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Re: No CT on PT, how to lower B+ ????

1/16/2004 1:48 PM
Dave H
Re: No CT on PT, how to lower B+ ????
PS. The bias current will need adjusting after lowering the screen voltage.
1/16/2004 1:25 PM
Steve Conner

"Would I be able to get away with the high plate voltage if I lower the screen voltage alot by creating a lower-voltage node off the HT rail"
IMo that is the best way to make EL34s happy. They can take silly plate voltages all day as long as the screen voltage is low. If you can get it down to 400V or less they will love you.  
Those 5W studmount zeners sound like they would work great to lower the screen voltage, or even the plate voltage if you decide to do that instead. But I vote for screen voltage.  
Steve C.
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1/16/2004 5:35 PM
Ian Anderson

Cheers for the responses everyone,  
I'd prefer to loose a little plate voltage as well for reliability in addition to the screen voltage.  
I'll try some zeners to bring about 50-75v off the plates (down to around 525v) and create a new node off the HT rail for the screens to bring em down to around 425v.  
It'll be interesting to see how the tone will change by bringing the screens around 100v lower than the plates.  
Anyone got any suggestions for a dropping resistor value to bring the screen supply down from around 600v to 425v? (sounds like alot to drop over one resistor and I'd imagine the wattage will need to be fairly hefty)  
Cheers... Ian :)
1/16/2004 7:12 PM
Jack Koochin

Wouldn't you be better off taking advantage of the zener's regulation characteristics for the screens, rather than a resistor? IIRC, screen regulation is kinda important. If you use a resistor, your screen voltage will go up and down a lot, varying with the signal. (Your total screen current may vary all the way from 20mA up to 85mA or so, depending on signal level.) If I'm wrong about this, someone please chime in here...  
1/16/2004 7:19 PM

I realize you're in the UK and this may not be directly helpful, however, 50W zeners are reasonable($14.18) through For example, a 526-NTE5269AK is a 33V/50W cathode lug zener. I believe they go all the way to 120V. They're not in the catalog though. You'll only find them on the website.  
I was just looking at the Music Man HD-130. They really created a normal sort of 360V supply and used a voltage doubler for the plates. So that amp had 720V on the plates and 360V on the screens.
1/16/2004 8:23 PM
Dave H

I wrote this before reading Jack's post so some of the info is repeated.  
EL34s have a large screen current change between zero and full power. A big resistor could introduce a lot of voltage sag and it would have to be 10W or more. I’d replace the 1k screen resistors with 150V 5W zeners or put them in series with the resistors if there’s room. The zeners are 25p each from Farnell. 1N5383B order code 933 545.  
1/16/2004 9:55 PM
Ian Anderson

I've already got some 68v/5W zeners - could I just string two of them together? These are stud-mount types and are supposed to be installed with the stud thru the chassis, would they be ok for using in series or do they really need to be bolted down?>  
Cheers... Ian :)
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