Bias & MV Qs

[Justin Thomas]

Hey All,

Building an AB165 Bassman clone but with some changes to the bias supply & adding a Master Volume (I know, that goes against my previously stated principles). I wanted to have a Bias Balance & Adjust but didn't have any tapped pots like the SF amps, so I opted instead for a Bias Adjust pot for each tube.

I followed the standard BF wiring of the pot, but I know there's a way to wire it so that if the pot fails, you have max negative voltage on the grids? I've got my best guess on my drawn picture but wanted to run it by here.

Both supplies are the same right now, so I'm thinking I can test changes on one & use the other as a "control," with no tubes installed. I'd like to stick with basic pots & resistors here, since I'm not even sure which way to draw a diode in circuit... I can tweak resistor values as needed; these are just starting values that seem pretty standard.

Also wanted to make sure my MV is safe here? I'm not concerned with "sonic results" at the moment, just want to make sure I won't cook my tubes. I'm thinking the only change I need to make on the MV is to not link the ground terminals on the dual pot, since I have individual bias pots?

Thanks,

Jusrin

[g1]

So the bottom drawing would be inserted instead of the bias arrangement in the upper drawing?

[Justin Thomas]

@g1 If it's safe to do so; it's just an idea. Part of my brain thinks it'd work, part says nope. Is that how you'd wire a bias pot to be safe if the wiper fails? I know it can be done but can't recall exactly how it was done. Like I said, I could leave one "normal" & muck about with the other...

Jusrin

[pdf64]

The variable resistor arrangement in the schematic at the bottom of the page is only applicable if there's an reasonable resistance in the supply side.
eg with 5F6a there's 15k feeding the 56k load, hence altering the value of the 56k will vary the bias supply output voltage. eg lowering the 56k to 15k will halve the bias voltage.
https://el34world.com/charts/Schemat...5f6a_schem.pdf
But the ab165 only has 470R, for a variable resistance load to drop voltage across that, the loading would be excessive, resistors would need to be beefy, ripple would be a problem.
But the value of the 470R can't be increased much before the cold bias end of the range becomes compromised.

For your arrangement, a 100k safety resistor, fitted between the bias pot wiper and supply lugs (by 'supply lug' I mean the one that connects to the 100uF/100V bias supply reservoir cap).

As per Merlin's R3 resistor http://www.valvewizard.co.uk/bias.html

[Justin Thomas]

Thanks, I'll do some comparisons & check that out.

Jusrin

[pdf64]

Thanks, I'll do some comparisons & check that out.

Jusrin

Regarding the bias pots, because the back up resistor causes the pot track value resistance to become effectively 10% lower at the hottest bias setting (smallest bias voltage), to get exactly the same range, the 15k to ground may need reducing a tad, eg 12 or 13k.

Your LarMar type master volume arrangement looks fine, though because the tracks are now totally independent but without their lug1 being decoupled, when set to minimum (CCW) volume, there may not be a total signal mute.
To get around that, add a eg 1-10uF between the bottom lug of each pot and ground to get a good signal decoupling node there.

Lastly, if you're not familiar with implementing this type of master volume, take care to maintain signal polarity, so that the correct LTP anode still feeds the correct 6L6 pin5 control grid.
If they get swapped around, the feedback loop becomes positive, and higher settings of the master will cause weird behaviour, possibly oscillation.

[Justin Thomas]

Thanks for the tips!

Jusrin

[Pedro Vecino]

This is the bias circuit I installed in my latest project. It also includes a master volume after the phase inverter. As pdf64 says, those two 10uF capacitors are essential for the master volume to be able to completely close the signal. The margins offered by this circuit are tuned so that it can handle any 6L6 or EL34 with a high voltage between 430 and 460v with a 60VAC bias secondary.

[Justin Thomas]

Thanks, Pedro!

AS it is, so far I've only added the 100K safety resistors; I'll work on the MV later on. I've currently got from -35V to -62V available. My only concern wascthat it seemed to take over a full minute for the bias to get to the full -62V. I swapped out my 100/100 cap for a 22/100 & I'll see if that helps at all maybe tomorrow afternoon or evening.

Thanks for the schemo to try out if I can't get what I've got to work!

Jusrin