I once upon a time had the pleasure to test out a all-tube bass amp, the Ampeg SVT-II Pro. I think it was back in the 90's, and a buddy of mine knew a bass player the owned one and was selling it, so I wanted to try it out just because I had the opportunity to do so. And what can I say... The sound of that amp! Incredible! Too bad I was poor as a church rat at the moment, so I had no way of buying it. Besides, the monster weighted a hefty 70 pounds (32kg) so I wasn't that keen on lugging it to gigs.

However, as a typical DIY guy the experience left a spark brewing in the back of my head, and years later I started to look for a electric schematic for the monster online. Didn't find the real thing, but something that looked close enough to be worth giving a shoot at. It included the schematic for the whole thing, including the BOM and the PCB layout.

So now I had to deside what sections to leave out that I didn't want/need to build the preamp section only. No power amp, no EQ section, no effects loop, no D.I. out and the mid controls selectable frequencies ditched. Only one frequency for the mid, all the other frequencies seemed too high, so the lowest one, 800Hz, was the most obvious choice.

Below a picture of the schematic.

Click on picture for a bigger version (opens in new tab)

So I believe it was back in 2011 I started ordering all the parts for the project and modifying the PCB, drawing and etching it. One of the most difficult parts to find was a suitable coil for the mid control and a chassi with the right dimensions. Found one but without a lid, so I had to get some aluminum plate for the lid.

Also a custom piece of wood had to be made for the three tube sockets to get them properly mounted to the chassi. Got all the components soldered to the PCB and, a lot of the wiring done and the chassi painted and preped for everything. Then I lost interest in the whole project, and the project ended up on the shelf for several years. Until autumn of 2017, when I finally had finished a bunch of other electronic projects I suddenly got the urge again to finish this one.

After hours of meticilous soldering and tedious cabel work I finally got it together far enough to fire it up for the first time. I was a little worried about this part because I hadn't done some DIY electronics with this high voltages (275V DC) before. So all precautions were made before I flicked the switch.

So what happened? Well... Nothing. No capacitor blew up or anything. No smoke, and the heaters in the tubes started glowing and all voltages seemed ok. The plate voltage seemed a bit low, but I still got signal going thru. All the controls seemed to work except the Ultra-Hi and Ultra-Lo switches and the sound wasn't sounding right. So I suspected something "funky" in the first section of the amp, around the Ultra-Lo/Ultra-Hi section. It took me a while to find the faulty wiring, three of the wires going to the first tube and the PCB was soldered to the wrong place. Re-soldered the wires and all was good.

And as I had anticipated some hum from the transformer was interfiering with the audio signal. So I decided to make a thin aluminum plate between the transformer and everything else, making shure I got it properly grounded to the chassi. That helped a lot, no audible hum or other AC related noises anymore.

Before assembly, the top aluminium plate was sanded with 150 grit sandpaper horizontally. Then some vertical strips was painted on the entire surface with a ordinary black permanent marker. That gave the surface a metallic-like black-striped look to it.

A flashing reb LED was connected to the bypass footswitch, so when the whole preamp is bypassed the LED will be flashing red.

Also, a mixture of super-bright and ordinary red LED's were epoxied to the underside of the top plate directed at the three tubes, thus giving them a fake "glow". Just to raise the coolness factor of it all.

So, how does it sound? Well, every bass that I plug into this preamp makes them "come alive". Can't "put my finger on it" but it shure does something to the sound that seems to be the work of the tubes.

The Ultra-Lo switch isn't really a "Ultra-Lo", more like a "shape". It reduces the low-mids and gives just a tiny boost at 40Hz. The circuit it probably designed to get rid of that boominess in 12 and 15" speaker cabinets.

The bass control is fairly subtle, smooth and "warm", while the mid control seems to effect a very wide spectrum of the mids. Works almost like a volume control, and the treble control is also quite subtle but very very good sounding. Not at all harsh and "squiky" as some other treble controls.

The Ultra-Hi switch is pretty useless, the effect is very subtle and does nothing more special than the treble control itself.

Time will tell how this project turns out. This preamp will probably not go with my on any gigs, too big and I feel a little afraid of breaking it in transit.

All in all, a very educational project. I learned a lot and had fun doing it...


Interested in vacuum tubes (or vacuum valve's, as they are called in the UK)? Check out this interesting article at IEEE Spectrum "The Cool Sound of Tubes".