• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

CJ MV55 questions and help.!

Hi there,

I have a MV-55 that I would love some help with; first thing is how can I lower the plate voltage so I can safely run Tung Sol 7581A’s ideally I would like to install a switch that would toggle between the two voltages, any suggestions on how to do this would be greatly appreciated…. Secondly would there be any other modifications required for the bias circuit if the above is doable?

GNFB, I’m struggling to find any information about the circuit and topology, can anyone tell me what level of NFB (if any) the amp has? I’ve obtained a schematic however it seems hand drawn and barely readable..!!

Output seems to be set at 4ohm, there are 2 conductors each (hot and cold) form the opt to binding post and one additional ground wire from the star point on the PCB to the cold BP terminal, also one conductor from the hot BP terminal to the PCB close to the front most bias pots..?? I would like to switch between 4 & 8 ohm if that’s possible with a modification?

Any help in understanding how this amplifier works would be appreciated.

Cheers.!
 
Moderator
Joined 2011
It would be difficult to lower the B+ voltage. Using a Variac would lower all of the filament voltages
and cause the voltage regulator to drop out. There are unknown taps on the dual primary windings
to better adjust the operation to the local line voltage.

There is no information in the schematic about changing the output connections.
If you connect the two secondaries in series, that would be suitable for 16 ohms.

Using the output as supplied would work ok for 8 ohms, with somewhat less power, but lower distortion.
Would not recommend changing the standard Dynaco type NFB circuit from stock.
 

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