Bias Adjustment From Outside of Chassis

[waggclan]

I am building a 50 Watt Lead Spec kit and would like to install the 3 input jacks on the back of the chassis for the multimeter probes, a bias adjustment knob on top of the chassis and buy the appropriate bias controller that interfaces with the board. Does anyone have a link to instructions regarding this type of bias adjustment mod? I know where I can buy the 3 input jacks and the 10:1 locking adjustment dial; but I need help finding where to buy the bias controller that interfaces with the board and connects to the outside adjustment knob.

Thanks.

[waggclan]

I realize this would be taking the build outside of "Period Correct" specs and all; but it would sure make tube swapping a lot quicker and easier.

Any comments, in support of, against, your own particular experience in doing this modification or any of the information requested in my original post will be greatly appreciated.

Thanks, Steve

[demonufo]

I always thought it a bad idea, no matter how it was implemented, locking or non-locking. Easier to put the pot on the top of the chassis rather than risk even a locking pot that could easily be overcome, but you'll still need to remove the back this way. The downside of your idea is that you have the bias resistors in there permanently. Just as easy to use bias probes as it is to plug your meter into your amp.

[waggclan]

I always thought it a bad idea, no matter how it was implemented, locking or non-locking. Easier to put the pot on the top of the chassis rather than risk even a locking pot that could easily be overcome, but you'll still need to remove the back this way. The downside of your idea is that you have the bias resistors in there permanently. Just as easy to use bias probes as it is to plug your meter into your amp.

First off, thanks a lot! I see Reinhardt and Komet Concordes using this type of biasing setup and it just looks very clean and convenient. Pulling the chassis out of the head each time I swap tubes or want to rebias is a relatively small price to pay. I mean, I'm not going to be swapping out tubes on a daily or even regular basis. I just thought it to be a rather clean looking and convenient way to get tube swapping and bias maintenance done. Your comments make a lot of sense.

Thanks, Steve

[Ted B]

I've done this:
http://forum.metroamp.com/viewtopic.php?f=44&t=36647

Retaining the 1-ohm resistors on the tube cathodes made no difference whatsoever in the sound of my amp, which is expected. What does alter the sound is when one biases using this method, simply because most of the time it leads to an erroneously high bias measurement. This is not the fault of the method, but of the measuring instrument. I witnessed this first-hand shortly after completing the amp. Unless you have a very good (read: expensive) multimeter, I would abandon this idea.

What I suggest doing is mounting a multiturn, linear bias pot with screw-driven, locking control (check Ebay) on the back panel, and using a conventional bias probe (e.g. Alessandro) to bias the amp. Not using a knob prevents an unintentional 'adjustment', and a conventional bias probe just plain works better with consumer-grade multimeters.

I also have a Komet 60w, and while it does also have this provision (as you mentioned), I still use a probe for the same reasons.

I hope this helps.