Nordic Breeds WA4VZQ wrote:
Read Lynn's post again. He said the carrier was strong but the audio was
quite weak. It is quite difficult to get over 95% modulation with a
suppressor modulator. With beam power tubes, you get far less than this.
Also you get lots of distortion because the beam forming plates do not
linearly control plate current, so you have to lower the modulation level
even further to make the audio readable by listeners. Hence the "strong
signal with very low modulation level" makes sense.

My experience was about 25% modulation, and that was with distortion that
was... well... kind of bad.

I would characterize this as pretty much the worst quality AM ever, worse
than screen grid modulation by a long shot.

Fortunately with
pentodes, it is easy to drive the output to zero (negative modulation).
The real problem occurs when the suppressor is driven very far into the
positive voltage region and it starts drawing current. Typically the
suppressor grid can only dissipate a few watts. Since current is drawn
during the positive peaks, an audio driver must present a low impedance.

And this also quickly becomes a distortion source unless the audio driver
stage is pretty hefty.

Just say no.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Fortunately with
pentodes, it is easy to drive the output to zero (negative modulation).
The real problem occurs when the suppressor is driven very far into the
positive voltage region and it starts drawing current. Typically the
suppressor grid can only dissipate a few watts. Since current is drawn
during the positive peaks, an audio driver must present a low impedance.

73, Barry WA4VZQ

Ah, yes, "(negative modulation)"!

You seem to be "hep" on old stuff, do you remember "negative peak
clipping"?
I thought I was in Heaven in about 1958 with a single 2E26 final on 15
Meters.
In class C, with a pair of 6L6 modulators, push-pull, class AB1, and used a
VR
tube across the modulation transformer secondary clipping the negative
peaks, while
allowing the positive peaks to go "over" 100%.

Technical gurus of the day poo-poo'd the scheme. It looked a little
rough on the
scope, but unsolicited signal reports said it "packed a lot of modulation".
I am thinking it must have been a little like more modern amateur
"speech
processing"

I also seem to remember those modified 1646's getting almost 20%
suppressor
grid modulation after tinkering around with power supply voltages.

Old Chief Lynn, W7LTQ

Antonio Vernucci wrote:
I am planning to build a suppressor-grid modulated AM transmitter.

In all the diagrams I have seen they use proper pentodes, i.e. tubes
that have a real suppressor grid (e.g. 803).

I have seen no diagram using beam power tubes (e.g. 813 or 814) which
have beam forming plates instead of the suppressor grid .

Before purchasing 803 tubes, I would like to be sure that 813s or 814s
are really unsuitable for suppressor-grid modulation.

My question is whether, with 813s or 814s, the plate current can be
actually controlled by varying the beam forming plates voltage.

Does anyone have experience on that issue?

73

Tony I0JX

Rome, Italy

A little late, but could be of interest:

Try the 2E22 valve.

I used to work with this excellent valve during my military service in
1968, repairing military equipment that used it as supresor modulated.

It was built especially for that purpose.

Here are the specifications:

2E22
Power Pentode

Base & Bulb
EIA Base 5J

Mechanical Data

EIA Base ...................................... 5J
Electrical Data

Heater Voltage ................................ 6.3 V
Heater Current ................................ 1.5 A
Direct Interelectrode Capacitances (approx)

Pentode
Input ......................................... 13 pf
Output ........................................ 8 pf
Grid to Plate ................................. 0.2 pf
Maximum Ratings (Design Center Values)

Pentode
Plate Voltage ................................. 750 V
Grid No. 2 Voltage ............................ 250 V
Plate Dissipation ............................. 30 W
Grid No. 2 Dissipation ........................ 10 W
Characteristics and Typical Operation

Class C Oscillator/Amplifier
Plate Voltage ................................. 750 V
Grid No. 3 Voltage ............................ 22.5 V
Grid No. 2 Voltage ............................ 250 V
Grid No. 1 Voltage ............................ -60 V
Grid No. 1 Current ............................ 6 mA
Plate Current ................................. 100 mA
Grid No. 2 Current ............................ 16 mA
Driving Power ................................. 0.55 W
Power Output (approx) ......................... 53 W
Characteristics and Typical Operation

Class C Modulated Amplifier (Supressor)
Plate Voltage ................................. 750 V
Grid No. 3 Voltage ............................ -90 V
Grid No. 2 Voltage ............................ 250 V
Grid No. 1 Voltage ............................ -90 V
Plate Current ................................. 55 mA
Grid No. 2 Current ............................ 29 mA
Power Output (approx) ......................... 16.25 W

Alejandro Lieber
LU1FCR
Rosario - Argentina

Real-Time F2-Layer Critical Frequency Map foF2 at:

http://1fcr.com.ar

Try the 2E22 valve.

I used to work with this excellent valve during my military service in 1968,
repairing military equipment that used it as supresor modulated.

Thanks for suggestion. I have some 2E22s, even some 2E24s (quick.heating
versions).

The problem with the 2E22 is that the output power would be small (probably 10W
or so) because, using suppressor screen modulation, the efficiency is rather low
(30%).

Using two 803s I can instead get 200W or perhaps 250W of carrier.

73

Tony I0JX

"Antonio Vernucci" wrote in message
. ..
Thanks for suggestion. I have some 2E22s, even some 2E24s
(quick.heating versions).

The problem with the 2E22 is that the output power would be small
(probably 10W or so) because, using suppressor screen modulation, the
efficiency is rather low (30%).

Using two 803s I can instead get 200W or perhaps 250W of carrier.

73

Tony I0JX

Hi Tony,

The 2E22 is a true pentode while the 2E24 is a quick heating filament
version of the 2E26. Both are beam power tetrodes and not suitable for
suppressor modulation. A pair of suppressor modulated 803's will deliver
approximately 100 watts of carrier.

73, Barry WA4VZQ

Hi Tony,

The 2E22 is a true pentode while the 2E24 is a quick heating filament
version of the 2E26. Both are beam power tetrodes and not suitable for
suppressor modulation. A pair of suppressor modulated 803's will deliver
approximately 100 watts of carrier.

73, Barry WA4VZQ

Hello,

Barry is right !

With 2 x 803, input power will be as far as 1,5 x 2 x 125 W = 375 W

and carrier power, about 125 W (33% of 375) only. No more !

All the best
Jean-Pierre (F6BGV)