I've got some 6dj8 tubes in a common-cathode configuration with a cap bypassed cathode resistor. I'd like to try battery biasing, but what would be the optimal voltage of the battery used? Also, if I use a battery with whatever voltage you guys sugest, what anode resistor should I use with a 180V B+? Thanks, but I'm still a little unsure when it comes to tubes, but I'm progressing...
2.5V to 3V is generally a nice voltage. A 3V lithium is actually closer to 3.6V. A charging pair of > 80mA/hr NiMH in series in the cathode will give about 3V. Alternatively, you could use grid bias with a pair of AA alkaline batteries in series with your grid leak resistor. Try 18k as your anode load, and it should draw about 4mA with 3V bias.
I hear many people warn against starving them of current, so I'll try your sugestion and see how it works, 4mA counds plenty. Thanks.
That's even better. The battery voltage will effectively set the anode voltage, so you can just choose it to maximise output swing.
Want to explain to me the AA grid bias? As I said, I'm a noobe to tubes... Also, what would I go about doing if I were to change the B+? I'd only change the anode resistor with proportion to the change in B+, right? Ah, so many questions...
have bought some ni-cd for used with battery biasing 6922. I am now charging them to full capacity. Once i install them can i leave them in the circuit continuesly connected? Or do i need a switch to disconnect them when i am not using the amp? As i read somewhere that what the amp is running the batteries get charge rite? Plus i am running the 6922 at 9ma hence the batteries should get charge by around this value rite? Your input on having a switch or without it please.
JoeBob,
you need a set of anode characteristics for your valve. This is a set of graphs that shows anode current against anode voltage for differing values of grid-cathode bias. An excellent source for these is:
http://home.planet.nl/~frank.philipse/frank/frank.html
Once you have these curves, you choose your HT or B+ voltage, and mark this on the voltage axis. Then, having chosen the resistance of your anode load, you divide this into the HT voltage to find the maximum current that could flow, and mark this current on the current axis. You now draw a straight line between your two marked points. This is your loadline, and shows how the anode voltage can vary with your chosen HT voltage and anode load. The operating point along this loadline is determined by the grid-cathode bias voltage you choose. This is necessarily rather abbreviated, but there are lots of currently available books that go into more detail - have a look at the component distributors.
NickC,
if you have your NiCd permanently connected in the cathode circuit with its positive terminal connected to the cathode, when you apply HT, current will flow into the NiCd to charge it. If the battery is already charged, this current heats the battery. Trickle chargers charge at C/10 (C is the battery capacity in Ahrs) because overcharging at this rate does not cause damage. Thus, a typical 0.5Ahr AA NiCd would be trickle charged at 50mA. Your battery is going to be rather warmer (it's near a valve), so I would recommend that you choose a battery with a rating of >20 your anode current = 20 x 0.009 = 0.18Ahr. An AA NiCd would be fine. Lastly, if you are going to buy new, buy NiMH rather than NiCd. One day, it will have to be disposed of, and cadmium is nasty stuff. NiMH not only offers greater capacity, but it's more environmentally friendly.
you need a set of anode characteristics for your valve. This is a set of graphs that shows anode current against anode voltage for differing values of grid-cathode bias. An excellent source for these is:
http://home.planet.nl/~frank.philipse/frank/frank.html
Once you have these curves, you choose your HT or B+ voltage, and mark this on the voltage axis. Then, having chosen the resistance of your anode load, you divide this into the HT voltage to find the maximum current that could flow, and mark this current on the current axis. You now draw a straight line between your two marked points. This is your loadline, and shows how the anode voltage can vary with your chosen HT voltage and anode load. The operating point along this loadline is determined by the grid-cathode bias voltage you choose. This is necessarily rather abbreviated, but there are lots of currently available books that go into more detail - have a look at the component distributors.
NickC,
if you have your NiCd permanently connected in the cathode circuit with its positive terminal connected to the cathode, when you apply HT, current will flow into the NiCd to charge it. If the battery is already charged, this current heats the battery. Trickle chargers charge at C/10 (C is the battery capacity in Ahrs) because overcharging at this rate does not cause damage. Thus, a typical 0.5Ahr AA NiCd would be trickle charged at 50mA. Your battery is going to be rather warmer (it's near a valve), so I would recommend that you choose a battery with a rating of >20 your anode current = 20 x 0.009 = 0.18Ahr. An AA NiCd would be fine. Lastly, if you are going to buy new, buy NiMH rather than NiCd. One day, it will have to be disposed of, and cadmium is nasty stuff. NiMH not only offers greater capacity, but it's more environmentally friendly.
ec8010 thanks for the help. However i bought a 600 mah ni-cd aready. I would like to say also thanks for the enviromental tip but it came abit late.
but i find the voltage to high 2.75 is too high for my circuit i need to reduce it to 2.4 or less would be better around 2.0 would be best.
i think i would use a voltage divider around 2k to get that 2 volts but i am worried that when the batteries are connected directly with the pot install that they will get drain faster unless i install relay to switch off the batteries supply
unless the trickle charge is more than the drain it should be fine
i think i would use a voltage divider around 2k to get that 2 volts but i am worried that when the batteries are connected directly with the pot install that they will get drain faster unless i install relay to switch off the batteries supply
unless the trickle charge is more than the drain it should be fine
If you add a potential divider, the cathode will develop a feedback voltage across its output resistance. With battery bias, you have to accept the battery voltage and change your circuit to suit.
EC8010 what you mean by
cathode will develop a feedback voltage across its output resistance
you mean that the resistance of the pot would be the cathode resistor and thus bias the tube?
cathode will develop a feedback voltage across its output resistance
you mean that the resistance of the pot would be the cathode resistor and thus bias the tube?
An unbypassed cathode resistor develops a feedback voltage, so we short-circuit it with a capacitor. A battery is already a short-circuit, so it prevents feedback. Adding a potential divider brings you back towards resistor bias. You have to use your battery and change the rest of the circuit to suit.
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