[AMRadio] Re:Reactor Wanted


jeremy-ca km1h at jeremy.mv.com
Mon Nov 12 19:55:04 EST 2007


----- Original Message ----- 
From: "Brett Gazdzinski" <brett.gazdzinski at verizon.net>
To: "Ben Dover" <quixote2 at ix.netcom.com>; "Discussion of AM Radio in the 
Amateur Service" <amradio at mailman.qth.net>
Sent: Monday, November 12, 2007 6:36 PM
Subject: Re: [AMRadio] Re:Reactor Wanted


>I suppose the added voltage makes the tubes run more as they like, and make
> much more power then needed.


Modulator or linear amp tubes (I treat them the same here) are designed to 
operate within certain parameters to maximize plate efficiency while 
maintaining acceptable distortion levels. Operating above those specs will 
increase power at the expense of tube life and increased distortion.


>
> Really, the power needed seems to be much more than 50% plus the
> loss in the mod iron,, I would recomend the same power out of the 
> modulator


If you read design info from the 30's that was the norm in many books and it 
specifically stressed circuit losses and having sufficient overhead. In 
later years the norm drifted to running modulators at their maximum ratings. 
I can only guess that as RF tubes that had ever increasing power 
capabilities came along many tried to justify the bare minimum of audio 
under so called "ideal" conditions.


I will be using 810's at 725W "advertised" audio to modulate a pair of 813's 
at 750W input. There will be a common PS providing ~ 2250V. Its a bit over 
spec for the 813's but nothing that a little air cant cure; the tubes hi-pot 
at well over modulation peak voltages.


> as
> RF input.
>
> The 4X150a deck can produce 650 watts of audio at 2000 volts and 500ma.
> I have done lots of expeiments over the years, with different mod iron,
> modified hysing,
> and since both the mod and rf deck are variac'd, lots of different power
> levels
> and voltages.
>
> Things always seem to work better reducing the RF current, and increasing
> the mod
> voltage.
>
> Its my theory that the extra mod voltage allows the tubes to run in the 
> low
> distortion range, and the RF tubes have reserve emission for modulation
> peaks.


The problem with that theory is that tetrodes have a higher level of IMD 
increase than triodes when run above their spec. Feedback and a low 
impedence regulated screen and grid supplies can gain back some of that IMD. 
Another tetrode problem at over voltage is secondary screen emission which 
contributes to even more IMD. Used tubes and especially FM pulls almost 
always have screen emission.

I worked extensively with the 4X150/250 family for decades on VHF where 
their cheap price resulted in high use in homebrew amps with resultant very 
poor IMD, often in the teens. It took several of us in the US and Europe to 
establish cause and effect and establish correct operating parameters.

Good 4CX250B & R tubes operated at 2500V in SSB service can deliver 28-32 dB 
IMD with the above criteria. Thats about equal to an average run of the mill 
triode. It still requires careful tuning and heavy loading which equates to 
using a correct mod xfmr for both audio and RF loads.

I dont see any published specs for 4X150A/7034 operation in either AB1 or 
AB2 at any voltage. Have you obtained that data in your actual setup? Youre 
not using the old glass 4X150A I hope.

>
> I also dont take current design (or old designs) as the best way to do
> something,
> only the cheapest way of doing something, that is why I have nothing but
> home brew
> stuff left in the shack.
> Never had a commercial or ham piece of gear where they did not cut corners
> to reduce costs. WRL made that an art....


Good performance comes by the pound. Tailoring the audio at low level is a 
seperate part of the problem and almost always inadequate in commercial ham 
rigs. My current rig is a Viking I but others will be homebrew using the 
best of the old designs and updated if necessary but not going overboard 
with high tech enhancements.

