Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jonas on October 05, 2019, 12:58:17 pm

Title: Output Impedance question
Post by: Jonas on October 05, 2019, 12:58:17 pm

Hi Everyone,

I need help clearing up some confusion...........

I have a push-pull amp with 4 tubes.  The OT is 2K primary and 4, 8, 16 ohm secondary with 8 ohm tap connected to 8 ohm speaker.
 
If I connect the 8 ohm tap to a 4 ohm speaker, would the tubes now "see" 4K?

If I pull (2) tubes, would I need to connect the 16 ohm tap to an 8 ohm speaker to get the tubes to see 4K?

Thanks for your help!!! 

Title: Re: Output Impedance question
Post by: shooter on October 05, 2019, 01:28:54 pm

the primary winding impedance is based on the load impedance (secondary)
so if you 1/2 the load (secondary), the reflected load (primary) with be 1/2

now pulling 2 tubes your design load is halved, so the actual load needs to be halved to "reflect" that change
I think, I'm a tin-knocker plumber at the moment  :icon_biggrin:

Title: Re: Output Impedance question
Post by: jjasilli on October 05, 2019, 01:33:17 pm

The impedance reflects back from the speaker (load) impedance.  If an 8 Ohm speaker load produces a 2 Ohm primary impedance; then a 4 Ohm load produces a 1 Ohm primary impedance.


Yes, an 8 Ohm load on the 4 Ohm secondary tap > 4 Ohm on this primary.  This matches the two tubes pulled scenario.


EDIT:  shooter -- pulling 2 tubes doubles the "output impedance of the remaining tubes.

Title: Re: Output Impedance question
Post by: PRR on October 05, 2019, 05:14:06 pm

> OT is 2K primary and 4, 8, 16 ohm secondary with 8 ohm tap connected to 8 ohm speaker. If I connect the 8 ohm tap to a 4 ohm speaker, would the tubes now "see" 4K? If I pull (2) tubes, would I need to connect the 16 ohm tap to an 8 ohm speaker to get the tubes to see 4K?

This is all *backward*.