ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Re: It works!! (now some questions)

:

1/7/1999 6:39 AM

Re: It works!! (now some questions)

I could be wrong but I don’t think you need to change the f/b resistor value or tap to keep the f/b the same as the original. If you have an 8R load on the 8R tap then the voltage on the 2R tap will be the same as it would be when using a 2R load on the 2R tap so the f/b ratio is unchanged (I think)

Is it a 5F6-A circuit ? With some Fender tone stacks you get no output with all the controls fully ccw but a 5F6-A Bassman should always have some output. With the treb pot working backwards there must still be a wiring error. The cw end of the pot goes to the 250p cap.

Dave

1/7/1999 8:23 AM

Dave:
As I understand it, the deal is that the -fb voltage is split between the Rfb and the presence pot. If you assume 50W, a 2-ohm tap is running about 10V, a 4-ohm should be about 14V, and an 8-ohm tap is cranking 20V (approx). That 27K resistor splitting the -fb with the 5K pot will give you a different amount of -fb with each tap you place it on (there may be no load, but a tranny's a tranny and the turns ratio is still in effect. You're delivering V, not V^2/R here.. ) You can clearly see a pattern in Leo's stuff; all the amps have their -fb loop tailored according to the output imp they run, for this reason (IMO). It is also good design practice to run the -fb from the tap you are using for more technical reasons I'm not as comfortable with. Maybe Dave Stork can pop in here with the heavy heavy tech stuff. ;-)

;-)

1/7/1999 8:54 AM

quote:
" If you assume 50W, a 2-ohm tap is running about 10V, a 4-ohm should be about 14V, and an 8-ohm tap is cranking 20V

there may be no load, but a tranny's a tranny and the turns ratio is still in effect"

That’s exactly what I’m trying to say ! At 50W the 2-ohm tap will have 10V on it whether you have an 8 ohm load on the 8-ohm tap, a 4 ohm load on the 4-ohm tap or a 2 ohm load on the 2-ohm tap so you can leave your 27k f/b resistor permanently connected to the 2-ohm tap, use any tap for the load and the f/b voltage will be the same.

Dave

1/7/1999 3:36 PM

...oops! you're absolutely right. I did mean the bit about other reasons though.. I've read some -fb articles/etc and talked with foks on teh newsgroups about it (Dave Stork & Randy Aiken come to mind, probably Mark Garvin as well) had points about why it is best for the -fb take off point to be on the same tap as the speaker which is in use.

Could someone with that body of obscure knowledge bust in here and throw me a rope? ;-)

;-)

Page 4 of 4