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Grommes C-40 Schematic?

L

labjr

Active Member
Grommes C-40.jpgSomeone brought in an EL84 Stereo Integrated amp. I've never seen this model and can't locate a schematic diagram. There is no model number on the chassis or the front panel. Just "Precision Electronics" on the rear panel. It appears to be identical to this one:


Appears to be similar to other EL84 circuits. Of course I'm never lucky enough to get something that hasn't been altered. It appears a metal oxide resistor was added between the screen grids and the cathode resistor. I presume they were trying to lower the screen grid voltage on the EL84's which are daisy chained together. The resistor which measures 9.4K, overheats when full line voltage is applied and starts to smoke. Not sure what the original resistor value was since the color bands are pretty much burned off.

The screen grid voltage is approximately 250VDC with the resistor in place. So it's trying to dissipate 6.65 Watts which is too much for that resistor. If I remove the resistor, the screen grid voltage jumps up to 326VDC. The plate voltage remains about the same at 370-375VDC. So not much change there.
 
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Probably a mix of lowering the screens, pulling up the cathode, and providing shunt regulation to the screens all in one shot. Its not the worst idea, but needs to have a suitably rated resistor for the job.


not sure how similar this one is, but the 40Pg has a 6973 screen regulator tube. This is a pretty interesting power supply design honestly but not going to side-track this with that if they are not at least mostly the same.

 
Thanks for the link. However, It seems pretty different and it's also mono.

I wonder if the 9.4K resistor replaced an original part is a modification? Because the screens are connected to a power supply node. I haven't measured everything but the 12AX77 plate voltages are also likely affected. Without a schematic, I have no idea what's normal.
 
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