Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: NSB_Chris on April 15, 2019, 08:17:54 pm
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Hello All! I have been planning a JMP 2204 clone build for some time and got a lot of crucial advice from this forum so I am positing the results. This was my most ambitious build so far. For the first time, I used relay switching, made the chassis from sheet metal, and built and telexed a cabinet. I tried to be true to the main circuit with only a few added features:
- Channel switching on the volume and master pots to have two gain settings
- I saw a lot of mods to tame the brightness by making a resistor and bright cap change between the first and second gain stages so I put that on a switch
- I put a switch in to change the second gain stage cathode resistor (Soldano setting)
- Because I can’t always get the power amp up into the sweet spot, the master ends up being just off zero. I switched in a resistor before the master volume in an attempt to provide a little more range for the master volume.
The channel switching does not alter the original circuit, but any of the other mods can be defeated so the original circuit is preserved.
I worked really hard on this amp for a long time and am really happy that it was not an epic fail. The added complexity of the mods I added made the circuit a bit complicated. As I was building it, I kept finding little oversights that increased my fear that there were undiscovered mistakes. I was so relieved when the amp made it through the start procedure! I decided not to post until I confirmed that I had not messed it up.
I am posting this on both of the forums that helped me on this amp and will add some pictures and other info on this in case others are interested. THANK YOU to those that gave me input! It really helped with the amp and to increase my understanding!
Chris
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The first pic is the rough cabinet before applying tolex
The second and third pictures are gut shots after wiring up the components.
The forth picture is with the chassis mounted, but without the front plate mounted.
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Well done, that looks nice :thumbsup:
I think a schematic and soundclips are called for :icon_biggrin:
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Here are the schematics. I will have to work on a recording of the amp.
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Attached are the measured voltages after setting bias to about 70% dissipation.
I used the PT with multiple secondary taps and used the lowest (560V) tap. The voltages are on the high side, but much closer to the target voltages than I have had on previous amps. Heater tap was at 6.61VAC.