Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bmccowan on June 14, 2019, 12:00:43 pm
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In the discussion on the
Gibson EH-185, I mentioned the Muzak amp and Brewdude asked if I could share the schematic.
I didn't want to hijack the other thread too much, so started this one.
My response to Brewdude was: It's a Muzak 908B made by Stromberg Carlson.
I pulled it from the shelf to make sure of the model # and it has 4 rather than 5 6SJ7 tubes. (faulty memory)
The only schematic I have is very aged - brown - glued to the base cover. I will try to take a photo of it later.
I will likely redraw the schematic in order to track circuit changes, and will share when I do.
Tube lineup is 4 6SJ7 - 2 6V6 - 5Y3.
It looks like it had an easy life - probably elevator music in a Macy's, eh?
There are a couple of Youtube vids of this model, and an expired Reverb listing for a 908A that I have seen.
I have now drafted a schematic - attached. I left out the tone stack as its upstream of the circuit; the additional high impedance outputs; and an optional resistor for dropping the B+ below 300v.
The EH-185 discussion influenced me to move this amp from the shelf to the bench. All is familiar to me except the use of a pentode as a phase inverter. This will be a trial and error(s) project. I will be trying some of the EH-185 preamp circuit, keeping the 6v6 power amp as is.
There is an excellent chance I have made errors in the schematic, so if you see any suspects...
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V2 has full G2 feed network, but is triode strapped.
It could be they always pentode-wired, but then for this model they wanted a lower-gain stage and added a P-G2 strap rather than re-build. However wired like that ALL the signal would be absorbed in the 0.22u cap. So there's an error somewhere.
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Ah! you are so right. Thanks!
replaced files in initial post
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I have now drafted a schematic - attached. I left out the tone stack as its upstream of the circuit; the additional high impedance outputs; and an optional resistor for dropping the B+ below 300v.
The EH-185 discussion influenced me to move this amp from the shelf to the bench. All is familiar to me except the use of a pentode as a phase inverter. This will be a trial and error(s) project. I will be trying some of the EH-185 preamp circuit, keeping the 6v6 power amp as is.
There is an excellent chance I have made errors in the schematic, so if you see any suspects...
How are you doing with this build @bmccowan?
I'm building this now - I have 2 x 908A chassis and transformers but the rest of the components have been stripped out.
I actually have a 240V version which used EF86 tubes, but couldn't find a schematic, so I'm going to build the 908B that you've made a schematic for.
I'm a bit of a beginner, but would you answer some questions for me?
The original schematic seem to use polarised caps everywhere, but you're using bi-polar in many places. Is there a reason for this?
The original schematic specifies 400V capacitors unless noted otherwise. Do i need to? there are a few small caps (10pF, 50pF) that I'm having trouble finding.
Thanks!
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The original schematic seem to use polarised caps everywhere, but you're using bi-polar in many places. Is there a reason for this?
It's highly unlikely that any cap smaller than 1µF is polarized. Can you show us the original schematic?
The original schematic specifies 400V capacitors unless noted otherwise. Do i need to? there are a few small caps (10pF, 50pF) that I'm having trouble finding.
You should use caps with at least 400v rating (unless noted otherwise). It's perfectly OK to use a 500v or 630v cap. Click the capacitor link at the bottom of this page to find all the caps you need, including the 10pF and 50pF. It's OK to use a 47pf instead of the 50pf.
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Hes21,
I'm happy to provide what info I can, but am confused by your questions. I converted mine to a Gibson EH-185 circuit, worked out great after I moved on from the arcane EH-185 bass control. That build is covered in another thread and Sluckey and PRR helped me figure some stuff out.
But, I do not sense that is what you are looking to do. And is that your thread on Reddit? If I had two of those amps, I would make a pair of monoblocs from them. I bet you could get a nice project that way.
Polarized caps? You are looking at the actual amp, not at a schematic, right? I bet you are confusing the identifying line for the outside foil on the signal caps. Most caps in those days had that line. Most modern signal caps do not. There are ways to figure it out, and they are chronicled here in the forum. I cannot hear the difference, so I do not bother. I just install them with the writing all in the same direction - very scientific. All electrolytic caps in that amp are polarized; cathode bypass and filter caps.
Voltage rating - what Sluckey said.