Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Madison on November 23, 2012, 04:21:50 am

Title: Mid pot addition to a Princeton (type) circuit. Searching for mojo sweep
Post by: Madison on November 23, 2012, 04:21:50 am

I want serious mojo power out of a mid pot on a tone stack.(a sweep? a swoop? whatever it is called now days)

I see there is a mod for the addition of a pot in series with the stock 6.8K resistor on a AA964.
Can somebody tell me what value pot to go with?
I have a 25KA I'd like to try but nothing lower in value in the parts bin at the moment.

For now, I am using a 1MA Bass, 22KB Mid,220KB Treble, 1MA Volume.

Peace

Title: Re: Mid pot addition to a Princeton (type) circuit. Searching for mojo sweep
Post by: worth on November 23, 2012, 07:40:20 am

David Allen uses a 100K... although he used to use a 1Meg. If your existing mid pot is 22K, then maybe a 25-50K pot would be enough to give you the boost you want.

Title: Re: Mid pot addition to a Princeton (type) circuit. Searching for mojo sweep
Post by: Ed_Chambley on November 23, 2012, 09:14:44 am

I simply replace the resistor with a 10k pot. Before wiring it up as a divider I measure the turn of the pot to read 6.8k and make a small mark which easily lets me return to stock value.

Check out fenderguru.com, the show the mod.  You can leave something like a 5k resistor if you want to get into vox territory chime.

Title: Re: Mid pot addition to a Princeton (type) circuit. Searching for mojo sweep
Post by: jjasilli on November 23, 2012, 09:38:51 am

Fender mid pot value is 10K.  If no mid pot Fender uses a 6.8K fixed resistor, assuming that this is where a mid pot would likely be dialed.  Marshall uses 25K which can further "boost" mids.  Some people prefer a 50K pot.  Note that the the mid pot replaces the fixed resistor. 

Then there's a RAW control:  100K - 500K pot.  This will definitely allow you to change the tempermeny of your amp. 

Your bass / treble pots seem to be of non-standard values.  Also tone cap value must be consdidered in terms of tone pot value (tone shaping). 

The value of the slope resistor and treble cap are each also critical.  Tubenit has posted lots of schematics.

Title: Re: Mid pot addition to a Princeton (type) circuit. Searching for mojo sweep
Post by: Madison on November 23, 2012, 06:57:54 pm

Super.

I'll first try increasing the pot.
Then I'll look into the RAW control.

The Bass and Treble pots are very effective and noticeable, the middle is weak.
I'd prefer the opposite.

This is the way I have it wired at the moment.
I am wondering if the caps should run to the bass pot and the mid pot just goes off the left lug if the bass pot directly to ground?

Kind of doing an AA964 pre.

Thanks.

Title: Re: Mid pot addition to a Princeton (type) circuit. Searching for mojo sweep
Post by: Madison on November 23, 2012, 09:33:47 pm

EDIT:

I rewired it, got it going.

Thanks all.