Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: oogabooga23 on March 22, 2023, 04:48:09 pm
-
I got me one of these at an antique store for roughly $30, and decided I would convert this into a guitar amp for funs. I switched out one of the speaker outputs with a 1/4 jack, and I hear crackling and such when I turn the knobs, so I assume the OT is working, but I am getting no sound from the input jacks. All the tubes seem to be working; I switched out the 6AQ5s since two were shorted and I also switched out the 5Y3GT and 12AX7s. My question is mainly about the 5879s: do I need to keep this in the circuit to work? I read somewhere that it was not needed, but I am unsure. I looked at the schematic and it seems to be bypassed to the 12AX7? Any help is appreciated.
-
So I realized I was reading a connection wrong (there was no connection), so the input goes to the grid of the 12AX7. I still have no sound, however, but I also have not replaced the caps yet. Could a leaky capacitor cause the input to not reach?
-
There is no power transformer on the schematic you posted.
-
That's odd, there is one. I was not attached to the chassis, however. I was instead attached to the top side of the cabinet it came in, with long leads that are covered. It seems it plugs into a socket to supply power.
-
I see, the power transformer is the socket right after the ON switch.
-
I'm picking one of these things up on Monday and will likely rewire the 5879 to take an EF86. There's a mess of input circuitry that I'd get rid of and instead use a single 1/4" jack. For these kinds of conversions a 15k grid stopper into the EF86 along with a 250pF cap and 1 Meg resistor to ground will make it friendlier for guitar... I'll try AC15 values for the plate load and coupling cap to the next stage. I'd also remove the existing tone network and keep things similar to the AC15. Rebuilding the power section with fresh filter caps will insure those aren't part of the problem... it will invariably reduce hum associated with 50-60-70 year old electrolytic caps! The transformers in these projectors are normally amazing but streamlining the preamp and cleaning up the B+ will make a huge difference.
These 202's are an oddball version because they had the ability to record a signal to a thin magnetic stripe on the film. This required extra circuitry but it means there's already a hole for a pilot light drilled into the face plate :)... Having the power transformer connected via an umbilical cable is also unique to this filmosound as most others were bolted to the chassis. I might think about rearranging things to make this possible as it will make it the amp more portable... either that or I'll build it into a B&H speaker cabinet that I picked up a while ago...
-
Try the 5879 out before you spend the time/money switching to ef86. 5879 is a great sounding tube!