Bias pot for princeton.

jacksparrow

Hi Folks, Any one know about putting a bias pot in without drilling any holes in the chassis of an original blackface Princeton reverb.

I am going to get the tech to replace the cap can also.  Anybody know where i can buy the suitable parts.

Thanks in advance.

RiftAmps

Modulus stocks the direct replacement cap can (Mallory) that works really well and any competent tech can install a non-invasive bias pot

https://modulusamplification.com/Capacitor-Electrolytic-20202020-uF-475-VDC-P5375685.aspx

ecc83

Overkill I am sure others will say but I would use a fully enclosed multiturn trimmer (Maplin USED to have them!) . Mounted on a piece of stripboard  (aka Vero) then fix that in using heavy gauge solid copper wire, internal of TV coax is perfek.

Make sure it is wired so that a fail "open" gives maximum bias volts not minimum!

Dave.

jacksparrow

Thanks guys, will order 2 x cap cans from modulus.

DJH83004

Just a quick point, the can capacitor used in the Princeton is multi section, (20/20/20/20 uF), you don't need two. 

Modulus_Amps

I fit Piher trim pots for bias adjustment, they are big enough to not be too fiddly, but small enough to be fitted directly on the board

RiftAmps

Why do you want to install a bias pot anyway? The amp has survived 50+ years without one and I bet it hasn’t suffered because of it

jacksparrow

I am getting the cap cans done, as they are the original ones, and just want to be able to bias my amps myself, with my bias probe.

Also one sounds a bit gravelly to me probably a cold bias.

@Modulus_Amps  ; do you sell them phier on your site?

ICBM

RiftAmps said:

Why do you want to install a bias pot anyway? The amp has survived 50+ years without one and I bet it hasn’t suffered because of it

Exactly.

The Princeton is not a high-stress circuit for 6V6s - by guitar amp standards anyway - and biasing is not critical as long as it’s in the right range.

RiftAmps

ICBM said:

RiftAmps said:

Why do you want to install a bias pot anyway? The amp has survived 50+ years without one and I bet it hasn’t suffered because of it

Exactly.

The Princeton is not a high-stress circuit for 6V6s - by guitar amp standards anyway - and biasing is not critical as long as it’s in the right range.

One side-effect of home biasing a PR is the inevitable e-mail to a tech enquiring why the tremolo is now weaker

Modulus_Amps

RiftAmps said:

ICBM said:

RiftAmps said:

Why do you want to install a bias pot anyway? The amp has survived 50+ years without one and I bet it hasn’t suffered because of it

Exactly.

The Princeton is not a high-stress circuit for 6V6s - by guitar amp standards anyway - and biasing is not critical as long as it’s in the right range.

One side-effect of home biasing a PR is the inevitable e-mail to a tech enquiring why the tremolo is now weaker

But how would you know until you have tried?

Most guys that build kits want adjustable bias, I suppose to satisfy themselves that they have the amp tweaked just the way they want it. there is no harm in doing it even if not needed and is part of the learning experience.

@jacksparrow yes I do have some Piher trimmers, they are also available at other electronic suppliers. depends on what value you want to fit.