Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: gtrbryan on September 29, 2013, 01:40:38 pm
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Someone gave me an old Bell and Howell amp/speaker. This was an amp and speaker for an old projector.
It appears that the signal goes through a 50M volume pot, then into half of a 6SL7, then into the other half of the 6SL7 where it splits into the 6L6 power tubes. I'm guessing that half a 6SL7 isn't enough to drive a guitar signal. What can I do with this? Would it be possible to drive this with a "preamp out" on my champ clone? I would gut it and make it a 6V6 combo, but the chassis is tiny and I would have a hard time getting another tube in there. Any thoughts appreciated.
Schematic and Pic attached for reference.
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I've looked at "amping" those a number of times. You're right, though, the chassis is inordinately tight. I couldn't see a way to do anything with the existing sheet metal that wouldn't be lots more work than starting from scratch.
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there's a very long thread i saw somewhere for b&h conversions.mostly the ones that came out of the projectors.Atmars conversions sounds awesome.But it is a small space to work in.Yours is the highest output they made at 25 watts i think.Very nice freebie and worth a good bit as is.Run a boost pedal into it and see what you get.
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read the desription
https://www.youtube.com/watch?v=GKywcYjIHsU (https://www.youtube.com/watch?v=GKywcYjIHsU)
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It appears that the signal goes through a 50M volume pot
This will probably be a 50k pot. The schematic also shows "100M" resistors which will be 100k. Components and schematics of that vintage used "M" as a 1000 times multiplier, unlike current standards where M is a million multiplier.
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It's just the power amp.
It is nearly-fine JUST the way it is. You might up the 22K NFB resistor to 47K, 100K, or open, for a less-tame more-raw sound.
Power output about 12 Watts. Replace the 5Z4 with 1N4007 diodes for about 20% more power.
Output seems to be 16 ohms, a less-common value. However the speaker seems to have an extension speaker jack, so impedance is probably not fussy. 8 ohms sure won't hurt it, though power may drop some.
You need a preamp. One Pentode no tone control, or two 12AX7 stages with tone control.
You can tap preamp power from this power supply. The B+ load sure won't hurt. The added 6V*0.3A or 2W heater load is about 0.25W added heat in the PT, which won't do the PT much harm.
If the 50K input pot offends you (is scratchy, loads your preamp, etc), rip-out and replace with 250K.
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See, this is a project that just screams, "Give me a decibel meter (http://www.el34world.com/Forum/index.php?topic=16097.0) to snack on!"
As in pair up an unloved piece of test gear (repurposed as a preamp with a regulated power supply) with a projector amplifier in need of some pre-amplification...
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Just for grins I would breadboard an FET preamp. What have you got to lose? Plug it up, see how it sounds. Cost you a couple of bucks. There are plenty of schematics out there. Ya, I know, its sand, a really horrible thing, but you'd be surprised how good they can sound especially when driving tubes. Plus you'd have fun.
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I built a 5e3 into one of these cabinets.
I had to rout the opening to make chassis fit.
I think I used a hammond chassis but I don't recall which one.
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I think a LND150 FET might be in order. Runs on high voltage, enough gain, even sounds not bad mildly overdriven.
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All interesting ideas. I think I'm going to test it and leave it basically as is, and run some preamp into it and do some experimentation. I've got all the stuff for the JFET booster lying around I'm sure, leftover from pedal builds. Also, thinking about a Octal Special 6SJ7 preamp might hit the spot. I'll keep everyone posted.