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 – I took out all the PCB’s, and put in a Marshall 2446 Vintage Modern clone in their place instead. This worked out great, since 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, however the PT came that way, and I did not want add another filament transformer to the chassis, so 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 lowered 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 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.
[Update – July 15, 2014] Well that did not last very long, after hearing clips of the JCM-145 (a special version of the JCM-1 made for Anderson Music’s 45th Anniversary), I just had to try it out. Using the schematic made by Matec here (thanks!) It was a short order to convert the 3Watter into a JCM-145 clone, it sounds pretty good considering I am using a surplus line-distribution transformer, with reflected impedance of 26k Ohms, which is quite a bit higher than the spec 20k Ohms.
[Update – October 30, 2014] The line-distribution transformer just did not cut it… When the amp is turned up, its core would saturate and cause severe distortion – not the right kind! So I finally replaced it with a rather large 10k:8 OPT (loaded with 16R speakers), which really is an overkill for this little amp. But the result is well worth it, now the amp sounds much fuller, and the LF is surprisingly loud coming off the little 12AU7.
After taking the Princeton clone apart, I decided to re-use the chassis for another build – and I settled on a mash-up of the Princeton and the AX84 4-4-0 Studio, it’s a very simple circuit – even though I re-use the TMB tonestack from the Princeton, the amp actually sounds much better with it bypassed, so there’s really not much between the input jack and the speaker – less is more!
Here is the schematic:
Most of the parts were mounted on a surplus terminal strip board.
The completed amp, note that another 6N8P/6SN7 was added after the shot was taken, that boosted the output power to a whoppin’ 3 Watts.
With the volume ~ 2 O’Clock (single 6N8P/6SN7), here is the scope shot of the output into a dummy load:
The soundclip was recorded with my mobilephone, the volume was around 1 O’Clock, I used DI tracks straight into the amp, each clip starts with the DI track played on my hi-fi speaker followed by the amp played through an Emience Ragin’ Cajun in the AX84 cabinet. Sorry about the poor recording quality – the amp sounds much fuller… anyhooo, here’s clip:
To me, this little amp sounds better than both the Princeton clone and the much larger AX84 SEL… It was a very quick and enjoyable build, since I had all the parts and the chassis ready to go, it’s definitely recommended for those looking for a small bedroom practice amp.
A follow-on to my earlier post on Spring Cleaning – how time flies… Since we moved into a smaller apartment, I need to consolidate my guitar amp collection, since most of them are in various stages of dis-repair – lacking proper cabinets and what nots. I plan to decommission all of them except for the 18Watt/Express build, since it sounds the best and was actually built with a proper chassis, I will be ordering a head cabinet shell for it soon.
So with some regret, the following builds will be decommissioned: AX84 SEL, Fender 5E3 Princeton, 6AF11 American Flyer, Tweed Overdrive Special and Otto Special. I have learned a lot since I built these amps, but none of them really blew me away, so it’s time to start over…
While browsing through the audio/diy forums and websites, I am constantly amazed by the depth of know-how in the hi-fi community, however, when it comes to the physical construction, I am equally disappointed by the general poor showing, including my own 😉 – many of the amplifiers are down right hideous, even if they sound great, I would never want to see them in my system – thus the inspiration for the title of this post – visual pollution! Look no further than the long-running diyaudio photo thread for some examples of what not-to-do.
On the other hand, I continue to be impressed by the designs and construction of Thomas Mayer at VinylSavor – everything he does is a thing of beauty – from the elegantly simple designs, high quality parts, meticulous wiring and construction details to the exotic wood chassis – truly great industrial design!
Time to clean house! I really got sick of looking at all the bare guitar amp chassis that I built over the past few years – they are ugly! And knowing what I know now, I think I can do a better job building them, as well as making some improvements to the designs. So in the next few months, I plan to disassemble and re-make some of them – the Fender Princeton clone, the Otto and the Tweed Overdrive Special. I will also scrap the American Flyer and re-use the 6AF11 in another design.
While all of them sound ok, to be honest, nothing really shouts “it’s a keeper” to me, so instead of letting them collect dust, I will try to do something else with them. In particular, the TOS will be made into a preamp, since that’s how it get most of its characteristic tone. So that leads naturally to a ‘studio preamp” project – perhaps fitting several channels of preamps into a single chassis a la Mesa Boogie, Demeter or CAE. Well at least that’s idea now – at a minimum, everything will sit inside a proper chassis for starters!
Some other “flavor of the month” designs that I may incorporate into the studio preamp are: the Thrifty Croaker series of pentode preamps from the Wattkins forum, Brown channel from the Bogner Fish. And if I am feeling really ambitious, may be adding MIDI channel switching would be nice.
To go along with the studio preamp, you guess it, I will also build a few power amps, low-to-high power, SE or PP, so the preamps and power amps can be mixed and matched – which is what the amp makers are doing anyway – pairing up various combo’s to get the “tone”.
Anyway, by putting all the parts behind rack-mount panels will save me from dealing with making cabinets for the amplifiers, which is both time-consuming and costly. Not to mention, I haven’t the clue or skill to make them as much as I want to, perhaps I will find a competent and cheap cabinet maker one day, then I will get some custom cabinets made.