MC30 schematic question

[cshumphrey]

FWIW, what I always felt was the best way to address the higher filament voltages due to the modern higher mains voltage from the era when these amplifiers were designed, was to simply change the power transformer`s factory default 117 volt tap to the alternate 126 V. tap that is provided to deal with higher mains voltage these amps might be connected to.

Easily done on the 30ties and their higher powered MC-60 brother, and by doing so on mine and every one of these 2 models that have come across my benches since 1975, and doing so have always provided very close to all the schematic`s factory`s voltage values, and the amps seem to appear to operate cooler by doing so, and avoids the insertion of selected low ohm "under the hood" waste heat generating 5 watt or higher sand resistors in the filament circuit feed off the power transformer.

Just a suggestive thought.

I did use the 126V tap at first, but the B+ voltage on the tubes was low--maybe 415V if I remember correctly. I liked the sound better with the higher voltages on the 6L6s that came with the 117V tap (about 430V, which is still below spec). As PRWW stated, It's probably not a big deal to run the filaments at 6.5V, so maybe I just leave well enough alone at this point.

On another note, the amp chassis is cooler with the choke. The choke doesn't seem to heat up at all, as opposed to the 150 ohm resistor that is stock for the power supply, which really puts off quite a bit of heat. The voltages on the tubes are identical with the choke vs the resistor. Bass response seems more pleasing with the choke, but that is subjective, of course. So, maybe the lower temperature inside the amp will allow the other components to last longer. This is speculative for sure.

Appreciate your comment, Bill.

 

[Bill Ferris]

Thanks for the suggestion. I did use the 126V tap at first, but the B+ voltage on the tubes was low--maybe 415V if I remember correctly. I liked the sound better with the higher voltages on the 6L6s that came with the 117V tap. As PRWW stated, It's probably not a big deal to run the filaments at 6.5V.

On another note, the amp runs cooler with the choke. The choke doesn't seem to heat up at all, as opposed to the 150 ohm resistor that is stock for the power supply, which really puts off quite a bit of heat. The voltages on the tubes are identical with the choke vs the resistor. Bass response seems more pleasing with the choke, but that is subjective, of course. So, maybe the lower temperature inside the amp will allow the other components to last longer. This is speculative for sure.

My incoming line voltages traditionally run on the higher side (closer to 125 V more often than not) so I felt more comfortable selecting the 126 V tap, even though the sunroom`s 30ties & the living room`s center channel MC-60 are AC powered via HQ surge/spike protected 20 amp rated AC line voltage regulators.

Any time an electrical/electronic device can be internally configured and sited in such a way that they will operate at cooler temperatures, they will undoubtably preform better and last longer, all other things effecting them remain the same, from my Opinion & yrs. of electrical/electronic experience.