Low-power tube bass amp

Snfoilhat

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Recently I found an Ampeg B-100R that could have been the very same I had when playing in bands 2000-2010. Missing back panel showing a burned up circuit board, sold for parts/repair $25. I wanted to keep the original chassis, so one major constraint is the clearance for tubes (red arrow), 3 inches. That's enough for any of the 12A_7s and their like, the 6DL5/EL95 (a sort of half-pint EL84 for car stereos), and -- just barely-- the 6BQ5/EL84.

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Laid out some parts. I think the push-push switches on the original are cool and with a little modification these Fender-style slide switches will be a good substitute in the same positions.

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Made some mock-ups of the new control panel, with some Ampeg-flavored choices to make about which controls to build (while not copying exactly the B-100R or B-15 or SB-12 or SVT).

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Modified the chassis to fit the new parts (full-size pots with 3/8" bushings, slide switches, 11/16" pilot).

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The original power transformer fit 2.5" x 2.0" Z-mount cutouts, so a Champ/Princeton sized PT dropped in. I added holes for a fuse holder, Marshall-style capacitor clamps, speaker jack, and three tube sockets with some old parts to check the fit. There's room for one more socket at the far right if need be. The output transformer may hang outside the chassis beneath the cap cans or may go inside the chassis above the speaker jack. Gotta find a scrap of blue checkerboard tolex to replace that missing back panel.
 

King Fan

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You had me at 'blue diamond check!' Plus I've missed seeing your builds. And I want to see any details on how you lay out and make the faceplate...
 

Snfoilhat

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Thanks for the interest, everyone!

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If the chassis deviated from the plan or there wasn't a plan to begin with, then I take some measurements to put into a drawing app. I filled the unneeded holes with JB Weld.

The finalized control panel graphics can go to some kind of professional. This will stay paper soaked in shellac, a sort of homemade sticker, while I figure the circuit out.

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Spending some time messing with these online tools to try to make more informed guesses.
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And here's a rough draft of a circuit that includes the features I'm trying to make. "Warm"? "Growly" but not fizzy/fuzzy? "Smooth"? :D

It's based on a guitar amp I have and like a lot but doesn't have great EQ for bass or cello. I'm hoping to figure something better out by trial and error.
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Snfoilhat

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Wired the primary side of the power transformer and the secondary side for the heaters. Starting to tuck things into place to get an idea of how much space remains.
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I always find this part challenging.

The next set of photos should be when it runs :)
 

Maguchi

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Recently I found an Ampeg B-100R that could have been the very same I had when playing in bands 2000-2010. Missing back panel showing a burned up circuit board, sold for parts/repair $25. I wanted to keep the original chassis, so one major constraint is the clearance for tubes (red arrow), 3 inches. That's enough for any of the 12A_7s and their like, the 6DL5/EL95 (a sort of half-pint EL84 for car stereos), and -- just barely-- the 6BQ5/EL84.

Laid out some parts. I think the push-push switches on the original are cool and with a little modification these Fender-style slide switches will be a good substitute in the same positions.

Made some mock-ups of the new control panel, with some Ampeg-flavored choices to make about which controls to build (while not copying exactly the B-100R or B-15 or SB-12 or SVT).

Modified the chassis to fit the new parts (full-size pots with 3/8" bushings, slide switches, 11/16" pilot).

The original power transformer fit 2.5" x 2.0" Z-mount cutouts, so a Champ/Princeton sized PT dropped in. I added holes for a fuse holder, Marshall-style capacitor clamps, speaker jack, and three tube sockets with some old parts to check the fit. There's room for one more socket at the far right if need be. The output transformer may hang outside the chassis beneath the cap cans or may go inside the chassis above the speaker jack. Gotta find a scrap of blue checkerboard tolex to replace that missing back panel.
Was that originally one of the Ampeg flip top tube bass combos? I had a newer reissue from around the 2000s, think it was called a B-15. 100 watts, 15" speaker. It was a great sounding monster bass amp with an all tube circuit.
 
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Snfoilhat

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@Maguchi, I don't know the history of the bass amp line very well, but I think this B-100R wasn't a solid state version of any of the old classic tube amps. It seems to have been its own thing, complicated preamp, really heavy particle board cab, not especially loud as I remember it, not too expensive. I've never played any of the flip-tops, just some of the SVT amps. Would love to some time.
 

Snfoilhat

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Use a faithful reproduction Champ power transformer and you can't be surprised when you reproduce Champ power supply issues :D

The voltages are higher than I hoped but manageable. The first draft came out like this. I think it sounds pretty good.
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I think shifting the mids just like 150 Hz is too subtle so that'll want some tweaking. There's really nowhere between the input and the power amp that cuts much highs automatically, making the treble control is really powerful across its sweep. The stock speaker seems to have a fair amount of top end too.
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Some matte finish lacquer stuff I found half a can of, didn't build up as deeply as shellac and looks too papery still. Maybe I can find a thin piece of plexiglass
 
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cobaltu

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There's really nowhere between the input and the power amp that cuts much highs automatically, making the treble control is really powerful across its sweep.
I know it might be a bit late for this suggestion, but - a variable high cut later in the circuit would be really useful. Especially on a bass amp.
 

Snfoilhat

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Thanks for the interest. This is the first amp work I've done since moving, and I'm still figuring some things out. I haven't even listened to it turned up past 5 to find out what the overdrive is like or where it tops out volume-wise, which I guess some people would be curious about in a bass amp. I'll try to get a clip together.

With blue checkerboard vinyl hard to find and asking like $50/yd, I opted for a different material for the back cover. I found this in the acrylic scraps at the art supply store -- 1/8" acrylic clad on both sides with a thin sheet of aluminum.
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David Barnett

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Kinda neat to see a cascode in an instrument amp, only other time I've come across that was one of the modules for a Seymour Duncan Convertible amp. A popular circuit in underground hifi circles in the late 1970s, it makes sense in a bass amp where we want clean and low noise.

Some fun things have been done with cascodes, such as replacing the bottom triode with an FET.
 
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printer2

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With that size PT I would have expected a Mosfet PI and another output tube. And you do realize, you could always recess the sockets in a little and easily pull larger tubes. Looking to see a MKII. ;)
 




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