Most vintage-sympathetic bias pot addition (1967 JMP)

[AML]

This 1967 JMP 50 (GZ34 and EL34s) has survived almost 60 years with no bias adjustment pot. It runs very hot - approx 50mA per valve and a plate voltage of 420V! Wonder how many output valves it went through in its life!

What in the forum's hive mind is the most sympathetic way to add reliable bias adjustment? I've seen various arrangements of trimmer pots soldered to the single turret that's available, some look a bit sloppy/ghetto.

Right now there's just the 15K-56k voltage divider (leftover from the JTM45-era, I think). I have -32V at the divider.



The other approach that I've seen is a larger resistor placed in series with the 15K (no pot).

 

[neikeel]

I use a not too small 50k paxolin skeleton type in series with the bias resistor. One limb to the bus turret, another to the bus wire and make a little hook on the third that I drop the bias resistor into. I often go down a size eg 56k to 47k of the original type. That gives me more range to get where I’m aiming. Looks better if the bias resistor has sharp dogleg to sit equidistant between the bias caps.
I’ll see if I have any spare pots (and an AB 47k cc) and can get it dropped around or can post.

 

[AML]

Cool - thanks for the suggestion and offer. I do have a small bag of Rs salvaged from other amps, so may have the R covered.

(I've ordered some Philips 10/160 caps to replace the modern/tiny Nichicons, too.)

 

[stickyfinger]

You could change the range resistor and add a pot to figure out what single resistor value you need. Then redo the bias circuit, replace the bias caps and add the single resistor so it looks stock.

 

[AML]

That's an idea and I might go for it if the new resistor value is available in the right value.

The pot makes sense for a future owner who wants to install some precious Mullards (etc) in the amp. At the moment it has Winged C EL34s.

 

[AML]

It's done and working. With a 47k resistor from the salvage box, and a small trimmer, plus some more vintage-looking caps:



Works a charm to get bias under control. 16W with the Winged-C valves that I am using for now, and I will retest and rebias with some old Mullards soon.

Only q left: whether to put a resistor from the wiper to ground as a fail-safe. There's space and I have a spare Piher 100k/0.5W, so I think I will.