A couple weeks ago one of the 12AT7s in my SRX Plus failed, and in troubleshooting it eventually took out both input sections. On analyzing the failure it appears that I may have been just a bit too much of a cheapskate. Therefore I have slightly revised the circuit so that at least a failure won’t take out both channels, and hopefully won’t even take out one.
The revision is pretty straightforward. Rather than supplying both input current source tails with a single resistor from B-, I substituted a 220k/500V rated resistor (270k/500V if you are using the original 300k plate resistors) from the B- rail to the input tail current source of each channel (which was previously marked on the schematic as being connected to -20v).
The revised schematic is shown (ignore the +/-340 volt numbers, they should be +/-325 v). This resistor takes the place of the 10k and100k resistor chain in the shunt power supply that ran from B- to -C. This means that C- in my original power supply is no longer connected to anything, and the only connections between the high voltage power supply and amp boards are B+, B- and ground.
For those using my shunt regulated current supply, you have to make a couple changes on the amp board. First, you have to cut the connection between the -20 volt terminal and one of the 100 ohm sense resistors, and then you have to cut the connection between that 100 ohm resistor and the other 100 ohm sense resistor. Then you have to connect each of those two sense resistors to a 220k resistor, which then runs to the B- rail – probably the closest place to do this on the amp board is the connecting point for the 100 ohm sense resistors on the output tail current sources – a bit of a kludge but it will work.
In addition, I substituted a 10M90S in place of the upper DN2540 on the input current source. This modestly improves its performance, but more importantly should prevent the input current source from failing due to overvoltage.
This modification does not need to be done for those of you who built the SRX Plus circuit using a Gilmore regulated PS, as that supplies the -15 volts that can be used for the input current sources.