Think of it this way: The tubes with filaments are giant NPN transistors and the 0B2 is a giant zener diode. The things marked "cathode" are really emitters and the control grids are really bases.

The emitter of the 6L6 is too high because the voltage going to the base of the 6BH6 is not getting there. If the 6BH6 had base drive, its emitter would dump the voltage on the base of the 6L6 to the zener 0B2 and shut down the 6L6.

 

I took voltages where the schematic tells me too here is what I get. Just to give you a better idea while i digest your explanation.

6L6GC

pin 4 - 674 V
pin 3 - 675 V
Pin 7 + Pin 8 - 526 V


Between R3 and R4 - 510 V

At P4 after C5 - 677 V

6b86

schematic shows measurement at pin 5 or pin 7 not sure which one so measured both.

6b86 pin 5 - 436V
6b86 pin 7 - 510V

 

Pin 7 of the 6BH6 is showing 510 volts on the 0B2. the 0B2 is not conducting. (The zener diode is open.)

 

Pin 7 of the 6BH6 is showing 510 volts on the 0B2. the 0B2 is not conducting. (The zener diode is open.)

Hmm so the tube is bad?

 

Either that or it has lost its ground. See if pins 2,4,7 of the 0B2 are grounded.

 

Okay I changed the tube and at least the (zener diode) is working here are my measurements. Ill check the ground

6l6gc

pin 4 - 668 v

pin 3 - 669

pin 7/8 - 515

6b86

pin 5 - 430 V

pin7 - 107 V (changed!)

 

Yes pin 2,4,7 seem to be grounded through the voltage pot. They connect to pin 3 of the pot. That should be good right? Since the pot is touching the chassis.

 

Now try to get some of that 515V on pin 7/8 of the 6L6 through the voltage control pot, on to the base of the 6BH6 and get the 6BH6 emitter to dump current through the 105 volt zener to ground.

 

NO. There is no part of the voltage control pot that is supposed to be connected to ground. This might be the real problem.

 

Hmm. Perhaps then I am describing the situation in front of me in correctly.

I also forgot to mention that the first time I powered up the unit it was working. The problem is not that the voltage is not present but it doesnt change when I turn the knob. It stays a constant 515V or so.

When i fired her up the first time, I put my multimeter between the DC + and DC- and got 100V and then as I turned the pot it went up to 350V.

However the second time I powered her up she was reading a straight 515V.

I did hear a slight sizzling sound coming from inside the chassis wasnt sure what it was.

So now All I get is 515V between the DC- and DC+ terminals and the pot doesnt do anything to change that.

Im attaching some pictures maybe in the hope that it can help.

 

Last pic of the pot itself.

Btw I really appreciate you helping me out. Thank you.

 

Go to post #28 and resume. The 500+ volts must go into the R7, R8, R9 voltage divider and apply a positive voltage to the grid (base) of the 6BH6 tube so its plate (collector) will dump the voltage on the grid (base) of the 6L6 tube through the 0B2 zener diode.

When the voltage control pot applies something around 105 volts to the grid of the 6BH6, that tube will lower the voltage on the grid of the 6L6. If the grid of the 6L6 does not go down, the voltage dumping path through the 6BH6 and the 0B2 is not conducting. Post #18 says the pot is providing 145 volts on the grid of the 6BH6.

Does the voltage on the grid of the 6L6 go down?

If it does go down and the output voltage does not fall, the 6L6 is possibly shorted. If the voltage on the grid of the 6L6 does not go down, the 6BH6 is not conducting the current through the 0B2.

If you are just tired, you can start again later.

 

Lol, yes I am getting tired and frustrated.

In my previous post I had said the pot was providing 145 however after swapping the OB2 tube now

when the pot is turned to the minimum it reads 295 V and when its at max its 145V

Im taking these measurements before the 1M resistor that goes into 6bh6.

Im going to continue with your directions and will report back

pin 4 and pin 5 would be the grid correct of the 6l6? Do i measure between them or to DC- and each pin as I turn the pot?

 

I'm going to watch TV for an hour. C U then.

 

Okay now I think I am getting the hang of it a bit.

I tested the grid of the 6l6 tube at pin 4,5,8 and neither changed all stayed at 500 or so volts when the pot was turned.

That means from what you have written I have a problem with the 6Hb6 tube and is not conducting it to the OB2.

At least we have narrowed it down. I am resoldering some old joints they are 30 years old im sure they could use a touch up.

After that I'll see if that makes a difference.

I was having trouble following you with the terminology I found some pictures of pentode tubes describing which pin goes to the grid and plate and cathode that makes more sense now.

 

Here's another idea. Put a 220k resistor from pin 5 of the 6L6 (same as pin 5 of the 6BH6) to ground (pins 2,4,7 of the 0B2). That should drop the output voltage by about half. If that lowers the voltage, it proves the 6L6 can take instructions and the current dump through the BH6 is not happening. Could be as simple as a burned out filament in the BH6.

 

Here's another idea. Put a 220k resistor from pin 5 of the 6L6 (same as pin 5 of the 6BH6) to ground (pins 2,4,7 of the 0B2). That should drop the output voltage by about half. If that lowers the voltage, it proves the 6L6 can take instructions and the current dump through the BH6 is not happening. Could be as simple as a burned out filament in the BH6.

You Right!

Voltage output dropped by half with the 220k resistors. So my 6l6 is okay i guess but not the bh6

New tube time?

However I did buy a replacement for the 6hb6 and neither seem to glow anymore when turned on.

Does a tube need to be glowing to work? Because there is no visible glow from the 6hb6 ( I have two of those tubes)

I also noticed the heater wires were not glowing in the 6hb6 so i tested the voltages on pins 3 and 4 (heater pins for 6hb6) and got nothing no volts on either. Could it be that the heaters are not being supplied by current?

 

You are getting very close to the final answer.

 

Time to check the filament voltage for the 6BH6. That would be pins 3 and 4. The drawing shows about 6.3 volts AC from green colored wires from the big power transformer.

While you're at it, track the green/yellow to see that it goes to pins 2,4,7 of the 0B2.

 

We have a Bingo. (Or, as the kids say, "We have wood".) You will need to get a small transformer to replace the failed winding in the power transformer. Look up the 6BH6 tube and see how much current it wants to heat its filaments.