You thoughts on Plate Voltage?

[StratTone]

My question comes from blackfacing an early silverface and the tone just not being right and with all things replaced to blackface, all caps replaced, lets just say it is right (haha) and the amp still don't have that soul or warmth of the blackface. The only thing off is higher plate voltages.

What is the typical plate voltage for a Blackface Super Reverb with todays wall voltages?

Anybody here adjust the ladder to bring the 5U4 supers voltages down to around the 450 460 range?

What is your thoughs on the importance of the pramps plate voltages being close to blackface specs?

[Jazz P Bass]

Blackface B+

I think the Blackface having a higher B+ transformer tap is THE biggest point that people miss when they try to convert a Silverface to a Blackface.

[booj]

agrreed. both the preamp and power amp voltages need to be the same. higher voltage is like a treble boost

[StratTone]

agrreed. both the preamp and power amp voltages need to be the same. higher voltage is like a treble boost

Cool that is where I'm at right now. I'm seeing about 480 at the plates with the GZ34. I'm thinking instead of putting the 5U4 back in that I'm going to put the 2.2k and 10k ladder resistors under the cap pan back? What do you guys find best?

Also what are the voltages you are looking for on the plates of the preamp and power tubes?

[soundmasterg]

If you blackface a silverface amp, you need to keep the 5U4 as the PT usually has a higher voltage since the 5U4 had a larger drop. When you do that, your voltages are close to the blackface versions, though the tone has a little more sag, but it still sounds good. If you put in the GZ34 your voltages are too high which changes the tone.

[stokes]

When BFing a SF amp by the usual methods explained on the 'net,you have to realize you are just getting the SF closer to the BF specs,you wont have anything close to a true BF.If you look at the schematics the voltages in the AB763 and AA763 (BF) are actually higher than the AA1069 (SF) so there goes that theory.

[StratTone]

Yeah it is close with that 5U4 back in there but close is the key I guess I will have to live with. Why did some Silverface Supers have the 2.2k and 10k resistor in the cap pan? (keep the voltage down?)

I also notice that some of the Silverface Supers have a 10k resistor on the bias supply board as well. (same reasoning?)

[wizard333]

Why did some Silverface Supers have the 2.2k and 10k resistor in the cap pan? (keep the voltage down?) 

Yes.

480 plate voltage on 6L6s is.......ambitious. I don't think I'd subject any modern tube to that other than a JJ. If you have to run PV that high, you might consider a KT66. It will be more forgiving.

[StratTone]

Why did some Silverface Supers have the 2.2k and 10k resistor in the cap pan? (keep the voltage down?) 

Yes.

480 plate voltage on 6L6s is.......ambitious. I don't think I'd subject any modern tube to that other than a JJ. If you have to run PV that high, you might consider a KT66. It will be more forgiving.

Cool thanks for the info. I have 460 on the plates right now with the 5U4 and the stock blackface cap pan resistors. 1K 4.7k.

What bout the 10k bias board resistor? Go to 470 ohms or only change if the desired bias can't be reached? Right now I think I'm good since I'm getting 460V on the plates of the power tubes and 270 on the plates of the preamp tubes.

[wizard333]

I wouldn't change the bias resistor if it biases correctly as-is.

270v on the plates of the pre-amp tubes is ambitious as well. Were talking AT THE PLATE right and not the B+ before the plate resistor? And this is with all tubes installed correct? I.e. under load? I've never seen anything close to that on a BF or SF Fender. If you're getting 270v at the plates on your pre-amp tubes under load, I'd really consider putting in a larger resistor(s) between your screen supply and your preamp filters in your power supply.

[StratTone]

I wouldn't change the bias resistor if it biases correctly as-is.

270v on the plates of the pre-amp tubes is ambitious as well. Were talking AT THE PLATE right and not the B+ before the plate resistor? And this is with all tubes installed correct? I.e. under load? I've never seen anything close to that on a BF or SF Fender. If you're getting 270v at the plates on your pre-amp tubes under load, I'd really consider putting in a larger resistor(s) between your screen supply and your preamp filters in your power supply.

Yeah 270 under load at the plates of the preamp tubes. (the ones in the 270 range anyway) I did change the dog house resistors back to the SF ones and it went down to 240 but changed them back to the BF values. I actually liked it better at 270. Sounded pretty damn good now. The GZ34 just made the plates too high but the 5U4 got it just were I need it. I'm sure I'll be happy for a while and then have to go back in there and do some changes anyway. The SF Super Trannies must be different cause it gets close to BF tones not just isn't going to happen I don't think. I'm going to be shopping for a BF Super since the prices are down.

[wizard333]

240-250 is more what I see commonly. The other side of that resistor should be around 380-400 range.

If you like the 270 better then leave it, it will be cleaner/brighter but it will wear out your tubes a bit faster.

[StratTone]

240-250 is more what I see commonly. The other side of that resistor should be around 380-400 range.

If you like the 270 better then leave it, it will be cleaner/brighter but it will wear out your tubes a bit faster.

Yeah I think before I replace the stock alnicos in the Super I'm going to leave it at 270. The lower voltage would have sounded better for sure but these alnicos are well worn.

[stokes]

Those 12AX7's can take the 270 v's with no problem.If you look at the schems one of the SF Super Rev shows just that.An AX7 can take 300v by design and most will whithstand a bit more.

[SoK66]

With the caution that I don't know exactly what you did when you "blackfaced" the amp, I'd just add that to get the amp to proportionally balance it's B+ you have to match the specs of the BF amp's dropping resistance in the cap pan. That is, along with removing the power tube grid caps & matching the PI resistance values to BF spec, and perhaps converting the amp to bias adjust vs bias balance (I do both), you need to match the cap pan dropping resistances to get things in balance.

If you're running the amp at today's line voltage (120 - 125vac) bear in mind the amp probably didn't see that kind of voltage when new. In fact, the old export amps had a selector switch for the various countries line voltage. The setting for North America was 117vac. Use that 117vdc as a guide when considering whether your amp's B+ is in the ballpark.

In general, when run at todays line voltages 50s, 60s & early 70s amps are going to run as much as 50vdc more B+ than shown on the schematics, and that's just not good. SS rectified amps are the most problematic. Although on tube rectified Fenders you can drop the B+ / plate voltage by selecting a different rated tube, this changes the amp's performance characteristics and does nothing about the higher heater voltages being generated.

There are two real fixes for this, either swap out the PT for a new one designed to operate at today's line voltages and render spec B+, or, use a variac, a buck transformer, etc. to drop the line voltage back to the 110 - 117vdc the amp was designed to see. You can make a simple buck transformer out of Radio Shack components, or spring for something like the Amp Preserver (do a Google) or the Weber Browner. One way or the other , if you get the amp to see spec internal voltages it will sound its best and tubes & components will last longer, too.

(All the above gleaned from experience, cursing, etc.....and, of course, JMHO!)