I've run into a snag with my Flot-A-Tone amp restoration. The 5U4G that came in it was a bit worn, according to my tube tester, and, when I installed a NOS 5U4G (not a GB), the B+ shot up about 50V.
It's a 2 x 6L6 amp with a 200 Ohm common cathode resistor. With a fresh 5U4G, it's runn ing 387V on the plates and 339V on the screens (measured from cathode, not ground). Cathode voltage is 27V. That means I'm seeing about 26W plate dissipation on an amp that was apparently made to use metal 6L6s or 5881s.
I'm guessing that part of it has to do with higher AC mains voltages. The high voltage secondary PT winding reads ~370-0-370VAC.
This is one of my least favorite problems in older amp repair--how to tame an amp that's running too hot. Any advice would be appreciated :-)
Would you play with the cathode resistor value? Try to bring down the B+ voltage somehow?
It's a 2 x 6L6 amp with a 200 Ohm common cathode resistor. With a fresh 5U4G, it's runn ing 387V on the plates and 339V on the screens (measured from cathode, not ground). Cathode voltage is 27V. That means I'm seeing about 26W plate dissipation on an amp that was apparently made to use metal 6L6s or 5881s.
I'm guessing that part of it has to do with higher AC mains voltages. The high voltage secondary PT winding reads ~370-0-370VAC.
This is one of my least favorite problems in older amp repair--how to tame an amp that's running too hot. Any advice would be appreciated :-)
Would you play with the cathode resistor value? Try to bring down the B+ voltage somehow?
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