Bias - Peavey

[Valvehead]

Which resistor should be changed to adjust bias.....R8 (47k) ?? R9 (470) ?

Peavey VTM 60

file:///C:/Users/km/Desktop/Peavey-VTM-60-VTM-120-Schematic.pdf

https://el34world.com/charts/Schemat...Schematics.htm

right now bias is 54ma per tube at 478v

[Jazz P Bass]

What voltage do you read for the bias?
- 56 Vdc should not be biasing the tubes to 54 ma's.

[Valvehead]

At tubes i have -52mv , pin 5

[Helmholtz]

At tubes i have -52mv , pin 5

As pin 5 is the grid, do you mean -52V (not mV)?

How do you measure idle current?
The unit for idle current is mA not mV.

[Valvehead]

sorry , yes it is -52v .

bias current measured at idle , shunt method at OT shows about 54ma per tube . This is the 60w model

[Jon Snell]

Using a DVM, what is the voltage on pin 5 of the output pair?
Be carefull!!!!!!!!

[Valvehead]

-52v

[The Dude]

What voltage do you read for the bias?
- 56 Vdc should not be biasing the tubes to 54 ma's.

^^^^^^That! You should not be reading such high bias current with your -52V measured bias voltage. Something doesn't seem right. I'm thinking either a measurement or calculation error. Or, maybe the amp is oscillatiing. Scope the output and see if you detect any oscillation.

[Helmholtz]

Also check your meter battery.

[Valvehead]

Also check your meter battery.

Ah ha....1 meter was at 8v. New battery in and i have =

-52v at pin 5

bias is 49ma / 44ma

All new electrolytics are correct values

[Valvehead]

^^^^^^That! You should not be reading such high bias current with your -52V measured bias voltage. Something doesn't seem right. I'm thinking either a measurement or calculation error. Or, maybe the amp is oscillatiing. Scope the output and see if you detect any oscillation.

Can you tell me the approximate settings for the scope? I do have a cheaper digital scope here

[The Dude]

Oscillation could be at any frequency or amplitude, so I can't predict scope settings. You'll just have to sort of "explore" time base and volts/division settings to see if anything shows up. If you spot a waveform, adjust settings for the best display and let us know what your settings are.

[Purgasound]

-56V is what the schematic shows leaving the bias circuit.

If you want adjustable bias in the VTM it can be done very easily. Change R9 to 3K and change R8 to a 22KL pot in series with a 10K resistor. Done.

[The Dude]

Well yes, of course you could modify the bias supply. However, IMO, there is something not right with the amp or with measurements. I think it best to find that "something" instead of sticking some bubblegum over the hole in the gas tank.

[Valvehead]

Im new to scopes so bear with me. Heres a pic , probe at the speaker jack. It didnt change with volumes on 0 or 10 .