Bassman100 Silverface Blues &

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aintpretty
Junior tube assistant
Posts: 1
(4/9/04 12:25 pm)
  Bassman100 Silverface Blues
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 I Recently acquired a Bassman 100 (70's era Tube amp) thinking that a bassman with 4 6L6s had to be too cool. yeah, Anyway - I re-tubed it with a matched quad of Teslas and it jus gots no punch - it sounds alright - nothing special but has no guts - my bandmaster is 10 times as loud. So I blackfaced it and made the normal channel like an AA864 and now it has even less power. The voltages all are close to the spec on the schematics but the driver is low 200 on pin 1, 67 on Pins 3&8 and 135 on pin 6. This is with a new 12AT7, a 12AU7 gives even lower voltage (154/79/105). before I did the mods the voltages were close to the schematic but it still had no punch. Does anyone have any thoughts??
Thanks.

Edited by: aintpretty at: 4/9/04 12:29 pm
 
Funkalicousgroove
Junior tube assistant
Posts: 46
(4/9/04 1:36 pm)
  Re: Bassman100 Silverface Blues
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 Steve ahola has a few cool mods for these @ www.blueguitar.org bassman 100's are "Ultra linear" and for the most part don't have that nice blackface "Mojo" alot of the tone has to do with the different tf's. Just my two cents for what it's worth. The lead dress is all screwed up on alot of these amps and to keep them from oscillating there are a couple of small value caps (Like 2000pf or so)on the power tube screen grid resistors that rob alot of the tone. www.ampwares.com/ffg/ has a schematic.
 
aintpretty
Junior tube assistant
Posts: 2
(4/9/04 3:04 pm)
  Re: Bassman100 Silverface Blues
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 You might have hit it right on the head! I did take those little guys out of the circuit because they weren't in the AA864 - I'll have to try adding them back - any ideas on how these could be 'tweaked' to restore some tone?
 
Funkalicousgroove
Junior tube assistant
Posts: 49
(4/9/04 5:27 pm)
  Re: Bassman100 Silverface Blues
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 fender (CBS) added these caps to help the noise and oscillation caused by the poor lead dress of these amps, they attenuate the highest frequencies; if the thing doesnt pick up radio or make a bunch of noise without them leave them out. what do you mean by punch? bass response? MID response? High end sparkle? Although I've never tried it, I've heard that changing the treble cap to 500pf,mid and bass caps to .022uf, and the mid pot to B25K really makes them growl and sing
 
aintpretty
Junior tube assistant
Posts: 3
(4/9/04 5:35 pm)
  Re: Bassman100 Silverface Blues
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 BY punch (or lack of it) I meant that with both the channel volume and the master volume dimed, I get volume slightly less than my 64 Vibrochamp and almost no breakup. I'm playing it through two 12" 8 ohm Eminence drivers in parallel.
 
Funkalicousgroove
Junior tube assistant
Posts: 50
(4/9/04 6:03 pm)
  Re: Bassman100 Silverface Blues
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 How are the tubes?? although alot of folks will tell you that you should leave em in if they test OK a tube tester will only run 100 volts or so and your tube amp runs them up to 400, I say change them if they don't sound good. have you confirmed all of your resistor values in the output section? carbon comp's tend toward more resistance. I suggest you check all values against the schematic with a VOM or multimeter and make sure nothing has drifted. Did it work before you removed those caps from the output tubes? if so the amp may just be oscillating. are your speakers good?
 
aintpretty
Junior tube assistant
Posts: 4
(4/9/04 7:19 pm)
  Re: Bassman100 Silverface Blues
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 Good questions all. The amp was not loud when I got it. The speakers work OK with other amps - Ive played the Champ through them and they sound great. I tried the head on another (8 ohm) cab and the results weren't much different. The power tubes are a new matched set of Teslas I tried an older conglomeration during the modifications/testing and the output hasn't changed much. It actually sounds pretty good just no volume and minor breakup. As I said earlier all the voltages seem right except at the driver (12AT7) which are all low. Would poor lead dress cause enough parasitic oscillation to cancel the signal? and would that lower the voltages on the driver? I have some mods from Gerald Webber that move the tone caps to the pots to eliminate the extra wire. I suspect that this may improve tone but may not appreciably affect the volume problem.
 
6G6
Junior tube assistant
Posts: 156
(4/10/04 8:31 am)
  Re: Bassman100 Silverface Blues
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 Have you changed any of the preamp tubes?
You might try moving a 12AX7 into the PI position to see if it drives the power tubes harder.
If that wakes it up, then you are due for a fresh set of preamp tubes.
One thing for sure, it should be WAY louder than your Champ.
 
ganzonimx
Hey get your own solder
Posts: 460
(4/10/04 10:20 am)
  Re: Bassman100 Silverface Blues
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 If it's a 100W Bassman it is not Ultralinear. The 100W Bassman should break up around Vol 4 to 5, the 135W UL stays a little cleaner up to 7.

The voltages of the PI are strange. You should get about 320V on the plates of a 12AT7 as PI with the original specs. If you changed to BF specs they are lower. If the PI does not work proper you wont hear the amp growl. As Funkalicousgroove said, check everything around the PI.

Cris


 
aintpretty
Junior tube assistant
Posts: 5
(4/11/04 7:14 pm)
  Re: Bassman100 Silverface Blues
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 Waaall(Blush), The good advice to check the restistors worked out pretty good. I had a 1M where a 100K should be going to the PI. That made things better but the output is still a bit wimpy. Still looking....
 
gvdrt
Junior tube assistant
Posts: 1
(4/11/04 10:39 pm)
  bassman
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My bassman has a 4 ohm cab. Sounds like your using 8 ohm. Hope this helps.
 
Funkalicousgroove
Junior tube assistant
Posts: 56
(4/12/04 2:53 pm)
  Re: bassman
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I'm guesing most of your problem is in the resistors, those ol' carbon comps drift when they get hot and unfortunately they drift toward more resistance. My good friend Bill taught me of the law of diminishing returns: I always use flame-proof metal film resistors because they are 1% or better tolerance, and DEAD QUIET, to me this out weighs the small improvement in tone and risk of fire you get with carbon comps. This is more for new builds than restoration of vintage pieces, but since a bassman 100 has no Real collectible value, and requires a lot of mods to make it "Sing" I might suggest (And it's just a suggestion before all you c-comp nuts send hate mail ) that you replace the bad ones with Carbon film or metal film.
 
aintpretty
Junior tube assistant
Posts: 6
(4/12/04 3:03 pm)
  Re: bassman
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Thanks! That does sound like good advice.
BTW, over the weekend I scored another Bassman 100 which has been heavily modded by Torres - extra gain stage with volume, effects loop, etc. AND all the 'new' resistors are flame-proof metal film and it sounds great.

Jus' tellin' it like it is...
 
Funkalicousgroove
Junior tube assistant
Posts: 58
(4/12/04 4:31 pm)
  Re: bassman
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Good stuff, the torrez that is, I've heard good things about his amps and mods. I've been wanting to try steve ahola's Hot-rod tweed mod for a SF bassman but I'm so busy building amps for my friends that I never have time to do any for m'self. Keep us posted as to what you come up with.
 
 
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