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I'm back working on my friend's 807 tube transmitter. I've had some previous posts on this and I'd like to thank all the people who responded for your help!

I bought an oscilloscope and a dummy load and almost instantly figured out the power problem. It's the 807 screen grids. With the current schematic, the 807 screen grids run at about 100 V. Power output calculated across the dummy load is 4 W RMS. If I increase the screen grid voltage to 250 V, the output jumps to more than 10 W RMS. The tube manual for dual 807s, class A, calls for 600 V plates (which I have) and 300 V screens.

Here's the problem: When I increase the screen grid voltage on the 807s, the modulation disappears. In the diagram, you'll notice the switch that controls the 807 screen grid voltage, also controls the EL84 audio amplifier plate voltage. 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. Input voltage to the jack and 12AX7 is about 1.5 Vp-p.

Any help would be appreciated! Schematic is below.

enter image description here

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  • \$\begingroup\$ "... 1.5 Epp." What does that mean? \$\endgroup\$
    – Transistor
    Commented Aug 5, 2019 at 14:14
  • \$\begingroup\$ Sorry, it means peak to peak voltage. \$\endgroup\$
    – McMurdo
    Commented Aug 5, 2019 at 14:16
  • \$\begingroup\$ You should probably write it as \$V_{p-p}\$. \$\endgroup\$
    – Hearth
    Commented Aug 5, 2019 at 14:58
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    \$\begingroup\$ A link to the construction article and/or a mention of the name of the design would be good, in case the problem you're having is a known issue with the type. \$\endgroup\$
    – TimWescott
    Commented Aug 5, 2019 at 17:09
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    \$\begingroup\$ If you have control of that schematic, please label the components (i.e., V1, V2, etc., for the tubes, D1, D2, etc., for the diodes, etc., for resistor, caps transformers, and, well, etc.). \$\endgroup\$
    – TimWescott
    Commented Aug 5, 2019 at 17:17

3 Answers 3

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(Caveat -- I'm a toob fan-boy, but I don't have many actual tube circuits under my belt. They're just vacuum-mode FETs to me.).

The final amplifier stage is screen modulated. 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.

If you look at this datasheet you'll see that the EL84 is not rated for positive grid voltage. It pulls a lot of current at 0V on the grid, in fact. I was going to say that this is a Bad Thing, and that the biasing of the amp is all messed up -- but then I noticed the diode on the grid of the EL84 in the lower right corner (now, wouldn't saying "D1" be easier?)

If that diode is doing what I think it's doing, then when the transmitter is idling, there will be very little voltage on the screens of the 807s, by design. When you input audio to the thing, the peaks of the audio waveform at the grid of that EL84 will tend to charge up the capacitor on the grid of that EL84. Then on the next down-swing of the audio, the EL84 will conduct less, the 807 screens will go high, and you'll actually see some transmit power.

So -- try feeding it with higher-level audio, see what happens. If you don't have enough audio out of your cell phone, build or by an external amplifier, or add another stage or two of amplification to the transmitter.

When you increase the screen voltage (presumably by reducing the resistance from the screen to B+), then you reduce the audio gain in the screen circuit. That would be why your modulation disappears -- you're essentially shorting out the final stage of audio amplification.

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  • \$\begingroup\$ Thanks Tim. I'm going to do some testing and try and figure out an inexpensive way to boost my line level. I've always wondered about that diode. What is odd though, is the author of the schematic claims a 35-40 watt carrier, and I'm only measuring 4 watts RMS at the output. When an AM transmitter (class A) is idling, it should have a full carrier, correct? If I remember correctly, carrier power should be 1/4 PEP. So I should be seeing around 20 watts carrier at the output. \$\endgroup\$
    – McMurdo
    Commented Aug 6, 2019 at 5:36
  • \$\begingroup\$ When a normal AM transmitter (class A, B or C) is idling, it should have full carrier. There are a number of different schemes out there (this is one, clearly) that mix that up a bit. \$\endgroup\$
    – TimWescott
    Commented Aug 6, 2019 at 15:10
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Drive the screens for full output. Replace the modulation circuit with a modulation transformer in series with the B+ Use plate modulation and modulate the 807s with a 50 Watt audio amp connected to the modulation transformer

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  • \$\begingroup\$ Thanks Wolf. Funny you should mention that. The schematic was available in a plate modulated version as well. I have a copy of the diagram. Now finding a modulation transformer...that's a different story. Ebay I guess. \$\endgroup\$
    – McMurdo
    Commented Feb 25, 2021 at 22:09
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    \$\begingroup\$ You can try using a filament transformer. I've used them with success in the past. Bigger the better. Connect the speaker output of your audio amp to the 6.3v winding of the filament transformer and connect the 115v winding in series with your B+. A 20 to 30 watt audio amp should give you some results. Worth a try as it's a simple connection. \$\endgroup\$
    – Wolf
    Commented Mar 17, 2021 at 5:19
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It's pretty simple.

  1. for an 807 in class A, the screen grid voltage is 200V maximum.
  2. You want to modulate the screen schedule. Beautiful. The EL84 should do that, but if you already supply a maximum screen grid voltage, the EL84 can't change that anymore. With a screen grid voltage of 200V, the EL84 can with a bit of luck produce a voltage swing of 100-300V.
  3. The EL84 needs an anode resistance of about 5-7K to the plus voltage. But at a low screen grid voltage of 150-200V, the anode resistance is 15k. EL84 then produces little power.

In short, this scheme requires the necessary adjustments, but with this information you should be able to get at least 10 watts including music :-). It's never going to be great with this schedule :-( Good luck, Koos

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