Bassman Micro conversion to fixed bias

[Hyakuya]

Hi all

Because my Bassman Micro isn't broken and works wonderfully I thought I would make some changes to it so fixed bias is the plan for no other reason than it's a learning curve and I enjoy it.

I put together a circuit to adjust the bias following the information @robrob kindly provides:

5F6A Mods

5F6A Bassman Amplifier Modifications

robrobinette.com

Top left. HV tap from the PT and a couple of Bourns trim pots connected to the 56K grid leak resistors on the right of the image.



Also, as this was in the back of my mind when I built the amp I have bias measuring jacks installed already:

Amp Mods

Generic Guitar Tube Amplifier Modifications

robrobinette.com

With cathode bias (fixed bias circuit disconnected) in place I measured one of the EF80's as follows:
Plate v (pin 7): 287
Cathode v (pin 1): 4.4
Plate to cathode v: 282.6
Plate current mA: 6.7

Using Robs Tube Bias Calc:
Plate disspation W: 1.9
Plate dissipation %: 75.7

A little on the cool side....

So I connected up the fixed bias circuit and bypassed the cathode resitor then wound up the trim pots to max resistance and powered up.

The plate current on the measuring jacks got to 24mA before I turned it off.... works out at about 270% dissipation if the plate and cathode voltages have remained the same which I think is a little on the hot side. The EF80's were certainly glowing....

From Robs website:

"The 27k bias resistor sets the maximum hot bias. If at full clockwise pot movement (max hot bias) you still need to bias hotter then reduce the value of the 27k resistor--try 24k or 20k. If at full counter-clockwise pot movement (min cool bias) you need to bias cooler then increase the value of the 27k resistor to 30k or 36k."

So I'm going to increase the 27K resistor and try again...

 

[King Fan]

Sounds like you're on the right track. What value dropping resistor do you have first on the bias board to knock down the HT voltage? Rob's 220K? What value trim pots, 50K? And you have two pots to make a bias adjust / balance setup? Bear with me if I missed something.

You may have seen it, but Merlin is a great read on bias. He does his a bit differently (as he says, there are a million bias schemes -- all to do the same simple thing). I especially like the way he shows us the max negative bias voltage needed -- which would be relevant here. Only issue is I'm not sure of the 'triode mu' of an EF80 -- I've seen 40 mentioned?

Knowing the max negative voltage that would chill out the tube, I found I could use voltage divider calculations to get close on the 'head' and 'tail' resistor values -- though as he says, it may still need some trial and error.

 

[Hyakuya]

The bias board is built to Robs spec so that's a 220K first. I see about 145VAC after that resistor but he does say that it's not fussy about the input voltage. I think the trim pots are 50k and yes, there are 2 so I can adjust the bias on each EF80 independently. If you look at the picture I posted you'll see that the grid leak resistors are not connected together at ground (unlike Robs layout and schematic) to allow independent adjustment.

Thanks for the Merlin link, I'll take a look....

 

[Hyakuya]

Hmmmm.... Upped the 27k resistor to 1M and I still get 15mA plate current with the trim pots wound right up to max resistance.

I noticed that the trim pots only read 25R at full resistance even though they are 50k pots. Will that be because they are wired in parallel ?

 

[King Fan]

I don't know if it helps, but I like to trace the negative bias voltage at each point as it goes through the bias circuit, with the pot(s) set to max negative voltage. And if it helps, here're a couple bias-startup nuggets I stole from super-users Sluckey and PRR on el34world....

Sluckey: No tubes in yet. If you have a fixed bias amp, power up and verify proper negative bias voltage is present on the control grid pin of every output tube socket. If adjustable, be sure the range is adequate and then set it for max negative voltage. DON'T PROCEED UNTIL THIS IS RIGHT!

PRR: Carefully use clip-leads to connect a voltmeter, black to ground and red to the point where the 220K resistors come together. Plug in, turn on. This bias voltage should go to negative 40V or 50V, and it should be adjustable with the trimmer. Don't linger, the unloaded voltage may be hard on the filter caps. But you MUST be sure you have LOTS of NEGATIVE bias voltage, before you put tubes in.

The only points I'd add are that 1) I hope unloaded voltage here *won't* be hard on your filter caps if it's only roughly 300V and 2) **if** the EF80's triode mu *is* 40, and say your screen voltage is ~280V, then Merlin's 280/40 = 7V max negative voltage, so -7V) might be enough at the 220K junction; PRR's -50V is for say 6L6s (triode mu 8) at screen voltage 400...