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!

[dehughes]

I just measured the plate voltages in this amp.....500v on the 6L6 plates! That's high for a Pro Reverb, yes?

[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...

[Alexo]

If it were up to me, and I had the time, I would take a stab at blackfacing it and really cleaning it up to "better than new."

[s2]

If you really want to cathode bias it, you should start with a 270R 10W+ resistor and a 50uF 100V cap if shared bias (assuming this is a pair of 6L6s). Or you can put a 500R 5W resistor on each power tube to bias it. You will likely have to adjust the resistor value based on voltage to get it into the right range.

But I would black face it and turn it into a really nice amp.

[dehughes]

If you really want to cathode bias it, you should start with a 270R 10W+ resistor and a 50uF 100V cap if shared bias (assuming this is a pair of 6L6s). Or you can put a 500R 5W resistor on each power tube to bias it. You will likely have to adjust the resistor value based on voltage to get it into the right range.

But I would black face it and turn it into a really nice amp.

Hmmm...thanks. So you think a 270 ohm Rk with 500v on the plate would put the (pair) of 6L6s where they should be? That's good to know...

I'll look into Blackfacing it. That'd make more sense, but that might be out of my skill range, what with the probably necessary tranny swaps, etc... I'll check into it, though...

[s2]

500v on the plate is pretty high. Maybe start with a 300R and take a measurement.

Do you have a copy of the bias spreadsheet I posted a while back? It shows the bias points for 50%, 60, 70, 80, 90, and 100% at every voltage from 200 to 600 for most common tubes.

If not, try searching for it and I'll try to remember to post it later tonight when I'm on my other 'puter. It might really help you out.

[dehughes]

500v on the plate is pretty high. Maybe start with a 300R and take a measurement.

Do you have a copy of the bias spreadsheet I posted a while back? It shows the bias points for 50%, 60, 70, 80, 90, and 100% at every voltage from 200 to 600 for most common tubes.

If not, try searching for it and I'll try to remember to post it later tonight when I'm on my other 'puter. It might really help you out.

Sweet man! Thanks...I'll go looking for it.

BTW, this amp has a SS rectifier, so is Blackfacing it really going to be worth it...do you think? I mean, wouldn't I need to get the voltages in line across the board?

[s2]

Here 'tis.

[dehughes]

KILLER. Thanks!