[AMRadio] Class C modulation

[John Coleman]

One of the most missed issues is that a lot of folks think that changing
tubes will give them more power, but the increased capability of the tube
upgrade is only realized if the voltage or load is changed to make use of
that capability.  In some cases, a tube upgrade, with more plate dissipation
and power handling capability will deliver less power than the original
because of one of the following reasons.

1. The MU of the new tube is less than the original, therefore requires more
drive.

2. The new tube is designed to work with higher voltages and less current,
requiring a load of higher impedence than the original.

3. The new tube can deliver the current change required but only with a
higher voltage.

4. Improper bias stabiliztion required by the new tube.

5. The new tube is not new, but is weak.

There may be other reasons that I haven't thought of, but I think that is
the most.

I can't really understand why any one would want to run a pair of 4-400s as
finals in class C.

Maybe Class B or AB but not Class C.

These tubes in pairs, or capable of 800 watts dissipation.  At 75%
efficiency they could be run up to 3200 Watts input with 2400 watts out in
intermittent service and ( this is a guess ) about 2400 input and 1800 watts
output for near continuous operation.

I think they would make a much better AB modulator for a single 833.

Just my opinion
73, John, WA5BXO

-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net]On Behalf Of wwd at netheaven.com
Sent: Tuesday, March 19, 2002 10:13 AM
To: amradio at mailman.qth.net
Subject: Re: [AMRadio] Class C modulation


Something is wrong with your configuration or maybe weak tubes if you are
seeing results like these.

The goal today (often) is a setup which can achieve well beyond 100%
modulation to allow room for positive peaks (male voice pattern) as well
as audio processing to maximize intelligibility AND full frequency range
impact. The last thing you want to transmit is a totally flat response
which shows up in the receiver as dull and lifeless. Anyone who has heard
Tim, WA1HLR, from his home QTH has a good idea of this concept brought to
perfection. To pick a well known example; there are a number of others.

So you really need TWICE the tubage in the modulator as in the Class C
final. With adequate mod-iron and power supply, too.

Bill        N2KQA


In <018e01c1cf5d$ccb051a0$0200a8c0 at hamhome.net>, on 03/19/2002
   at 09:50 AM, "w5omr" <w5omr at yahoo.com> said:


>: Question:  Have you ever built a transmitter using 4-400's modulated with
>: 810's or should I use 250TH tubes to modulate a pair of 4-400A's.  Thank
>: you.

>I've had a pair of 250TH's in the final of my homebrew rig, since I've
>had it.

>I've modulated that thing with a pair of 811's, 810's, 4 810's in p-p
>parallel, and 250TH's.  the next incarnation of "The Titanic" will be
>with a pair of 450TL's (if I can ever slow down at work, enough to get
>started on it!)

>So far, the 250TH's have been superior, modulating a pair.  However, do
>NOT discount a pair of 811's.  Those dudes put out more audio than a pair
>of 810's did.

>Sometimes you just gotta forget what the Handbooks say, and go with  what
>works.

>I switched from the 811's to the 810s, because the handbook said 810's in
>class B, have like 700w output.  A far cry from the 811 classification,
>but the kicker was, there was more audio out of a pair of 811's, hands
>down.

>73 = Best Regards,
>-=Jeff/W5OMR=-

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