Gents - All this talk about using ten ohm resistors as fuses leads me to a better idea. How about putting REAL fuses in there! I always fuse the B+in my tube amps. Once the amp is running properly, turn it off, discharge it, pull one B+ lead and insert a DC current meter in series. Then run the amp to near clipping and read the current. Say it's 250 ma. I double it and use a 500ma, Fast-Blo fuse. Don't depend on a resistor to burn out, it's too slow a process. And don't depend on the mains fuse to protect anything either. It's there simply to prevent a fire from starting. The mains fuse has to be chosen to pass the high voltage currents and the heater currents as well. So it may not blow just because one tube red-plates. Repeat the process for the other amp channel if it's stereo.I use 10 ohm 1/2W fusible resistors, they have saved output transformers and 5AR4 when a tube goes runaway like the JJ 7591 likes to do during the first half hour of burn in. After all the resistor is a cheap safety for the rest of the amp..
Given that, a 1/4 watt 10 ohm resistor works better as a fuse
Is this the board you've got? If so, have you done the bias fix? https://aa100restoration.wordpress.com/Any comments on this circuit design? It is from a gentleman that is selling replacement circuit boards for the AA-100. I have purchased and installed the board which is very nicely designed and documented. But in my case I cannot get the bias voltage to come down on 3 of my tubes. They are reading around 48mv with the 10k pot at minimum. He suggested getting the tubes tested which I will do shortly. But I am curious if anyone has an opinion on another possible solution. My tubes are original Daystrom tubes and I would love to keep them. Some notes: the 220k resistors R91,92,85,86 were supplied as 200k. The voltage going into P40 is -17 volts. My power supply is putting out 488vdc.
Once you get the bias supply sorted out you should be able to bias your tubes okay. I also have this board and I can bias my mixed bag of 7591s correctly (after the bias fix).Yes I did the bias fix. He included an instruction sheet for that process. I can adjust the bias upward but cannot get it down to the 33ma recommended.
-17v at P40 isn't enough. That should be at least -20v.
Is the bias rectifier a silicon diode (NOT the selenium rectifier anymore)? If it has not been changed to a silicon diode, then it NEEDS to be.
If the diode has already been replaced, and the 3.3K resistor (R106) is already gone (which seems to be the recommendation as part of the bias mod)- then replace the 8.2K series resistor (R103) in the bias supply, with something like a 6.8K. That should get you up to near -20v at P40. Then, you should be able to adjust the bias to a sensible value (less than 35ma per tube).
If someone is going to make new boards for the output amp- then I would suggest making them work with the 6GH8 for the driver tube, instead of the 7199. This has been done with many amps that used the 7199, with good success. The 6GH8 is pennies on the dollar compared to the 7199, and seems to work just as well.
This is just speculation, but I suspect the participants here would enjoy having access to some sort of universally applicable PCB for this style of power amplifier.What do you guys need?
The first run of PCBs is nearly gone (thank you!). I would be glad to make more AA-100 parts if there are enough people who want them.