ruffrecords
Well-known member
When I started designing tube mic pres some years ago I initially used ac heaters and got into all sorts of trouble with hum. I quickly moved to dc heaters, raised the voltage to 12.6V dc for easier, more efficient regulation and used tubes that either had a 12.6V heater or could be wired in series to operate on a 12.6V heater supply. Since then I have never had hum problems caused by heaters.
The current range of designs all use this 12.6V dc heater supply. However, because the main output tube used is the 6922, and this only has a 6.3V heater, they always have to be used in pairs. Recently there have been a couple of occasions when it would have been very convenient to use a single 6922 on a board but to be compatible with the 12V dc heater system you would need to use a wasteful heat generating dropper resistor/regulator of some sort or provide an additional 6.3V heater supply. There is a 12.6V heater version of the 6922 but it is extremely rare and very expensive and therefore not a viable option.
As a result of all this, I began to wonder whether 6.3V heaters, even ac ones, might be viable. For a completely unrelated reason I had been looking at the schematics of the Altec 250SU tube console and in particular its power supply, the 535A. The power supply can provide 6.3V roughly smoothed dc at an incredible 13 amps with a ripple of only 1.5V pp (this is better than 20dB below 6.3VAC). This supplies a mixer whose mic pres achieve a specified EIN of -126dBu, so hum is clearly not an issue. It looks like the ancients knew a thing or to.
I therefore decided to test this out on my standard Eurocard mic pre. I modified the heater wiring on one I had been using on the lunch box project so it could be powered from 6.3V. I then connected it to an old power supply I built a long time ago that has my standard HT supply and just a 6.3VAC heater winding output. I took no special precautions; the extra heater wires were just strung across the underside of the PCB and the PCB just sat in the open on my bench (no screening at all). The heater wires from the power supply to the board were not even twisted.
I powered it up and measured the gain from mic input at maximum gain to transformer balanced output of the first amplifier. the results were:
Gain: 51,4dB into 600 ohm load
Noise at max gain, input shorted: -69dBu
This means the EIN is -120.5dBu
I had measured this same board the day before in the screened lunch box with dc heaters and under the same source, gain and load conditions and the output noise was only 1dB better at -70dBu. I conclude that using 6.3V ac heaters seems to make no real difference to this board. This is surprisingly encouraging.
There are a couple of downsides. Running a 6.3V ac hefty current along the back plane of a mixer or lunch box could possibly induce interference insensitive inputs if proper attention is not paid to screens In that event, the use of roughly smoothed 6.3V ac might be better. The Altec 13A supply uses 70,000uF of smoothing to achieve its 1.5V pp ripple at 13A. Today, 47,000uF 16V capacitors are readily available so providing 100,000uF of smoothing should be straightforward. For something like a lunch box which would take only 4A at 6.3V, this amount of smoothing would reduce ripple to 0.4V pp.
This makes for a really simple power supply. The only problem with this simplicity is that there is no regulation so the voltage would vary with load. Altec overcame this by having a series of primary taps on the heater transformer so the output voltage could be adjusted. I am seriously toying with the idea of moving the Mark3 to 6.3V heaters.
Cheers
Ian
The current range of designs all use this 12.6V dc heater supply. However, because the main output tube used is the 6922, and this only has a 6.3V heater, they always have to be used in pairs. Recently there have been a couple of occasions when it would have been very convenient to use a single 6922 on a board but to be compatible with the 12V dc heater system you would need to use a wasteful heat generating dropper resistor/regulator of some sort or provide an additional 6.3V heater supply. There is a 12.6V heater version of the 6922 but it is extremely rare and very expensive and therefore not a viable option.
As a result of all this, I began to wonder whether 6.3V heaters, even ac ones, might be viable. For a completely unrelated reason I had been looking at the schematics of the Altec 250SU tube console and in particular its power supply, the 535A. The power supply can provide 6.3V roughly smoothed dc at an incredible 13 amps with a ripple of only 1.5V pp (this is better than 20dB below 6.3VAC). This supplies a mixer whose mic pres achieve a specified EIN of -126dBu, so hum is clearly not an issue. It looks like the ancients knew a thing or to.
I therefore decided to test this out on my standard Eurocard mic pre. I modified the heater wiring on one I had been using on the lunch box project so it could be powered from 6.3V. I then connected it to an old power supply I built a long time ago that has my standard HT supply and just a 6.3VAC heater winding output. I took no special precautions; the extra heater wires were just strung across the underside of the PCB and the PCB just sat in the open on my bench (no screening at all). The heater wires from the power supply to the board were not even twisted.
I powered it up and measured the gain from mic input at maximum gain to transformer balanced output of the first amplifier. the results were:
Gain: 51,4dB into 600 ohm load
Noise at max gain, input shorted: -69dBu
This means the EIN is -120.5dBu
I had measured this same board the day before in the screened lunch box with dc heaters and under the same source, gain and load conditions and the output noise was only 1dB better at -70dBu. I conclude that using 6.3V ac heaters seems to make no real difference to this board. This is surprisingly encouraging.
There are a couple of downsides. Running a 6.3V ac hefty current along the back plane of a mixer or lunch box could possibly induce interference insensitive inputs if proper attention is not paid to screens In that event, the use of roughly smoothed 6.3V ac might be better. The Altec 13A supply uses 70,000uF of smoothing to achieve its 1.5V pp ripple at 13A. Today, 47,000uF 16V capacitors are readily available so providing 100,000uF of smoothing should be straightforward. For something like a lunch box which would take only 4A at 6.3V, this amount of smoothing would reduce ripple to 0.4V pp.
This makes for a really simple power supply. The only problem with this simplicity is that there is no regulation so the voltage would vary with load. Altec overcame this by having a series of primary taps on the heater transformer so the output voltage could be adjusted. I am seriously toying with the idea of moving the Mark3 to 6.3V heaters.
Cheers
Ian
