Tube turntable score - Bogen VP20

[mtjo62]

After finding the Motorola SH12S last week, I found a Bogen VP20 turntable/PA today for $10. I haven't opened it yet to actually inspect the tube assortment, but it has a Jensen C12R in it. The unit powers up and plays beautifully. The Jensen sounds loud and fat, even lid mounted. I think I have gained an addiction to old iron now.

[Gnobuddy]

I know this is a two year old thread. However, I was given most of a Bogen VP20 a while ago (everything but the actual turntable mechanism). It has been sitting while I learned something about tube amps and safety issues associated with vintage amps in particular, and also thought about what I wanted to accomplish with it.

I recently stumbled across the 18 watt Marshall design, and realised that the Bogen VP20 looks like an excellent starting point to mod into one of these. Almost all the major components (tubes, transformers, chassis, knobs, pots) needed for the simpler versions of the 18 watter are already in the VP20. Adding a few more parts will let you modify to one of the modified versions of the 18-watter (Treble/Mid/Bass tone control or TMB version, dual channel version,etc).

While www.18watt.com appears to be gone for good, searching the 'web for "18 watt Marshall" will give you a wealth of information, including schematics and build pics for a variety of variations on the original 18 watt Marshall theme.

-Gnobuddy

[printer2]

18watt.com is still around, a new post today. Give Wattkins.com a shot if you can not get to 16watt.

[Gnobuddy]

Originally Posted by printer2

18watt.com is still around, a new post today.

Thanks for that info, printer2. I would not have guessed it from the following message that's been displayed on the 18watt home page for nearly a year now:

NEW UPDATE 4-25-2011: The 18Watt site is down for an upgrade so we are not adding any new members until the upgrade is complete.
<snip>
Please check back over the next few days.
<snip>

Originally Posted by printer2

Give Wattkins.com a shot if you can not get to 16watt.

Thanks for that suggestion also. I did stumble across Wattkins.com during my hunt for information, but I haven't tried to sign up to any of those forums yet.

I started work on my Bogen VP20 today. First priority is electrical safety, so I removed all the tubes, wrapped them in paper towels, put them in a safe place, then started to modify the chassis to accept a fused, three-pin IEC inlet. Once that's done, the switch is moved to the live AC wire (instead of neutral) and the chassis is safely grounded, I'll move on with the rest of the project.

Working with a steel chassis is slow going. It took me a couple of hours to first come up with a plan and then cut a five-sided opening for the fused IEC inlet into the rear of the steel chassis with a Dremel tool and a few cutoff wheels. Before I could cut the new opening I first ground down the rivets holding a little 2-pin AC outlet to the rear of the chassis (to power the turntable motor), removed that AC outlet, then used the cutoff wheels to modify and enlarge that opening to accept the IEC inlet.

The VP20 is more of a candidate for a full tear-down and rebuild than an easy mod to a guitar amp. Most of the passive components are potted - sealed into long rectangular blocks of some reddish material with only the wire leads emerging. It looks as though the components were laid out and soldered together in four separate arrays, then each of these arrays was potted in whatever that red material is.

So I can't see what's inside the blocks, the only schematic I have is blurry and incomplete, and some of the stuff on the schematic seems to be very old engineering practice - grid leak biasing on the 12A*7 tubes, for instance, with what appears to be 3.3 M ohm grid leak resistors and no cathode resistor. That had me scratching my head for a while before I realised what I was looking at.

So it won't be easy to make minor mods to the VP20 (remove the RIAA phono equalisation, the scratch and rumble filters, modify the tone controls to suit guitar frequencies, and so on). Either I use it essentially as-is, or pretty much strip it and rebuild with new passive components. I'm going with the latter plan. Sink or swim!

-Gnobuddy