Hi, I'm looking for a low-power modulation transformer. I'd like to build a QRP (5W?) tube AM transmitter for the 10m band. Thanks, 73s, IT9XXS

"it9xxs" wrote in message
...

Hi, I'm looking for a low-power modulation transformer. I'd like to
build a QRP (5W?) tube AM transmitter for the 10m band. Thanks, 73s,
IT9XXS


Are there any old tube type CB radios around?

I suppose size might matter, but sometimes old tube type
power transformers can come up with a "close enough"
impedance ratio.

Another old time solution was to find a tube type speaker
output transformer, with the secondary on an outside winding
which could be stripped away, making room to add a new
secondary. (keep a log of turns removed on the old
secondary, so a good estimate of new turns required)
(to get impedances fairly close.. precision not required)

Old Chief Lynn, W7LTQ

On Fri, 20 Jan 2012, coffelt2 wrote:


"it9xxs" wrote in message
...

Hi, I'm looking for a low-power modulation transformer. I'd like to
build a QRP (5W?) tube AM transmitter for the 10m band. Thanks, 73s,
IT9XXS


Are there any old tube type CB radios around?

I suppose size might matter, but sometimes old tube type
power transformers can come up with a "close enough"
impedance ratio.

I barely see solid state CB sets around, so I suspect the
tube ones have long disappeared.

Besides, he's not in North America, and CB was mostly a North American
thing.

Another old time solution was to find a tube type speaker
output transformer, with the secondary on an outside winding
which could be stripped away, making room to add a new
secondary. (keep a log of turns removed on the old
secondary, so a good estimate of new turns required)
(to get impedances fairly close.. precision not required)

Or find a 400Hz transformer, nobody wanted them so at one time they were
not only available but cheap. Various articles said they worked fine,
since voice didn't have to go down to 60Hz.

Michael VE2BVW

In article ,
it9xxs wrote:

Hi, I'm looking for a low-power modulation transformer. I'd like to
build a QRP (5W?) tube AM transmitter for the 10m band. Thanks, 73s,
IT9XXS

One technique was to use only the primary of a center tapped (push pull)
audio output transformer. The modulator tube, running class A, was on
one side, and the other side was used as the supply for the RF output
stage, with the B+ to the center tap.


Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)

On Jan 21, 9:56*am, Michael Black wrote:
On Fri, 20 Jan 2012, coffelt2 wrote:

"it9xxs" wrote in message
...

Hi, I'm looking for a low-power modulation transformer. I'd like to
build a QRP (5W?) tube AM transmitter for the 10m band. Thanks, 73s,
IT9XXS

Are there any old tube type CB radios around?

I suppose size might matter, but sometimes old tube type
power transformers can come up with a "close enough"
impedance ratio.

I barely see solid state CB *sets around, so I suspect the
tube ones have long disappeared.

Besides, he's not in North America, and CB was mostly a North American
thing.

Another old time solution was to find a tube type speaker
output transformer, with the secondary on an outside winding
which could be stripped away, making room to add a new
secondary. (keep a log of turns removed on the old
secondary, so a good estimate of new turns required)
(to get impedances fairly close.. precision not required)

Or find a 400Hz transformer, nobody wanted them so at one time they were
not only available but cheap. *Various articles said they worked fine,
since voice didn't have to go down to 60Hz.

* * Michael *VE2BVW

Get a single ended triode type of audio output transformer and drive
it from a hi-fi amp.

Mark Zenier wrote:

One technique was to use only the primary of a center tapped (push pull)
audio output transformer. ...

I tried it (many years ago, when I was a boy...) but it doesn't work
well: most push-pull output transformers are designed for a null total
cc flux (currents in the two halves of primary winding should be equal
and opposite) and, when driven in single-ended, the core saturates and
severe degradation of audio response occurs.

