Hello,

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 r/electronics didn't seem to have too many posts about vacuum tubes.

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.

The receiver circuit is shown here:

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https://preview.redd.it/0egltj0224041.jpg?width=1200&format=pjpg&auto=webp&s=e735c0c55563c2b271853acbedd778e52a2fee69

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.

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. The tap that has yielded the best results so far 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, 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."