lowering HT B+

[Xander8280]

Long story short, my power transformer is 350-0-350 and I am seeking out a way to get 600VDC.
Currently with a doubler center tapped circuit it is putting out 1005VDC and 502VDC at the center tap.

Would a voltage divider change with current draw changes?

Would a big 50W-100W potentiometer as a divider work?

If there a 'safe' way to possibly add a negative voltage to the HT to bring is down? or would that cause a short of some sort?

I did a divider and the calculated voltage output was far off. Instead of 595VDC I was getting out 350VDC. Not sure if I made an error.



Any help would be great, drop resistors just seem so far out of sight.

[cbass]

Are you sure it's a 350-0-350 ?
Was it designed to be used with a voltage doubler?

A 350-0-350 pt would give arpind 500vdc with a regular full wave rectifier . if you used it as 350-0 with a vd you would end up around a 1000 volts biut your current capacity would be severely limited.
As far as a good way to get 600volts a fwb choke input would get you around 650 or so but the price of a properly rated choke might be close to new pt.

This page helped me understand voltage doublers better.
http://tubes.nekhbet.com/power.shtml

[Xander8280]

The PT is a Hammond 373jx, it is listed as a 700v power transformer, so I imagine it is designed to run with a doubler. The square root of 2 making 700v into 1000v.

Thanks for that link!

[cbass]

So it is a 350-0-350 with lots of current available . what tubes are you wanting to run?
Like I said you could use a normal full wav e recto and end up with 490 or so vdc.
Use the pt as a 350-350 (dont use th ct) with a full wave bridge recto for around 980 or so vdc with less current available.
A choke input fwb would get you close to 625 vdc. With full rated current available but I! Not sure where you would get a choke for this application i imagine it would have to be pretty huge and expensive

[Xander8280]

Running a quad of kt120, so max plate current 180mA x 4 = 720. Plus some 12AX7's... I will look into the choke, I doubt a huge resistor could substitute for the choke. I was hoping to get up into the higher 500's on the voltage. I'll have to try the build out with 495vdc and see how I like it. Tired of staring out this unfinished amp!

[martin manning]

The PT is a Hammond 373jx, it is listed as a 700v power transformer, so I imagine it is designed to run with a doubler. The square root of 2 making 700v into 1000v.

With a FW SS rectifier, you could get ~490V. For KT120, I'd put the screens at 390V, and run a quad of them into a 1k load. That would get you around 300W output power.

[Phil_S]

Running a quad of kt120, so max plate current 180mA x 4 = 720. Plus some 12AX7's... I will look into the choke, I doubt a huge resistor could substitute for the choke. I was hoping to get up into the higher 500's on the voltage. I'll have to try the build out with 495vdc and see how I like it. Tired of staring out this unfinished amp!

There is no free lunch. If you run it through a doubler, you get "twice" the voltage (really 2.8x) but only half the current. You won't have what you need to feed 720mA+.

What is the magic of 600VDC? Use what you have and see if you like what it does.

[R.G.]

Power supplies are fun!

You're wanting 600vdc at about 3/4 of an amp. So power = 600*0.75 = 450W.

Making 600Vdc requires some thinking about the AC to be rectified and the rectifiers. I'm not too adept with remembering tube rectifier specs. You could get the power and voltage done by using some of the IXYS 1200V 10A FREDs.

If you use tube rectifiers with a ... what, 50V drop at power, you'll need 600V peak plus the rectifier drop of 50V, or 650V peak out of the power transformer secondary. That means the secondary voltage is 650/1.414 = 459Vac RMS. Or 920Vac CT. The transformer current is about 1.4 times the DC for FWCT, so 920Vac CT @ 1A, or 1.8 times if you do a bridge, or 460Vac @ 1.35A. Big trannies.

Here's the deal: if your power transformer is not about the same size and weight of a looked-up-on-the-net transformer of the same volts times amps, it won't be able to put out the power. Transformers with about the same mass and temperature limits put out about the same power.

A good, standard transformer to get you to about this range is a 120Vac to 440Vac step-up; this is a semi-standard item, and can be found many places. You could certainly find something to compare poundages.

They won't have heater windings, though.

The Hammond 717 comes close: 1020Vct @500ma, 750ma for consumer/non industrial service. 17+ pounds.

There's a 440:120 trannie on ebay for $70 that's rated at 750va that would do it. This is some serious iron.

[Stevem]

Let's take some steps back for a minute!
Why is it you need this much output wattage in the first place, is this to be a Bass amp?
If not just remember that you will only gain 2 X the clean head room in the output stage only when you go from a 50 watt to 100 watt output stage all things being equal, but not twice the volume output!