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In Reply to: RE: 211 Bias overload....... posted by 2395 on July 26, 2024 at 02:53:37:
This was less of a problem 20 years ago before 211 prices shot up. Back then a $100 NOS tube that might be lost due to a chance failure every 10 or 20 years wasn't such a big financial injury. Now the NOS tubes (all I use) are almost prohibitively expensive, due in part to a major collecting effort on the part of Audio Note and others.Anyway, there are two issues that I'm aware of regarding fusing the output stage. First is of course the high anode current that results from a simple loss of bias (grid goes to zero volts or floats). The latter might occur if the socket loses contact with the grid pin. I've seen this happen if the curved pins of the socket style in your amp become bent or misaligned. After working with these a long time, I have concluded that the type of socket with a flat spring tab under each tube pin is probably more reliable. The sockets themselves are somewhat fragile and can crack if mishandled, but in normal use the tabs can't lose contact unless they're very dirty. Unfortunately, I sold all those off years ago before I realized this (and before I built my amps). The Johnson sockets in my monos are like yours, and I spent a lot of time testing them for reliable contact by twisting the tubes slightly in various directions to be sure contact couldn't be lost during actual operation. Not all the sockets I tested were OK in this regard, and this ultimately provided the incentive to add a 211 cathode fuse to my amps.
The second area of possible damage is the 211 grid. Excess grid current is a real danger in a direct coupled amplifier where loss of the negative supply might move the grid positive. The simplest solution to this that I know of would be a second small value fuse in series with the grid. I haven't done this, but I did install a fuse in series with the driver tube B+ power supply. In my amp, driver tube current is controlled by a negative voltage applied to the driver tube grid, and this voltage is tapped off the main bias supply. So, if the bias supply goes to zero, the excessive driver current will cause its B+ fuse to open, and this will prevent the application of a positive voltage from the driver cathode to the 211 grid.
The schematic below shows how I fused the output stage of my monos. B+ on the output is +1050V. The purpose of the 82V Zener is to prevent the cathode from floating to a destructively high voltage level if the fuse opens:
Edits: 07/26/24 07/26/24 07/26/24
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Follow Ups
- Output Fuse - Triode_Kingdom 10:30:35 07/26/24 (0)
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