9/18/2023 0 Comments 807 tube audio amplifier schematic![]() If you look at this datasheet you'll see that the EL84 is not rated for positive grid voltage. Either someone is mistreating the EL84 in the lower right corner of the schematic, or they're being very clever and you are under-driving the transmitter. The final amplifier stage is screen modulated. (Caveat - I'm a toob fan-boy, but I don't have many actual tube circuits under my belt. Input voltage to the jack and 12AX7 is about 1.5 Epp.Īny help would be appreciated! Schematic is below. What is the correlation here? What can I do to fix it? Do I need more peak to peak voltage on my input? I'm currently using an old cell phone with a tone generator for testing. In the diagram, you'll notice the switch that controls the 807 screen grid voltage, also controls the EL84 audio amplifier plate voltage. Here's the problem: When I increase the screen grid voltage on the 807s, the modulation disappears. The tube manual for dual 807s, class A, calls for 600v plates (which I have) and 300v screens. If I increase the screen grid voltage to 250v, the output jumps to more than 10 watts RMS. Power output calculated across the dummy load is 4 watts RMS. With the current schematic, the 807 screen grids run at about 100v. I bought an oscilloscope and a dummy load and almost instantly figured out the power problem. ![]() I've had some previous posts on this and I'd like to thank all the people who responded for your help! I'm back working on my friend's 807 tube transmitter.
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