red plating power tube

[robthefisher]

v5 on my 18 watt tmb richie layout keeps red plating at full volume.it has solid state rectification.ive checked the solder joints and such but still happens.when every thing is turned up to 10 after about 15 min it starts red plating.my voltages are v4=1:0v,2:14.2mv,3:11.6v,4heater,5 heater,6:0v,7:378v,8:0v,9:361.4v v2=1:0v,2:330mv,3:11.2v,4:heater,5:heater,6:0v,7:380.5v,8:0v,9:361.4v.i have a ppi master volume la mar type on it,any ideas on problem?thanks.not very good at taking pictures...im old lol.

[JMPGuitars]

You wrote v2 instead of v5, but here's my thoughts:

Pin 2 mV reading tells you how good your path to ground is. v4 pin 2 you wrote 14.2mV, which is exactly where it should be. V5 pin 2 you wrote 330mV, which means that path to ground is poorly soldered, or has something else happening (usually it's the soldering).

Do you have separate bias resistors for each of the 2 tubes? I assume that's why pin 3 has 2 different values.

After correcting the soldering to bring down the voltage on v5 pin 2, I would consider changing the resistor to bring pin 3 voltages closer to 12V...and then recheck pin 2.

Your pin 7 and 9 voltages are also about 40 volts too high. My guess is your B+ is higher than normal. We target 340 to 345 V for B+. What's your B+?

Thanks,
Josh

[robthefisher]

thanks Josh i appreciate it.my b+ is 388 volts.im gonna go over my solder joints to ground.i only have one bias resistor for v4 and v5 so ill check that solder joint too.to lower b+ would you raise the reistor across the can capacitor?thanks again.this is the second one ive built.never had no troubles with first...lol.thanks again

[robthefisher]

resoldered all the ground wires and connections now when i turn it on the voltage on v4 pin 2 14.5 and the same on v5 but after a couple of min it raised to 173 mv on pin 2 v4 and .75 volt on pin 2 of v5.when i ohm to ground it reads good.could it be the master volume?it uses a daul 250k pot and 2 2.2 meg reistors.

[zuceno]

Like Josh said, your B+ is well over the top. Solid state rectification does it again. How your first amp survived I don't know, but it must have some good stuff in there.

Whatever, you are trying to get tubes designed for lower voltages to work over specification. As I see things you have two choices. Either you take the voltage down with resistors or Zener diodes, or you use other tubes that can stand the voltage. I know there is a dual pentode, like a pair of EL84, which can live with these sort of potentials - I don't remember the code.

[robthefisher]

resoldered all the ground wires and connections now when i turn it on the voltage on v4 pin 2 14.5 and the same on v5 but after a couple of min it raised to 173 mv on pin 2 v4 and .75 volt on pin 2 of v5.when i ohm to ground it reads good.could it be the master volume?it uses a daul 250k pot and 2 2.2 meg reistors.gonna drop b+ to 340.first amp was tube rectified.thanks every one.

[zuceno]

first amp was tube rectified.

That explains a lot - 6DZ7 is the number of the "magic tube" which might stand the abuse (I remembered where I had noted that down).

Then again, you are not going to release the 18watt magic when the B+ is high - that is going to propel you in the clean direction. This thang is supposed to growl at people.

[robthefisher]

lowered b+ to 340 now doing good.3 hours wide open and no red plating!not as noisy too.sounds great thanks again....

[dgately]

lowered b+ to 340 now doing good.3 hours wide open and no red plating!not as noisy too.sounds great thanks again....

What method did you use to lower the voltage of b+ to 340 (resistor or zener diode)? Thx

[robthefisher]

sorry i took so long to answer.i used a 250 ohm 10 watt wirewound ceramic resistor.b+ is 345 now.epoxyed resistor to chassis.