Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Fresh_Start on March 23, 2011, 11:55:36 am

Title: Interesting AX84 Design
Post by: Fresh_Start on March 23, 2011, 11:55:36 am

I thought you all might like to look at a recent AX84 design:
Comanchero Club (http://www.chappellamps.com/amplifiers/comanchero/Comanchero_Club_100621.pdf)

What got my interest is that it has Bass, Mid and Treble controls, but they're in series through the preamp with each taking a different form.  A traditional TMB tone stack it is not!  The "Load" control also is interesting - I assume it's a form of volume control. 

I can't say that the sound clips I heard blew me away, but the design just looked cool to me.  Efficient use of two 12AX7s and two EL-84s IMHO.

OK, back to work now...

Chip

Title: Re: Interesting AX84 Design
Post by: jeff on March 23, 2011, 12:05:22 pm

 Not sure what load  is but look at the note (1) in the upper left. It says that the load is intended to be a trimmer pot although it could be mounted on the front pannel.

Title: Re: Interesting AX84 Design
Post by: bluesbear on March 23, 2011, 12:53:27 pm

"Load" looks like a pre-PI master volume to me. It is an odd tone section. It warrants looking at... and trying to understand.
Dave

Title: Re: Interesting AX84 Design
Post by: Fresh_Start on March 23, 2011, 02:33:39 pm

Quote from: jeff on March 23, 2011, 12:05:22 pm

Not sure what load  is but look at the note (1) in the upper left. It says that the load is intended to be a trimmer pot although it could be mounted on the front pannel.

Ever heard the expression "Can't see the forest for the trees"?  You're right, although I saw at least one build with a knob on the front panel called "Load".

There also were comments regarding the Mid pot having a big effect on gain.  I'm not familiar with this form of Mid control.  Looks like it takes advantage of local negative feedback though.

Rock on!

Chip

Title: Re: Interesting AX84 Design
Post by: jjasilli on March 23, 2011, 09:52:34 pm

There also were comments regarding the Mid pot having a big effect on gain.  I'm not familiar with this form of Mid control.  Looks like it takes advantage of local negative feedback though.

Yes, but this is above my pay grade.  If there are frequencies in the gjuitar range that are large (bass) enough to survive bleeding off to ground through the dual cathode caps (one switched), then it looks like 2-way feedback.  The cathode is driving the bottom of the "mid-stack"; and the mid pot is feeding back to the cathode from the plate's signal from the top of the "mid-stack".  My guess is that this is too complicated; and that the cathode caps are bleeding away all this potential feedback to ground. 

Title: Re: Interesting AX84 Design
Post by: tubeswell on March 24, 2011, 03:56:01 am

The load is a gain control that you can't turn all the way down - sort of like a clean-crunch control

Methinks the tone stack is something of a mid-scoop shift control