0D3 shunt regulator for single ended duty - diyAudio
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Old 29th October 2007, 07:06 PM   #1
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Default 0D3 shunt regulator for single ended duty

I've seen this sort of setup used for push-pull amps before, but does anyone have an opinion of using it for single ended designs? Also, does this necessitate additional components -- a small cap to parallel the 0D3, a large PS cap after the regulator, etc? (the tube in the schematic is a 7788 which runs at about 18mA when biased to 2.3V with a 160V B+)
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Old 29th October 2007, 07:51 PM   #2
rdf is offline rdf  Canada
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That 680 ohm resistor wrecks the regulation across the 0D3's full current range. It specs 4 volts variance between 5ma and 40 ma for around ~115 ohms effective internal resistance. The series resistor varies a little over 23 volts over that same range. I'ld ditch it and play with different LEDs to hit the desired plate currrent. The static current through the regulator also seems low. Whatever the tube isn't drawing gets shunted through the regulator. When/if the tube pulls instantaneous current exceeds 25 ma the regulator current will fall below rated 5 ma minimum and presumably stop regulating. Otherwise it's a circuit I'm considering elsewhere.
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Old 29th October 2007, 08:50 PM   #3
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I used an 0D3 in a 27 preamp I did. You can find it here. I think it works well, but honestly I don't have a vast frame of reference. The tube does need to be bypassed, either by a small cap (max .1uf) or a RC.

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Old 29th October 2007, 08:54 PM   #4
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A resistor, of some sort, is necessary, though, correct? I don't think there is anything magical about the 160V operating point, so dropping the resistor to half that value, which only drops things by 4 or 5 volts, and which is where I have seen others set it, would be fine. For this application, driving some headphones, I don't imagine that I'll exceed the current, but I can certainly bump it up a little more if need be.

How about the cap situation? With the resistor between the VR and ground, presumably the small cap goes from anode to cathode, rather than from anode to ground?
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Old 29th October 2007, 09:39 PM   #5
rdf is offline rdf  Canada
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I don't think so. I'm no glowtube wizard but a series resistance is normally employed between the last big cap and the glowtube to prevent exceeding the regulator's current limit while still allowing voltages to rise high enough for 'ignition' to occur. That's a beauty of the CCS feed, as long as it's set below the tube's upper current limit, over-curent is an impossibility. Another huge benefit of the CCS is the voltage divider ratio looking into the circuit from the power supply tranny becomes something like 2Gohm:115, or a 'theoretical' 140+ dB of even, broadband attenuation of PS noise without considering the bypass cap. Maybe the resistor has a function I don't know but nothing I see so far would have me use it.

Since I haven't seen a glowtube used like that the best way to bypass is a guess. It might be possible to use higher than the normally recommended max of 0.1 uF (across the tube) from B+ to ground with it in place. Play around until it oscillates.
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Old 30th October 2007, 03:46 AM   #6
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My understanding is the resistor helps limit current during strike, not current from the current source, but from the load capacitance, depending on what you have downstream. Take a look at Lynn Olsen's designs for some examples:

http://www.nutshellhifi.com/IT-Triode-Amp.gif
http://www.nutshellhifi.com/Raven-MarkII.gif

Yes, these are not SE circuits, but they do illustrate the capacitance downstream. Before the gas regulator strikes, the CCS charges the load capacitance up to the firing voltage of the regulator. Once this is reached, the regulator essentially provides a near zero impedance, discharging the load capacitance down to the regulated voltage.

I use 100 ohm resistors, and regulation is actually pretty good, but again, PP circuits.
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Old 30th October 2007, 04:31 AM   #7
rdf is offline rdf  Canada
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Thx zigzagflux. It's an interesting application however Lynn follows the reg in one case with 80uF in series with an uspecified resistor (chosen by tube) and in the other with 20uF in series with 87 ohms. The rated maximum for an 0D3 is 0.1uF so his application can be said to be very atypical.
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Old 30th October 2007, 05:44 AM   #8
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Drop the I source down to about 10mA or so and buffer the OD3 with a high voltage MOSFET configured as a source follower.
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Old 30th October 2007, 11:48 AM   #9
Merlinb is offline Merlinb  United Kingdom
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Given that a single ended stage causes zero average modulation of the HT current, what's the pioint in voltage regulating it?
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Old 30th October 2007, 04:25 PM   #10
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Quote:
Originally posted by Merlinb
Given that a single ended stage causes zero average modulation of the HT current, what's the pioint in voltage regulating it?
At least three possible reasons:

Consist supply source impedance across the audio range, most typical unregulated supplies have rising source impedance at both ends of the frequency spectrum. In particular with good output transformers this helps to maintain what little damping factor is available down to the performance limits of the output transformer.

Noise performance - regulated supplies can offer extremely low noise levels without requiring huge amounts of capacitance.

(These two advantages are probably irrelevant in a parafeed SE amplifier, but are relevant with conventional designs.)

In amplifiers using fixed bias (regulated obviously) this allows you to set an operating point and have it stick with significant variations in line voltage.
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