Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: uki on April 14, 2019, 01:36:50 pm

Title: Body control pot
Post by: uki on April 14, 2019, 01:36:50 pm

What this body control does (see video) ? Couldn't find any schematics.

Title: Re: Body control pot
Post by: tubenit on April 14, 2019, 02:25:49 pm

Kevin O'Connor has a "body control" idea using a pot to turn a push/pull amp into a pseudo single ended amp.

Having said that, it will not surprise me if other builders use/d the term "body control" and mean something totally different.

With respect, Tubenit

Title: Re: Body control pot
Post by: d95err on April 14, 2019, 02:49:15 pm

There's a gutshot of the amp on their site: http://bcaudio.com/bel-air.html

Most likely (haven't analyzed the gutshot), the pot gradually fades in a second coupling capacitor in parallell with the first coupling cap. I.e. it's a variable highpass filter, most likely before the volume pot.

Title: Re: Body control pot
Post by: uki on April 14, 2019, 06:13:28 pm

Quote from: d95err on April 14, 2019, 02:49:15 pm

There's a gutshot of the amp on their site: http://bcaudio.com/bel-air.html

Most likely (haven't analyzed the gutshot), the pot gradually fades in a second coupling capacitor in parallell with the first coupling cap. I.e. it's a variable highpass filter, most likely before the volume pot.

I did got the pic already been looking at that but can't figure out what is going on there, very interesting feature !

Title: Re: Body control pot
Post by: shooter on April 14, 2019, 06:59:17 pm

best guess from your snippit, you guess values  :icon_biggrin:


updated, connect the dots version  :icon_biggrin:

Title: Re: Body control pot
Post by: uki on April 14, 2019, 07:39:08 pm

Quote from: shooter on April 14, 2019, 06:59:17 pm

best guess from your snippit, you guess values  :icon_biggrin:

There is a bright control switch there behind the body pot. I think you missed that.

Title: Re: Body control pot
Post by: d95err on April 15, 2019, 04:13:51 am

This is what it looks like to me. Interesting non-traditional bright-switch arrangement.

EDIT: Oups - mistake! The switch should go to the volume pot wiper, not the input.

Title: Re: Body control pot
Post by: uki on April 15, 2019, 06:56:42 am

Yeah I think we got it !

Check it out !

Where is the tone stack connected?

Title: Re: Body control pot
Post by: VMS on April 15, 2019, 07:24:42 am

Quote from: uki on April 15, 2019, 06:56:42 am

Where is the tone stack connected?

My guess is that tone stack is between second gain stage and phase inverter.

Title: Re: Body control pot
Post by: tubeswell on April 15, 2019, 01:53:24 pm

Or it could be resonance control in the NFB loop, or a mid-hump/mid-scoop control in the TS? or something else? 


(Edit: Oops - I see uki has a picture. Cool sales video too)

Title: Re: Body control pot
Post by: uki on April 15, 2019, 03:41:58 pm

Any guesses for the components values on the body control schem guys ?

I got a few

Title: Re: Body control pot
Post by: VMS on April 15, 2019, 04:17:40 pm

the cap on the pot looks smaller than .022


KOC uses in his soma 84 amp values 22nf and 1nf


body control works so that in one end of pot coupling cap is the .1 cap and on the other end .1 cap in series with the smaller cap.
so effectively the value of the smaller cap, which means thinner sound