Bias Pot for 1960 Fender concert amp

[Underwood]

Hello,
I was looking for some advice on a bias adjust pot for my 1960 Fender Concert amp. This amp is 100% bone stock and am definitley leaning toward the small 50k or the 10k pot that hoffman sells. I do not want to drill any holes for a cts pot. Even the tubes and all caps are 100% original. But, and I really mean but, I intend to play this amp and not put it on display. Ok, it will be on display, but it is going to be played a lot. Any advice would be appreciated. The bias circuit presently has 2 - 8uf 150V caps and a 56k r. Also a 10K r coming from the diode.  THANKS

[eleventeen]

The no-driling answer is to get or make a small right angle bracket and anchor one leg under a PT bolt. You could use a piece of junk folded sheet metal or get a brass 90 degree angle bracket at a hardware store. Talking about a 1/2" wide piece of metal, each wing is maybe 1-1/2" long, measured from the angle. Make sure you use a star washer to hold it down so it doesn't come loose.


Have you worked out the values for the divider resistors you will need? It's certainly desirable NOT to be able to dial the bias to ZERO. Thus, you want a resistor between the "cold" side of the bias pot and ground. You have that: the 56K. The bias control carries very small current so wattage is not important.


It looks to me like you could simply replace the 15K resistor with your 10K pot, the pot wired as a rheostat (two neighboring lugs shorted together) I would kind of prefer a 20 or 25K pot there but a 10K should work fine. I find, that with old caps, the bias supply does not put out as much volts as with new parts. Hence, you will tend to want to move the wiper closer to the bias supply output (essentially the anode of the single diode) to get "more volts". Of course, this is a negative voltage supply, so I am talking about "more negative volts".

[sluckey]

Get a 50K pot and a 22K half watt resistor. Replace the 56K resistor with the 50K pot and 22K resistor connected in series. This will allow you to take the bias voltage up and down from the original 'fixed' voltage. No danger of setting the bias to zero.

[Underwood]

Thanks! That is what I was thinking if doing. I did a 62 Bandmaster like that and it worked good. I was worrying slightly about the 50K not dialing in very accurately. Any advice on the capacitors. Should I change out all the electrolytics?

[sluckey]

The 50K pot will be fine. The caps are 55 years old. I would change them if you will be gigging the amp.