SP3a1 tube map.

DonH50

Member Sponsor & WBF Technical Expert
Jun 23, 2010
3,577
26
48
Monument, CO
#4
NP. I wish I had taken advantage of the upgrade program ARC did a few years ago, but it is not in my system now and has not been for a while, and I had modified it to use a DC source for filaments and to regulate the B+. ARC said it would have to be stock to do the upgrade even though they were replacing the old board so I passed.

Many years ago (late 70's/early 80's) I measured mine and distortion and noise was very, very low. It also had ridiculously high phono overload and line out capability. The only bad thing I found was that the zener bias string was not temp-compensated so bias varied for a while (tens of minutes) until it stabilized, and it had very large slow output ripples during the first minute or three. I built a timer circuit to delay powering on the amp for a minute after getting a SS amp; the tube amp I had with it (ARC D-79) took long enough to come up on its own that it wasn't an issue. I almost had a heart attack the first time I hooked up a sub (SS amp) and saw what the cone did in the first minute or two... I think the B and C versions corrected the start-up output (ground loop issue causing motorboating inside the preamp) and I made that mod to mine at some point (was a simple resistor fix but don't ask exactly what after all these years!)

My current SS setup is cleaner and far more powerful but I still miss that magical tube sound.
 

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