Sound City help

[WORMDIRT]

Ok, I have a sound city 50 plus, mark 4
3 things
1) What do I need to do change to get the bias to go lower, the lowest setting on the pots is 49ma per tube
2) There is a third potentiometer, in between the bias pots, what does is it for exactly? there are 2 are for each individual power tube's bias, and the third?
3)Can this thing run 6550's? It has the original partridge tranny in it.

Schems

http://soundcitysite.com/sc_lb50plus_schemo_left.jpg
http://soundcitysite.com/sc_lb50plus_schemo_right.jpg

[xtian]

You want to increase the value of R43 in order to generate more negative voltage on the grids, giving you cooler running tubes.

I do not see the "third" pot in the bias schematic. Perhaps you have a balance control? Naw...that would be redundant with the individual bias pots.

[Leo_Gnardo]

Third pot might also be a hum balance. If it's connected to the filament supply, with the wiper going to ground, then that's what it is.

If you choose to run 6550's, you will have to run the bias voltage higher yet.

[WORMDIRT]

Third pot might also be a hum balance. If it's connected to the filament supply, with the wiper going to ground, then that's what it is.

If you choose to run 6550's, you will have to run the bias voltage higher yet.

Higher? The wiper goes up pretty high, like 150ma maybe more.

[cbass]

Third pot might also be a hum balance. If it's connected to the filament supply, with the wiper going to ground, then that's what it is.

If you choose to run 6550's, you will have to run the bias voltage higher yet.

Higher? The wiper goes up pretty high, like 150ma maybe more.

I believe Leo means more negative voltage which you would need for 6550s not higher current.

[Leo_Gnardo]

Third pot might also be a hum balance. If it's connected to the filament supply, with the wiper going to ground, then that's what it is.

If you choose to run 6550's, you will have to run the bias voltage higher yet.

Higher? The wiper goes up pretty high, like 150ma maybe more.

I believe Leo means more negative voltage which you would need for 6550s not higher current.

Thanks again cbass, yes bias voltage higher in the "absolute value" sense, more negative voltage. Probably around -60V, maybe more.

[cbass]

Higher? The wiper goes up pretty high, like 150ma maybe more.

I believe Leo means more negative voltage which you would need for 6550s not higher current.

Thanks again cbass, yes bias voltage higher in the "absolute value" sense, more negative voltage. Probably around -60V, maybe more.

Its gettin pretty bad if you need someone with my 6th grade vocabulary to clarify for you. I make up for my lack of words by makin ,up new words n my mind.then I know words that even educated geniuses don't know.

[WORMDIRT]

I believe Leo means more negative voltage which you would need for 6550s not higher current.

Thanks again cbass, yes bias voltage higher in the "absolute value" sense, more negative voltage. Probably around -60V, maybe more.

Its gettin pretty bad if you need someone with my 6th grade vocabulary to clarify for you. I make up for my lack of words by makin ,up new words n my mind.then I know words that even educated geniuses don't know.

Muh bad, I read your reply like an ass, instead of actually trying to comprehend what you were saying.
Soo, changing R43 to a higher ohm will let in more voltage to the grid, thus allowing me to install 6550s.
2 things
How much negative voltage will each tube need, exactly?
And how can I know if the tranny can handle the 35w per tube?

[Leo_Gnardo]

Muh bad, I read your reply like an ass, instead of actually trying to comprehend what you were saying.
Soo, changing R43 to a higher ohm will let in more voltage to the grid, thus allowing me to install 6550s.
2 things
How much negative voltage will each tube need, exactly?
And how can I know if the tranny can handle the 35w per tube?

Not to worry. Sometimes my answers are worthy of a donkey too...

Can't be certain exactly what -V you will need, just prepare for around -45 to -55V. Replace R43 with a larger value resistor to put you in that area and adjust R7 and R8 bias trim pots properly once you have the 6550's in place. Start with the bias voltage at the highest setting - meaning the most negative. I think the hi voltage supply will work just fine. But you'll be drawing an extra 1 ampere current on the filament line. I doubt the power transformer will complain. If you see the filament voltage fall much below 6.3V that would be the cue to quit trying 6550 and go back to EL34. You don't have to bias 6550 all the way to 35W. You'll probably find the crossover notch disappear at 20-25W, I'm sure your tough old Partridge PT will handle that.

I revised my guess for bias voltage down from -60V after seeing the hi voltage will likely be in the 420-460V range.

- - - - - - - - - -

cbass, you're a smart guy and know your stuff in amps & guitars and lots more besides. No excuses! I'm glad you got my back. Thanks again!

[shane]

Theres a 22K trimpot in the schematic on V3b by the gain switch if you look carefully. Its under the 2K7 R32 and across the gain switch.

Cheers
Shane

[Stevem]

With 420 volts on the plates of the output tubes 49 ma is not at all too hot, in fact 55 ma would not be too hot.
Why do you want to drop the tubes idle point, less early output stage distortion?

[cbass]

With 420 volts on the plates of the output tubes 49 ma is not at all too hot, in fact 55 ma would not be too hot.
Why do you want to drop the tubes idle point, less early output stage distortion?

I believe he is still using el34s and inquired aqbout using 6550s
I wouldn't push the 34s past 40mA at that voltage

[WORMDIRT]

Theres a 22K trimpot in the schematic on V3b by the gain switch if you look carefully. Its under the 2K7 R32 and across the gain switch.

Cheers
Shane

Ok, so what does it do?