Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on December 28, 2013, 07:39:08 am

Title: Mo' Rock Special
Post by: tubenit on December 28, 2013, 07:39:08 am

Decided to start a new thread.  After trying out a 5879 bootstrapped mosfet CF,  I discovered that idea may have great potential for a overdriven crunchy harmonically rich tone?

So, I mentioned to SILVERGUN that this idea may be worth considering?  

Here are some ideas to consider.  The B+ rail is on the 2nd page of the SCH file schematic.

Not sure what some of the values should be?  So I tried to mark what you may need to experiment with.

I strongly advise building it with the active FX  which works wonders as a master volume and also adds some gain besides working well with delay and reverb pedals.

With respect, Tubenit

Title: Re: Mo' Rock Special
Post by: tubenit on December 28, 2013, 07:44:55 am

OR .................... one could do a "lead boost" relay switched version to go between crunchy rhythm and lead.

The "trim pot" for the boost gain stage could be on the layout board.  The "trim pot" into the 5879 would be a full sized pot on the
front panel.

With respect, Tubenit

Title: Re: Mo' Rock Special
Post by: terminalgs on December 28, 2013, 01:13:07 pm

(http://i.imgur.com/nNagDqB.jpg)

You might do something like this with the dpdt switch. There is no reason not to send a signal to the driver Triode when switched to clean. Doing so also  frees up half a dpdt for something else.

R1/R2 are your trimmed voltage dividers. R3 can be high like 2M, it is there to bleed stored DC so the SW doesn't pop.

Use the other half of the dpdt SW to switch a tuned resistor in and out of the top of your post tone stack volume control pot.  This counteracts the enormous gain jump from the OD Triode.  You can tune R1/R2, and R4 all such that (1) the output signal is the same +dB gain from input to output and (2) you get a nice juicy pentode OD in the middle.

In tuning, don't be surprised if R4 needs to be a big R (like 1M to 2M) to attenuate the ODed signal down to the same +dB of the clean SW position.

In general, I think this arrangement you've posted is the one to work on (Triode drives pentode into OD) rather than the first one you posted (post pentode OD).

Title: Re: Mo' Rock Special
Post by: terminalgs on December 29, 2013, 08:51:32 am

Maybe make R2 a pot "overdrive", so player can tweak the crunch. With R2-pot cranked, You can dial R4 into to be some max +dB level you are comfortable sending to PI.

In my crude drawing I left the MOSFET out, but you know where to put it.