[AMRadio] Re:Reactor Wanted

Brett Gazdzinski brett.gazdzinski at verizon.net
Mon Nov 12 20:42:25 EST 2007

> 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.
I am running them at 2000 volts which is where the books say to run them.
> I will be using 810's at 725W "advertised" audio to modulate a pair of
> 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
> at well over modulation peak voltages.
I have experimented with mine at 2500+ volts (the deck has filtered forced
air cooling)
and heavy modulation without problems.

> 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
> 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
> poor IMD, often in the teens. It took several of us in the US and Europe
> establish cause and effect and establish correct operating parameters.
> Good 4CX250B & R tubes operated at 2500V in SSB service can deliver 28-32
> IMD with the above criteria. Thats about equal to an average run of the
> triode. It still requires careful tuning and heavy loading which equates
> 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?
> not using the old glass 4X150A I hope.

I have been using new 4x150a/7034's, which I thought were equal to a 4cx250
type tube except for the vhf/uhf zone where the ceramic did better then the
The RCA book lists the 3rd order dist products level as 30db and 5th order
as 35 db down,
nor sure how that ties into THD or IM distortion.
The distortion gets lower with higher plate voltages (in the book) up to
volts, which is as high as I run them.
The screen voltage and the bias voltage are regulated and adjustable.
Except for the frequency ratings, both types seem the same.
Switching between a pair of 4x150a's and 4cx250b's does not change
anything as far as I can hear.
This deck sounds cleaner then any class B deck I have built.

> 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
> 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.

The 813 rig takes up two 6 foot rack cabinets, its very heavy.
The 4 UTC cg108's and 109's must weigh 100 pounds each, plus 2
large power transformers, 2 large variac's, all the oil filled caps, etc....
I have used and abused this rig for over 20 years without a crapout.
(now having said that....)


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