Masco MA-25 power supply question

[luthierwnc]

Hi All,  This really builds off a thread from a couple years ago: https://el34world.com/Forum/index.php?topic=24930.msg269548#msg269548  and https://el34world.com/Forum/index.php?topic=21115.msg223742#msg223742

I am rehabbing one of these amps.  It is the version with two 7B4 input tubes and two 7N7s feeding a pair of 6L6s.   I am getting 425VDC at the rectifier and 412VDC after the choke.  After the 5k screen resistor the volts drop to 240.  The inverter plates see 70 and 55 volts respectively and the preamp plates come in at 95.  I did replace the 10k voltage divider at the bottom of the screen node with a 25 watt aluminum unit that bolts to the chassis.  It is warm but not real hot.  Both the screen and cathode resistors are original and measure 4k8 and 210R respectively.  I did use series 22uf caps on the first two nodes with 100k resistors.  All other values are stock.

From the schematic in the second thread I should have another 100 volts on the screens.  5k between the choke and screens seems reasonable for an LC circuit so I'm guessing that 10k resistor on the bottom of the voltage divider needs to come up. 

FWIW, the amp actually sounds fairly good but volume is much less than it ought to be so I'm hoping those preamp tubes are doing OK.  I did test it with a known-good pair of TAD small-bottles and they sounded about the same as the RCAs that came with it.
Any thoughts on making this more competitive in the 25-watt weight-class?  Thanks, as always, Skip

[PRR]

....From the schematic in the second thread I should have another 100 volts on the screens.  ....

I believe that is wrong. Yes, remove the 10K, use 6L6GC, and see what happens.

[luthierwnc]

Thanks PRR.  I was pretty deliberate in removing components and didn't recall seeing anything of the mass needed to dump that much juice to ground.  The amp wasn't operational.  It came with four 7N7s in the loctal sockets so someone must have just thrown whatever they had in the spare-box to make it look complete.  One power tube is a Sylvania and the other is an identical-looking Westinghouse glass with black plates.  The 5U4 and all the loctals are Sylvania as well. 

I'll pull the bleeder and report back.  Cheers, sh

[luthierwnc]

That did it.  Grounds for divorce when cranked.  I need to tame it down a little but the voltages are good with 420 on the 5U4, 412 on the OT, 365 on the screens and an assortment on the preamp grids.  As always. thanks for everyone who looked and best with your projects.  I haven't always been a big fan of 15s but this might like an open-back P15N or something in that family.  sh

[luthierwnc]

Nutha question on power:  Using the same schematic values I'm getting 415VDC on the B+, 404 on the plates, 322 on the screens, 216 on the fourth node and 176 on the fifth.  All of the 7N7s are running about 70VDC on the plates.  I got four with the amp.  One was bad but the others are consistent.  70 volts seems pretty low and the amp distorts sooner than I think it should.  Any more ideas on the rail?
Thanks amigos, Skip

[PRR]

> the amp distorts sooner than I think it should.

It is a weak microphone amp, not a hot guitar amp. They quit using that type input stage for guitars 60+ years ago. Wire Fender-like grid and cathode resistors.