I'm glad you enjoyed the photos and post!
As far as the Spyder Valve goes, I have a MkII HD100 head. I recently repaired it (bad tube) and it seems to work flawlessly but I haven't made time to play through it. I want to take it out to a local jam I am a regular at to try it for real but I have to get "checked out" on it first!
I set the bias on the HT40 using a "bias probe" tool and measured the cathode current directly with that. There ARE several test points on the PC board in the area of the bias pots that must be the ones used to check bias and balance. However, I was in a hurry and didn't try biasing the amp with only a meter using these points:
This photo was taken from the rear of the amp. The trimpot closest you (most rearward) in the photo is labeled PR1 and this is the Bias Balance adjustment. The trimpot towards the top of the photo (most forward) is labeled PR2 and this is the actual bias adjustment. There are 3 Test Points (pads on the PCB) to the left of the pots in the photo labelled TP10, TP13 and TP18. TP10 (connected to one end of the 1-ohm resistor) is the "Total Bias" measurement point. Measuring mV DC, each mV represents 1mA of cathode current.
Let's say you wanted to set each EL34 to 30mA of bias current. with your meter set to measure millivolts (mV), you would put your black meter lead on chassis ground and the red lead on TP10. Adjust trimpot PR2 to read 60mV (which represents 60mA, the total bias for both tubes). I BELIEVE (not sure) that TP13 and TP18 may be individual tube measurements and trimpot PR1 is adjusted to make these two test point measurements identical. Then you touch up the Bias then re-balance. I didn't use all of these test points so I am not sure how close I am. Remember, you MUST insert a 1/4" Phone plug or guitar cable into the input jack (and put the Standby switch in the "operate" position) to turn the tubes "On" so you can measure and set the bias. Turn the Master volume control all the down to prevent stray signals and noise from influencing your measurement.
I found the factory setting was about 25mA per tube (50mA total) and I only increased this a little bit. The amp might sound a little better at 30 or 35mA but I thought I would leave it in a safer "colder" range for now.
I am not sure if I was being asked for control settings or the modifications I did to the amp.
The Amp: It is a 1984 Marshall 4104 50W 2x12 JCM800 combo. I bought it brand new in 1985 and modified the amp while it was less than a month old!
The Mods: I did 3 things to make the amp more usable for me in live music situations. I installed a small PC board with 2 DPDT relays on it. Each relay is switched by its own footswitch making connections through 2 1/4" phone jacks mounted on the rear chassis into which I plugged a dual footswitch (Roland) that had 2 1/4" mono Phone plugs on the end of the cable.
Relay #1 switches between 2 different Gain controls and two different Master Volume controls. I added an extra set to the front panel. THis is like a channel switch. I can preset each gain and its matching Master volume control differently then footswitch between them.
Relay #2 and footswitch takes a 25uF electrolytic bypass cap I added across the cathode resistor of the first input tube stage and switches it in and out of the circuit. This increases the gain (this acts as a foot-switchable boost) and makes the following stages louder and/or more over-driven.
The settings: Whatever I need and whatever sounds the best.
Yeah, I know it's a classic amp and you shouldn't mod them blah,blah,blah.... but I modified it to my tastes after I bought it brand new BEFORE it was a classic. My mods are classic as well!
Gotta run and get set up for the gig tonight! (Portland International Raceway in Portland Oregon, tonight 5:00pm to 8:30pm if you are in the area!)