express external bias-pot option question?

[analog_stud]

express BOM shows a locking-bushing pot for the externally mounted biasing option. Better, of course, but this isn't absolutely necessary is it? I don't see a problem using a non-locking pot, unless there is? Was thinking of using this: http://sensing.honeywell.com/index.cfm? ... medium=SEP

[analog_stud]

oops, shaft seems a little long on that one. (get that complaint all the time )

[paulster]

The only thing I'd really say is that, compared to a trimmer, potentiometers are generally very easy to turn, which means that your bias could get messed up very easily. If the amp is going to live inside a headshell where the pot can't get knocked then it may not be an issue but if you use a standard open-back Trainwreck-style shell and reach over the back to plug in or unplug a speaker cabinet there's always the risk of knocking it, depending on where it's placed.

That's why I'd recommend either a locking pot or a multi-turn pot. Or even better is a multi-turn pot with a locking dial attached, which is what I always use.

[analog_stud]

Cool. Decided I'm gonna mount it on a bracket, inside the chassis, so the tip of the shaft is just inside the hole on the chassis. Looks a little cleaner (and safe).

[sliberty]

I used a multi turn pot similar to the one below:

http://www.mouser.com/ProductDetail/Bou ... lWxZROuwTU

There is an accessorie available for pots like these which enables you to mount them to a panel. The screw adjustment ends up recessed in the attachment, so it cannot be turned without a screw driver. Worked out very well.

A Fender style bias pot is also viable, but I would prefer to mount it to the top of the chassis, not the back.

[M Fowler]

Nice I see they have the panel mount version as well.

Mark

[analog_stud]

hmmmmm..........multi-turn, much easier for fine bias adjustment, AND a panel-mount option? I think I'm gonna go that option. Thanks fellas!

Now, if I could just find the best way to install this option. Can't seem to find a layout or instructions on this mod Did I overlook this in the files here?

[sliberty]

The Express layout has a trim pot on the power supply board. Simply run wires from the solder points on the board to the panel mounted pot.

You might also want to install the 1ohm resistors and make test points available on the back panel. But if you use a bias meter (I have a CompuBias), you won't need these.

[analog_stud]

I'm putting the test-points outside also, I guess I've been spoiled. Coulda swore I ran across someone selling wreck clones with this option and had proper instructions. (placement, wiring, etc...)

[sliberty]

Maybe this will help. It is not a total clone - it's my UniWreck, which used a donor chassis. But you'll see how I handled it.

[analog_stud]

NICE! Yes, that's exactly what I want to do. I know Ken didn't do it, but soo damn easier. If I don't majorly screw this build up, I'd like it to be my gigging amp so any added convenience is just that: convenient.

I like where you put the resistors.

Off topic, but are those the Pacific transformers? Did you get them from Rj? I was thinking of going that route and wondered if they made THAT big of a difference compared to something like Magnetic Components "Classictone" offerings

[sliberty]

This build is several years old, and used the Heyboer transformers that Moose was selling back then. I have no experience with the Pacifics.

[guitarsnguns04]

Ive tried several varietys of trannys...for the express i prefer the pacifics....heyboers a close 2nd

[Gibsonman63]

Good thread. I plan to do a little work on my Express once I finish my Liverpool head cabinet. I was thinking about doing a separate bias adjustment for each tube or maybe an overall bias adjust with a bias balance. Any thoughts or recommendations?

[M Fowler]

Separate bias for each tube since the matched tubes we get today don't stay matched for long.