--
73 es 51 de i3hev, op. mario

Non è Radioamatore, se non gli fuma il saldatore!
- Campagna 2006 "Il Radioamatore non è uno che ascolta la radio"

it.hobby.radioamatori.moderato
http://digilander.libero.it/hamweb
http://digilander.libero.it/esperantovenezia

Hi OM Giovanni,

You could use an audio amplifier tube as the modulator with its matching audio output transformer for choke modulation.

Here's how I did it.

http://nandustips.blogspot.com/2011/...ansmitter.html

73

Nandu.

"it9xxs" wrote in message
...

Hi, I'm looking for a low-power modulation transformer. I'd like to
build a QRP (5W?) tube AM transmitter for the 10m band. Thanks, 73s,
IT9XXS


The guys presented a lot of great ideas.. If a conventional plate
modulation
transformer is found, and 100% modulation of the final RF stages plate
voltage is achieved, the next logical step (I think) is to introduce
"Negative
Peak Clipping". A highly controversial method that clipped a little of the
negative modulation envelope, and added that same amplitude to the
positive envelope peaks.
100% PLUS modulation! Oh, how the purists did cry! Fact is, that
oscilloscope observations were convincing, over the air observations
while switching the "clipper" in and out were totally convincing. It
worked! There was a little commercial AM broadcast testing, with
FCC strictly against such abhorrent practices, but it did work. I swear.
On the receiving end, the audio sounded much like the current
practice of Television stations "enhancing" commercial advertising.
The received audio amplitude would increase startlingly.
Detractors "proved" mathematically that this distortion of the AM
envelope introduced undesirable sidebands, but hey, with my
10 watts input to a 2E26 final, nobody ever complained.
Time frame, AM glory days 1955 to 1958.

Old Chief Lynn, W7LTQ

On 01/20/2012 05:08 PM, it9xxs wrote:
Hi, I'm looking for a low-power modulation transformer. I'd like to
build a QRP (5W?) tube AM transmitter for the 10m band. Thanks, 73s,
IT9XXS




Use a filament transformer. Primary winding to the plate circuit,
secondary winding driven by the output of a PA amplifier.

On Jan 22, 1:33*pm, "i3hev, mario" wrote:
Mark Zenier wrote:
One technique was to use only the primary of a center tapped (push pull)
audio output transformer. *...

I tried it (many years ago, when I was a boy...) but it doesn't work
well: most push-pull output transformers are designed for a null total
cc flux (currents in the two halves of primary winding should be equal
and opposite) and, when driven in single-ended, the core saturates and
severe degradation of audio response occurs.

--
73 es 51 de i3hev, op. mario

Non è Radioamatore, se non gli fuma il saldatore!
- Campagna 2006 "Il Radioamatore non è uno che ascolta la radio"

it.hobby.radioamatori.moderatohttp://digilander.libero.it/hamwebhttp://digilander.libero.it/esperantovenezia

A solution to that is to make it more like they used to do AM
broadcast transmitters: feed the RF stage its DC through an audio
choke, and capacitor-couple the audio into the RF deck end of the
choke (from one of the plate connections on the audio transformer).
If the audio amplifier and the RF deck use the same plate voltage, the
capacitor doesn't need to handle the full DC voltage, but it should be
non-polar. The problem then becomes one of finding (or winding) an
audio choke with enough inductance and that can handle the current.
In AM broadcast transmitters, the modulation choke was typically the
largest component. Since for voice you should only need to get down
to 300Hz or so, 10 henries inductance should be OK (about 20k ohms
impedance), and you might be OK with less. A 4.7uF coupling capacitor
should work OK, as it would be just over 100 ohms reactance at 300Hz.
4.7uF film capacitors aren't unreasonable to find. You wouldn't ever
get to 100% modulation, since the audio side doesn't go to zero volts
on the plate of the conducting side, but you could add the voice coil
winding in the proper phase to get a bit more modulating voltage.

I've also seen a design where the modulator was single-ended but used
a push-pull transformer; the RF amp was fed its DC through the other
side of the center tapped winding. That allowed reasonable balance of
the DC in the transformer, and worked decently.

Cheers,
Tom