Valco Early Model 1215 Voltages

[W5FH]

Hello-
I have been studying the (attached) schematic for the early version Valco 1215. Working on a clone build and looking at biasing of some of the preamp stages. Am having trouble with estimates of voltages at B+ nodes of original Valco design. There are no voltages shown on schematic. Assuming the plate voltage rating of 6L6-G's is not exceeded I estimated a B+ starting voltage of 370 vdc with all tubes idling to use in my calculations.
      I used these estimated idling current draws from stages:
6L6-G plates = 88 mA
6L6-G screens = 2.5 mA (10K dropping resistor)
6SN7 P/I = 1 mA
6SL7 Active Tone = .5 mA
6SN7 Active Mixer = 1 mA
6SJ7 Mic Amp = .4 mA
6SJ7 Instr Amp = .3 mA
     Working my way down the B+ nodes starting with 370 vdc upstream of 10K/10w dropping resistor I get these voltage drops:
57 volt drop (5.7 mA total) through 10K/10 watt resistor for 313 v at 6L6-G screens
32 volt drop (3.2 mA total) through 10K/1 watt resistor for 281 v at 6SN7 phase inverter
132 volt drop (2.3 mA total) through (10K/1 watt and 50K/0.5 watt resistors) for 149 v at 6SL7 active tone amp
34 volt drop (1.7 mA total) through 20K/0.5 watt resistor for 115 v at each 6SJ7 input and 6SN7 active mixer.
     If anyone has records of voltages at nodes in this amp I would surely appreciate it. Also, if anyone will scrutinize my calculations I would appreciate that too. I would like to nail the voltages down a little better before deciding on power transformer.
Thanks,
Byron W5FH

[bmccowan]

Hey Byron,
Have you looked at this thread? https://el34world.com/Forum/index.php?topic=25082.0 The poster was determining PT spec for the same amp. A bunch of smart people jumped in, and then there was me.
I think you are being far more analytical than Valco ever was. You will see that echoed in that thread. I have repaired 3 old Valcos from that time period. I looked through my files and found that I did not record full voltage sets on any of them. I did record a B+ of 307 at the first node of a 1201 National Sportsman using a 5Y3 rectifier. Its a 2x6v6 amp. Older 6L6 Valcos often have B+ voltage below 400 IME, so you are likely in the right ballpark.
Magnetic Components/Classic Tone used to make PTs that were perfect for these amps. But they went out of business. I would think that selecting Hammond with somewhat lower spec than a Fender 2x6L6 will work out. I know...not very scientific, eh?
There is a You Tuber named Leon C who has a collection of nice old Valcos, including a 1215. You may want to ring him up.

[W5FH]

Thanks for info bmccowan-
     Yes, I discovered and studied that 2019 thread in my recent research on the amp. Leon C said he did not own this particular amp.
     My plans with this build is once the amp is up and running then to experiment with re-biasing of the 6SJ7's and 6SN7 common anode mixer. I thought it would be good to know the original voltages at B+ nodes as it appears these tubes are biased for lower idling currents than seen in designs today. I am sure I will need to lower some of the B+ dropping resistors if I rebias the 6SJ7's and 6SN7 mixer. Besides the sound from pentode input tubes I have interest in the active treble and bass circuits of the 6SL7 stage; am wishing to know the B+ supply voltage to this stage. Also curious about the anode load resistors of 6SL7 - the treble side uses split load 100K resistors to tap off the treble signal. The bass triode has a whopping 500K anode load resistor. Reducing gain at treble frequencies and increasing gain at bass frequencies?? Just wanting to understand the design of these two triode stages. I would imagine you don't want to overdrive this 6SL7 tone amp to the point of collapse of the feedback loops.
    My plans are to use a larger higher quality output transformer and old USA stock 5881's so I am not going for the complete original sound; don't want the output transformer saturation but do want the character of the old octal preamp tubes. Staying with cathode bias of output tubes.
Byron W5FH