Blencowes bootstrapping for more gain?

[Mark]

http://www.freewebs.com/valvewizard1/dccf.htm

Has anyone tried this on their Marshall builds?

[Colossal]

Hi Mark,

Yes, I have tried bootstrapping a CF. It's difficult to give it an unbiased review on its effects since the goal of the amp I tried it in was about stacking gain and little tweaks to build a monster. But, as advertised, you can hear a boost in gain with the cap switched in. You can change the cap's value as well as the split load ratios (within reason). I think it's important to consider what Rk(Ck) you have preceding the CF since the CF adds more gain to this stage by sourcing current while following. This has a tonal effect. I think the BS'd CF could be a good addition for an amp that is mostly or all stock, but you are looking to bring out "a little more". I like CFs for their tone and smoothing and of course their function to drive a load.

It's a pretty simple mod which you can put on a switch (recommended) so you can A/B it. Ultimately, I did not keep it in the amp in question because with its already high gain, the CF BS only made it one louder. But I would keep that tool in the drawer for another day as it has utility.

Sorry I can't help more.

[chief mushroom cloud]

I put it in a 'voxish' top boost 15W build I'm doing.....not a hi gain amp but stock 'ac-30 tb' preamp circuit w/ this mod...I'll report back when I get it up and running (and take comparative measurements w/ the mod bypassed)

[Noval_novice]

I did this to my ax84 HO build, which is basically a JCM800 into a SE output section. I also decreased the value of the resistor on cathode of the CF. The combination does increase compression more so than gain to my ears.

[chief mushroom cloud]

ok, I measure an increase of 2.8W at the output....so it's not chump change.

'because with its already high gain, the CF BS only made it one louder'

That's just about my result.
I also decreased follower cathode resistor, but to 47K...since the TB is normally a 56k
Definite added compression when driven.....I like it.
There is definitely some boost and just as Merlin sez/shows in his book.....some very soft clipping but sounds like compression....
I'm very happy w/ this amp, and will post sch on Hiwatt/Vox room.

[LeftyStrat]

ok, I measure an increase of 2.8W at the output....so it's not chump change.

'because with its already high gain, the CF BS only made it one louder'

That's just about my result.
I also decreased follower cathode resistor, but to 47K...since the TB is normally a 56k
Definite added compression when driven.....I like it.
There is definitely some boost and just as Merlin sez/shows in his book.....some very soft clipping but sounds like compression....
I'm very happy w/ this amp, and will post sch on Hiwatt/Vox room.

I'd love to see this schematic. I've been wanting to do something Voxish/Rocketish in the 15 watt range.