67' 1986 board

stokesdead · Post by **stokesdead** » Tue May 22, 2007 10:17 am

Ok I have a thread in the 50 watt section but am just trying to get as many eyes as possible on this thing before I install it. I wired it like the early models with 2 16 uf caps on the board (no 3rd can). Please look let me know if you see any goof ups. Particularly the bias part of the circuit. I am not quite sure that I have it right.

Thanks,
Chris Stokes

stokesdead · Post by **stokesdead** » Tue May 22, 2007 10:18 am

Bias pot close ups:

darrell · Post by **darrell** » Sat May 26, 2007 5:05 am

Flames1950 · Post by **Flames1950** » Sat May 26, 2007 8:22 am

Bias pot is wired just fine.

http://forum.metroamp.com/viewtopic.php?t=11812

neikeel · Post by **neikeel** » Sat May 26, 2007 8:26 am

Are you SURE the lugs need to come off?

The lugs on the body need to be earthed as does one of the main lugs. The wiper goes to the bias resistor (I believe).

To check put you DMM across from earth to far side of bias resistor and see what range of adjustment you get.

The body of the pot must be earthed.

darrell · Post by **darrell** » Sat May 26, 2007 6:16 pm

Flames1950 · Post by **Flames1950** » Sat May 26, 2007 6:32 pm

darrell wrote:Its done wrong and wont work like that.. Take a any pot and touch the leads to the 2 outter ends of it and turn the pot you will notice that it doesnt change any..Now take one lead and touch it too the inner lug and one ot the outter and turn the pot the ohms resistance will change.. You need one part Outter lug to the turrett and the inner lug to the resistor.. I dont know what will happen the way you have it but I wouldnt use good tubes in it to find out..
I can post a pic of mine if you like..

It is not wrong, and it is the way George detailed to wire it in the thread link I provided. The leg with the resistor still varies with respect to the other grounded lugs (the two on the bottom are part of the same plate so it makes no difference that they are grounded to a different lug each.) It is the same way the '69 board's bias pot is wired as well, with two contacts grounded.

I re-hosted George's pic form his thread for you though.

stokesdead · Post by **stokesdead** » Sat May 26, 2007 6:44 pm

where should the ground wire coming off the bias pot be grounded?

darrell · Post by **darrell** » Sat May 26, 2007 6:48 pm

darrell · Post by **darrell** » Sat May 26, 2007 7:06 pm

Flames1950 · Post by **Flames1950** » Sat May 26, 2007 7:09 pm

And that's what's been done in the above pics. Grounding the bottom plate's two lugs and the third leg of the bias pot essentially turn it into a two-wire device, in at the resistor and out to ground. And the resistance still varies between the first leg and ground.

stokesdead · Post by **stokesdead** » Sat May 26, 2007 7:13 pm

Hey flames, Where should I wire the ground on the bias pot? Is there a specific ground point to use?
Thanks,
Chris Stokes

darrell · Post by **darrell** » Sat May 26, 2007 7:18 pm

stokesdead · Post by **stokesdead** » Sat May 26, 2007 7:38 pm

That pic you just posted is the same as I have wired if you keep in mind the lugs are jumpered with buss wire.

stokesdead · Post by **stokesdead** » Sat May 26, 2007 7:42 pm

I believe this is what you are not seeing. Correct me if I am wrong guys.