Hey, this is a silver face champ model aa764. It has no vibrato. the voltage on the plate of the vibrato tube (bottom one on schematic) is 170 which is far too low. I have replaced all electrolytics and most other caps in the thing. No other components look burnt. This model should not need the pedal for the vibrato to be on or am i just dumb? thanks! i have the proper 340 volts coming from the power supply also. its happening between the power supply and the tube

 

On which pin of that valve are you measuring 170V - 1 or 6?

 

There is nothing but a wire from the 340 VDC at the capacitor to the pin 6 plate, but the voltages are different, therefore you have a problem with the alleged wire. Albert also has a point. Are you measuring in the right places?

 

On which pin of that valve are you measuring 170V - 1 or 6?

There is nothing but a wire from the 340 VDC at the capacitor to the pin 6 plate, but the voltages are different, therefore you have a problem with the alleged wire. Albert also has a point. Are you measuring in the right places?

Let me go take measurements of each pin

 

On which pin of that valve are you measuring 170V - 1 or 6?

There is nothing but a wire from the 340 VDC at the capacitor to the pin 6 plate, but the voltages are different, therefore you have a problem with the alleged wire. Albert also has a point. Are you measuring in the right places?

Pin 1: .68v
Pin 2: .88v
Pin3: 0v
Pin4: 0
Pin5 : 0
(Those two are connected)
Pin 6: 369
Pin7: .7
Pin 8: 9
Pin 9: 0

That 170v reading must've been a fluke

Could that 470k resistor be at fault

 

Could that 470k resistor be at fault

Certainly the first thing to check.

 

Twist the speed knob back and forth while watching the DC on pin 7. That will tell you if the DC is leaking out through the 0.02 capacitor. The answer should be, "zero changes". If the speed knob changes the DC, you have a bad capacitor.

 

If the speed knob changes the DC, you have a bad capacitor.

Even if that capacitor was short there should still be about 50V on pin 1.

 

Twist the speed knob back and forth while watching the DC on pin 7. That will tell you if the DC is leaking out through the 0.02 capacitor. The answer should be, "zero changes". If the speed knob changes the DC, you have a bad capacitor.

Nope. No change.

 

Even if that capacitor was short there should still be about 50V on pin 1.

Yeah there is no change anyway. So check the 470k resistor first? What would be after that.

 

So check the 470k resistor first? What would be after that.

Yep.
After that? That depends on the actual resistance of the supposed 470k.

 

Even if that capacitor was short there should still be about 50V on pin 1.

You're right. That 100k would support about 1/6 x 369V.
I was just looking for an easy way to eliminate one capacitor.
(Darn, missed again.)

 

Yeah there is no change anyway. So check the 470k resistor first? What would be after that.

There ain't no, "after that" until you actually check it!
You are asking for crystal ball work. Try doing the real work.

 

There ain't no, "after that" until you actually check it!
You are asking for crystal ball work. Try doing the real work.

I meant that because I don't wanna make 40 trips to the electronics store for each component. I have already replaced the 2x .01 caps and the .02 In the vibrato section

 

Yep.
After that? That depends on the actual resistance of the supposed 470k.

Cool cool. I'll measure it and see what we have got

 

I meant that because I don't wanna make 40 trips to the electronics store for each component

You don't have to drive to the store to measure a resistor or pull the tube out of the socket to see if it's internally shorted.

 

You don't have to drive to the store to measure a resistor or pull the tube out of the socket to see if it's internally shorted.

Yep.
After that? That depends on the actual resistance of the supposed 470k.

Tested and it was wide open. Replaced with a good one. Now when I turn the vibrato knob up, it cuts volume. What the hell lol I'll compare voltages on pins soon.

 

Tested and it was wide open. Replaced with a good one. Now when I turn the vibrato knob up, it cuts volume. What the hell lol I'll compare voltages on pins soon.

It's acting as a amplifier volume knob.

 

Check the two 1M resistors around the left hand part of that valve.

 

Check the two 1M resistors around the left hand part of that valve.

Already did. THey are good. I'm replacing the 46 ohm one. It's acting as a master volume but backwards because the pot is a reverse audio