Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: The_Gaz on August 15, 2010, 10:53:44 pm

Title: Cap in Hoffman "Meat" circuit
Post by: The_Gaz on August 15, 2010, 10:53:44 pm

http://www.el34world.com/Hoffman/images/H50pre.gif (http://www.el34world.com/Hoffman/images/H50pre.gif)

I know that the meat control is a variable NFB circuit, and the 22uf cap blocks DC from the pot, but I was wondering about how I can determine the voltage rating the cap should be? I know it's a low voltage there, but I don't have an amp on hand to check the voltage in that spot. Also does polarity matter in this context, or is it noted on the schem to show that the cap is electrolytic? Thanks.

Title: Re: Cap in Hoffman "Meat" circuit
Post by: topbrent on August 16, 2010, 01:23:22 am

I thought that the "meat" control is for resonance, not a "variable NFB circuit."

Title: Re: Cap in Hoffman "Meat" circuit
Post by: sluckey on August 16, 2010, 07:22:13 am

Polarity does matter since the cap eventually connects to the cathodes of the PI. Connect it as Doug shows it. 100vdc rating is probably overkill. You could probably even use a 50vdc rating. Measure the cathode voltage to be sure.

Title: Re: Cap in Hoffman "Meat" circuit
Post by: EL34 on August 16, 2010, 10:06:10 am

I came up with that circuit after hearing and seeing the 5150 resonance control (If my memory is correct)

The polarity is shown on the diagram
I don't remember what sort of voltage appears across that cap
As Sluckey said, 50V will probably be fine

Title: Re: Cap in Hoffman "Meat" circuit
Post by: The_Gaz on August 16, 2010, 01:49:05 pm

Thanks guys!

Title: Re: Cap in Hoffman "Meat" circuit
Post by: The_Gaz on August 16, 2010, 01:50:18 pm

..and the control should work like variable NFB because the 22uf cap passes all audio frequencies.

Title: Re: Cap in Hoffman "Meat" circuit
Post by: jeff on August 18, 2010, 01:22:30 pm

I always thought a in "meat" control a cap, usally a high one, goes between lug 2 and 3. That way you're always sending the highs(through the cap) to the Phase Inverter but the variable resistor is limiting the lows getting to the PI making it "fatter". But maybe that's called something else. Maybe you could put a small cap in series with a spst switch between lug 2 and 3 and experiment.