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Bassman -- any ideas?

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12/18/1997 11:15 AM

Bassman -- any ideas?

I've just tuned into this site, and after checking around a bit, I'm wondering if anybody can give me some basic direction. I've got a early 70s Bassman, which at the moment is in a combo case with a single 12" JBL, and to tell you the truth, it's never sounded so good (I have run it with 2 12s and it sounds about the same, so I don't think it's the output impedance). Normally I play a Les Paul w. P90s, which sounds quite decent through other amps. Anyway, the tone just sounds kinda flat, without much warmth in the mids, and though it does put out some decent relatively clean lower end, it can get a bit boomy if not set right. Ultimately, what I'd like is a bit more bite in the early Freddie King mode. I'd be interested on suggestions on mods, and can do them, though its been a while, but also some suggestions on rounding up the usual suspects to just get a relatively decent stock tone, as I think this must be a bit below normal. Of course, we are talking the wonder years of CBS here. Thanks.

R L

12/18/1997 12:52 PM

For an early Freddie King sound, I recommend completely rewiring your Bassman as a late-50's Tweed Bassman circuit. You would not need to alter the chassis at all (unless you wanted to add a tube rectifier). There's already 4 preamp tubes (you only need 3), 4 input jacks, and 6 pots (although some of the values will need to be changed). Also, there are more than enough eyelets on the tag (circuit) board to implement the circuit. Another thing: Fender bassman heads come with very good quality transformers.

I would also recommend rewiring the output balance control as a true bias adjust, and also remov (if they are there) the .002 capacitors from control grid to ground on the 6L6 sockets.
Also, replace all original electrolytic caps, including the filter caps. You might also want to replace, rather than reuse) all of the coupling caps.

I promise, if you decide to do this project and you do it competently, you will wind up with a very impressive tone machine.

12/18/1997 5:15 PM
David Axt	Jim. >> There's already 4 preamp tubes (you only need 3)<< If I am not mistaken there is only 2 preamp tubes (12AX7) and 1 phase splitter tube (12AT7) in early 70's silver face bassmen. David Axt

12/18/1997 7:43 PM
Karl Zwengel	Actually, in most circles, at least the ones I travel in, the driver tube is considered part of the pre-amp. Or if you want to be more generic you could say "non-power tubes".

12/19/1997 12:54 PM
Pete	I'll have to take issue with you on this one by asking you one simple question When you buy "preamp" at the store, does the preamp have multiple outputs for single ended or push-pull configurations? No

12/19/1997 3:13 PM

Here I was speaking in terms of the general circuit topologies. I've never used a rack mounted preamp myself so I don't know much about them. As far as I can tell from the advertising, and God knows advertisers NEVER mis-represent products, it seems that these are a totally different beast. Lot's of DSP going on here.

Basically what I was trying to get across was that what tubes are where is a moot point. Here's a neat little example;

Heathkit used to make a stereo power amp that used a 12AX7 for a differential phase splitter. It was then fed into a pair of EL84's in a cathode follower configuration to drive the power tubes from a low impedance source.

It was a really nice circuit but it doesn't exactly fall into the pre-concieved idea of a power amp. So for convienince you just say that the pre-amp stops where the preamp tubes stop. Not real scientific but it works.

That's enough running off at the keyboard for now.

Have a great weekend !
Karl

12/22/1997 12:29 PM

Thanks for your suggestions. Any idea where I could find the Tweed schematic? What are thought to be the caps of choice these days for this work?
I thought I might give you a bit more info on the amp as well, in case you have any further suggestions. It has A53075 stamped on the back, 3 12AX7, 1 12AT7, and 2 6L6GCs. It's pre-amp circuitry seems relatively standard for the Bassmans, though the cap/resister values on the bass input circuits seem a bit different than the various schematics I have. There's no master volume, and the feedback circuit values are 100pf/470k. The power tube circuit is different than any schematic I have for a Fender: it has 150 resisters on the cathode to ground lines, and a cap between the two cathodes. Any idea what that's all about? Would it make sense to begin by going to the more conventional wiring here (direct to ground)?

I suppose it might also make sense to replace the tubes, which are what came in it second-hand back in about 78 or 79 when I bought it. The 6L6s are Sylvanias, the AT is Black Magic, or something like that, and the AXs are just marked ECC83.

One of the other replies suggested that I start by replacing the speaker I have, and I was wondering if you had any thoughts about that.

Thanks again for your help. By the way, any opinion on the current line of Ampeg retro amps?

RL

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