Progress on the transmitter so far... - UK Vintage Radio Repair and Restoration Discussion Forum

geo_VE3GZB · 2nd Jan 2009, 3:15 am

More at:
http://99.239.146.96/Projects/Transm...ransmitter_V2/

I posted this also at AM Phone forum for other interested amateurs. Thought I'd share it here too!

I got this all up and running the evening of Dec 31.

The tubes are all 1920s battery tubes - British Cossor brand. The oscillator and the first speech amp are Cossor 210HL Triodes, the buffer is a Cossor 215SG Tetrode and the final PA is a push-pull pair of Cossor 215P Power Triodes, neutralized.

The American equivalents to these tubes would be an '01A Triode, a '22 Tetrode and a '71A Power Triode.

I'm probably going to change the speech amp to be another 215SG Tetrode to be followed by another 215P Power triode for Heising modulation.

In the photos, one triode is only receiving filament power now, it's not wired in for speech yet - that' the one I'll probably change to be another Tetrode, it'll give me more gain and simplify the modulator.

Yes, the coils are homemade. I have about 10,000 feet of #26 wire, some of which I around a boiled-in-wax cardboard tube, sealed with wax (I melted spare unscented Ikea white candles in a pot and dipped the coils into this). I like making coils this way, it gives me more control over the circuit. I have my small L/C meter which I use to wind the coils precisely.

I used this same approach in making my mini-loop receiver where I made my own variocoupler). I used the same British tubes in this receiver as well and I've received broadcasts from Russia, Cuba, China, Romania and Taiwan on it).

I'm not on the air yet, I have no antenna set up and there's simply too much snow and ice to do anything like this now....I was thinking of making a loop antenna for transmitting purposes but I'm budgeting my time over Xmas/New Year holidays.

I used Brass strips, stapled down to the pine base, as my conductors. In a way it sort of resembles a modern PC board with it's flat conductors. The coils are all hand-made from cardboard tubing, held together and sealed with wax from molten candles. The copper tubing is suspended with some plastic chopsticks Epoxied into place.

I was having a problem with parasitics initially. Neutralization was tricky as well since the parasitics were getting in the way of doing a proper job of neutralization.

What could I do to isolate and halt the parasitics? Were they in the buffer (which is high gain)? In the oscillator? In some intercoupling between the final tank and the hand-made coils? Would I have to erect metal screening between these stages?

Well, I found that I COULD squelch the parasitics when I removed the buffer Tetrode and shorted the plate lead to B+. That told me the parasitics were confined to the final PA!

Then I tried 150 Ohms in series between the LC circuit and the grid and that really stabilized things but reduced drive. Then I put 56 Ohms in on each PA grid and found that 99.999% of parasitics were gone and drive levels were up now! Funny, I've never seen anything in any of my older publications about putting resistance in with the grid to aid stabiliztion, only in later publications from the 40s and on have I seen this, but it really works to help stabilize the final in the old transmitter too!

I had tried using a Triode as buffer but found that I needed an extra tuned circuit on the plate-side of the coupling RF transformer to keep levels nice and Sinusoidal. Switching to a Tetrode eliminated any need for a tuned primary on the plate-side of the buffer. Perhaps the very high plate resistance of the Tetrode helps the final jump into instability whereas the low plate resistance of a Triode buffer ahead of the Push-Pull PA would squelch parasitics in the grids in older designs? Hmmm!

I experimented for about a week in my home lab - while I have my boss' Spectrum Analyzer over the holidays - and found that keeping the grid-leak capacitance as low as possible in the master-oscillator contributed to stability and a very low harmonic content, about -45dB below the fundamental. So I'm using 8.2pF in the grid leak, along with 15k+1.2mH to discharge.

A higher grid-leak capacitance I found forms a nice time-constant which encourages "squegging" at some frequencies and draws grid-current from the oscillator tank, contributing to significantly higher harmonic content through asymmetrical loading, current draught and ultimately distortion on the oscillator tank.

I tried running it on higher voltages than normal....these Cossor tubes are rated to maximum 120 volt but I managed to get them up to 180 volt successfully.

With increased bias in the final (25 volts instead of around 10-ish), plate-current draw was no higher than running at 120 volts. But I didn't want to push these tubes so hard....the next step would be to build-in a power supply into the bottom, including a modulator. I want to separate the buffer and oscillator supply from the final PA supply.

