Tubes for Battery Power Applications.

For true battery operation, low plate voltage as well as low filament requirements are necessary. Fortunately you can still obtain reasonable output characteristics with these tubes. Common wisdom suggests that low plate supply equates to poor sound. I have not found that to be the case. I'll characterize two reasonable candidates for battery power operation: the CK512AX and the 6088 in both pentode and triode connection.

CK512AX

Pinout: 1(red dot)=anode, 2=screen, 3=filament, 4=g1, 5=filament Vf=0.625V, If=20mA. Va max=45V, Vsg=45V. No power limitation, since you can't draw enough current to do anything bad.

Pentode mode: gm=160uS. Triode mode:gm=250uS, Ra=35k, MU=9.

Reasonable amplifier bias points:

Pentode: 0.7V bias. Va=22.5V, Vsg=27V, Anode supply=45V. anode resistor=180k. Ia=.12mA, Isg=.06mA.

Triode: 0.6V bias. Va+sg=22.5V, Anode supply=45V, load resistor=100k, Ia+sg=.2mA.

I found this device to be quite musical in triode mode, although slightly noisier than the 6088. My devices did not exhibit much in the way of microphonics. Even with low anode load resistor values, the sonic qualities remained OK. It's not bad for a wimpy little device.

6088

Pinout the same as 512AX. Vf=1.25V at 20mA. Va max=67.5V, Vsg=67.5V. There is no power limitation, you can't draw enough current to get the anode nor the screen too hot.

Pentode mode: gm=650uS. Triode mode: gm=860uS, Ra=15k, MU=13.

Reasonable amplifier bias points:

Pentode: 1.5V bias, Va=30V, Vsupply=45V, anode resistor=30k, Vsg=45V. Ia= 0.5mA, Isg=.2mA

Triode: 1V bias, Va+Vsg=30V Vsupply=45V, anode resistor=22k. Ia+sg=.7mA.

This is an interesting device. It sounds quite nice in either pentode or triode mode, and very linear in triode mode. With 45V on the screen the anode "knee" is 1.7mA and 12 volts for zero bias. So with a 54 volt battery in a RC amplifier, pentode mode, 30k plate resistor, (with 1.3V bias and 45V on the screen, the plate voltage will be about 30 volts) you can actually get over 12VRMS before clip!

In triode mode, with a 22k plate load resistor, the output impedance will be less than 10k, and you can get about 5VRMS at low distortion. (1V bias). Thus, you can get a reasonable drive to your PA.

The several of these devices I have (all Military box Raytheon) were all surprisingly quiet, suggesting that they might actually make a kind of novel battery powered phono preamp.

Biasing

There are several ways of biasing these amplifiers. For the 512AX, if the filament + is connected to +1.5V, and the filament - is connected thru a 39 ohm resistor to ground, returning the grid resistor to ground provides an effective bias of -0.8 volts, which is perfectly adequate. For 512AX stages, you could also "series" 2 filaments across 1.5V battery (with 15 ohms from the "lower" - filament to ground) to provide a bias of .9 volt for the "upper" tube and .3 volt for the "lower" tube. Both bias points "work". You could also use a very high grid bias resistor on the lower stage to provide some "contact" bias as well.

You can also use a 1.5V cell to provide a fixed bias as well.

For the 6088, since the filament voltage is rated at 1.25V, only .25 volt is available from the filament battery as bias. Therefore, either contact potential (4.7 to 10M grid resistor) or separate bias battery is appropriate.

Battery Life

Let's consider a working example of a "full featured" stereo preamplifier. The tube complement might be:

2 x6088 Input stage phono (stage gain about 40) (one tube for each channel)

2x512AX second stage phono  (stage gain about 25)

2x512AX triode mode line amp (stage gain about 4)

2x6088 triode mode line output (stage gain about 8)

The combined phono gain (1000) is fine for the phono section as 20 dB loss is had from the RIAA equalization, leaving a overall gain of 100 (40 dB) at 1kHz. The combined gain of 32 (30dB) in the line amp section provides plenty of gain for tone or other effects if desired.

The filament requirements are assumed to be taken care of by a single "D" cell. The total filament requirement is 120 mA. So a D cell, assuming an alkaline battery ,will be about 130 hours. (or 4 hours per day for a month). The Bias can be taken care of using a single AA cell, lasting indefinitely. The B+ can be 5 or 6  9V alkaline batteries in series. The total B+ current for the system will be about 5 mA, so the 9 volt batteries will also last about 130 hours.

And no power supply hum to worry about ;-)

Steve