Checking pin six will only tell you the negative bias voltage for the tube, which is useful but not sufficient (it ought to be around what the schematic shows, but the schematic target is approximate: -19V. It should be in the ballpark, but this is not meant to be exact). Depending on the condition of each tube, the actual tube current draw, which IS the most important thing, will vary, so you need to determine the actual current draw (cathode current) of each tube. Here's how:
If your unit is stock, there is a 10 ohm 5W resistor between the cathodes (the number 5 pins) and chassis ground for each pair of 7591s. The actual resistors are located between the output transformers underneath. To determine the tube cathode current draw, measure at the hot end (non-ground) of the resistor and divide that voltage by 10 to determine mA for each PAIR of tubes. Divide that by 2 and you have the average current draw for each tube in the pair. As I recall, the resistors are wired the same as the tubes when viewed from underneath--that is, the left resistor is for the left hand 7591 pair, and the right for the right pair (double check the wiring to verify).
Before setting the bias levels, warm up the amp with no signal applied and set the DC balance for each pair. The simplest way is to to set your DMM for a low voltage range (1-V unless it's auto-range) and hook one lead to the plate pin 3 of one tube in a pair and one lead to it's companion plate pin 3 and adjust the DC balance control for that pair for a difference of zero volts (as close as you can get). BE CAREFUL--you're measuring at the tube plates, which are operating over 400VDC here and you don't want to get that across your body! Carefully use clip leads to connect so you're not trying to juggle two probes and the bias pot adjustment at the same time.
Then, set the bias level for the pair using the bias level control for each pair, again with no signal applied to the amp. A good target for 7591 cathode current per tube in this amp would be 35mA per tube, or 70 mA per pair. To set this, you would measure across the 10 ohm resistor for the pair and adjust the bias level for 700 mV for the pair. Divide that by 10 and you have the current for the pair via Ohm's Law: 0.700V / 10 Ohms = 0.07A, or 70 mA. Divided by 2, that's 35mA for each tube.
The bias settings are interactive, so you may need to go back and forth a couple times to get the tubes set where you want them, as one adjustment can affect the other and you are 'tuning it in' with successive measurements.
A 35mA cathode current draw less about 3mA for screen current draw would give an idle plate dissipation of 0.032A x 440V plate voltage = 14.1W, or about 74% of the maximum spec anode (plate) dissipation rating for the 7591 of 19W.
Again, take care when measuring and setting the bias in this way to avoid a painful and possibly fatal shock!
Dave
