SSS cathod follower PI on Fender AB763

[bluescaster]

Hi,

i would like to add post CF on my AB763 amp (40w). I draw this schematic from SSS #2 (see attachment).

My questions are: What is the goal of 470k resistors after CF (i saw a SSS layout without this ones)? Is there any mods to do on bias circuit to be ok ?

Regards.

[ToneMerc]

As far the bias you will need -135 to 140 volts on the grids of the PI. Thus, you need to have a fairly high negative DC voltage hitting that 220K/820K junction.

For the raw bias supply, instead of using one leg of the HT winding like HAD, I had a PT made with a dedicated high current/high voltage bias winding. I also used a 2W pot/resistor network to fine balance the CF follower, in addition to a 1W resistor/75K 2W pot network instead of the 130K bias tail resistor.

TM

[talbany]

As far the bias you will need -135 to 140 volts on the grids of the PI. Thus, you need to have a fairly high negative DC voltage hitting that 220K/820K junction.

For the raw bias supply, instead of using one leg of the HT winding like HAD, I had a PT made with a dedicated high current/high voltage bias winding. I also used a 2W pot/resistor network to fine balance the CF follower, in addition to a 1W resistor/75K 2W pot network instead of the 130K bias tail resistor.

TM

So Mike am I reading this correctly?..For the balance pot on the CF you used a 75K..Curious if that gave you much swing?..I ended up going with a 250k..I tried a 100k in mine at first and couldn't get my BH7 to balance..

Curious How did you wire yours up?

Also how did you configure your STBY..Are you interrupting the screens and bias voltage?

All The Best

Tony

[ToneMerc]

As far the bias you will need -135 to 140 volts on the grids of the PI. Thus, you need to have a fairly high negative DC voltage hitting that 220K/820K junction.

For the raw bias supply, instead of using one leg of the HT winding like HAD, I had a PT made with a dedicated high current/high voltage bias winding. I also used a 2W pot/resistor network to fine balance the CF follower, in addition to a 1W resistor/75K 2W pot network instead of the 130K bias tail resistor.

TM

So Mike am I reading this correctly?..For the balance pot on the CF you used a 75K..Curious if that gave you much swing?..I ended up going with a 250k..I tried a 100k in mine at first and couldn't get my BH7 to balance..

Curious How did you wire yours up?

Also how did you configure your STBY..Are you interrupting the screens and bias voltage?


All The Best

Tony

Tony, no for CF I used a 1W 62K and a 100K 2W pot, I'm running KT88's. I substitued the 130K bias tail with a 91K resistor and 75K pot .

Standby is the on/off switch

TM

[talbany]

As far the bias you will need -135 to 140 volts on the grids of the PI. Thus, you need to have a fairly high negative DC voltage hitting that 220K/820K junction.

For the raw bias supply, instead of using one leg of the HT winding like HAD, I had a PT made with a dedicated high current/high voltage bias winding. I also used a 2W pot/resistor network to fine balance the CF follower, in addition to a 1W resistor/75K 2W pot network instead of the 130K bias tail resistor.

TM

So Mike am I reading this correctly?..For the balance pot on the CF you used a 75K..Curious if that gave you much swing?..I ended up going with a 250k..I tried a 100k in mine at first and couldn't get my BH7 to balance..

Curious How did you wire yours up?

Also how did you configure your STBY..Are you interrupting the screens and bias voltage?


All The Best

Tony

Tony, no for CF I used a 1W 62K and a 100K 2W pot, I'm running KT88's. I substitued the 130K bias tail with a 91K resistor and 75K pot .

Standby is the on/off switch

TM

Thanks Mike..I guess each one is different (mine is 100w 4X 7581's)and BH7's are hard to find close to balance..I did end up going with a 12AT7 driver and a 7025 in the PI
Mine too does not have the STBY yet..To be honest I have not missed it..

T

[wjdunham]

I wire the standby by lifting the power tube cathodes with a 1M resistor. Very similar to the way the 50W/100W switch on the ODS amps turns off two of the power tubes. I use 1M as 100K did not completely silence the power amp. This is a much safer approach than switching on and off the high voltage supply, especially if your negative bias on the cathode of CF driver is still on. That tube will be conducting all the time, and switching on it's plates at the same time you switch on the voltage to the power tubes will cause all sorts of bad things to happen.

I had a very hard time finding balanced 12BH7's, and even then they were never as close as a balanced 12AT7 for the CF.

Bill

www.sebagosound.com