Excellent little transmitter you got there, Dave.

I'll have to give it a try myself. Yeah, your transformer is a bit wimpy for the circuit you have, such as the filaments are pulling 1.2 amps on that 1 amp winding. (I was surprised the 6C4 only pulls 150 milliamps!) but it should work ok. I have a similar situation on a little amplifier I built.

http://userpages.bright.net/~geary/projects/index.html

The transformer is also a bit too wimpy, but I already cut the hole, so i couldn't choose a bigger one. Though, it does get a bit warm after some use. I added a separate filament transformer for the 12AX7s to help take the load off the main transformer.

When I have more time, I'll build one of those transmitters myself. Thanks!

-Steve

I thought I might need to substitute a 6BH6 for the 6AU6 but the transformer must be very conservatively rated since it just barely gets warm after several hours of use. The total circuit draws about 17 ma from the B+ supply. With 125 volts applied to the plate of the oscillator tube (modulator disconnected), the tube draws about 5 ma.

Steve - I like the wooden aprons on your stuff!

Dave

Hi Dave

Do you have specs for a "new band" coil??? I don't own a set with the old band and have no plans to buy one...

The AM guy

I don't remember the exact specs but if you start out with about half of what is needed for the old band, it should get you in the ballpark. It will probably be less than half so you can do some experimenting by taking a turn off at a time and/or spreading the spacing between windings.

Dave

Cool info Dave, thanks!

I was thinking, is it possible to convert this to crystal control? Or is it reasonably stable as it is?

The problem with crystal control would be the need for a +/- 75 kHz modulation swing. It can be done with PLL circuits but that gets complicated. Stability of my unit seems good enough. Once warmed up, I don't notice any drift.

Dave

Did anyone successfully build this for the new FM band? Just curious to see how it worked out.

-Dan

Very nice work. I would probably have to modify it for the new FM band too. I wonder what the maximum range would be for this transmitter?

It'd been done, believe me, though I'm not sure you'd really want to try it. Here's a link to an RCA BTF-10D broadcast transmitter that used the BTE-10B exciter which does what you've proposed: http://louise.hallikainen.org/Broadcast ... tf-10d.pdf It has the reactance modulators plus crystal control. Caution- it's a large file to download. The relevant info is near the bottom as you scroll down. There are pictures of the exciter and a schematic of it. Except for a few solid state diodes for the DC and a couple in the AFC, it's all done with tubes. There's an even older exciter that uses a similar topology but I don't have any links to a schematic for it.

Scott Todd

Ham magazines and handbooks of the roughly 1947-57 period had small 10M (and 11M)FM mobile transmitter articles that could easily have the RF coils scaled to ~42 MHz. The fundamental crystal frequency was in the 7MHz area and higher and lower frequency non hamband crystals are almost free. A tube lineup could be 6AU6 reactance modulator, 6AU6 oscillator/doubler, 6AU6 doubler, 6AQ5 doubler/amplifier; and using a 5.25mc FT-243 surplus crystal or VFO. The somewhat high output can be padded to be legal and also facilitate a forced match to a random length antenna.

Carl

I consider my work on this project to be relatively sloppy since there were many trials and errors involved. The range depends largely on the antenna and how well it is matched to the frequency. I found the range to be more than adequate with a 3' length of wire dangling from my workbench. Covers the whole house and yard with a strong signal.

To modify for the new band would require experimentation with the coil size but I would start with about half the number of turns as the one I showed. It also depends on wire size, coil diameter and space between turns. Transmitting in either band is at your own risk since part 15 rules for these frequencies are very stringent.

Dave

Hmmmm......I suppose that I was a little hesitant to build this FM transmitter as both of the two AM transmitters that I had earlier built produced such fantastic sound. In the case of the two tube 6AB4-6888 AM transmitter, I originally modified it for a 14.31818 Mhz carrier. The audio quality on that SW band was superb, and it sounded exactly the same when I eventually converted it over to the AM band (1.0 Mhz).

I think that I'm going to get started and build this FM transmitter. I previously had no doubt whatsoever as to this transmitter's performance; it's just that I thought that three great sounding transmitters in my collection was going to be redundant. Nice work Dave, and thanks a heap for sharing the schematic with us.

Let us know of your progress and the coil dimensions you wind up with.

Dave

I would have built a little tuneable downconverter using a 6J6 or similar

Nothing to listen to here on FM except NPR and one classical station. The FM dial is loaded with commercial stations but their programming is awful. I already transmit internet jazz stations using a homebrew (solid state) new-band FM transmitter so I thought I'd try something I built 50 years ago but for the old band.

BTW - if anyone is looking for a cheap FM module to use as an exciter for a new-band project, these $4.00 units work great.

http://www.ebay.com/itm/New-200-Channel ... 3cb7232728

"Let us know of your progress and the coil dimensions you wind up with."

Well, it will probably be Sunday (after the Thanksgiving holiday) at the earliest when I begin gathering up the materials, and about a week or so thereafter when I'm through scavenging for parts.

On the FM band, there are a couple of classical stations on the lower end of the dial with the rest being predominantly hard or punk rock in my area. On the AM band, the line up on most stations is usually talk radio.

Luckily, many ARF members are fortunate enough to possess their own transmitters so their receivers don't wind up being slaves to such junk broadcasts.

Dave: One question. Is the "pre-emphasis" rheostat on the 6AU6 critical?

No...not critical. Anything from 10k to 100k should work fine. I used 25k because I had one handy and it seemed to give the range I wanted. 100k will give a narrow adjustment range, though. You could eliminate it entirely and experiment with fixed values for the resistor and cap. I didn't do measurements to come up with a 75 us pre-emphasis curve. I just adjust the pot to give it a bit of treble boost so it sounds right.

I did find the 1.5k cathode resistor on the 6C4 to be somewhat critical to produce a low distortion, wide-band (75 kHz) modulation swing.

Dave

Thanks Dave. That's what I wanted to know.

Hmmmm......I didn't happen to have anything smaller than #24 enameled wire in the back room for this project so I elected instead to purchase a 0.4uH RF coil (Part # TOR.4) from SurplusSales.com. I'm planning on using this coil with a 3-15pf compression trimmer capacitor so that should get me somewhere near the 101MHz ballpark.

I'm currently getting the parts together, but it looks like it's going to be a couple of weeks before I get them all scrounged up.

101Mhz was the preferred operating frequency for the "New Band" FM transmitter that I'm getting ready to build as it offered the best window for the 'new station'. There are a horde of stations that are crammed side by side like sardines throughout that entire band.

I may have to tune that transmitter mostly by "ear" as my 50Mhz scope will be well out of range at that frequency, but that's another day.