[Niteshade]

What is your screen voltage? Try a voltage of around 150v. At your B+ voltage, anything much over 200v will most likely be too much. What is your bias voltage? You will have to use voltage dividers to get the proper screen voltage. To reduce plate voltage, try a 100 ohm 10w wire wound resistor from the rectifier to your first filter capacitor.

For best sound, you'll have to experiment w/ screen & bias voltages and maybe a resistor between the rectifier & first filter cap (as mentioned above).

Good luck! Sounds like fun. 

[trem]

Yes, all the tubes are in.   I have some high voltages on parts of the PI/Compensator, or whatever that 6U8 tube is.   Scott gives a rather lengthy bias procedure for this amp that is kind of over my head.   Turning the bias pots gives about a 10 volt change on the pot output,  but it does not have any affect on the power tubes.    I may have reached the limits of my abilities at this point, but I will just keep doing the best I can.
Thanks Again

[jrebman]

Wish I could be more help, but I'm sure you'll get it working the way you want sooner or later.

Good luck,

Jim

[Hank]

I have my LK48B that I built from the kit when I was about 17.  It has the selenium rectifier (which I will replace with a diode bridge).  I've got a couple of 222C's (factory built version of the LK48B) that I bought off eBay and they've got seleniums.  Selenium's go bad over time and should be replaced with diodes.  Because the voltage drop is different across the two rectifier types, the voltage out of a diode bridge is higher and you have to increase the resistance of the following resistor (the 2 watt 10-ohm) so that your bias voltage doesn't go too high.  There's a recommended resistor value that slips my mind.  I'll try to look it up, but try a 15-ohm and see what that does.

[smbrown]

First off, I'd be concerened that you don't drop any volts across that 20ohm resistor, which is what I think you posted. It should be somewhere around 3v, I'd guess (20ohms * 150ma). The 5AR4 might be part of the problem. I often find plate voltages on vintage amps too high because they're built for 110v line, not 120v and the 5AR4 has about the lowest voltage drop across it you'll find. An extra 10v on the primary should give you maybe 40v extra B+ if I'm doing my math right. You might try to find a rectifier with a higher forward voltage drop (5U4, 5V4) but be careful because they tend to pull more heater current, which might compromise your power transformer. Are you finding all the B+ are too high, or just on the output tubes? That 10 ohm resistor on the 7189 should drop around 35ma*2*10 = 0.7v. As a last resort, you could change that 20ohm resistor off the 5AR4 to 390 ohm, at least 20w.

[trem]

Thanks for all of the continued help.   I realize how hard it is ti diagnose via the INTERNET.  I will digest this latest info for a few days and post back with some detailed voltage readings.
Thanks Again

[Niteshade]

I just finished working on a Bogen El84 based amplifier and wanted to share the voltage spec's for the output tubes:

B+ : 390V
Bias: -15v
Screen: 270v

I know yours is different- but this gives you something to go by. The amp works VERY well at these voltages and operates at low temps.

[trem]

I just finished working on a Bogen El84 based amplifier and wanted to share the voltage spec's for the output tubes:

B+ : 390V
Bias: -15v
Screen: 270v

I know yours is different- but this gives you something to go by. The amp works VERY well at these voltages and operates at low temps.

Thanks for the info.   I have had to put the Scott on hold for a few weeks.
But are you saying you have 270 on the screens and close to 390 on the plates?
Thanks

[Niteshade]

That's right. Since your B+ is higher, make the screen voltage lower and the bias more negative. It will take some experimentation.

___________________________

"Thanks for the info.   I have had to put the Scott on hold for a few weeks.
But are you saying you have 270 on the screens and close to 390 on the plates?"