Boogie .50 Cal + bias circuit mod

[Chevy]

Guys,

I have a .50 caliber + that had the factory non-adjustable fixed bias, and wanted to add a stable, reliable bias adjustment feature. Hoping I can get some input from someone to help out with my circuit design.
I was following the schematic for the amp that seems to be circulating around (first image, below), and found that my amp's bias cct. was a little different (as shown in second image, "cct trace"). What I did was add a 20K trimpot between the bias supply diode and the paralled pair of 82K/68K resistors in the "cct trace" circuit (sorry, no image included for this mod). This seems to work fine, but I'm concerned about stability... if the trimpot has an intermittent wiper contact or something, the output tubes will be wide open.
So I came up with the circuit shown in the third image "new bias", a rather old-school type of cct. with a 22K pull-up R in case the pot wiper goes open for some reason. Any comments on this proposed change? (I assume the -1.8V "F" output will change, but what the output V will be, I don't know yet.)
One thing I just don't get is why anyone would design the original cct. the way it is... makes no sense to me. Any ideas? Why use an 82K R and 68K R in parallel ?
Another thing is about the long-tail pair design... supposed to be at -1.8V cathode bias, but more like +1.8 volts actual in reality. Huh? How to design the cct. to accomodate the long-tail pair in this case? Should I make another similar adjustable cct. parallel to the one in the proposed changes, and use it's output to get the -1.8V?
One other question... I haven't much experience with this... would you possibly recommend that I just leave the circuit as I have it now, with the 20K trimpot in place, and just hope it doesn't ever go "open"? I've heard vibration and such can cause this to happen. Anyhow, the amp seems to run fine now, just concerned about longevity.

Any comments or ideas would be appreciated.
Best,
Chevy

[Enzo]

Myself, I never draw bias voltage through a wiper.

Ignoring the 22k parallel part, you have a voltage divider comprising the 4.7k, 20k and 27k. In my work, I would take the bias from the top of the 20k pot, where it meets the 4.7k, then wire the wiper to one end so the pot becomes a variable resistor, rather than a variable divider. That way if your wiper ever lost contact, you'd still have the 20k there, and the bias would revert to the highest voltage. I consider that the safest way.

Having said that, I can;t recall the last time I found a bias control lose its wiper. So some of this concern is like keeping an elephant gun in my car in case I run into one charging on the interstate.

I have no idea what was on their design minds, but in amps where you might need adjustments, but where there is no trim pot, they COULD have mounted two resistors thinking they could snip one out if the bias wound up to high. They would be different so you had a choice. Alternatively they make a 37k total, which is a non-standard value, so maybe they wanted 37k and didn't want to source precision resistors. Not that anything else in there is precision, but you never know. Sacrificial "adjustment" parts are not as uncommon as you might think.

Phase inverter. Look at th big picture, don;t stick your nose on the cathode and wonder. That isn;t -1.8v cathode bias, that is a tube stage spanning +417v and -47v. 464v. SO instead of those cathodes sitting at 50v or whatever we might normally expect, dropping the "ground" of that stage to -47v gave them more headroom. SO the difference between -1.8v and +1.8v is less than 4v... out of almost 50v. COnsider an AB763 Twin Reverb, with 100v on the PI cathode. If we then found one with only about 90v sitting there, I doubt we'd think twice about it.

[Chevy]

Thanks much for the feedback, Enzo.

Hey... you really know your stuff! Excellent comments and ideas... will follow your suggestions.

Actually, I can use the 82K//68K R's as is, for 37K as the bottom fixed R in the new voltage divider, and just add the 4.7k R and the 20K pot to complete the voltage divider; take the bias off the top of the pot as you suggested, and should be good to go.

The PI circuit threw me, as I don't quite follow the biasing and stuff, but it came much clearer when you mentioned that the -47 volts was the negative rail for the PI... good stuff... thanks kindly for your time.
Best,
Chevy

[Chevy]

Bias on the PI question

Just an afterthought...
The point "F" on the Boogie .50 cal + schematic specifies -1.8V at the cathodes of the PI tube. But I actually measured something around +1.8V there. Could this be a typo, in that it should really read +1.8V?

I dont' understand this PI configuration, but it seems to be somehow "auto-biasing" around the grids, which are held at ground potential by the associated resistors/pots. So... +1.8V on the cathodes makes sense, since this would put the grids at -1.8V relative to the cathodes.

Does this make sense? Just trying to understand how this works.

Thanks for your help,
Chevy

[loudthud]

That's a typo, it should be +1.8V. This type bias uses a negative supply for the tail resistor. It eliminates two capacitors because the PI's grids are at ground potential.

Warning: Some of the schematics show a 4.7K connected to the diode in the bias power supply. I think you'll find that resistor should be 47K at least, probably a 2W. The bias supply is shown connected to the high voltage winding, probably something over 300VAC.

Your trace of the circuit makes sense but there is an interaction between the operating point of the PI and the bias voltage supplied to the output tubes. And it's not the first time I've seen a mistake in a Mesa schematic. I wonder if they do that on purpose. Anyway, the voltage divider pot with a safety resistor is probably the best choice and check that the PI plates are near the correct voltage and that the output stage has adequate adjustment range.

[Chevy]

Great, good stuff. Thanks kindly!

I'll try remember to post my finished, tested and working circuit schematic when I'm done, to help out anyone else who has the same questions.

[Zouto]

Hi Chevy, I got the same amp on the bench. How did you do the bias control finally?
Thanks for informations,
Zouto

[gtr-fxr]

Yeah im in the same boat, seeing how the pot is connected would help.
Thanks