Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Planobilly on August 22, 2016, 01:57:11 pm

Title: Hoffman AB763 2 channel Master Volume question
Post by: Planobilly on August 22, 2016, 01:57:11 pm

Hi,

I am sure this question has been answered somewhere on the site...just can't find it.

Is there a schematic for installing a post phase inverter master volume control on a  Hoffman AB763 2 channel amp? Are there any known issues doing this?

Thanks,

Billy

Title: Re: Hoffman AB763 2 channel Master Volume question
Post by: MakerDP on August 22, 2016, 03:53:36 pm

Cool thing about PPIMV circuits is they are very easy to implement and pretty universally useable in any push pull amp.

Read up on them in The Trainwreck Pages...

https://robrobinette.com/The_Trainwreck_Pages.htm (https://robrobinette.com/The_Trainwreck_Pages.htm)

Title: Re: Hoffman AB763 2 channel Master Volume question
Post by: Planobilly on August 22, 2016, 06:48:32 pm

Thanks MakerDP,

I was considering using a Bruce Collins style PPIMV. It is pretty typical. 1M dual pots. He uses  two additional .1uf caps to prevent the bias voltage from entering the pots. What I don't understand is why they would both be .1uf. That would change the total capacitance per side from 100 nanofarads to 50 nanofarads.

I can post a schematic if needed.

Cheers,

Billy 

Title: Re: Hoffman AB763 2 channel Master Volume question
Post by: MakerDP on August 22, 2016, 10:02:29 pm

When axding ppimv to an existing amp start with the easiest and if it's not what you'd hoped it would be try the next type.

So try the type 3 first. You may love it and save yourself a ton of work.