Problem using new 300B tubes

[glynnw]

After using a pair of JJ 300B tubes for about a year with no problems, sometimes playing VERY loud, I decided to spend some money and ordered a set of Sophia Princess 300Bs (meshplates).  They sound great (I had to turn down the subs because there was so much new bass) but when I turn the volume up (yet not as loud as I get with the JJs) I reach a point where I hear a loud pop and one or both channels drop to a very low volume, but it still keeps playing.  Turning the amps off and then on again somehow resets them and they are then OK again.  I have done this with both my extended Foreplay III and a vintage Sansui preamp I have, the volume cutting each time at about the same level.  My speakers are Basszillas which are supposed to by 97 dB efficient - I know they are more efficient than my Straight 8s.  My planned solution is to simply go back to the JJs - but I am open to any suggestions.  It looks like the Sophia Princess tubes just cannot handle a large input level.

[Paul Birkeland]

Measure voltages and post them here.  I would suspect you have less voltage at the big black cathode resistors than you should have, which would indicate low emission and inability to apply full signal level to the tubes.

If this is the case, you have a dead set of tubes, sorry.

[glynnw]

Before I list the readings, I want to make another observation.  I have been using my Paramounts with a passive preamp, and it played as loudly as I wished.  I noticed when I first put in the Sophia Princess tubes and used the passive pre, that it did not go nearly as loud with the same input.  So when I used the Extended Foreplay and tried to play at loud levels, I obviously had the Foreplay turned up higher than I ever do with the JJ 300B tubes.  I use a remote volume control that gives me no visual indication of how much it is turned up. After all this, I went back to passive and the Sophia Princess tubes still played as before, so I don't think they're broken.
Also be aware that Ed Fallon installed a delay turn on circuit in my amps, so the readings may vary from standard because of what he changed.  I tested both tubes and got similar readings, so I will just list 1 set.  Where there are 2 answers, it is because at these points the 2 tubes had different readings

1   456, 5  51 & 60, 6  51 & 60, 9 203, 10 0, 11 283, 12 208, 13 0, 14 136, 15  64,  16 477, 17 0, 18 0, 19 201, 20 0, A1 54 & 61, A2 452, A3 .02, A4 52.1 & 55.3,
B1 201, B2 0, B3 2.5, B4 1.17, B5 1.17, B6 356, B7 0, B8 4.95, B9 1.27.

Let me know if this indicates the tubes are no good.  I am hoping they are simply a bad match for these amps.

Thanks

[Paul Joppa]

The A1 and A4 (filament/cathode) voltages are quite low, indicating either low emission or high gain (mu). Since you said the loudness was reduced when you first swapped in the Sophia tubes, it is more likely to be low emission.

It may just be that the cathodes are not yet fully formed - I've seen this in other new-production tubes over the years. If that's it, then letting them run for a hundred hours or so should make a big difference. Since we're still unsure exactly what is going on, I'd not run them when nobody is home to keep an eye on them, but you won't need to be playing music - just running the steady DC current through the cathode is what's needed.

If you already have a few hundred hours on them, then it's likely to be something else.

[glynnw]

Thanks - they have about 4 hours on them  I'll just let them cook for a week or so and then see what happens.

[Rick58]

I e-mailed Glynn, but thought I would post my readings for other interested parties ...

I have notes from when I built these in 2006. My AC was 123V then, and I used the EH 300Bs that came with.

A1 measured 77V on one (R) amp, 75 on the (L) other. A4 was 72 on R and 71 on L.

With the Princess 300B (purchased in April, 2009 for my birthday!) on the "R" amp today (and my AC is down to 119.6V ... maybe time of day, load on the lines, etc.), when I first turned on the amp, A1 read only about 60VDC! but was creeping up 0.1V every few seconds.

I measured a couple more times, and after about 10 minutes it reached 72.8VDC and seemed pretty stable (maybe creeping 0.1V after a minute or something ...). Maybe after 1/2 hour it would reach 75V.

A4 is harder to get to, as I have shoehorned in a 3.3uF Mundorf Silver/oil parafeed cap! but I did measure it after it warmed up for a few minutes at 68.3V. It may reach 70 after 1/2 hour or something?

I didn't measure the L amp, but the channels seem very stable WRT balance, and I am VERY picky about balance and soundstage. That's one reason I splurged on the McIntosh C220 preamp, it has very fine remote control over balance and volume. (Not that the B'head preamps don't SOUND as good (or maybe better) than the Mc! but I am lazy and I had a windfall ...).

I can't tell you what the Princesses measured when I got them ... I listen at rather low levels. If the gain/sensitivity changed over the first 100 hours or something, I didn't notice. I did have the C220 when I got the Princesses, so it must not have changed much. But the OP only noticed 'issues' when running them hard, which I don't do.

When I first got the 300Bs, Sophia did say:

On Apr 2, 2009, at 9:20 AM, Sophia Electric wrote:
Dear Rick,
 
Thanks for your kind email.  The tubes need to have 24 - 48 hours to

[Doc B.]

but please do not keep your amplifier on for more than 6 hours per session to have longer tube lifespan.  Turn off your amp for about 20

[Rick58]

WTF?

I donno ... I figured this was during the breakin phase ...?  I never listen for more than 6 hours at a time, so I never gave this a second thought! 

[Paul Birkeland]

That makes no sense.  Power cycles are pretty hard on tubes. 

20V of shortage across a 1k resistor is 20ma of current that isn't being drawn.  IMO that is a very weak pair of tubes, or a failed pair from the factory.  The "should" be factory tested, and certainly would be rejected or left on the burner if that was happening.  If that voltage doesn't come up after 10 hours of constant use, I doubt it ever will.

We never did establish where the tubes came from, they could be used or factory seconds? 

[glynnw]

The Princess tubes have not been used again since my initial problems, so there has not been any time spent burning them in to see if that improves them.  The tubes were purchased directly from Sophia through eBay.  Sophia is being very good about this, offering to replace the tubes if my attempt to run them 100 hours doesn't fix them.  I will report back in a couple of weeks on whether this fixes the problem.  The JJ 300Bs were put back in the amps and voltages checked and they were almost textbook perfect, so the amps are not a problem.

[Steve_in_NV]

I have those exact same tubes and never had a problem with them.  I have since upgraded to a 2A3 configuration and am not looking back.

[glynnw]

After 100 hours of burning in these babies, one came up to almost correct output and the other declined to only half the correct voltage reading.  I have sent them back to Sophia and hope the new pair will be in spec.

[Grainger49]

That is disappointing.  The mesh plates are wonderful sounding tubes.  I hope to have some one day.

[glynnw]

I am glad to say that I received a replacement pair from Sophia that work perfectly and sound great.