Sure Marcel, I am by no means an expert. When I saw the bias locations I figured it had to be pretty easy. I have biased an amp or two in the past. Here is the steps I used in biasing  the DT25 Head:

1. Remove the upper panel on the back. It is six screws and then you might have to gently use a screw driver or some other object to pull the panel out. The tolex is sticky.

2. Remove the 4 screws on the bottom.

3. You have to use some force here but be careful and gently wiggle the amp out of the chasis. You might want to remove the tubes but it isn't required. Push from the front and pull from the back. Again the sticky tolex makes it difficult but it will finally come out. 

4. You will see round holes identified as Bias V2, Bias V3, Ground and Adjust Bias V2 and V3

5. Set your multimeter to DCV 200m or volts 200m

6. This is where I had to be creative. My multimeter tips were not long enough to protrude through the hole and touch the measurement point so 1 cut the tips off and stripped the wire back so about an inch was exposed. I then was able to stick the wire into the measurement hole. You will see there is a small goldish color hole down inside the hole cutout that you can stick the wire in. I stuck a black in the groud and red in the V2 and powered on the head. I let it warm up and then after making sure I had a speaker load and the other end hooked to my guitar I took it out of standby.

7. Make sure you are in AB mode and you should get a reading close to 25.0. If it's off, turn the adjustment screw with a small screwdriver until it is at 25 +- 3. I then put it in standby and moved the red lead to the V3 side and did the same.

So here is where are questions I have. The stock tubes were close to same voltage and within spec at around 25. I stuck in a matched set of Groove Tubes and it read around 29 on one side and 33 on the other. I adjusted them so one side was around 28 and the other 25. I then tried a couple other sets of matched EL84 tubes and had similar readings as the stock tubes. The tubes sound in my opinion was better the higer the voltage. Anyway for the questions:

1. Is it normal for matched tubes to be a couple volts off from one another?

2. Is biasing it up around 35 enough to cause amp damage?

3. I read Class A is Cathode and AB is Fixed. Does this mean Fixed is more sensitive to the bias readings?

4. I swear my amp is louder in Class A mode but Class A is supposed to be 10watts and AB 25 Watts. Am I missing something?

5. What are the advantages of running Class A over AB or vise versa.

Thanks everyone!

Hey

I've just switched out the stock EL84s for JJ-Tesla cryo EL84s (a matched pair from Watford valves, UK). This was done to try and solve a "crackle and pop" problem which I deduced was probably due to the poweramp tubes.

The crackling problem is completely resolved so it seems the problem was indeed faulty stock poweramp valves (after approx. 7months use).

However, I'm new to tube amps and am now wondering if I should have the amp re-biased.... I don't think I'd want to venture this far myself so would probably pay for an amp tech to do it.

Is this really necessary and am I putting the amp and/or tubes at risk if I don't have it done? The amp sounds great following the tube swap.

Any advice would be much appreciated.

Thanks.