance.

Reply with quote  #11 

Get the JJ 300b tubes from a reputable dealer. They'll meet or beat the 300b standard specifications and sound excellent.

I've had nothing but problems with the GL PX300B in both the Pinnacle and the Masterpiece. Blown fuses and bias drift out of control as Wolfgang mentioned. I think I've found the root cause. Two points:

1. The PX300B tube sheet states that the cathode has a 1.0 amp capacity. I called New Sensor and they told me that this is an "absolute" limit. I read this as meaning the actual safe current is a good deal lower with certain percentage as a margin, MAYBE up to 1 amp. I call B.S. on their datasheet.
The Masterpiece runs the cathode at 1 amp, and the Pinnacle has -315Vdc B voltage on the cathode of one pair. Two tubes developed cathode to grid shorts within a few minutes. My Pinnacles were blowing fuses almost instantly, which tells me the pathetic cathodes can't take the power they should. The blown fuses are a testiment to Bruce's design and are trying to stop tube damage.
My theory is that the coating is poor on some tubes but not others within a "platinum matched" set. Some are arcing over, some are not--it's a crap shoot.

2. New Sensor's Russian manufacturer doesn't throw anything away that they can sell. Their power tubes show a wide amount of variation and they all go on the market. You have to get their power tubes from a quality seller that burns them in on a rack then re-matches them. But burning them in won't change the fact that the cathodes often times may not meet specifications.

I did modify the Masterpiece slightly by changing the current supply setting resistor. If you want to use the GL PX300B in the preamp it can be easily done. I adjusted it one Ohm higher, but I think I should have gone one Ohm lower. I had a 13 Ohm but not an 11.

Reply with quote  #12 
...And I shouldn't have said that PX300Bs were a crap shoot. More like playing Russian Roulette.

Pardon the pun.

Reply with quote  #13 
And I made a mistake. When the power tubes are in the positive cycle of push-pull, that's when they act as a cathode follower in the Pinnacle.

I haven't reached chapter 13 in Bruce's book yet--that's my excuse. Heck, I still find myself going back to chapter 4, AC circuit theory.

Robin H
Reply with quote  #14 
Thank you so much for sharing your experience and finding.
I think that answers my questions and it confirms my concern that PX 300B won’t work here!
My tubes are claimed factory matched with close test numbers. But it just doesn’t work although bias is fine.
I have tried as Wolfgan suggested to swap tubes in the amp. Still, it did not help at all.

Now I’ll have to go get some JJ tubes.

Thank you all.

Reply with quote  #15 

Please let us know what happens when you change tubes to the JJ's.  I need to buy tubes for my Pinnacle build, and I certainly want to get those that give the best results.


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