I have, in my infinite wisdom (please dont tell my wife), acquired 2 Counterpoint SA-12s. One has a blown mosfet in the left channel, the other has a blown mosfet in the right channel. Is it nuts to think about running the amps as one channel amps. Since the rail fuses and gate resistors are blown or fried, one channel is inoperative. I am assuming each channel produces approximately 40 watts. Should I also remove the 6 DJ8s from the inoperative channel? and the output fuse (s). Tell me why I should not, must not, can not do this?
You can run only one channel per amplifier, but insert shorting plugs in the unused inputs.
If there are separate rail fuses for each channel, remove the fuses in the unused channels.
It appears you can remove the unused tubes also, though the B+ will rise some.
If there are separate rail fuses for each channel, remove the fuses in the unused channels.
It appears you can remove the unused tubes also, though the B+ will rise some.
Not at all. All the channels in these amps worked but i did exactly what you suggest — i did remove the extra channel.
https://www.t-linespeakers.org/FALL/Q303.html
dave
https://www.t-linespeakers.org/FALL/Q303.html
dave
There is zero reason to limp along with a blown channel. You can modify the SA-12's to take regular lateral MOSFETs from Profusion https://gb.profusion.uk/uk/ecf10n20 - I know they are EOL, but there is still stock available. Just modify the bias circuit to have a spread of ~1.2V between the gates. I did this 10 years or so ago. Forget the the values used, but it is simple Ohms Law voltage dividers. Have fun.
Vilfort, I understand what you are saying, and while Im sure I could do the replacement procedure, modifying the bias circuit and adjusting things afterward is beyond me. I built, or modified, a pair of Dynaco MK3s with Joe Curcio's kits years ago, but I really just followed the wiring diagrams with the kits. According to the service manual, adjusting bias on the Counterpoint is pretty tricky. Actually, assuming the SA 6 🙂) ) us putting out around 38-40 watts that is all I really need for the top end of my bi amp system. Also, with your 303s, if you remove a channel, the amp is putting out half the power, not twice the power is it not?
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That was mentioned above. It seems to me its safer to just leave old but serviceable tubes in the dead channel.
The B+ is less than 400VDC, so unless the tube section capacitor DC voltage ratings are less than 400VDC,
which is unlikely, it should be ok to remove the tubes. You could check the ratings.
Duh! I read the thread and I missed that.🙈
Gotta start paying more attention.
I am up and running. The two amps are different versions. One is 1985, one is 1987. The 1987 heat sinks are considerably hotter than the 1985 amp, should I assume bias is low on the 85? I also notice the 85 has had the right channel mosfets replaced. That channel does work, but there is a slight hum. When both channels are running the heat sinks on the new side dont warm at all. Despite the differences I hear no audible difference channel to channel and I have a fairly resolving system.