Vibrato Question

[TIMBO]

Hi guys, Got a vibrato circuit same as the 6G9A http://el34world.com/charts/Schematics/files/fender/Fender_TREMOLUX_6G9A.pdf
My voltages are a bit higher than what on the 6G9a schem but I don't think that matters.

My question is....
With the oscillator off, I have the bias set so that there is approx. 42mA (.042v) in both valves across the 10ohm resistors
With the oscillator on, the reading across the 10ohm resistors is as low as 62mA (.062v) - 85mA (.085v)

Does sound right, as I have not measured the effects of the vibrato on the power valves bias.

[sluckey]

42mA (.042v) in both valves across the 10ohm resistors

Are you sure about the decimal point? .042v across a 10Ω resistor calculates to only 4.2mA, not 42mA. That would be extremely cold.

[Blind Lemon]

And with the Trem on isn't it varying the bias anyway, so with your DMM you wouldn't get a steady accurate bias reading anyway.

BL

[PRR]

How about Trem On but turned to "zero"?

Heavy trem *will* increase the average current in the power tubes. The increase on up-swing exceeds the decrease on down-swing. If the bias is "hot" this can be 20% or more. 40mA is fairly cool. It is probably swinging to flat-zero on down-swing, can't get lower than that. So the up-swing becomes the major effect.

Your meter may also be confused by wildly asymmetric swing.

Will it idle at full trem without distress? Does it make odd whucks when playing gently at full trem? If nothing is "wrong", I would take the increased current as "OK".

[TIMBO]

Yes sluckey, your correct.
Measured across 10ohm resistor .42v/valve and plate is 490v
What I am finding is when the intensity is turned up, the cathode voltage goes up as well, in the vicinity of .6-.8v
With the trem on 0 the cathode voltage stays stable around .42v
Getting some buzz on the wavering notes, this is when the intensity is on 6-10 on the dial but this is where the tremolo starts to kick in. 

[TIMBO]

Hi guys, The trem seems to be working fine.
Just getting some static on the pulsating notes mainly on bass notes or if you dig in a bit.
Swapped a couple of tubes to see if it could be that, but it's still there.