Two Rectifiers, One transformer

[jkeny]

Just a quickie - I need two different voltages 250V & 130V and I've only one transformer giving 115V AC

I can get the 250V with a voltage doubler rectifier (295V regulated down to 250V)

Can I use full bridge rectification on the same transformer to get a regulated 130V without causing problems.

[nitrate]

Yep, you can get the base voltage from the transformer plus the same again times two using two bridges. This is what i have done here http://www.diyaudio.com/forums/showt...48#post1568248

Regards

Leigh

[kevinkr]

Originally posted by jkeny
Just a quickie - I need two different voltages 250V & 130V and I've only one transformer giving 115V AC

I can get the 250V with a voltage doubler rectifier (295V regulated down to 250V)

Can I use full bridge rectification on the same transformer to get a regulated 130V without causing problems.

Is this a center tapped type with 115-0-115 or just an untapped 115V winding?

With CT you can bridge rectify the 115-0-115, ground negative terminal of rectifier and get half the raw supply voltage off of the CT.

Otherwise a FW doubler would work and you can pull some current from the first (lower) cap in the doubler.

[jkeny]

Here's the schematic - it's from a Rogers Cadet III integrated amp into which I'm putting a BDT preamp - I need 250 & 130V regulated supplies for BDT. So I could take the lower voltage off the bottom cap!

[kevinkr]

Originally posted by jkeny
Here's the schematic - it's from a Rogers Cadet III integrated amp into which I'm putting a BDT preamp - I need 250 & 130V regulated supplies for BDT. So I could take the lower voltage off the bottom cap!

How much current do you actually need at 130V - if only a few mA I would probably pull it from the regular supply rail..

[jkeny]

But if I pull it from the regular supply rail I'm worried about the dissipation issues as I'll be dropping a lot of voltage from ~300 down to 130V at 24mA

[Tweeker]

Well, you could avoid dropping it all in the reg and drop some in another RC section.

[kevinkr]

Originally posted by jkeny
But if I pull it from the regular supply rail I'm worried about the dissipation issues as I'll be dropping a lot of voltage from ~300 down to 130V at 24mA

I suspect that the additional 24mA is probably going to be an issue for the power transformer. And I believe even taking it as 1/2 wave between the caps is going to cause a problem with excessive heating as well, not to mention poor performance in the doubler.. I can't imagine that the existing transformer was that oversized. Any possibility of fitting a small transformer to supply the additional B+ required or to off load the filament load?

[jkeny]

Would off-loading the filament load unburden the trafo enough?

The 24mA is the tube current when at max vol - I don't think this will be the case - don't know yet what the normal operating point will be.

This is for the John Swenson's BDT preamp based on 6AR8 Beam Deflection Tubes which are supplied by two voltages, 250 & 130V. The 250 supplies the accelerators and the 130V supplies the bias for the cascode on the plates.

According to JS

The accelerators are driven with 250V and the transformer primary (B+) gets 130V. The B+ can be anywhere from 24mA per tube to less than 1mA dependant on the volume control setting

I'll probably try it with existing trafo & see how the load is. I've removed 3 ECC807 tubes which were the preamp in the Rogers & replacing them with 2 6AR8 BDT tubes so maybe the trafo is up to the job?

[kevinkr]

I'm not really sure whether eliminating the filament load on this transformer will do the trick or not as it is very dependent on the winding gauge of that HV secondary. The additional 24mA load at maximum volume is pretty significant..

The plate current those ECC807 used is probably on the order of just a few mA so their elimination is not going to buy you much.

Not trying to rain on your parade, the only thing you can really do is try it and see whether or not the power transformer gets too hot. The safest bet might be a very compact 230VCT transformer to supply just the BDT pre-amp plate and deflection electrode voltages. I'm assuming there are small toroids with split primaries and secondaries that might fit. A 20Va type ought to do it just fine.