I took out the JBL crossover PCB, stripped everything off it and installed the new components on it. Here are the before and after shots:
I replaced the stock speaker with the JBL 2118H that I took out from the 4311A cabinet – what a difference! Compared with the stock speaker, the 2118H has a much improved frequency response, in fact,the amp can’t be cranked up full, because the bass rattles the poor little combo cabinet, this is a 200W speaker designed for heavy duty PA work after all…
I got this amp awhile back, I finally had a chance to work on it this week – starting with a few simple mods to the output stage:
- remove the triode/pentode switch and install a Zener dropper for the Low Power mode;
- install an adjustible bias for the cathode bias circuit with test points, so I can try out different output tubes.
Since the amp was designed with modding in mind, it is pretty easy to work on. I put a 6P3P in place of the 6V6 for the test, to make sure the basic function of the mods are working, and everything is fine. Although, the tone is pretty boring. Well what a difference a tube make, this morning I swaped out the 6P3P with an EL34, re-biased it to around 51mA and plug in an 8-Ohm spealer to the 16-Ohm jack, the sound was much improved, beside more low end, it just sounds fuller.
Here are two “JCM” mods I found on the net (Ultimate Guitar and Peavey Forum):
Both of these mods bypass the V2a gain stage when the “JCM” mode is selected, and retain the anode driven tonestack. I wonder if keeping V2a in the loop and converting V2b to a CF to drive the tonestack would make the amp more “JCM”-like? Perhaps something like this:
Mod 3 (tonestack mods not shown):
There are a few options to consider:
1) DC-coupled cathodyne, which could add a bit more harmonic richness when over-driven:
2) A more traditional LTPI used by Marshall:
Both are doable, but the LTPI would involve adding more parts and re-wiring on the PCB, while the DC-coupled cathodyne should be a relatively easy mod – just need to cut a trace and add a few parts.
One of the “must-do” modifications on the VK212/100 is the adjustable bias. I plan to add another twist into the mod – the high-low power selector. In Peavey’s design, the amplifier utilizes a potentiometer (the “Texture” control) to vary between Class A/Class AB opeation. At one extreme of the setting, one side of the push-pull pair is shut off completely, resulting in Class A operation, while at the other extreme, both halves of the push-pull pair operate normally for Class AB operation. The design is fine, I think it is even patented, but who wants Class A operation in a guitar amp?!
Instead, I plan to remove the Texture circuit, and install a DPDT switch in its place, the switch can be used to disconnect one pair of the output tubes from the PI, so the output power is halved. You may ask why not just remove two tubes like in many other 100W amps? Well, in this particular amplifier, all of the tubes’ filaments are wired in series, so removing any tube would render the whole amplifier inoperative, the switch is a workaround and easy to implement.
I also plan to add a bit of refinement to the adjustable bias mod that you find on the Net for the VK. I plan to add a DC balance control so the upper and lower half of the push-pull pair could be adjusted for a perfect balance or some imbalance as preferred.
Here is the proposed schematic for the modification: