hi,
In the past I have used a 'hum dinger' pot across the heaters, with the pot slider to chassis, trim to null any hum.
E
Look here:
http://www.valvewizard.co.uk/heater.html
extract:
Humdinger
The ways in which hum are induced into the audio circuit are rarely perfectly symmetrical. Often you will find that minimum hum occurs with an 'off-centre' tap. This can be done using a trimpot with its wiper grounded to create the artificial centre tap. Again, a low value is preferable, and a 500 ohm pot will dissipate less than 80mW at 6.3V. This 'humdinger' pot can then be adjusted on test for minimum audible hum, which seldom occurs at the exact centre setting.
Neazoi, that's a strange way to power it up! It looks like the incoming AC is stepped down to 6.3V then stepped back up again for the HT supply. Why not feed the AC directly to two transformers, one for the heaters and one for the HT (or combine them in to one transformer), it would be far more efficient.
As for floating the heaters, it really depends on the capacitive coupling between the heating wires and the cathode. As one cathode is grounded and the other is at very low impedance to mains frequency, it shouldn't really matter whether you ground the heater supply or not. For safety, I would either ground one side directly or through a resistor (~10K) just to prevent any high voltages from the transformer leakage building up on the heater supply. The "humdinger" or similar circuits are a good idea on sensitive audio pre-amps but not really essential for RF applications.
If this isn't a tried an tested circuit, I would be careful about the current allowed to flow into the anodes, there is no bias on either triode so if it doesn't oscillate for any reason, the current will be very high.
Brian.
The choke should be fine, all it has to do is present a high impedance at the oscillation frequency so you can probably put more turns on it to make it work better. The wire and it's resistance isn't the issue, what concerned me was what would happen if it didn't oscillate for some reason. The 12AU7 may be specified to work at 250V and 10mA in normal operation but then there would normally be a negative voltage on the control grid (actually 0V on the grid but some positive voltage on the cathode). In that schematic, both triodes are run without grid bias or any cathode resistors so the current could go much higher. The oscillation in the first triode will limit it's current by developing a negative grid voltage (positive half cycles will be conducted from the grid to cathode as though it was a diode) and the signal reaching the second triode will similarly be rectified to produce a negative grid voltage. If the oscillation stopped the grid bias would be zero and they would both conduct as hard as they could!
Brian.
That would be 1W input power not 1W output. Provided the oscillator runs, it should be OK on 250V but personally, I would add a resistor in the anode of the oscillator stage, even if only 10K to limit the current and I would add a resistor of say 100 Ohms, bypassed with 100nF in the cathode of the output stage. It shouldn't change performance much but it would offer some protection.I said 250v as I thought it would produce more output power. The original circuit works on 120v and the amplifier tube will draw 8.3mA (i.e 1W output). Should I stick to this lower voltage better for protection do you think?
About this you say: "to prevent any high voltages from the transformer leakage building up on the heater supply." Would the hum reducing potentiometer circuit help on that?
That would be 1W input power not 1W output. Provided the oscillator runs, it should be OK on 250V but personally, I would add a resistor in the anode of the oscillator stage, even if only 10K to limit the current and I would add a resistor of say 100 Ohms, bypassed with 100nF in the cathode of the output stage. It shouldn't change performance much but it would offer some protection.
It would because it provides a leakage path to ground but I doubt you would notice any hum in that schematic anyway. It really matters in sensitive audio pre-amplifiers where even the tiniest of pick up would be audible after amplification. The reason I suggest fitting a resistor or grounding one side is that otherwise the heater circuit is 'floating' and could potentially rise up to quite a high voltage because of capacitive leakage through the transformers. The performance is not influenced by the heater connection because it is not connected internally in the 12AU7 but there is a risk if the heater floats to high voltage that it could flash over (arc or spark) to the cathode. Adding a resistor or ground link would leak away any charge before it could reach damaging voltages. For almost zero cost you add extra protection.
Brian.
That is exactly what I meant. I still think you can ground one side of the heater supply directly or through a resistor, I doubt you would notice any difference at all if you adjusted the potentiometer.
Keying the cathode will still work but do it this way: Keep the capacitor permanently connected from the cathode to ground, put the key in series with the ground side of the resistor. If you notice key clicks, wire another resistor and capacitor in series across the key (suggest 100nF & 100 Ohms). What you want to avoid is a very suddent rise or fall in cathode current and a high voltage across the key contacts, wiring it as I suggest should fix both problems. I would advise against connecting a capacitor directly across the key contacts (without its series resistor) as it will charge to ~25V with the contacts open then be shorted out as they close, you might get sparking and contact erosion.
Brian.
Thats correct.
If you want to further improve stability, you could stabilize the oscillator supply voltage by adding a Zener diode (100V to 150V) across the capacitor between the oscillator choke and load resistor. At the moment, as you close the key the supply will drop slightly and pull the oscillator frequency a little, the Zener would make sure the oscillator supply stayed constant. If you do that, calculate the load resistor so the Zener passes about 2mA, taking into account the current through the oscillator anode as well. so for example, if the oscillator consumed 5mA and the Zener passed 2mA the total through the resistor would be 7mA and if it dropped say 250V to 150V Zener voltage, it would be R=V/I =(250 - 150)/.007 = 14.285K or 15K as the nearest standard value.
If you want to avoid semiconductors (for the fun of it - although you are using a semiconductor bridge rectifier already) you could use a gas tube voltage stabilizer such as the 0A2. Not as efficient, much bigger, more expensive, but with a nice blue glow to look at!
Brian.
The 0A2 should be connected to the junction of the choke and resistor and to ground. It works best with a little more than 5mA flowing through it so adjust the resistor value accordingly. The capacitor across it must not be greater than 100nF, I would suggest 10nF. If you use one too big, the 0A2 will oscillate by itself at a few Hz!
What frequency are you using this at? I might be able to listen out for it on the air!
Brian.
I do not think it can radiate that far, it is only 1W or so and Greece to UK is a long distance. It can be easily done with a few 10s of W though. Once I have spoken to an operator in uk with a simple wire and antenna tuner.
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