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

1/15/2004 9:28 PM
Ian Anderson
No CT on PT, how to lower B+ ????
Got a Laney PA head that is eating EL34s (600v on the plates). I want to bring the B+ down to around 500v for reliability but I don't think the PT has a CT. Anyone got any ideas on how to lower the B+ in this situation?  
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?  
Cheers... Ian :)
1/15/2004 9:41 PM

You have several options. you could try a choke-input filter, if you don't have one already. this will drop the voltage to some extent. another thing you could try is one or several 100v zeners in parallel, cathodes facing the rectifier. this would drop your overall supply voltage by about 100 volts. short of doing those mods, another thing you might try is dropping the bias current through the tubes. this will always make them last longer, and allows them to tolerate higher plate voltages.
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1/15/2004 11:04 PM

Better make those zeners about 10 watts, eh? Wouldn't be a bad idea to heat sink them too.
1/16/2004 12:40 AM
Don Symes

Uhh ... diodes don't parallel well ... unless you put a 10-ohm resistor in series with each.  
Orrrrr... a 120:120VAC transformer set up to 'buck' your HV secondary down by 120V. See R.G.Keen's Vintage Voltage Adapter for some explanation.  
1/15/2004 9:48 PM

"want to bring the B+ down to around 500v for reliability but I don't think the PT has a CT."
Is it a bridge? If so, the BA zener goes at the ground connection.
1/16/2004 12:35 PM
Ian Anderson

I've got some 63v/5w stud-mount zeners - could I use them? 63v would probably be sufficient btw.  
The transformer idea is a good one, but I'd prefer to keep the cost down if possible (a few zeners sound £best to me)  
... Ian :)
1/16/2004 1:45 PM
Dave H

It's got four EL34s hasn't it? If so it would draw 400mA flat out. That's 400mA x 63V = 25W in that 5W zener. A string of 10V or less 5W zeners would do it but a bucking transformer is probably a better bet in a 100W amp (less heat). It would only cost about £10 but the simplest thing to do is to use the zeners to lower the screen voltage like Steve suggested. They won’t get too hot with only the screen current through them.  
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