5F6A Volume Interaction?

[robrob]

I don't understand why there is no interaction between the two volume controls in the 5F6A Bassman.

Here's the schematic with a thick grey highlight of the voltage divider that is formed when the unused Normal channel's volume is at minimum. The two 270K mixing resistors form a voltage divider that cuts the Bright channel guitar signal by 50%.



If you turn the unused Normal channel volume to max the lower leg of the voltage divider increases from 270K to 1.27M and the attenuation should drop from 50% to 18% but I do not notice an actual volume change when doing this on my 5F6A.

I'm using the standard R2/(R1+R2) to calculate the voltage divider attenuation:

Min Normal volume = .270M / (.27M + .27M) = .5 or 50% attenuation.

Max Normal volume = 1.27M / (.27M + 1.27M) = .82 or 18% attenuation

What am I doing wrong?

[Leo_Gnardo]

What am I doing wrong?

Expecting a major volume change? Not so much because of those mixer resistors. I do hear a tone alteration, a change in brightness and "fullness" as the volume is brought up from zero on the channel I'm not plugged into. Something to be considered, and used to get a good recording or live tone on these amps, a "secret" tone control.

Now on old style Deluxe tweeds where the "mix" is accomplished by wiring the pots directly together . . . there's a major volume change.

[robrob]

That may be it but it seems like a 64% increase in signal voltage would be clearly audible.

[pdf64]

When dealing with topics related to perception, I find it helpful to convert things to dB, to help put the numbers into perspective.
As they're voltage circuits, 0.5 is -6dB, 0.82 is -1.7dB.
So yes, that degree of change (ie 4dB) should be apparent, as the unused channel volume control is swept over its range.

However, I don't think that your circuit analysis has taken account of the full picture, eg plate impedance of the input stage, source impedance of the active channels volume control at the setting used, changing grid circuit impedance acting with the 2nd stage's miller capacitance to roll of treble.

Think of the preceding plate impedance as being equivilant to a ~10k (12AY7) or 40k (if 12AX7) resistor to 0V; so the max resistance of the unused channel volume to 0V won't exceed ~500k.
And the source impedance of the active channels volume control will add to the 270k, eg when electrically halfway, ~500k, so total of 770k!

So in reality, the change will be somewhat less, perhaps < 2dB; maybe try measuring it at various frequencies and settings of the active and unused channels controls? As it gets too hard to calculate it all out!

[Dave H]

If you turn the unused Normal channel volume to max the lower leg of the voltage divider increases from 270K to 1.27M and the attenuation should drop from 50% to 18% but I do not notice an actual volume change when doing this on my 5F6A.

I'm using the standard R2/(R1+R2) to calculate the voltage divider attenuation:

Min Normal volume = .270M / (.27M + .27M) = .5 or 50% attenuation.

Max Normal volume = 1.27M / (.27M + 1.27M) = .82 or 18% attenuation

What am I doing wrong?

It's as Pete said. You have not taken into account the output impedance of the Normal channel. At max Normal volume the output impedance will look like about 40k (to ground) so that 1.27M above should really be 0.27M + 0.04M or 0.31M

[robrob]

However, I don't think that your circuit analysis has taken account of the full picture, eg plate impedance of the input stage, source impedance of the active channels volume control at the setting used, changing grid circuit impedance acting with the 2nd stage's miller capacitance to roll of treble.

Think of the preceding plate impedance as being equivilant to a ~10k (12AY7) or 40k (if 12AX7) resistor to 0V; so the max resistance of the unused channel volume to 0V won't exceed ~500k.
And the source impedance of the active channels volume control will add to the 270k, eg when electrically halfway, ~500k, so total of 770k!

So in reality, the change will be somewhat less, perhaps < 2dB; maybe try measuring it at various frequencies and settings of the active and unused channels controls? As it gets too hard to calculate it all out!

That's a great explanation of the input impedance I was not accounting for. Mystery solved, thanks Pete.