GuitarLab Tube Bass Preamp

[Daveze]

As the astute will have gathered, the above is a Tube based bass guitar preamp, which I recently purchased off the Evilbay (for the princely sum of 1 bargain, shipped). Being reasonably rough 20 years since its creation and having paid only a relatively small amount, I'm most pleased to report that it actually does work. I'm yet to test the balanced outputs but so far the main signal path seems to work well.

While I say 'well', I mean simply that it works. Which has led me to my first steps of further work (and questions in red):
- Replace the crackly pots (Whats a 'C' type pot? Its got what I assume are A and B taper but also a C taper);
- Replace the dodgy power lead (Its currently just got the lead through a gromit, with a seperate fuse, is it worth moving to an IEC connection if I've got the real estate?)
- Shield the signal section from the power section (Whats the best way to do this? I was thinking I could just shield the tube but that won't help other sections of the amp. Or am is it more likely that the buzz from somewhere else?)
- Replace the sockets with something more substantial than the existing plastic ones and rusted XLR's
- Replace the existing balancing transformer with a Jensen (How likely am I to be able to drop in a different transformer and have it just work? Or, how likely is it that one of the Jensen's will drop straight in, what do I have to look for?)
- Replace the existing opamps with something of higher quality. The tops have been filed down but I have a suspicion that 5532/4 are going to be the logical replacements but I'll have to have a look at the circuit to see

Just wondering what people's thoughts are.

Cheers,
Josh D

[Daveze]

Close-up of the left


And of the right

[Nigel Goodwin]

Originally posted by Daveze
[B]As the astute will have gathered, the above is a Tube based bass guitar preamp, which I recently purchased off the Evilbay (for the princely sum of 1 bargain, shipped).

Actually it's an opamp based preamp, with a valve bodged in so they could charge four times what it's worth!.

Can't help on the pots, but they are usually log, anti-log or linear, best thing to do is draw the circuit out and you should be able to work out what they should be.

Power lead? - fit a socket if you want!.

Buzz - find out what's causing it.

Balanced output - try it first and see how good it sounds, or add a more modern electronic balanced output (less prone to hum pickup than a transformer).

Opamps - any crappy old opamp you can find is already far higher quality than the valve you have in the ciruit, so it's really not going to make much difference. But opamps are usually pretty standard connections - you just need to find if they are single or doubles. Again, draw the circuit out - there's not much to it, it should be a pretty easy job.

[wxn]

Overdrive the tube!

[Daveze]

I did try that but it wasn't all that successful, only minimal fuzziness at maximum input gain...

I also managed to get in contact with the fellow that designed it, so we'll see what he can offer.

Nigel:
So we'll call it a tube-containing preamp? Is your statement based on experience with this model, or similar designs? Given the price I paid, I wasn't really expecting class A pure tone, silver wiring and gold plated knobs, I'm actually just pleased that it makes most of the right noises.

On the buzz front, what would cause it to get louder as you turn the master volume down? I've read a smattering tube theory but practical knowledge is limited, any tips on how to go about diagnosing the problem?

Cheers,
Josh Davies

[Artie]

Isn't this the same company that became Labsystems? My info is only hearsay, from other guitarists/bass players, but I was under the impression that this was top-notch, hand-built stuff. Albeit, an old one.

I'd think it would be worth restoring . . . and enjoying.

Artie

[Daveze]

Correct. GuitarLab became Labsystems.

I've actually been in contact with the fellow that designed it back in 1982.

He's offered to service it for me (at a cost, plus parts), an offer I'm increasingly likely to take him up on.

I can't speak for its quality that much, other than the fact that it has had a fairly rough 25 years of service and still [mostly] works. I can probly pick a few problems with the design, heaters are AC straight off the transformer, which I understand can lead to a little more noise. Also, (you can see it in the right photo) one of the resistors in the power section of the tone circuit looks like its had a fair bit of heat through it...Oh, and it has this buzz that gets louder when you turn the master volume down.

Josh D

[Nigel Goodwin]

Originally posted by Artie
My info is only hearsay, from other guitarists/bass players, but I was under the impression that this was top-notch, hand-built stuff. Albeit, an old one.

Certainly from the pictures it's not 'top notch', it looks of cheap non-descript manufacture?. This isn't to say it's not OK, but it looks to only be a cheaply made item?.

Repairing it should be pretty trivial, assuming you can get replacement pots for any that might be duff (and won't clean).

As with any repair job, you need to try and isolate it in to sections, and find out which section the fault is in. Anything like this with buzzing noises, would immediately make me look round the valve section.

If you can get a circuit diagram, that would be very helpful to you, if not, it shouldn't take long to draw out.

[Brett]

So you're who bought it. I looked at bidding, then decided I'd rather do a scratch build instead of fixing up an old product that probably wouldn't do what I wanted anyway.