Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rdrgtr on November 10, 2010, 10:15:16 pm

Title: too much B+
Post by: rdrgtr on November 10, 2010, 10:15:16 pm

I just finished the power supply in my new build.  I have installed a switch to select between a tube rectifier and a solid state rectifier.  I have 540vdc on the solid state rec. and 537vdc on the 5u4.  These readings are with the transformer, switch, rectifiers, and filter caps.  I pretty much wired this just like the drawing in the link below, but I left off that secong stage of filter caps on the solid state side. (only 2 220uf caps instead of 4)  I am not really worried about the voltage for my caps because they are all rated at 400vdc(series). My question is, will the tube rectifier drop more with a load than the solid state rectifier?  I was under the impression that it would drop more even without a load.  If my B+ is now 540vdc, what would it be with a load?  Oh, by the way, this is a marshall 50w build with 2 el34 output tubes.

http://www.eventcom.net/catalog/tube2web.pdf

Title: Re: too much B+
Post by: sluckey on November 10, 2010, 10:32:20 pm

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My question is, will the tube rectifier drop more with a load than the solid state rectifier?

Yes

Quote

I was under the impression that it would drop more even without a load.

No. With no load, ie, current draw, no voltage will be dropped.

Quote

If my B+ is now 540vdc, what would it be with a load?

That depends on how much load you put on it, the internal loss of PT, and the type of rectifier you use. Also, the load will change as you change the bias of the output tubes.

Title: Re: too much B+
Post by: PRR on November 11, 2010, 06:40:02 pm

FWIW: With _NO_ load, a vacuum rectifier has even less drop than a sand rectifier.

Your 4V lower suggests there is some tiny load, probably typical electrolytic leakage.

Put a load on it. Silicon, 0.5V to 0.9V drop. Vacuum, 0.1V to 70V drop. For any "useful" load, vacuum rectifier losses are larger than Silicon.

> marshall 50w

Then it should stand 540V long enough to smoke-test. Unless your PT is obscenely over-weight, with nice warm bias it will sag 10%-20% under load, putting you below 490V, maybe 440V with 5U4 and FULL ROAR signal. These are reasonable.