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Old 22nd July 2013, 11:51 PM   #1
BRSHiFi is offline BRSHiFi  United States
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Default Putting Stock In Carver?

I am messing around in Spice with DC clipper (restorer) circuit topologies. Carver claims they reduce crossover distortion and allows for cooler tube operation. I analyzed his circuit and see how a 6AL5 potentially pulls this off.

Since the 6AL5 is no longer in production, what type of diode do you recommend I substitute? My spice work so far yields no effectiveness. Does that circuit really work or am I not doing something right? My main motive is more efficient operation (less heat and longer tube life).
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Old 22nd July 2013, 11:55 PM   #2
roline is offline roline  United States
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6AL5's and 12AL5's are still a low $ tube and available in quantity, but not current production..
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Old 23rd July 2013, 02:45 AM   #3
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The diodes need to be reversed. The idea is for them to conduct on the opposite (negative) signal swing, at the same level as the internal U1 or U4 grid1/cathode effective diode does on the positive swing. That way the coupling capacitor does not charge up over many cycles. Both diode polarities
should present similar series resistance as well for symmetrical charging currents.
I've seen 6AL5 for $0.50 in quantity.
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Old 23rd July 2013, 03:19 AM   #4
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As I understand it, the DC restorer is a clamp circuit that prevents the output tubes from being biased too negatively on strong signals. This allows them to be operated at a lower idling current, to a point. The diodes in your circuit should be reversed and not be zener diodes. Carver uses regular 1N4148 switching diodes. And the normal negative bias voltage for the tube grids is seperate from the diodes source. Here is a link to Carver's own hand drawn modification of the Citation II.
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Old 23rd July 2013, 04:03 AM   #5
BRSHiFi is offline BRSHiFi  United States
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I dropped in some 1N4148 diodes in spice. I cranked the bias way back and the distortion readings are hardly influenced, both with and without the clippers. Does that mean the circuit is a hoax or do I need to tweak for 6L6 usage? I figure less than 8 watts static plate dissipation with the clipper should be what Carver claims should work.
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Old 23rd July 2013, 04:33 AM   #6
BRSHiFi is offline BRSHiFi  United States
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I am continuing to tinker, but 1N4148 diodes seem to exacerbate notch distortion.
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Old 23rd July 2013, 04:52 AM   #7
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Make R3 equal to R9 and R8 (actually R3 needs to be even higher due to the internal resistance of the grid1/cathode diode). (The effective series resistance of both diode sets should be equal.)
BiasA should be twice BiasB.
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Old 23rd July 2013, 05:09 AM   #8
BRSHiFi is offline BRSHiFi  United States
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Weird, taking out the diodes and putting them in and the distortion is the same in spice when making R3 2.2k ohms.
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Old 23rd July 2013, 05:23 AM   #9
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The diodes don't do anything unless the signal peaks exceed BiasB. Then the SS diodes and the internal tube diodes draw (hopefully) equal opposite currents, so that the coupling capacitor does not charge up and shift the DC operating bias level.

With no shift of DC operating bias for large signals, the design idle current of the tubes can be decreased, since they won't cut off from bias drift.
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Old 23rd July 2013, 05:32 AM   #10
BRSHiFi is offline BRSHiFi  United States
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Why does carver boast the amps can be biased much cooler? In spice, with a continuous waveform, this does not happen without large amounts of notch distortion? The diodes are "supposed" to eliminate some of this. Hmmm.

I have the spice experiment attached.
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