Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on December 02, 2011, 03:07:23 am

Title: OT impedance - I've find a schematic/formula to measure it but need explanations
Post by: kagliostro on December 02, 2011, 03:07:23 am

I've find this schematic

(http://i81.photobucket.com/albums/j207/merlinblencowe/OutputImpedance.jpg)

I'm correct [ referring to the formula  -  Output Impedance = (Rload x V1) / V2  ] if I say that

Rload = Dummy load 10W min. ?

and if so which value is better to use for the Dummy load ?

Thanks

Kagliostro

p.s.: be patient I've no study on the matter

Title: Re: OT impedance - I've find a schematic/formula to measure it but need explanations
Post by: PRR on December 02, 2011, 09:33:44 pm

That is the hard way.

It also only tests at 50/60Hz.

Get a "normal load" dummy, say 8 ohms, and one 10X higher, 80 ohms.

Load with the 80 ohms and bring the output up to a convenient level far below clipping. 1V is a good value.

Now parallel the 8 ohms and measure voltage.

If still 1V, the output impedance is zero. This is the expected result for most direct-coupled transistor amps.

If the output sags to 0.5V, Zout is about 8 ohms (actualy 80||8= 7.27r). This ballpark is typical for classic Fenders.

If the output sags to 0.1V, Zout is about 10 times 8 ohms (actually 9 times 7.27r). This is typical for no-NFB pentodes.

Since the power is at-most 1V in 8 ohms, 1/4 resistor is plenty.

Title: Re: OT impedance - I've find a schematic/formula to measure it but need explanations
Post by: kagliostro on December 03, 2011, 12:46:53 pm

Thanks PRR

Kagliostro