I was thinking about modifying the VK212 when I first got the amplifier a few months ago, and came up with some potential modifications. But at the end, I abandoned the idea and went with a near-scratch build – so I took out all the PCB’s, and built put in a Marshall 2446 Vintage Modern clone instead. This worked out great, the chasiss has quite large so I had ample room to work with. Of course, I re-used the PT and OPT which were perfectly fine. I also saved the PCB’s from the VK in case if I ever want to put them back (which I doubt seriously). The only tricky bit was to modify the heater supply, since the VM uses one extra 12AX7 tube than the Vk212.
I don’t really like the series heater arrangement in the VK, but since the PT came that way, and I did not want add another filament transformer to the chassis, it would have to do. Of course, the disavantage is if one of the tubes go bad, the whole amp will be out of commission. Since it is just for home use, it is not a really big deal, but I would not want to take it on the road.
Instead of the 6L6’s that came with the amplifier, the output tubes have been changed to the 5881’s that I have laying around, so the output power was lowerer to about 75W. I also took the opportunity to add a bias adjustment board, which is the small board mounted above the power tubes that you see in the photo. It came from a scrap’d hi-fi amplifier. With the board, each of the output tubes could be biased individually, which was sorely missing from the original VK212.
With the existing VK speakers, the amp sounded a bit bright to me, then again, I still haven’t spend much time with it…
Here are some photos from the build:
I completed the ODS build a few months ago, here are some photos. I got the chassis from RJ Guitars, and the purple head shell from Glassworks, thanks guys.
This is a follow-up to the Trainwreck Express post.
As mentioned earlier, the amp turned out to be pretty noisy… While working on it over the weekend, I got it much quieter by re-wiring the PS, but somehow it also sounded much worse with obvious blocking distortion which wasn’t there before… After fussing with it for awhile, I decided to restore the whole darn thing back to the original 18W design but with a cascade switch added for some flexibility. I will try some new EL84s soon, so I can compare them to the Chinese 6P14s that are currently in the amp.
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.