JMP -1 12AX7 tube voltages.

[rockon1]

Recently picked up a Marshall JMP-1 pre amp. Sounds great,works great! Curiousity got me to checking the two 12AX7 tube voltages. The heater current on both tubes is a bit above 9 volts. The plate voltage on one is around 91 volts and the other about 128 volts. Ive googled it but cant come up with anything. Anyway to determine if this is normal from the schematic? Or perhaps someone knows something about this pre? Thanks Bob

http://www.drtube.com/schematics/mar...jmp1-61-04.pdf

[Enzo]

9V. I bet you measured it from the chassis. Put your meter on pins 4 and 9 and measure. NOT chassis to pin 4 and chassis to pin 9.

if it sounds great, then let's assume whatever the voltages are, they are close enough.

[rockon1]

Originally Posted by Enzo

9V. I bet you measured it from the chassis. Put your meter on pins 4 and 9 and measure. NOT chassis to pin 4 and chassis to pin 9.

if it sounds great, then let's assume whatever the voltages are, they are close enough.

Indeed I did measure from the chassis. More curious than anything else. I was replacing the tubes and treating it with old stock Mullard long plates and figured while I had the top off I'd just check the voltages on them since the heaters glow much dimmer than usual no matter what tubes are in it. I imagine they are running them are lower voltages or something since Ive read that its more SS than tube. No matter though really-it does sound great! Thanks Enzo,Bob

[Gregg]

Page 2 of the schematic says "12V unregulated D.C. to valve heaters".
As far as I know anode voltage should be ~300V. Maybe that's why HT caps are rated at 350V.

[dobrota]

I found somewhere, that HT is only 160V DC. Who knows... I didn't measure it.
But if I see schematic, it's strange - using IC12b (4053 IC) for switching signal from V2 valve after divider 330k/680k. Because of maximal signal for this analog switch is only about +-7.5V before damage and maximal signal in valve input from diode distortion circuit is about +- 3*0.8V and its gain is about from 20 to 30. So it seems there's a mistake in divider values or HT is maybe lower (as in new valvestate series)
Please Rockon1 can You measure HT DC and all plate voltage values for us ? Thanks.