[AMRadio] AM and power

[Bernie Doran]

Yep, it does need to be a quiet band, sometimes I have to go to the 4KC and 
tune off center to pick up the sideband with the least QRM.

Too bad the guys that made the modulator tube change from TLs to TH are not 
around to ask. I would lean toward the TL and run them AB1, but it does take 
a lot of voltage swing to drive them, then again the TH take a fair amount 
of grid current as the grids have to be driven significantly more positive. 
I really like the way Gates did it, the 807 drivers are cathode followers, 
connected directly to the 833 grids, no transformers, no caps.

For modulator service the square law does not really apply.  however for the 
RF stage it does.  If one looks at the plate curves for a Tetrode, it can be 
seen that with a constant screen voltage doubling the plate voltage has 
almost no effect on plate current, the same for a 3-1000A hi mu triode, 
833As medium mu show a significant plate current increase, but not quite 
double, the 304TL  mu =12 more than doubles with a doubling of the plate 
voltage.      I have often wondered if using a TL on the positive side of 
the modulator and a TH on the negative peak side would/could provide a 
smooth limiting of the neg peak.

Regarding the change from TL to TH, I think I just found the answer!!  the 
75TH requires 175 Volt grid swing and the TL requires 300 Volts. they both 
produce  about the same power 300 watt for the TH and 350 for the TL. But 
look at this!!the TH has a THD of only 2 % while the TL is clear up to 6%. 
That is the first time that I have seen Eimac publish distortion data.   I 
will have to go and check the 100T and 250T , but I sure do not remember 
ever seeing distortion data.  That 75TH and then the 304TH is a Hi Fi tube. 
If I could reduce distortion by 4% just by changing tubes. It would not take 
very long to make up my mind!!!  bet the same does apply to the others also 
but may not be published.

Further evidence that  the old goats really did know of what they did!!!


----- Original Message ----- 
From: "Todd, KA1KAQ" <ka1kaq at gmail.com>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Thursday, January 26, 2012 11:03 AM
Subject: Re: [AMRadio] AM and power


On Thu, Jan 26, 2012 at 4:44 AM, Bernie Doran
<qedconsultants at embarqmail.com> wrote:

> It took me some time to find a receiver
> that would pass that bandwidth and audio. The R390A with modified audio
> seems to meet what I wanted. It is usually set in the 8 KC position ( the
> actual filter passes 11 KC) , that sounds very good with a transmitter 
> that
> is flat out to that or more. on very rare occasions The 16 KC position 
> shows
> a slight improvement, usually means a home built or broadcast transmitter.

And this applies to the nights that are relatively quiet enough to
listen in the wide positions. Unless I have a blowtorch signal nearby
on a noisy night like K4HX, the improved fidelity is lost in the
static crashes or adjacent frequency interference.

> That brings up Triodes VS Tetrodes. I still believe that
> low or medium mu Triodes simply have the ability to modulate( plate
> modulate) much better. Especially the low mu tubes. Hi mu Triodes and
> Tetrodes do not even come close to the square law plate characters needed.

Interesting. Collins built the early 30Ks with 75TLs for modulators
but changed to the TH. I've seen the TH used over the TL in other
places too, can't recall where. BC-610 maybe? Perhaps done
intentionally to throttle the modulation a bit at the gate?

~ Todd,  KA1KAQ/4
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