Cathode bias cap/resistor value for a Silverface Pro Reverb?

[dehughes]

Hello again,

I have a friend's SS rectified, late '70s Silverface Pro Reverb and it is a messy mess of tinkery modding. Oh my goodness....so many people have had their hands inside of this thing...and to top it off, it is a later model with the hum balance pots (no bias adjustment), a master volume (worst ever), and midrange pots (interesting). At this point I may as well try anything my little heart desires, as it would most certainly be an improvement!

I figure that since this thing is 1) not the ideal Pro Reverb circuit anyway 2) has been modded to bits, and 3) isn't going to be his main amp for guitar, I may as well try running a cathode bias setup for the 6L6s, especially as he plays this thing up LOUD and could probably benefit from the extra give that a cathode bias would provide. Plus, then we won't have to deal with the "output tube matching" pot lameness.

What cap/resistor values should I start with? Is there anything else to consider in making this particular model run a cathode biased power section?

Thanks!

[Vince]

[...]
3) isn't going to be his main amp for guitar, I may as well try running a cathode bias setup for the 6L6s, especially as he plays this thing up LOUD and could probably benefit from the extra give that a cathode bias would provide. Plus, then we won't have to deal with the "output tube matching" pot lameness.

What cap/resistor values should I start with? Is there anything else to consider in making this particular model run a cathode biased power section?

Thanks!

Try 220R/20W res and 50-200uF/50V cap.
If B+ is big - try 270R-330R resistor.
You will LOVE cathode bias

regards
V.

[dehughes]

SWEET! Thanks... I wager that the B+ is pretty big being as this is a later model SFPR.....maybe it is running ultralinear? I'm not too up that....but my point is, I wager the B+ will be big indeed. I'll measure it and find out.

I have some nice quality BMI 250uf 100v caps lying about from my AC30 build, so that'd work just fine, yes?

THANKS!

[David Root]

What is the circuit number? It should be on the tube chart in the cabinet. If it is an ultralinear OT, why not try two KT66s for some real beef? Should get about 30W or so flat out. Looks like about 250 ohm 25 or 50W Rk.

[dehughes]

What is the circuit number? It should be on the tube chart in the cabinet. If it is an ultralinear OT, why not try two KT66s for some real beef? Should get about 30W or so flat out. Looks like about 250 ohm 25 or 50W Rk.

I couldn't find a circuit number on the tube chart....this amp has been PLAYED so there isn't much tube chart left. It has also been MODDED (!) so there's not much circuit left! It will state that it has a master volume, and midrange controls, and small ceramic caps across the SS diodes in the recifier section.

250 ohm...okay, that sounds like an early tweed value for 6L6s....would that work with these high plate voltages? I just measured 500v (!) on the 6L6 plate...yikes...

[dehughes]

You know, I may be in over my head with the cathode bias power section idea...as the vibrato circuit is tied into the hum-balance pots and I'm not sure I'd be a good enough tech to figure out how to cathode bias the 6L6s and still keep the vibrato working...

[rhinson]

You know, I may be in over my head with the cathode bias power section idea...as the vibrato circuit is tied into the hum-balance pots and I'm not sure I'd be a good enough tech to figure out how to cathode bias the 6L6s and still keep the vibrato working...

well it's not really a hum balance, it's the old bias balance instead of the normal blackface style bias adjust (which btw i'd change if you weren't going to cathode bias it) part of the neg. voltage runs to the trem--just leave it hooked up---it's got nothing to do with cathode biasing the amp. all you have to do to cathode bias is: unhook the neg voltage supply from the 220k resistors and hook them to ground instead; choose your big cathode resistor/cap combo and hook them up to pins 8 of the power tubes---that's it. you just built an ac30----no difference in the style of how to do it.

btw, a normal hum balance control is a 100k pot across the filament winding leads (since it won't have a ct) in lieu of the 2 100ohm resistor thing. i've founded several of these bad in fenders over the yrs---i now just yank them in favor of the 2 100ohmers. rh

[dehughes]

Wow. Thanks man! Maybe I'll try this then...if it isn't to complicated... So, when you say leave the negative bias voltage hooked up to the trem....what exactly does that look like? I'm still not getting it...

My mental picture is that I leave the 1k5 grid resistors in place, and then connect them to a set of 220k resistors that go to ground on one end and on the other end connect to the PI caps. Attatch a cathode resistor and cap to pin 8 on the 6L6s and I'm done.

Do I then unsolder the two leads that go to the hum balance pot? I'm a bit confused....I'm not used to working with tremolo circuits...

Thanks!

[rhinson]

Wow. Thanks man! Maybe I'll try this then...if it isn't to complicated... So, when you say leave the negative bias voltage hooked up to the trem....what exactly does that look like? I'm still not getting it...

My mental picture is that I leave the 1k5 grid resistors in place, and then connect them to a set of 220k resistors that go to ground on one end and on the other end connect to the PI caps. Attatch a cathode resistor and cap to pin 8 on the 6L6s and I'm done.

Do I then unsolder the two leads that go to the hum balance pot? I'm a bit confused....I'm not used to working with tremolo circuits...

Thanks!

the hum balance pot has nothing to do with the trem-----it's just a ground reference for the filament winding since it doesn't have a ct---it functions just like the 2 100ohm resistor setup you see in many amps.

the other "balance" pot is the bias balance pot -----it's not a true bias adjustment like in blackface amps--which is why people change it to a true bias adjustment, like the blackface circuit. just make the top part of your pi circuit look just like you said: 2 220k load resistors to each coupling cap, their centers to ground (remove any of the fixed bias supply that is attached in that part of the board). don't worry about the trem---it's got nothing to do with any of this. you need a schematic so you can see what's going on if you can't tell from the board. btw, find the 2.2m resistor in the trem circuit--that's where the neg. voltage supply ties in to it.

i'd personally do what these guys have alluded to---change the whole thing to the blackface circuit values---it really doesn't take much. just go to schematic heaven and get any ab763 or aa763 of any of the fender models from deluxe on up thru twins---it's all the same.

http://www.schematicheaven.com/fenderam ... tr_rev.pdf

chances are this is the amp circuit you currently have from what details you've described. the caps across the diodes are not a mod---this is stock fender from this era and you see others do it as well. it's to help get rid of the diode switching noise if indeed it was a problem. rh

[dehughes]

Hmmm...okay, Blackface it. That makes sense.... Thanks. I'll have to do some studying and reading...

Regarding that schematic....it isn't quite this amp, as the amp I have here does not have a tube recto....and, the 1st filter caps are two 220uf caps run in series.