Re: Looking for one low-power tube modulation transformer

[coffelt2]

"it9xxs" <it9xxs....@radiobanter.com> wrote in message
news:it9xxs....@radiobanter.com...
>
> 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

[Michael Black]

On Fri, 20 Jan 2012, coffelt2 wrote:

>
> "it9xxs" <it9xxs....@radiobanter.com> wrote in message
> news:it9xxs....@radiobanter.com...
>>
>> 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

[rrus...@hotmail.com]

On Jan 21, 9:56 am, Michael Black <et...@ncf.ca> wrote:
> On Fri, 20 Jan 2012, coffelt2 wrote:
>

> > "it9xxs" <it9xxs.95fa...@radiobanter.com> wrote in message

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

[Mark Zenier]

In article <it9xxs....@radiobanter.com>,

it9xxs <it9xxs....@radiobanter.com> 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 mze...@eskimo.com
Googleproofaddress(account:mzenier provider:eskimo domain:com)

[i3hev, mario]

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

[vu2nan]

it9xxs;786901 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

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/2011/02/my-first-hf-transmitter.html

73

Nandu.




--
vu2nan

[coffelt2]

"it9xxs" <it9xxs....@radiobanter.com> wrote in message
news:it9xxs....@radiobanter.com...
>

> 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

[Kenneth Scharf]

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.

[K7ITM]

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

[Ian Jackson]

In message
<59cd79e2-eab8-4564...@x6g2000pbk.googlegroups.com>,
K7ITM <k7...@msn.com> writes

Have a look at the Codar AT5 schematic:
http://www.vmarsmanuals.co.uk/new/at5.htm
http://www.vmarsmanuals.co.uk/new/at5.pdf
--
Ian

[K7ITM]

On Feb 12, 2:57 am, Ian Jackson
<ianREMOVETHISjack...@g3ohx.demon.co.uk> wrote:
> In message
> <59cd79e2-eab8-4564-8698-c3015672f...@x6g2000pbk.googlegroups.com>,

Thanks for sharing that, Ian. The webpage says "1960's." You
wouldn't happen to know any closer than that, would you? The one I'm
thinking of was a homebrew unit that as far as I know was developed by
Dick "Mitch" Mitchell in Washington state. I forget his call. He
developed it before I became a ham, so it must have been mid 1950's.
He used a 6146 RF PA, and 6146 modulator output. It was a minimalist
design, using a carbon mic with a mic transformer to directly drive
the modulator grid. I think he had a couple tubes in the RF chain
before the 6146, but I don't remember. It was very compact, built
entirely inside a 3" high aluminum chassis box, probably 10" x 12".
It used an external power supply: a dynamotor in mobile applications,
typically. We typically used one on Field Day, and given how much
power was dissipated inside that small box, I was a little surprised
that we pretty much never had any problem with it. I don't recall now
if he built them as single-band units, or had some band switching.

Cheers,
Tom

[Ian Jackson]

In message
<d29fc18e-5bd7-4d70...@vh10g2000pbc.googlegroups.com>,

K7ITM <k7...@msn.com> writes
>On Feb 12, 2:57 am, Ian Jackson
><ianREMOVETHISjack...@g3ohx.demon.co.uk> wrote:
>> In message
>> <59cd79e2-eab8-4564-8698-c3015672f...@x6g2000pbk.googlegroups.com>,
>> K7ITM <k7...@msn.com> writes
>>
>>


>>

>> >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.
>>
>> Have a look at the Codar AT5
>>schematic:http://www.vmarsmanuals.co.uk/new/at5.htmhttp://www.vmarsmanu
>>als.co.uk/new/at5.pdf
>> --
>> Ian
>
>Thanks for sharing that, Ian. The webpage says "1960's." You
>wouldn't happen to know any closer than that, would you?

No really. It was just that the mention of the centre-tapped modulation
choke immediately reminded me of the Codar AT5 and the matching
receiver, the matching receiver, the T28. I used the pair mobile back in
the 1960s, and although such equipment is hardly up to today's
more-exacting standards, I had great fun with them. You occasionally see
the odd one (or pair) on sale at the various amateur radio rallies, for
around $50.
--
Ian

[i3hev, mario]

K7ITM wrote:

> A solution to that is to... feed the RF stage its DC through an audio
> choke, and capacitor-couple the audio into the RF deck...

this is of course a perfectly good solution, at least from a technical
point of view, but... audio chokes are nowadays precious rare modern art
masterpieces :) and the only practical way to get a suitable one is to
wind it yourself, which is more or less the same as winding your own
transformer :)

> ...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. ...

been there, done that - it sure works, in a fashion... but not very
satisfactorily. At the time when I tried it, I reasoned that using a
single ended audio tube with a plate current more or less similar to
that of the RF amp tube, the total core flux in the transformer should
be more or less balanced - what I did not know then (I was *very*
young...) was that average plate currents mean very little, saturation
occurs because of peaks! :)

--
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"

[highlandham]

===================================================
To my knowledge it is called Heising modulation.As a young
(radio-unlicenced) boy I made a single 6K7 valve(tube) oscillator for
the 80m band with the anode fed by the anode voltage of a single EL3
output valve 4 Watts audio amplifier meant to feed a loudspeaker. With
a long wire antenna the results were an amazingly clean AM signal
getting out well in excess of 100 km
Those were the days before SSB.

Frank GM0CSZ / KN6WH only licenced since 1985.