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What do you think of this circuit
What do you think of this circuit
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Old 6th December 2017, 04:39 PM   #11
petertub is offline petertub  Sweden
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Quote:
Originally Posted by Doppler9000 View Post
Is the circuit dangerous?
The 300B lives outside it's designed area. It still might work. But you cannot
complain if the tube goes into thermal runaway.

There is no point in using the tube outside it's spec, one could easily change
the grid resistor to be within design limits.
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Old 8th December 2017, 12:21 AM   #12
Miles Prower is offline Miles Prower  United States
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Quote:
Originally Posted by Doppler9000 View Post
Hello,

I came across this circuit and wondered what people thought before I dig in further on the design. It is a regulated 300B SE unit.

TIA
There are a few problems:

1) The series capacitors in the power supply need 100K equalizing resistors. The tolerances of electrolytics isn't tight, and the DC will divide unevenly. This could lead to overvoltage across one or the other in the series string.

2) There are two capacitor couplings, both with 0.1uF capacitors. That will cluster the low frequency phase shift, possibly leading to low frequency stability problems once the gNFB loop is closed. Capacitor couplings need to be staggered to prevent that.

3) The input isn't protected against DC offset. You need an input coupling capacitor unless you're certain you won't be connecting to anything that could have DC offset.

4) The spec sheet for the 300B lists 250K as the max value for a grid return resistor. Here, you have almost twice that value. This could lead to a run-away condition. That's why I'd like to see a separate grid driver. A follower eliminates that problem, as well as the heavy load on the final gain stage. Triode gain stages don't drive heavy loads gracefully without distortion.

Also, complaints about disappointing sonic performance of 300B's is almost always due to designs with inadequate grid drive. Even if you're nominally operating in Class *1, these power triodes need adequate grid drive both for Ci + CMiller, but also for grid current, as grid current will flow even though Vgk < 0V.

5) I would lose that head jack and its associated switch. A failure there could leave the secondary of the OPT without a load, and that's a disaster waiting to happen. A plate current cutoff will let the OPT free wheel like a TV HD xfmr. Unlike a TV HD coupling xfmr, the OPT wasn't designed to stand up to K-volts, and you could lose the OPT. Also, decide what your load is, and connect the gNFB loop to that point. Connecting to the top of a multi-tap secondary with only part of it connected across the load gives stability problems.

Quote:
Originally Posted by petertub View Post
unnedded Standby switch could be eleminated.
Not for a solid state power supply. The voltage will come up very fast even before the cathodes have a chance to warm up. It's better to allow for the application of heater power, let the cathodes warm up, before turning on the HV. That's how I do designs where SS power supplies are used. You could avoid the problem if the HV supply were redesigned to use hollow state power diodes.
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Last edited by Miles Prower; 8th December 2017 at 12:28 AM.
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