Something about these old tubes, their filament resistance between them isn't consistent after these years. I'm running filament power from 5VDC, dropping through series resistances on each socket. I worked out 30 Ohms for the Triodes and 20 Ohms for the Tetrode based on dropping 3 volts at their rated filament current (0.1A for the Triodes, 0.15A for the Tetrode) and found discrepancies not connected to oxidation (the pins on these tubes must be silver-plated, they had a dark/black oxide layer which I cleaned off with 200 grit sand paper). So I ended up tailoring a few resistances to each tube in lieu of any rheostats.

Frequency stability is good, at the low frequency end (80 meters) I'm seeing drift in the order of a few hundred hertz over perhaps an hour. At the top end, it's a little more, like about 1kHz over 45 minutes or so. My power supply drifts a bit too so I know it's FM'ing my oscillator a bit.

These are all battery-operated tubes BTW, the schematic doesn't show that because I have only 12AX7 and 6AQ5 in the CAD library. I found that being the battery-operated type, they run much cooler since they have no heater/cathode mass as AC operated tubes do, so I think that helps stability!

The spectrum analyzer screen shot shows the fundamental at the far left, the second harmonic toward the middle and the third harmonic towards the right. Each vertical division represents -10dB. Not bad! And there are no Crystals in the circuit!

I find it awesome that such old basic technology can STILL serve a good useful purpose and actually function decently given care in the details! No damned esoteric, high-tech micro-miniature materials are really needed!

geo_VE3GZB · 2nd Jan 2009, 4:35 am

I'm getting about 4 watts out when I crank it up to 180 volts, continuous at 120 volts I get about 2 1/2 watts out, perfect for QRP operation!

An interesting footnote.....I put the design into a computer simulation program here. My simulation program has vacuum tubes in among it's device-models.

According to the simulation, the design doesn't work! Ha ha!!

That is why I'll never have much respect for this darned computer-generated nonsense and design, I think the software in my brain is better than any bit of digital rubbish!

Dr-Watts · 3rd Jan 2009, 3:21 pm

Quote:

Originally Posted by geo_VE3GZB

That is why I'll never have much respect for this darned computer-generated nonsense and design, I think the software in my brain is better than any bit of digital rubbish!

Hear hear !

Racalfanatic · 3rd Jan 2009, 5:23 pm

Hi,
Very interested to see that you are using Heising modulation.
I'd be very interested in your results with this. I beleive the Codar AT5 transmitter uses Heising modulation and they always sound very clear and well modulated.

