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NS PT a tad powerful, eh?


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9/23/2000 6:59 PM
BMann
NS PT a tad powerful, eh?
I had no idea the Marshall 50-watt transformer New Sensor sells pushes out the firepower. I just recently started doing the finishing touch work on my 2203/Soldano clone, and first thought, nyahhh, the meter's out of calibration, after all, it has seen the floor a few times...but a different and newer meter put that idea to rest...  
 
What worries me are the plate and screen voltages, 485 and 482 respectively, on Svetlana EL34's. These drive Speedracer's "plexy" transformer (which sounds killer!). He says the OT can take this punishment and then some, so that's not the worry.  
 
The worries are such as, what's a good bias figure, can the screens take that much power before meltdown, etc. No, I incorporated a master volume, haven't dimed it yet, but sheesh! Even on 2 this amp's monstrous!  
 
Advice anyone? Especially on what PT to use _next_ time?
 
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9/23/2000 8:16 PM
Ken Gilbert

screen stoppers >1k will serve as adequate protection for all but the most misaligned and/or wimpy screen grids.  
 
personally i use 2k2.  
 
kg
 
9/23/2000 8:33 PM
Bruce

I'd set the bias at about 30ma with 5 watt 1K to 1K5 screen resistors and just play it.  
Bruce
 
9/23/2000 10:25 PM
Rick Erickson

quote:
"What worries me are the plate and screen voltages, 485 and 482 respectively."
 
 
I was thinking of dropping one of those in my 4010 50W. the stock p/t barely produces 400V on the plates and the output is around 36WRMS. Not one of the more powerful Marshalls out there. With the voltage that high a series resistor, maybe 150 ohms 25W or so would provide a bit of tube recto type sag  
for more of a JTM-45 type effect, hopefully without losing too much punch.  
 
RE
 
9/24/2000 4:00 AM
Don Symes

If you're really nervous, you can knock 50V or so off your B+ by implementing the 'zener trick'  
 
Who did we wind up naming that after?  
 
Anyway - www.aikenamps.com somewhere under Tech Info is teh, um, info.
 
9/25/2000 4:22 PM
Gil Ayan

quote:
"What worries me are the plate and screen voltages, 485 and 482 respectively, on Svetlana EL34's. "
 
 
I am not sure what makes you feel uneasy about this? The rated plate voltage of the EL34s is 800VDC, so what's 480VDC? Not much to speak of.... Put it this way, any Fender twin with 450VDC on the plates is a lot closer to the 6L6's 500VDC maximum plate voltage than your EL34s... :)  
 
Actually, those are low voltages in comparison to some other amps out there. For one thing, the real Plexis we hot, and for example the Groove Tubes Soul-O 75 amp has a whoppinh 520VDC on the plates... and they "advertise" their amp can be used with 6L6s as well. Of course, it can be used with 6L6s without any reported problems that I am aware of.  
 
quote:
"The worries are such as, what's a good bias figure, can the screens take that much power before meltdown, etc. No, I incorporated a master volume, haven't dimed it yet, but sheesh! Even on 2 this amp's monstrous!  
"
 
 
Again, I am not sure whether you meant to write that your plates have 580VDC instad of 480VDC? Put it this way, if you were expecting 450VDC, you have increased the voltage by 6.7%. Any "target idling current" you had in mind for the expected plate voltage would have to be decreased by the same amount to get the same idling power figure....  
 
I would say that since the dissipation of the EL34s is around 25W, to get half poer at idling you want to aim for around 26mA or so.  
 
quote:
"Advice anyone? Especially on what PT to use _next_ time?"
 
 
It all depends what you're after... As a Plexi replacment, I believe the New Sensor is a GREAT piece of iron -- I used a New Sensor MPT 100W in one of my amps, and it just so happens to be the one I think sounds best. :)  
 
Gil
 
9/26/2000 10:35 PM
BMann

I just never recalled working with plate voltages quite so high in 50's, the ones I worked with were usually around 400 (thereabouts. The 100-watt units were a different story, of course, around 550 if I recall.)  
 
The other "problem" is that different spec sheets quote different Esg values, some parking the limit at 400vdc. Svetlana sets theirs @500vdc for their EL34.  
 
Overall, I do appreciate the feedback from you guys, it's been years since I built a tube system from scratch, and the knowledge gained here helped immensely, and I do appreciate it. Now, I'm just waiting for Speedracer to cough up his 100-watt version OT....ooooohhh baby!  
 
-BMann
 

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