• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Requesting assistance building a low voltage output stage

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I am currently trying to build an audio amplifier stage to couple to a regenerative receiver that I have built. I know that this is straying slightly into the RF world but hopefully it is still on topic enough.

In essence, I am working trying to build a battery powered tube portable radio receiver. This includes a vacuum tube powered amplifier stage. Due to its portable nature and limited space in the enclosure that I plan to use I can only fit so many batteries inside. Given that, I'd like to keep B+ voltages at 36-40vdc. One of my goals is to be able to use normal (~50-100 ohm) headphones with this circuit. If you might be able to suggest any other 6 volt miniature tubes that have reasonable filament current draw that may work better for this project I'd love to hear about them. I've thought about perhaps a 6U8 due to it have a triode and pentode section or perhaps a 6GM8 due to it being designed specifically for low plate voltages.

The receiver circuit is shown as an attachment to this topic.

Here are some of my notes from my experiments:

"Okay, so I've spent some more time experimenting and I've also recently managed to construct and operate the circuit shown above.

After trialing some different types of tubes I've found good results with the EF86 or 6267 type tube. Due to its high transconductance it seems to work reasonably well at low voltages and amplifying relatively small signals. So far I have it running at about 30vdc on the plate and screen.

My primary goal is to use this amplifier circuit with the second circuit. Ideally, I'd like to take the signal from the plate of the 6AK5 and run it straight into the EF86. I currently have the circuit configured where the signal is taken from pin 5 (plate) of the 6AK5 and then is coupled to the control grid of the EF86 through a 0.1uf capacitor. The control grid of the EF86 has a 1 meg resistor to ground. The plate and screen are being fed 30vdc. The suppressor grid is tied to cathode.

I am using a Bogen T-725 matching transformer on the output with the transformer tap being the one selected for loudest volume. This corresponds to about a 40k impedance on the primary and an 8 ohm impedance on the secondary.

The issue that I am running into is that if I tap the signal from the plate of the 6AK5 the resultant signal through the headphones on the output of the EF86 is quite quiet. If I tap the signal off of the collector of T2, a BC547 NPN transistor, the resultant signal is much louder which is to expected given that the BC547 operates as a preamp. So, is this to be expected? Is there any way to either better optimize the circuit of my EF86 output stage and/or better optimize the signal coupling between the 6AK5 and the EF86?

I may try experimenting with the 6U8 tube and use input the signal from the 6AK5 into the grid of the triode of the 6U8 and then connect that to the control grid of the 6U8's pentode and use the plate of the pentode section as my output. Perhaps the triode stage will give me enough gain to allow me to tap the signal from the plate of the 6AK5 and not have to tap the signal from T2 further on down the signal chain."


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Make the B+ 45 V. and you can use an O/P tube intended for portable radio service, the 3S4. You get considerable life from a B+ pack of 5X "006P" alkaline batteries and 2X alkaline "D" cells. 3X AAA cells take care of grid bias.

FWIW, the portable superhet complement is 1R5 converter, 1U4 IF, 1S5 detector/AF amp, and 3S4 "final".

Yes, the small signal pentode in the 1S5 is OK for a regenerative detector. The "textbook" 250 pF./2.2 Mohm grid leak network is fine. Use Armstrong oscillator topology. Large inductance and small capacitance in the "tank" circuit may allow eliminating a long wire antenna, as a good earth ground just might prove sufficient.
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