Hello phantum,
thanks for your advices!
I've done the tube change and re-biasing recently by myself and I'm still alive!!!
What I've used:
Two DMM's, two bias probe adapters, safety leads with crocodile clamps, a safety philips screwdriver, lots of amp tech literature and forum research, Ohm's law and a calculator.
How I've done it:
[THIS IS NO HOW-TO-GUIDE OR ANY OFFICIAL ADVICE!!! THIS IS JUST A DESCRIPTION ABOUT HOW I'VE DONE IT AND WHAT HAPPENED. I DO NOT GUARANTEE FOR THIS AS BEING SAFETY OR WILL TAKE ANY RESPONSIBILITY FOR THIS!]
- After last playing I turned it off directly (without activating Standby) to drain caps fast. When the tubes were cold I removed every cables and the chassis.
- On my cleaned up workbench I first checked voltage between pin 3 (red lead) and the chassis (black lead) with the DMM (auto range VDC) - it was already around 0-1 V. I removed all tubes and put the new ones in.
- I reconnected one speaker, a cable to the input (to deactivate bypass), connected the leads of the DMM again to pin 3 and chassis, connected the power cable, checked EVERY connection twice to be secure and only then turned power on. The voltage went to 529VDC.
- After some minutes of warming up (and checking that the tubes for non glowing plates) I deactivated Standby and the Voltage dropped to around 508-510VDC (later I used this value to calculate the 50% and 70% values of the tubes).
- Again I directly turned of power and watched the caps draining on the DMM. It took a while until it was again between 0-1V and I removed the power cable for security.
- With the measured value of the plate voltage I calculated 50% (25Watt x 0,5 / 510VDC) and 70% (25Watt x 0,7 / 510VDC) as my biasing range an decided to go a little more to the hotter side around 64-66% what would be around 31-32mA.
- After the tubes were cold again I installed the bias probes for the power tubes, connected them to the DMM's and removed the leads from pin3 and the chassis before I reconnected the power cable.
- I switched the DMM's again to VDC (autorange in my case) and turned power on. In this case there is no voltage in standby where the bias probe measures (OF COURSE THERE WILL BE HIGH VOLTAGE IN THE CIRCUIT!!!).
- After deactivating Standby I've measured Voltages around 30mV (what is equal to mA because of the circuit of the used bias probes) on both sides. In the first minutes the values fluctuate a lot around a half mV/mA until it gets more stable. First I lowered the value a bit to see what happens over time and visibly checked the power tubes from time to time.
- Then I began to higher the value in small steps. This is not as easy as the bias pot range is VERY sensitive!!! When values get stable I've also adjusted the balance pot slightly. After every change I checked also the sound and humming by dialing in Volume and Tone controls of channel 1. I ended up at 32mV/mA after a while and let it settle over about 30 minutes.
- Again I turned the power off directly, removed the power cable and waited some minutes before I checked voltage between pin3 and chassis again to be safe before removing the bias probes.
- I reinstalled the chassis into the amp housing and that's it...
Have I forgotten something or calculated wrong?
I'm very satisfied with the sound right now!
One thing I've recognized afterwards (or maybe never checked before): the amp housing is warming up very high in the area where OD1/OD2 EQ is (especially the chassis screw there). It's not in a "melting" area but still very very warm. Do you recognize this on your amps too?