Pre-amp after Inverter in PP?

[bluefoxicy]

Has anyone tried a preamp-after-inverter topology, ever? I'm thinking the following:

(S)               ------------{Va1}----{Vb1}------{EL84}---    -- (speaker)
 |             [+]|            ^        ^                  |  |
{12AU7}  -{Inverter}      (12AX7 1)   (12AX7 2)            3||E (OT)
 |      /      [-]|            v        v                  |  |
{Tone}--          ------------{Va2}----{Vb2}------{EL84}---    --

Obviously, the 12AX7 pre-amp tube(s) would be in Class AB1 push-pull mode. This might take some of the smooth pre-amp drive out, causing the amp to sound harsh... I'm unsure.

Note the need for a preamp tube (12AU7 here) still to drive a tone stack.

Thoughts?

[Steve Conner]

Well, one reason not to do this would be that it would cancel the even harmonics and reinforce the odd ones. Even harmonic distortion sweetens the sound, whereas odd harmonics tend to make it more aggressive and discordant.

If you like aggressive distortion, though, it might work for you.

I have a hi-fi amp that uses the popular Williamson topology, with a gain stage after the PI.
http://scopeboy.com/scopeblog/?page_id=37
http://scopeboy.com/scopeblog/wp-con...photofact1.jpg

[bluefoxicy]

Well, one reason not to do this would be that it would cancel the even harmonics and reinforce the odd ones. Even harmonic distortion sweetens the sound, whereas odd harmonics tend to make it more aggressive and discordant.

Yeah that's what I was thinking. Otherwise, technically doable....

[gmoon]

The Kalamazoo "Bass 30" used that topology. Not a hi-gain amp by any means, but sounds darn good with guitar..

http://schematicheaven.com/bargainbi...zoo_bass30.pdf

[MWJB]

I've built a couple of amps like this, a split load PI as the input tube...it's not really a new idea, Red Bank NJ Danelectros had a similar topology in some of their accordian amps from 1952 onwards. Though theirs had a passive tone circuit before the input triode & a second volume between the PI triodes. They also had more conventional amps with input>tone stack recovery>PI>additional gain stage...much like the K'zoo GMoon references (really just an additional PP triode stage, rather than a "PP preamp").

One of the amps I built is, in effect, a single channel 5E3, built "back to front" (input at the PI>vol & tone controls x2>2nd 12AX7>cathode biased 6V6s)...killer clean tone, scooped mid, but heavy, rich, "gunmetal" tone...think "Hey Joe".

Not ideal if you like crunch & rasp, great for smooth overdriven sounds. Not bad for acoustic guitars too...as long as you like a slightly compressed, tubey warmth with them.

You wouldn't need to use a 12AU7 as V1. 3 preamp tubes might be pushing it, especially with EL84, assuming that you are just using tone controls & volumes (I have 56K/56K split load plate resistor to dump some gain at the first triode but regular 100K will work too.

The other version had 2 BF style tone stacks, 6L6/6550 outputs with fixed/cathode bias switch. Again rich, round, full, smooth BF style tone. Still an awful lot of gain with 3 preamp tubes...I think I ended up with the input triode's cathode unbypassed (amp was eventually rewired with a different topology)?

Reasons for not doing it might twice the parts count in pots & tone circuits (unless you tried a simple 'low loss' tone control & vol between the PI triodes, remember you need a coupling cap before V1B grid to keep dc off the pots), tricky to apply reverb etc, tricky to apply NFB (though neither of the amps I built needed NFB), twice the parts to faff about with if you want to mod anything after the PI, you would need to be more careful with layout.