Increasing Plexi 6v6 gain

[Big_Mike]

Hey guys -

I built a Hoffman Plexi 6V6 earlier this year, and I really like the amp, especially the cleans.  I am interested, though, in some possible mods to the amp to increase preamp gain and distortion.  I find it hard to get a good distorted sound with single coils.  Humbuckers are better for this, but I find I need to get a good pedal to give the amp some "drive.". I am curious if there are any mods out there that would give the amp more drive, but retain the good clean character of the amp.

[SILVERGUN]

It would be a simple experiment to try lifting the tone stack by disconnecting the mid pot lug from ground.
Tone stacks are inherently lossy and this will definitely increase gain. You could either make that switchable or install a raw pot for more control.

[Big_Mike]

Could I increase the plate resistor values?  Something like 120K or 150K instead of the stock 100K?

[SILVERGUN]

Could I increase the plate resistor values?  Something like 120K or 150K instead of the stock 100K?

Yes, that will also increase gain
You wont notice much difference at 120K.
However, keeping the front end clean might be the key to keeping your cleans where you want them.
You wont know til you know.

[SILVERGUN]

I've been waiting for a spot to experiment with this from the Valve Wizard's site
Scroll to the bottom of the page and see "Bootstrapping for more gain"

http://www.valvewizard.co.uk/dccf.html

[Auke Jolman]

I've done the mod mentioned above. There's a lot more gain added fhen I anticipated. I removed the bypass cap on the cathode resistor from the previous stage.

I guess it depends what you're looking for.

[Big_Mike]

In layman's terms, what is the theory on why bootstrapping adds more gain?

[SILVERGUN]

In layman's terms, what is the theory on why bootstrapping adds more gain?

I can't say it any better than Merlin
Quote from link: The cathode follower then effectively multiplies or bootstraps the value of the lower anode resistor R2, making it look like a constant-current source. This will increase the gain of the previous stage so it becomes almost equal to the mu of the valve!

http://www.learningaboutelectronics.com/Articles/What-is-a-constant-current-source.php

[2deaf]

In layman's terms, what is the theory on why bootstrapping adds more gain?

Sometimes it helps to plug in some numbers (and sometimes it doesn't).  Referring to Valve Wizard's drawing, let's say we have a 10V change at the plate of V1.  Without bootstrapping it would take a change of 10V / 94K = 106uA to get the 10V change at the plate (Ohm's law).

Now let's say that a 10V change at the plate of V1 causes a 9.5V change at the cathode of V2.  This 9.5V change is coupled to the junction of R1 and R2 by C1, so that end of R2 has a change of 9.5V.  One end of R2 has a 10V change and the other end has a 9.5V change, both changes are in the same direction, so there is a change of 0.5V across R2.  0.5V / 47K = 10.6uA change.  Already we are getting a sense of increased gain because it only takes 10.6uA to get a 10V change with bootstrapping whereas it took 106uA without bootstrapping.

By Ohm's law, 10V / 10.6uA = 940K.  So R2 appears to be 940K to the plate of V1 with bootstrapping.  Say the mu of V1 is 100 and the internal plate resistance is 60K.  A voltage divider is formed by the 940K and the 60K.  When mu is spread across this voltage divider, most of it is across the 940K.  If the cathode were fully bypassed, the gain would be mu times the voltage divider:  100 x (940K / 1000K) = 94.

This explanation is subject to peer review.

[Hellawatt]

Would this bootstrapping trick work on an amp with only two preamp tubes? Like a Hoffman stout? Could you go triode 1 > cathode follower > tone stack > PI ? I have another Hammond AO35 chassis I'm looking at preamp ideas for...

As to the original question, I built a 6V6 plexi a couple years ago, and I ended up converting the first preamp tube from parallel to cascading, so it is more of a JCM800 now. I was surprised that it only took a few small component changes to go from plexi to JCM! Its not super heavy metal high gain like a 5150, but it can do AC/DC and GNR tones, and it still has good cleans.