Jim Kelly/ Ampeg V4 active midrange circuit question

[beasleybodyshop]

Hey everyone,

I've been doing a bit of research lately into the Jim Kelly FACS/Single Channel and the Ampeg V4 circuits.

Essentially, the bones of the Jim Kelly circuit are based on the Ampeg SB-12. Ampeg used the same basic James/Baxandall EQ for what seems like forever - and didn't seem to tweak it much until we get to the Ampeg SVT or V4. Those circuits went one step further and added an inductor based active midrange control to the output of the classic James/Bax circuit.

So i've been tinkering with building an amp that combines the Jim Kelly single channel circuit with the active midrange of the Ampeg V4. Schematic added to post below.

So, now for my questions:

1.) I see that in the Ampeg schematics for the active midrange, they decouple the James/Bax circuit to the input stage of the active midrange with a .01uF cap. Would any of you suspect this was done to attenuate the low frequencies going into the active EQ portion or to serve as part of the biasing of the active EQ input gain stage? The reason I ask is, my first idea didn't have the EQ AC coupled and I don't know how critical it is to decouple the stages from each other. Granted I could "just build it and see" but wanted to get some clarification here.

2.) In the Jim Kelly circuit, the reverb wet signal path is split off the output of the James/Bax EQ output. The V4 uses a pretty unique Ampeg style direct AC coupled reverb circuit that while novel, I don't want to use. The Jim Kelly amp uses essentially a Fender style reverb circuit with a few minor tweaks. would it make more sense to split off the reverb wet signal BEFORE the active EQ or AFTER?

Ampeg V4 Active Midrange.PNG

[PaisleyTube]

PS

B+ 485 V, cathode-bias and 6V6s........ wouldn't do that

[DIY4Life]

DSB, I am very interested in this build. Are you posting progress somewhere? This looks great, but I know nothing!

[beasleybodyshop]

Sorry I forgot to mention the voltages on this schematic are incorrect - I need to cross reference them with the Jim Kelly amp I had in for service a few years back. That being said, the original Kelly amps did indeed have like 400+ plate voltage on a quad of 6V6s - I intend on using the JJ 6V6 which can handle the higher plate voltage and what was used in the Jim Kelly reissue amps from a few years ago.

DIY, No I haven't posted the build here yet. It was a project that I started a few years ago and life got in the way - Although I have more or less built out 90 percent of this amp.

After mulling this over some, I think I will tap off the Reverb wet supply AFTER the midrange circuit; this would seem to make the most sense as the Fender circuit has the split before the 3.3M resistor and the return immediately after.

[jml77]

I think that without the 10nf decouple cap it will mess up the bias of the cathodyne stage that follows and the mid circuit won't work as effectively without it.
Without it there's a DC path to ground via the Bax 120k resistor and the bass pot, which will tie the grid to ground, preventing the cathodyne from working properly.

[stephenl]

What is the function of the 1n914 diodes from grid to ground - clipping?

[teemuk]

The diodes enforce symmetric clipping distortion. Grid conduction clips the positive lobe of the signal at threshold equivalent to approximately DC bias (cathode) voltage and some. The diodes clip negative lobe at approximately same threshold. Relying to plate clipping would produce traditional asymmetric clipping.

Jim Kelley design overall favours tone of symmetric clipping and odd order harmonic distortion. e.g. push-pull output stage clipping, differential "Line Amp" power amp emulation clipping, and overall preamp design enforcing symmetric clipping.

[stephenl]

Thanks for the education, much appreciated!

[beasleybodyshop]

I think that without the 10nf decouple cap it will mess up the bias of the cathodyne stage that follows and the mid circuit won't work as effectively without it.
Without it there's a DC path to ground via the Bax 120k resistor and the bass pot, which will tie the grid to ground, preventing the cathodyne from working properly.

Thats a good observation! I think I will stick with the coupling cap between the stages