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Different voltages from the one tapped transformer

4/16/2003 1:37 PM
PeteRH Different voltages from the one tapped transformer
I am planning to build an amp with 300 volts to an 4/el84 power section and 400 volts to the preamp. I have only seen this done once before and the tranny windings were completely separate. ie. two coils 300-0-300 and 250-0-250 ac.  
The tranny i am looking at has 300-250-0-250-300 which can give me appx 400 for the preamp and 300 for the power amp. Probably a silly question but does this matter that they are on the same coil??  
Are these trannies generally designed for you to use one tap or the other and not both?  
The PT is rated at 250ma which is heaps for 4 el84 and a couple of 12ax7's.  
4/17/2003 2:17 AM
John Culp

Pete, why do you want 400V on the preamp? 12AX7s are rarely run over 300v plate supply. Seems like 330V's the official maximum. No doubt they can be run higher, but it seems as though they'd work fine at the voltage you're going to run your EL84s on, or a little less if you follow the udsual power supply topology with a decoupling/dropping resistor and filter cap after the power tubes' plate and screen supplies.  
That is way more DC capacity than you'll be using from the tranny, meaning the output voltage will be somewhat higher than the rated value as well. Output voltage is normally given at the full rated output. You might end up with 475V or some such. Of course, you can add resistance in series with the rectifiers to drop that.
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4/17/2003 9:04 AM
Hi John  
My express clone (or what started as a clone) sounded better running higher plate voltages on the phase inverter and preamp, especially when running 6V6 outputs. I was running 400 volts on the pi and preamp. Now this probably doesn't sound like an express was supposed to sound but I liked it.  
I would like to try el84's in a similar fashion but 400 volts is just too much for these little fellas.  
With this tranny, if the 400 volts doesnt suit the preamp/poweramp combination I can just run it off the 300 volt rail so I thought I couldn't loose. The multi tap tranny appears basically win win......unless you can't or shouldn't run both taps with this type of transformer arrangement.  
ps. I agree that the 400 volts is not standard but the original AA864 Bassman, which is very close to the wreck and clones, ran these voltages. Similarly the Alembic preamps and Showmans ran 400 volts.  
pps The big plus ........400 on the preamp and 300 on the plates hasn't been (commercially) done before!!!
4/17/2003 12:43 PM

Why not try Heyboer(  
Interestingly they make a PT that, if they really are separate windings as opposed to taps in the same winding, looks as if it may fill the bill. If it's the same winding perhaps they have others with separate windings.From another thread & quoting Chris Gondoli;  
"...All I asked them was if they ever wound any transformers for Ken Fischer,and if they did would they wind me up a set...They told me they wound three different transformers for Trainwreck amps,1power with 2different secondarys one for solid state,one for atube and 2 outputs 1thirty watt,1 thiry-five watt..."  
"...Power transformer 120volts in, 300-260-0-260-300,@300ma,6.3volt @7.5amps ct ,5volt@3.0amps.there is no bias tap...."  
By my math, which could be wrong!, thats 390 & 340 vdc respectively.  
Good Fortune  
4/17/2003 1:48 PM
Thanks Philip,  
I checked with Heyboer but theirs is also on the one tap....not two separate windings. I cant seem to find one with two completely separate taps....maybe(big ??) they dont need to be?  
Where is KG when you need him??  
4/18/2003 3:15 AM
John Culp

Okeydoke, Pete. Not arguing, just wondering. Thought it'd make your life simpler if you only needed a single voltage PT. With your PT's current rating, shouldn't be a problem using it as you suggest.  
I want to put 400V on some preamp 6SL7s myself, but I'm actually going to drop it down to about 360V at standard line voltage so I don't overvoltage the 400V filter cap at a 125V line voltage. My arrangement's a standard one with higher voltage on the output stage, though.
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