Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Baguette on May 01, 2011, 06:20:01 pm

Title: "simulating" a cathode follower
Post by: Baguette on May 01, 2011, 06:20:01 pm

Hello guys,

Basically, I want to build a Plexi / 5F6A / AC30 Top Boost type amp without the cathode follower... but still retain the original tone (I'd like to free up the triode for something else).
The tone stack would be driven from the plate of the 2nd gain stage.

Is there some tuning I could do to the amp to simulate the tone of a regular FMV amp without the cathode follower? Like tayloring the tone stack values... I'm thinking raising the slope and mid resistor to reduce the loading.

Anyone tried it and got successful?

Thanks for your kind help!

Title: Re: "simulating" a cathode follower
Post by: Fresh_Start on May 01, 2011, 10:21:54 pm

Merlin Blencowe makes a fairly convincing case to back up the theory that the cathode follower driven tone stack is a key part of the tone of Plexi/Bassman based circuits:
http://www.freewebs.com/valvewizard1/dccf.htm

It is possible to use a MOSFET in place of a vacuum tube triode for the cathode follower.  The MOSFET can drive the tone stack the same way; however, the "natural" compression of the vacuum tube triode would be absent:
http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

Also look at the recent thread started by tubenit about "cathode follower resistors".

I'm not trying to be discouraging - it justs seems that you are after a "Holy Grail" that may in fact be mythical.

Cheers,

Chip

Title: Re: "simulating" a cathode follower
Post by: tommytornado on May 02, 2011, 10:57:38 am

Check out the Trainwreck schematics, they follow an "input > V1A > tonestack > V1B > V2A > PI > power tubes" topology.   http://www.blueguitar.org/new/schem/trainwreck/wreckxpr.pdf

Also, if you want an extra triode.. why not use one of the V1 triodes and make a one channel amp?

The CF in a 5F6A or AC30 type amp is critical to how the amp's tone stack hits the next gain stage/PI.  Driving the TS from the V2 plate, a high imp. source will drop way to much gain and effect the tone/drive of the amp.  There are others on this board who can explain it way better than I can.   

Title: Re: "simulating" a cathode follower
Post by: tubeswell on May 09, 2011, 02:27:09 pm

The direct DC-coupled CF stage in the 5F6A steals current from the plate of the preceding (inverting) stage so as to make the wave form more assymetrical (which increases the harmonic distortion), and also produces more gradual entry and exit to clipping (softer clipping), as well as serving to drive the TMB tonestack with a good level of current. So it is a big part of the sound of a tweed bassman (and every Marshall amp that copied it)