Setting aside the voltage regulator option, what would be the target for ripple voltage on DC heaters in a phono preamp?
Assuming an indirectly heated cathode, a quality tube with low filament to cathode leakage, and good wiring placement, you might get away with 1 or 2 millivolts rms (or maybe even more ripple).
I built brute force CRC filters for DC powered 300B filaments and had 1 or 2 mV rms on the 1.25 Amp 5V filaments.
But remember, the 300B is a Direct Heated Triode output tube.
A 300B output tube is not the same as an indirectly heated phono preamp tube. The preamp tube gets input signals in the microvolt range (5uV to 5mV from the cartridge is 60dB dynamic range). Or for a 14 dB louder record or cartridge, 25uV to 25mV, that is also 60dB dynamic range.
Getting the filament ripple down to 1 or 2 mV is not hard to do; preventing hum ground loops in the phono stages is the major problem.
I built brute force CRC filters for DC powered 300B filaments and had 1 or 2 mV rms on the 1.25 Amp 5V filaments.
But remember, the 300B is a Direct Heated Triode output tube.
A 300B output tube is not the same as an indirectly heated phono preamp tube. The preamp tube gets input signals in the microvolt range (5uV to 5mV from the cartridge is 60dB dynamic range). Or for a 14 dB louder record or cartridge, 25uV to 25mV, that is also 60dB dynamic range.
Getting the filament ripple down to 1 or 2 mV is not hard to do; preventing hum ground loops in the phono stages is the major problem.
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The ripple is not important seriously, the tubes work both with ac or dc.
Most DC heaters are very deficient in terms of rectification if not used with a regulator.
The target for ripple is preventing any levels which could interfere with the signals, like the wires inside the amplifier and the regulator module proximity to signals.
In my phono amp I use separate over-head twisted wires for DC heaters, and they are regulated on a separate board.
Some phono tubes are capacitative more than others, experimentation must be used with wire placement and parts, signal wiring etc.
So there is no straight answer except the tube specifications for DC ripple if it mention it.
Most DC heaters are very deficient in terms of rectification if not used with a regulator.
The target for ripple is preventing any levels which could interfere with the signals, like the wires inside the amplifier and the regulator module proximity to signals.
In my phono amp I use separate over-head twisted wires for DC heaters, and they are regulated on a separate board.
Some phono tubes are capacitative more than others, experimentation must be used with wire placement and parts, signal wiring etc.
So there is no straight answer except the tube specifications for DC ripple if it mention it.
I have used DC once to get around hum and it didn't make any difference.
Since then I always use AC heaters and haven't had any problems with pre amps or main amps.
If you really need to use DC then find a good regulator and heat sink.
Plenty of smoothing and it should be fine.
Since then I always use AC heaters and haven't had any problems with pre amps or main amps.
If you really need to use DC then find a good regulator and heat sink.
Plenty of smoothing and it should be fine.
The phono preamp in question will be 12A_7s. I would like to use AC heaters if possible for simplicity but given almost 65dB of gain in the bass region I am trying to think ahead in case hum becomes and issue. I did a guitar amp which is also high gain with no trouble but of course that was a more limited bandwidth application.
On the one hand I guess that AC heaters are essentially a 12 or 6 volt ripple so even a mediocre filtering on DC will be quite a bit less ripple (but an octave higher). It seems that reaching a few hundred mV would be pretty easy. Single digits would be much harder making the prospect of an LM317 and heat sink a bit more palatable.
On the one hand I guess that AC heaters are essentially a 12 or 6 volt ripple so even a mediocre filtering on DC will be quite a bit less ripple (but an octave higher). It seems that reaching a few hundred mV would be pretty easy. Single digits would be much harder making the prospect of an LM317 and heat sink a bit more palatable.
If you did use DC heaters, and still could not get around the hum,
then it was likely one of these things:
Most likely:
Ground Loops: Include Signal; Cathode and Grid; B+ ground loops (in that order of importance).
Next most likely:
B+ ripple
Next most likely:
Magnetic fields
Next most likely:
Attention to detail of the DC filament supply, and again those ugly ground loops.
As I have said before: "Grounds are Commonly Misunderstood"
then it was likely one of these things:
Most likely:
Ground Loops: Include Signal; Cathode and Grid; B+ ground loops (in that order of importance).
Next most likely:
B+ ripple
Next most likely:
Magnetic fields
Next most likely:
Attention to detail of the DC filament supply, and again those ugly ground loops.
As I have said before: "Grounds are Commonly Misunderstood"
I do have a 25.2V 2A PT that I could filter down for 12V heaters (need 750mA) and I might even have an LM317 lying around from my solid state days in the 70s.
Does it have a centre tap? If you you can use full wave rectification to get 16V or so, and a simple RC filter. If not, run the tubes in series @25.2V?
In my phono amp, I'm using a 12V SMPS laptop style brick for heaters and DC boost for B+. I run the 6V heaters in series/parallel. I use the chassis as return. No noise, no buzz, no hum and you save a little bit of wire, too.
In my phono amp, I'm using a 12V SMPS laptop style brick for heaters and DC boost for B+. I run the 6V heaters in series/parallel. I use the chassis as return. No noise, no buzz, no hum and you save a little bit of wire, too.
You may not have a center tap,
But if you do have two 120V primary windings, just wire the two primaries in series (careful, make sure the phase is correct, or you will let the smoke out).
Two 120V primaries in series on 120V gives 12.6Vrms out, or about 17.6V peak.
That is enough to run a bridge (prefer Schottky), and one of two things:
1. CRC filter with a large R to give enough voltage drop (and lots of filtering) for very clean 12.6VDC.
Or . . .
2. CRC filter and an LM317 for regulated 12.6VDC.
Remember, an LM317 requires up to 3V input to output drop.
Schottky diodes have lower voltage drop, so lots of voltage to give 1 above the ability to use a larger R in the CRC filter (better filtering, lower ripple for the same C);
Schottky diodes lower voltage drop, so lots of voltage out to give the LM317 more than 3V drop to work with.
But if you do have two 120V primary windings, just wire the two primaries in series (careful, make sure the phase is correct, or you will let the smoke out).
Two 120V primaries in series on 120V gives 12.6Vrms out, or about 17.6V peak.
That is enough to run a bridge (prefer Schottky), and one of two things:
1. CRC filter with a large R to give enough voltage drop (and lots of filtering) for very clean 12.6VDC.
Or . . .
2. CRC filter and an LM317 for regulated 12.6VDC.
Remember, an LM317 requires up to 3V input to output drop.
Schottky diodes have lower voltage drop, so lots of voltage to give 1 above the ability to use a larger R in the CRC filter (better filtering, lower ripple for the same C);
Schottky diodes lower voltage drop, so lots of voltage out to give the LM317 more than 3V drop to work with.
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