Mississippi Historical Radio and Broadcasting Society

515 Frequently Asked Questions & Comments

Some general notes and comments:

1) The circuit is from a Kit made by Graymark - their model # 515

2) It is an easy to build circuit that performs well. Not great. Just well. It can be made with readily available parts (a single AA5 plus an extra 50C5 provide just about everything) -- and it is EASILY modified to output near 1W RF. It runs around 100mw as published - but there is an easy way to push that on up with only a couple of parts. We don't publish the mods as they are not legal here in the U.S. - but they can be found with a bit of effort on the Web.

3) Some people note that Heising modulation (the rf choke on the plate) does work better. This is true - but only marginally - if you try to push either circuit past 50 - 60 % modulation - you start getting some serious FM artifacts - the by-product of modulating an Oscillator.

4) if you are really serious about 50C5 transmitters - there is a circuit called the "Widow Maker" designed by Mike Murphy and covered in Electric Radio #101 September 1997. This is a serious transmitter: 12BA6 Osc. 12AX7 pre-amp/driver a 50C5 heising modulator and a 50C5 PA. Oh and 300VDC B+ supplied by a line-powered doubler. Two points 1) it is a heising so it is a 1:1 modulation scheme - so it can't reach 100% - but 2) since you are not modulating the Oscillator - you can get real close to 100%... and it's pretty clean. As this "Widow Maker" is built - it outputs a fairly clean 2W.

Just how far can a "50C5" transmitter be pushed (practically here, not just in theory)... We'll just quote part of the info we have: "Tom, K1JJ, built a rig (160 meters - just above the AM band) using 50C5s - but used two in the final, and four (!!) in the modulator, with a real modulation transformer. He also used an embarrassingly high plate voltage on the tubes: 300VDC on the finals and 600VDC on the modulators. This gets you into the 25W class. Just how much voltage will a 50C5 plate take in amateur service? The RCA Receiving tube manual lists the 50C5 Plate voltage at 150V and the screen at 120V (the 50L6 is rated 200V plate and 125V screen). Most users report that these ratings can be exceeded by a factor of 2 to 3."

Usual disclaimers:
1) these are line powered devices - and can easily deliver fatal electric shocks. Use all safety precautions.
2) Making any modification to a part 15 transmitter is in violation of the law and it makes the FCC really unhappy.
3) Using an Amateur transmitter within the Broadcast Band is in violation of the law.
4) We have no connection whatsoever with ER - other than as subscribers / readers.

Electric Radio (ER) can be contacted:
Barry R. Wise man N6CSW
Electric Radio
14643 County Road G
Cortez, CO 81321-9575

Some Questions and answers:

Q: I'm getting a lot of distortion. How come?

A: There are two primary causes for distortion in a properly built 515.
1) The pre-amp has a lot of gain - and it's easy to "overdrive" that stage - and cause the modulator tube to be run from saturation to cutoff - which it should never (quite) do. The best way to check these stages is to use an o'scope and look at the wave form at the plate of the lower (modulator) 50C5. If it clips - and /or there are "gaps" in the waveform - the audio is too high. Many people are "driving" the input stage with CD players with pretty high outputs - and they are overdriving the modulator. Adding a "padding network" on the input usually solves the problem. Remember - the purpose of the modulator is to cause the oscillator to vary it's strength from nearly 0 to nearly twice the signal it has with no input.

2) Wrong Frequency. Many people use an oscillator coil salvaged from a broadcast receiver - which is fine - but sometimes they forget to check the resulting frequency - and when they try to tune the signal - they find a weak - or poor sounding signal. Most radios use an oscillator that is above the received frequency by the IF frequency. So to receive a station at say 1000 on the dial - the oscillator is at 1455. If a coil is running "high" -- it's frequency could be "above" the AM band - and what you are receiving is not the actual "signal" but the IF image. Here is an example: Say the 515 is actually running at 1800. An AM radio could receive that signal at 890 on it's dial - but the strength will be down alot - and may sound real distorted - because even though it's in the IF's pass band - the RF section of the radio is tuned to 890 - and trying real hard to not receive that signal. An o'scope - or frequency counter (or a radio with a 1.7 - 2Mhz tuning range) can be used to see if this is the problem. Using a larger value for the tank capacitor (C5) will bring the frequency down.

Another less common source of distortion is the coil itself. Some salvaged coils are wound with an extremely small gauge of wire. This wire has enough resistance to cause losses in the oscilliator circuit. There are two tests to determine if this is the problem. First, using an o'scope - check to see if the oscillator is running ok - the waveform should be a reasonable sinewave. Also - the cathode of the oscillator should be approximately 8 Volts. Then carefully connect pin 1 of the Oscillator tube - through a millammeter - to ground. The meter should indicate approximately 6ma. --- and the waveform should nearly double - but remain a realitively clean sine wave. If the output wave "flatens out" or does some other wierd thing - there is a problem in the 515 - and the oscillator coil may be suspect.

Q: Where can I get a good coil?

A: AES has some "universal" osc. coils - we are going to get a couple and check them out - we hope these will work well.

Reminder: Keep in mind - the 515 is running on 120VAC wall current - and though if wired as reccomended the chassis is not hot - there are still dangerous voltages and currents on many points. BE CAREFUL!
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Copyright © 1997 Randy Guttery