Express Voltages - Please take a look at these

[shaunf]

Hi guys,

I've attached my amp's voltage chart that I've just finished measuring. These measurements are with all tubes installed and amp in "play mode"

Some of the voltages, particularly B+3 - B+5 look too low, as do the preamp tube plate voltages.

Is this a problem, and if so, how would you recommend I go about fixing it? My noob brain tells me that the B+ dropping resistors are dropping B+3 - B+5 too much, and these are then resulting it the lower plate voltages on the preamp tubes.

Are there any other voltages I should be concerned about?

Thanks guys.

Shaun

[rock_mumbles]

The thing that seems odd to me is you have about twice the cathode voltage on V1a as V1b (expected with v1a 2.7k cathode resistor and v1b 1.5k cathode resistor) but the plate voltages are the same ???

[martin manning]

Looks to me like V1a is drawing a bit more current than expected, which will pull its plate voltage down, as well as the others on that end of the power supply. Are its plate and cathode resistors the correct values? Note that the B+ voltages on the Express schematic are 395, 380, 295, 282, and 267; closer to what you have. Try another tube in the V1 socket, or swapping the preamp tubes around and see what effect that has.

FWIW, just so you can see what variation there might be, my voltages using rj's Pacific transformers are:

Line 120vac
B+ 390, 375, 308, 290, 274

V1b 1.47, 175 (first triode in the signal path)
V1a 1.88, 203
V2a 3.07, 257
V3b 48.4, 223 (PI bias and tail resistors are non-std)
V3a 48.4, 232
V4 -30.7, 369, 388
V5 -30.7, 369, 389