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5E3 rectifier

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1/16/2000 7:20 PM

5E3 rectifier

I'm using a 275-0-275 VAC power tranny for my homebrew 5E3 (I used this transformer on the suggestion of some people that said at those voltages, I could use different rectifier tubes to get different voltages on the plates) my question is, what will the voltage be after the rectifier if I'm using a 5Y3? and what other tubes can I use for the rectifier and what will the voltage be with them? -ricky

1/16/2000 7:48 PM

It depends on a few things Ricky.
Who is the tranny made by, what is the line voltage into the PT, what is the current rating of the PT and how much are you loading the PT down. etc.
Unloaded with no tubes, I bet that tranny will charge your filter caps up to close 390 to 420vdc.
If you have a 120v primary...275vac/100ma secondary PT and your line voltage is around 120v, and you are using it with your 5E3 tweed clone idling at around 36ma-40ma each 6V6GT, I'd expect to see about 330v-340v dc.
If you use this with a 5AR4, I think you'll jump about 20 to 25 volts up.
Solid state will be a little higher again, maybe another
10-12vdc higher then the 5AR4.
I build a little power supply that uses a 270vac/120ma tranny with solid state rectifier and it runs the B+ up to about 360vdc under a 76ma total load.
Perfect for quite a few different, small under 20 watt amps.

By the way, in these little low powered amps, it has been my experience that a beam forming power tubes like a 6V6GTs or 6L6GCs, don't seem so voltage dependent for good solid power output once you get the plate up over 320v.

Bruce

1/16/2000 8:24 PM
Ricky	this transformer has 150mA of current, does this make much difference? -Ricky

1/17/2000 1:38 AM
Bruce	Yes if it's like a Hammond,or any other well made piece, it will work great in a tweed Deluxe type amp. The B+ will hold up well with current flow and you'll have true power tube distortion with good power output and not an absurdly high plate voltage. Bruce

1/17/2000 7:01 PM
Ricky	will the 150mA (as opposed to 100mA) of current decrease the amount of rectifier "sag"? or, is this not related? -Ricky

1/18/2000 6:18 AM

Well Ricky it's not really related.
It will help your overall sag though.
A vacuum tube has a forward voltage drop when it is turned on and that voltage drop is greater when there is more current flowing throuogh the tube.
In a given amp, once the drop is established at idle the only real difference when playing is how much extra current the power tubes are pulling during high volume peaks.
All the preamp tubes are running class A and their average current is constant.
Roughly speaking:
If your PT can't keep up with those peaks and the current load from the depletion of stored energy in the first main filter cap, (the main filer capacitor is like a battery too), then the PT will start to cave in a little (and maybe get pretty warm) while the B+ is going to sag even more.
That's independent of the rectifier.
If your PT is of decent quality and rated at 275vac@100ma, I think you'll still be OK in a little tweed Deluxe.
Use a 5AR4 to experiment or (and) use a slightly larger first filter cap value to compensate a little too.
I've used my share of 40uf to 50uF caps connected to 5Y3s with NO damage to the rectifier tube.
I have a power supply test jig at the shop that has a single 100uF/500v E-cap and I've never blown a 5Y3 rectifer with it.
It has two 50 watt 1000 ohm power resistors on it that can be shunted right across the filter cap to ground.
That means with one of them switched in, your 275v@100ma PT would have to handle 275ma of current to stay at 275vac on the secondary.
Most decent PTs will out do their ratings.
Many PTs, that I've given their true life test, have survived this for 24 hours or more!
I've had good trannies so hot, I could cook lunch on them.
After they've cooled down for a day or so, they seemed exactly the same as new!
Yes, I've let the smoke out of a few too.
It's pretty unlikely you'll be running the amp at full clipped volume all the time anyhow so I think you're OK.

Bruce

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