Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => AmpTools/Tech Tips => Topic started by: AHeck on October 25, 2021, 07:52:16 pm

Title: I Built It, Help Me Use It
Post by: AHeck on October 25, 2021, 07:52:16 pm

I built a rectifying circuit to help approximate the useable DC from the several PTs I’ve collected or hoarded.  The issue I have is computing the load I should use for any given circuit.  If someone could rough it out for me or suggest reasonable estimates for different power sections,!id be very appreciative.  The circuit at hand is a pair of EL84s in a PP configuration driven by two 12AX7s.  Is there more information I need to come up with a load value?  Thanks very much.

Title: Re: I Built It, Help Me Use It
Post by: PRR on October 25, 2021, 08:24:59 pm

Please do not electrocute yourself on that rig!!

> ... pair of EL84s in a PP configuration driven by two 12AX7s.  Is there more information I need......

Rectifier!! Silicon, you only need the loaded AC voltage. Tube rectifier adds a significant drop. Although, for commercial economy, you use the smallest rectifier which won't die (in 90 days) and then the VDC may be only a hair above the VAC.

A couple of 12W bottles (EL84, 6V6) with typical bias and loading will like 350VDC at 100mA. The little bottles never matter if the big bottles are more than a couple Watts.

This would seem to be 250VAC@0.2A, or 1250Ω one one 250VAC winding, or 5000Ω across a 500VCT winding. The nominal power is 35 Watts, but since you might connect higher-voltage windings, 100W would be a lot safer.

It really is better to TAKE NOTES when you salvage old iron. But nobody does.

Title: Re: I Built It, Help Me Use It
Post by: AHeck on October 25, 2021, 10:42:38 pm

Right, now I’ve test driven it, it gets fused and a Plexi home with an external ground connection and the needed probe points all accessible behind glass.  Plus I’m going to earth ground the plate and tie it in, too. Thank you for the advice and I can see it looks fill tilt sketchy in the picture. Just wanted to make sure it worked.
Also thanks for the reply.  I was pretty’s much there but I was way off on the percent increase given two windings.  So, if I have a center tap with say 200-0-200, at .1A, even though I’m only using one winding that reflects 2KΩ , the PT sees 8KΩ ?

Title: Re: I Built It, Help Me Use It
Post by: AHeck on October 25, 2021, 10:52:33 pm

Wait.  So 200-0-200 would be (200/.1)*2? Or 400/.1?

Or am I basing the circuit resistance on the DC voltage?