2nd Jan 2009, 3:15 am	#1
geo_VE3GZB Retired Dormant Member Join Date: Nov 2008 Location: Grand Valley, Ontario, Canada Posts: 8	Progress on the transmitter so far... More at: http://99.239.146.96/Projects/Transm...ransmitter_V2/ I posted this also at AM Phone forum for other interested amateurs. Thought I'd share it here too! I got this all up and running the evening of Dec 31. The tubes are all 1920s battery tubes - British Cossor brand. The oscillator and the first speech amp are Cossor 210HL Triodes, the buffer is a Cossor 215SG Tetrode and the final PA is a push-pull pair of Cossor 215P Power Triodes, neutralized. The American equivalents to these tubes would be an '01A Triode, a '22 Tetrode and a '71A Power Triode. I'm probably going to change the speech amp to be another 215SG Tetrode to be followed by another 215P Power triode for Heising modulation. In the photos, one triode is only receiving filament power now, it's not wired in for speech yet - that' the one I'll probably change to be another Tetrode, it'll give me more gain and simplify the modulator. Yes, the coils are homemade. I have about 10,000 feet of #26 wire, some of which I around a boiled-in-wax cardboard tube, sealed with wax (I melted spare unscented Ikea white candles in a pot and dipped the coils into this). I like making coils this way, it gives me more control over the circuit. I have my small L/C meter which I use to wind the coils precisely. I used this same approach in making my mini-loop receiver where I made my own variocoupler). I used the same British tubes in this receiver as well and I've received broadcasts from Russia, Cuba, China, Romania and Taiwan on it). I'm not on the air yet, I have no antenna set up and there's simply too much snow and ice to do anything like this now....I was thinking of making a loop antenna for transmitting purposes but I'm budgeting my time over Xmas/New Year holidays. I used Brass strips, stapled down to the pine base, as my conductors. In a way it sort of resembles a modern PC board with it's flat conductors. The coils are all hand-made from cardboard tubing, held together and sealed with wax from molten candles. The copper tubing is suspended with some plastic chopsticks Epoxied into place. I was having a problem with parasitics initially. Neutralization was tricky as well since the parasitics were getting in the way of doing a proper job of neutralization. What could I do to isolate and halt the parasitics? Were they in the buffer (which is high gain)? In the oscillator? In some intercoupling between the final tank and the hand-made coils? Would I have to erect metal screening between these stages? Well, I found that I COULD squelch the parasitics when I removed the buffer Tetrode and shorted the plate lead to B+. That told me the parasitics were confined to the final PA! Then I tried 150 Ohms in series between the LC circuit and the grid and that really stabilized things but reduced drive. Then I put 56 Ohms in on each PA grid and found that 99.999% of parasitics were gone and drive levels were up now! Funny, I've never seen anything in any of my older publications about putting resistance in with the grid to aid stabiliztion, only in later publications from the 40s and on have I seen this, but it really works to help stabilize the final in the old transmitter too! I had tried using a Triode as buffer but found that I needed an extra tuned circuit on the plate-side of the coupling RF transformer to keep levels nice and Sinusoidal. Switching to a Tetrode eliminated any need for a tuned primary on the plate-side of the buffer. Perhaps the very high plate resistance of the Tetrode helps the final jump into instability whereas the low plate resistance of a Triode buffer ahead of the Push-Pull PA would squelch parasitics in the grids in older designs? Hmmm! I experimented for about a week in my home lab - while I have my boss' Spectrum Analyzer over the holidays - and found that keeping the grid-leak capacitance as low as possible in the master-oscillator contributed to stability and a very low harmonic content, about -45dB below the fundamental. So I'm using 8.2pF in the grid leak, along with 15k+1.2mH to discharge. A higher grid-leak capacitance I found forms a nice time-constant which encourages "squegging" at some frequencies and draws grid-current from the oscillator tank, contributing to significantly higher harmonic content through asymmetrical loading, current draught and ultimately distortion on the oscillator tank. I tried running it on higher voltages than normal....these Cossor tubes are rated to maximum 120 volt but I managed to get them up to 180 volt successfully. With increased bias in the final (25 volts instead of around 10-ish), plate-current draw was no higher than running at 120 volts. But I didn't want to push these tubes so hard....the next step would be to build-in a power supply into the bottom, including a modulator. I want to separate the buffer and oscillator supply from the final PA supply. Something about these old tubes, their filament resistance between them isn't consistent after these years. I'm running filament power from 5VDC, dropping through series resistances on each socket. I worked out 30 Ohms for the Triodes and 20 Ohms for the Tetrode based on dropping 3 volts at their rated filament current (0.1A for the Triodes, 0.15A for the Tetrode) and found discrepancies not connected to oxidation (the pins on these tubes must be silver-plated, they had a dark/black oxide layer which I cleaned off with 200 grit sand paper). So I ended up tailoring a few resistances to each tube in lieu of any rheostats. Frequency stability is good, at the low frequency end (80 meters) I'm seeing drift in the order of a few hundred hertz over perhaps an hour. At the top end, it's a little more, like about 1kHz over 45 minutes or so. My power supply drifts a bit too so I know it's FM'ing my oscillator a bit. These are all battery-operated tubes BTW, the schematic doesn't show that because I have only 12AX7 and 6AQ5 in the CAD library. I found that being the battery-operated type, they run much cooler since they have no heater/cathode mass as AC operated tubes do, so I think that helps stability! The spectrum analyzer screen shot shows the fundamental at the far left, the second harmonic toward the middle and the third harmonic towards the right. Each vertical division represents -10dB. Not bad! And there are no Crystals in the circuit! I find it awesome that such old basic technology can STILL serve a good useful purpose and actually function decently given care in the details! No damned esoteric, high-tech micro-miniature materials are really needed! Attached Thumbnails Last edited by Brian R Pateman; 2nd Jan 2009 at 9:05 am. Reason: Enormous image removed from post & converted to thumbnail.

3rd Jan 2009, 5:23 pm	#4
Racalfanatic Retired Dormant Member Join Date: Aug 2007 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 105	Re: Progress on the transmitter so far... Hi, Very interested to see that you are using Heising modulation. I'd be very interested in your results with this. I beleive the Codar AT5 transmitter uses Heising modulation and they always sound very clear and well modulated.

2nd Jan 2009, 4:35 am	#2
geo_VE3GZB Retired Dormant Member Join Date: Nov 2008 Location: Grand Valley, Ontario, Canada Posts: 8	Re: Progress on the transmitter so far... I'm getting about 4 watts out when I crank it up to 180 volts, continuous at 120 volts I get about 2 1/2 watts out, perfect for QRP operation! An interesting footnote.....I put the design into a computer simulation program here. My simulation program has vacuum tubes in among it's device-models. According to the simulation, the design doesn't work! Ha ha!! That is why I'll never have much respect for this darned computer-generated nonsense and design, I think the software in my brain is better than any bit of digital rubbish!