22uf/ 470ohm going to ground off pin 8 ?

[jamme61]

Have a Marshall studio 15 that I changed all the filter caps on. I noticed on pin 8 on the 6v6's there's a 22uf parallel with 270 ohm resistor to ground. My first question is what does the cap do? the amp was made in 1986 - so far I changed all the filter caps and the bias caps (the two 10uf caps) amp sounds great but should I replace those caps off of pin 8? thanks for any help

[thejaf]

cathode bias

[martin manning]

It's mixed fixed and cathode bias. I would change them given the age and hot environment they are in.

[jamme61]

great! thanks for the help- I have it open so Ill replace them. didn't know you could have a mixed fixed/cathode bias- have to hit the search engine about that one. all the best

[Blackburn]

Check out the Vox AC50...

http://www.voxac50.org.uk/images/misc/c ... ark_03.jpg

[jamme61]

Check out the Vox AC50...

http://www.voxac50.org.uk/images/misc/c ... ark_03.jpg

hey thanks great stuff

[gingertube]

That VOX AC50:
Note that the 47 Ohm Cathode Resistors have no bypass capacitors.

The designers intent here (I'm guessing of-course) was not so much to have a mix of cathode and fixed bias, but rather to de-generate the output tube gm (its a form of negative feedback) to make them a better match as far as their AC signal handling goes, the bias is mostly fixed bias.

Its effect will be to reduce the amount of 3rd and other odd harmonic distortions produced in the output stage.

It is a reasonably common thing in tube HiFi Push Pull Amps and is "bog standard" in transistor differential amps where there are small individual de-generation resistors in each emitter which then connect together to the "tail" resistor.

Cheers,
Ian

[matt h]

(deleted)

[jamme61]

what does the bypass capacitors do in the studio 15? is it there to add bass?
does a lot of voltage pass through there (the resistor/cap )? The resistor is pretty big but the cap is only rated at 25 volts? just curious thanks.


added pictures to show it - not my amp

[martin manning]

The caps allow AC (the signal) to bypass the cathode resistors, which prevents any negative feedback from occuring. They will charge up over some small time, though, and cause the bias point to shift downward. This is what creates the sound and feel of a cathode biased power amp.

[jamme61]

The caps allow AC (the signal) to bypass the cathode resistors, which prevents any negative feedback from occuring. They will charge up over some small time, though, and cause the bias point to shift downward. This is what creates the sound and feel of a cathode biased power amp.

thanks for all the info