Carl
KM1H



>
> Brett
> N2DTS
>
>
>
>
>
>
> ----- Original Message ----- 
> From: "Ben Dover" <quixote2 at ix.netcom.com>
> To: "Brett Gazdzinski" <brett.gazdzinski at verizon.net>; "Discussion of AM
> Radio in the Amateur Service" <amradio at mailman.qth.net>; "Discussion of AM
> Radio in the Amateur Service" <amradio at mailman.qth.net>
> Sent: Monday, November 12, 2007 2:00 PM
> Subject: Re: [AMRadio] Re:Reactor Wanted
>
>
>>
>>
>> -----Original Message-----
>> >From: Brett Gazdzinski <brett.gazdzinski at verizon.net>
>> >Sent: Nov 12, 2007 12:14 PM
>> >To: Discussion of AM Radio in the Amateur Service
> <amradio at mailman.qth.net>
>> >Subject: Re: [AMRadio] Re:Reactor Wanted
>> >
>> >I also run 2000 volts on the plates of the modulator, 4x150a's, and
> usualy
>> >1500
>> >volts on the pair of 813's in the final.
>> >
>> >It seems to like it that way, i have done experiments as both supplies
>> >have variacs, things always seem to modulate better, and sound better
>> >with more mod voltage.
>> >
>> >Brett
>> >N2DTS
>> >
>>
>>
>> Howdy, Brett!
>>
>> Interesting indeed.
>>
>> That approach flies in the face of decades of design convention; as a
> rule, AM
>> rigs either use the same supply for mods and final, or as the rig gets
> bigger,
>> the mod has it's own supply which is usually a lower voltage than the PA
> plate
>> supply.
>>
>> The only reason I can see for improved performance using this approach
> would be
>> that when used conventionally the modulator operating parameters were
> incorrectly
>> chosen for low distortion AT THE POWER LEVEL REQUIRED. I've noted over 
>> the
> years
>> that most ham designers tend to be very optimistic when it comes to
> estimating the
>> power that they'll get out of a modulator, and tend to not recognize what
> they'll
>> lose in the transformer, especially those losses that come from a
> significant
>> impedance mismatch between the mod plates and the PA load impedances.
>>
>> The usual result is that they can't get to 100% modulation, or if they're
> using a
>> pair of pentodes or tetrodes as modulators, they find it necessary to run
> the tubes
>> well into the nonlinear area of the curves caused by the action of the
> screen grid
>> in order to get enough power.
>>
>> Jacking up the plate voltage to the modulator is ONE way to get the
> additional watts
>> needed without correcting the design defects that caused the problems in
> the first
>> place. In addition to giving more audio power, raising mod plate voltage
> will also
>> increase the amount of low distortion headroom the modulator has
> available.
>>
>> Other than that, I can't see ANY reason, either in terms of physics or
> intuition,
>> for raising mod plate voltage above the PA causing an inherent increase 
>> in
> audio
>> quality.
>>
>>
>> On the other hand...  if the modulator parameters have been properly
> chosen in the
>> design, AND losses in the mod transformer and due to the (almost
> inevitable) minor
>> impedance mismatch are taken into account (so that it's not necessary to
> drive the
>> mod tubes into nonlinearity) when the mod is run at the same B+ voltage 
>> as
> the PA
>> or less, jacking up modulator plate voltage shouldn't make ANY difference
> whatever
>> in resulting signal quality...  except INCREASING the distortion, caused
> by the
>> change in mod tube plate to plate impedance creating a mismatch with the
> PA load!
>>
>>
>>
>> What it boils down to is this...
>>
>> It's GOOD to design a modulator that's capable of producing MORE than 50%
> of the PA's
>> input (i.e., a mod that has some headroom) no matter WHAT the plate
> voltage of the
>> modulator is relative to the PA. If it's got headroom, it's gonna distort
> less.
>>
>> It's BAD to underestimate mod transformer losses (figure 20% or more of
> the audio you
>> put into it ain't gonna come back out). It's even WORSE to pull another
> common ham
>> stunt, driving your mod transformer core toward saturation (face it... 
>> mod
> transformers
>> are getting harder and harder to find, and they AIN'T CHEAP anymore. 
>> Using
> too small
>> a transformer for the design is VERY tempting sometimes).
>>
>>
>>
>> 73's,
>>
>> Mr. T., W9LBB
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