voltage doubler,rectifier

[gary sanders]

I have a power transformer I need to make a doubler for and a bridge rectifier.Anyone have any ideas or done this before?The leads,no load,were at 180V,no CT on this transformer.I need about 400V

It was originally wired with a doubler in an organ amp.I only know this because I found an old discussion about it on another forum,but I couldnt figure it out,it was all point to point.Looked like a phustercluck to me.If I had a schematic I could have made sense of it.Ive done a few multipliers before succesfully just not on high voltage like this.

[Structo]

Here is a simple voltage doubler.

Not real sure what the output will be with 180vac on the secondary.

If you haven't read it Merlin Belcowe has a great book on power supplies, called Designing Power Supplies for Tube Amplifiers.

He covers all that and more in the book.
Excellent resource.

[ToneMerc]

Not real sure what the output will be with 180vac on the secondary.

Should be around 508VDC unloaded.

TM

[gary sanders]

I was in a hurry and quickly scanning Merlins rectifier and other valve wiz site on this today.I think His web guy was doing some maintenance cause there were images missing so without pictures,Im hopeless.His books are on my letter to Santa

Thanks for the diagram,and calculation,will give it a try.I need around 500V for the 7868s.

(edit) One more thing,do the diodes in this act as a rectifier or do I need to use a bridge between this and the reservoir cap?Im guessing it does but in a way it also looks as if the diodes would cut the AC as well

[Structo]

No they rectify the current but I seem to remember there was something funky about the current demand of the transformer.

The diagram I posted is a full wave doubler.

[gary sanders]

Thanks again,Structo.Im on it.This is going to be a crazy amp.Ive posted quite a few different topics lately about different things but Im about to put them all in one chassis and see what happens.Its no fun if you do what everyone else is always doing

[ToneMerc]

No they rectify the current but I seem to remember there was something funky about the current demand of the transformer.

It's HT current capacity rating is almost cut in half when configured as a doubler, so the PT needs to be rated at twice the norm. A good example is the MusicMan 4 output tube power transformer which is setup as a doubler, it's rating is 265V@1A.

TM

[gary sanders]

Im searching for a schematic on that just for reference but no luck.Do you have one you could post?

[ToneMerc]

Two different configs, the same outcome. When I built my high voltage D clone I used a modified version if scheme #2.

[gary sanders]

Would 40uf 450V caps be ok since Im using a bit less voltage?

[Phil_S]

The doubler will produce about 2.8x input voltage. Someone said 504V and I didn't double check that math, but it sure seems right to me; besides, actual results will vary due to the loading of the circuit. If you think you can knock it down to 450V, good luck with that -- there are a number of ways to reduce voltage. I would want to use 600V rated caps as I would want to spec the caps for what that might see peak voltage, not for ultimate end result.

[marcoloco961]

Here is a schematic for an old Silvertone amp I have. It uses a double voltage doubler...LOL. Two very low voltage secondaries doubled, then stacked together. Just ignore one of the secondaries and you will see how it was implemented. Hope this helps.

[ToneMerc]

Would 40uf 450V caps be ok since Im using a bit less voltage?

Well as far as voltage you could get by with 350V caps with a 180v secondary. The first node will see the two caps in series, which will be rated for 700V. My build with 122.5 VAC in with a 268V secondary yielded 765V unloaded and I used two 450V caps in series. The two series caps will half value so I would raise the amount of capacitance on the front end and filter the screens from the choke as depicted. I would use at least two 100uf 350V caps.

I'm not sure of your exact circuit, but you need to have an idea of your power trans HT current rating. Now as far dropping voltage, you can use a big power resistor(with or without a bleeder resistor) or zener diodes.

TM

[ToneMerc]

Would 40uf 450V caps be ok since Im using a bit less voltage?

Also, the next question is how you will you implement the bias circuit?

TM

[gary sanders]

Would 40uf 450V caps be ok since Im using a bit less voltage?

Also, the next question is how you will you implement the bias circuit?

TM

Its already there,it is two (stereo) pair of 7868,cathode biased.There are other leads around 25V,if I remember correctly,that powered the solid state preamp and I was considering using one of them for a bias supply,I just found a Bogen schematic with a doubler and fixed bias 7868 quad.They are also using 100uf/300V.

As far as the current goes I am assuming the PT will be fine because a half wave doubler was the original circuit that not only run the power amp but also about 30 12au7 oscillators,i gut the power supply section because it was a PTP mess with old can caps and 45 year old diodes.Doing so also gave me room for the new preamp.

In your reply about using at least 100uf,I will just try the original 100uf/300V cans to get it going and install new ones when the mail gets here.May give the JJ cans a try.

(This is just another one of my salvage builds where i try to make chicken salad out of chicken sh*t )