Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: morton on August 26, 2016, 08:10:04 pm

Title: Bias Pot Question
Post by: morton on August 26, 2016, 08:10:04 pm

Is there any problem with using a regular linear taper pot (example 50kl) for bias adjustment? Asking because my next project is a 5F10 and I would like to be able toadjust the bias externally and mount the pot on the back of the chassis.

Thanks,
Gary

Title: Re: Bias Pot Question
Post by: eleventeen on August 26, 2016, 08:37:42 pm

Linear is exactly what you want.

Title: Re: Bias Pot Question
Post by: sluckey on August 26, 2016, 08:41:12 pm

If you put a twisty thing on the outside of a guitar amp chassis, someone will twist it. Could be expensive.

Title: Re: Bias Pot Question
Post by: morton on August 26, 2016, 08:54:17 pm

It would just be me, actually I'm going to use your layout from the Harvard you redid. Thanks by the way, the schemo and layout helped greatlly. Weber had a locking 50kl, but it looks like they've been out of stock for q while. They have a locking 25kl, but that would reduce the range. Might just end up using the 50kl that Doug offers. Once I get the board layout started I'll need some pointers on the bias pot connections.

Thanks,
Gary

Title: Re: Bias Pot Question
Post by: sluckey on August 26, 2016, 11:52:48 pm

I like the RV6 style locking pots and have used them in several of my builds...

http://www.ebay.com/itm/50K-OHM-MINIATURE-POTENTIOMETER-CTS-RV6-SIZE-LOCKING-NOS-1-PCS/331940074134?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D38804%26meid%3D142e5d09c5b042cc8cbcde7ccf148f5f%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D232049549153 (http://www.ebay.com/itm/50K-OHM-MINIATURE-POTENTIOMETER-CTS-RV6-SIZE-LOCKING-NOS-1-PCS/331940074134?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D38804%26meid%3D142e5d09c5b042cc8cbcde7ccf148f5f%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D232049549153)

Title: Re: Bias Pot Question
Post by: morton on August 27, 2016, 06:09:55 am

Thanks, those are just what I was looking for. Just ordered 3. Is the install the same as when you're using the mini 50kl trimmer pot that Doug sells?

Thanks,
Gary

Title: Re: Bias Pot Question
Post by: sluckey on August 27, 2016, 06:29:20 am

Wiring is the same. You may have to use your ohm meter to figure out which terminal is the wiper.

Title: Re: Bias Pot Question
Post by: morton on August 27, 2016, 08:15:52 am

Thanks, just to make sure I get my turrets placed right, on Doug's bias link with the trimmer, right leg not used, left leg to the 24k and ground on your layout, wiper to the negative of the 100uf/diode then to the 220k resistors. Or do I use the standard Fender circuit with left leg to 24k/ground, center wiper to the junction of the 220k resistors, right leg to the 100uf diode?

Thanks,
Gary

Title: Re: Bias Pot Question
Post by: sluckey on August 27, 2016, 09:13:49 am

I would do it like this. Install one more turret...

Title: Re: Bias Pot Question
Post by: morton on August 27, 2016, 09:18:20 am

Thanks, appreciate it. I should get started on it next week. Post some pics when it's complete.