Wednesday, August 4, 2010

AM Transmitter, 813 Style, Part 2 (Modulator Deck)

This post (Part 2 in a series of three parts) describes the modulator stage of my 813-based 75-meter AM transmitter. (Part 1 is here and Part 3 is here).

The modulator uses a pair of 813s wired as triodes and in a push-pull configuration. It's driven by an external audio driver (in this case, the audio from a Johnson Ranger). The modulation transformer is mounted on the Power Supply Deck, and can be found in the schematics posted in Part 3 of this design.

(Click on image to enlarge)
Here are some pictures.


Using a scrap chassis from the junkbox. Just a few extra holes!



Notes on the Modulator Deck


1. John Staples' "Electric Radio" article (issue #57, January, 1994) mentioned the use of a small amount of negative voltage (around -1.5 volts) as grid bias for the modulator tubes. I had originally designed a DC supply into the Modulator Deck to provide some small amount of negative voltage for biasing these grids, but, during testing, I discovered that it wasn't very "stiff" and would fluctuate significantly with voice peaks.

A quick test revealed, though, that 0 volts bias actually worked pretty well, and it had the added advantage that I could eliminate all of the bias supply components!

I left the ability to add a negative bias supply externally, though. An external bias supply can be connected between pins 6 and 7 of the Octal Jack on the back panel of the Modulator Chassis. If no external bias supply is used, then the bias must be set to 0 volts by shorting out these two pins.

(Note: with 1800 VDC plate voltage and 0 volts grid bias, idle plate current is about 23 mA through each 813 (connected as triodes), which equates to about 45 watts plate dissipation per tube.

2. I'm not sure if the suppressor grids of the 813s should be connected to ground or connected to the other two grids in the tube when using the 813's as triodes. I followed W6BM's example (and for which he plotted his curves) and connected all three grids together, as mentioned in his article and per his schematics which he kindly sent to me (these were not published in Electric Radio).

But I noticed that K1JJ (see the K7JEB website below), as well as W7XXX in his Electric Radio article (ER # 125), connected the suppressor grids to ground. The 1959 Edition of the Radio Handbook discusses "Zero Bias Tetrode Modulators" (section 30-8) and also shows a pair of 813's in push-pull with the suppressor grids grounded and the control grid and screen grid connected together. And W7XXX, in his Electric Radio article, alludes to potential stability issues (and the modulator becoming an unwelcome generator of RF), but unfortunately he doesn't provide any references.

(Digging further, I found a discussion on the AMFONE forum (here), and some mention of stability and hi-mu versus low-mu configurations, but again, nothing that I, as an engineer, would consider definitive.)

So -- is there anything wrong with connecting the suppressor grid to the other grids? Frankly, I don't know, and my research hasn't yet revealed any adequate explanations as to why this approach might be bad. But at least John, W6BM, plotted his tube characteristics using this connection configuration: so there is some data associated with it, and I decided to follow his approach. So far there hasn't been a problem...

3. When I first designed the Modulator Deck, I didn't have resistors in series with the grids of the two 813s, and I discovered during testing that modulator plate current would suddenly sky-rocket, tripping my over-current protection circuitry in the power supply. Adding 100 ohm resistors in series with the grids of each tube calmed them down. (Note: W6BM used 56 ohm resistors, but I didn't have this value in my junkbox, and 100 ohms seems to work fine.)

4. The 2K ohm resistors across each grid are supposed to provide a more constant load for the Ranger driving this Modulator Deck, per John, W6BM. John used 1.6K ohm resistors, but I had 2K's in the junkbox, so in they went instead. Are they really needed? I don't know -- a distortion test made with and without these resistors would answer that question.

5. I had originally added the transformer-coupled "audio sample" so that I could do trapezoidal monitoring of the transmitter's performance, and my thought was that sampling the audio prior to the modulator would be best, because then I could see if there were any non-linearities introduced by the modulator/mod-transformer. Well, it was a nice idea in concept, but it doesn't work in practice. Because of the time-delay through the tubes, you really must sample the audio at the output of the modulation transformer. Otherwise, you get a "phase distorted" trapezoid.

Old ARRL handbooks have photographs showing this type of distortion on the trapezoid waveform. For example, from the "Amplitude Modulation" chapter of the 1955 ARRL handbook:


Anyway -- rather than add another audio sampling circuit at the modulation transformer (and its high voltages), I decided that it would be sufficient to just monitor the RF itself using my already-existing "RF Sample" port on the PA Deck, and simply adjust the audio gain by look for "zero-lining" on the RF waveform -- after all, this was how I monitored the performance of my other AM transmitters, and it seems to work well. So my "Audio Sample" port on the Modulator Deck really isn't needed.

6. I added some 1 ohm resistors and test points (i.e. feed-thru caps) to allow me to measure the cathode current of each 813 independently, as well as overall grid current. Measuring each tube's cathode current allowed me to easily find a matched-pair of 813s.

7. For experimenting with grid-bias voltages, an external DC supply could be connected between pins 6 and 7 on the octal plug. For 0-volts grid bias, these two pins should be shorted together.

8. Although I grounded the bases of the 813 tubes in the PA Deck (using fingerstock), I did not ground the bases of the 813s in the modulator deck.


How does it sound on the air?

You can listen to a clip of the 813 Transmitter on W6THW's website here. It's the track labeled "K6JCA 813 RIG (AM)".

(The rig was putting out about 300 watts, carrier power. Mic is a Heil PR-40 run through a Beringer 802 Mixer/EQ box, which feeds the Johnson Ranger's microphone input.)


References

Articles:
  • "A Modern One Kilowatt AM Transmitter," W6BM, Electric Radio, #15, July, 1990
  • "813 Triodes as Modulators," W6BM, Electric Radio, #57, January, 1994
  • "Triple X 813 Homebrew Transmitter, Part Two," W7XXX, Electric Radio, #125, Sept., 1999
  • "Zero Bias Tetrode Modulators," Radio Handbook, 1959 Edition, Editors and Engineers, page 662
  • "Checking Transmitter Performance," ARRL Handbook, 1955 Edition, ARRL, pages 271-273
Websites, Modulator with 813s as Triodes:

Caveats!


Standard warnings apply:

First, I may have made mistakes when writing this post or in my design. I cannot guarantee everything is correct.

Second (and most importantly), this design uses high voltages that can kill you. Be cautious and BEWARE!

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