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[Amps] Alpha 8410 using 4CX1500B - why?

Subject: [Amps] Alpha 8410 using 4CX1500B - why?
From: John Lyles <>
Date: Fri, 31 Oct 2014 00:22:28 -0600
List-post: <">>
Don't be concerned over idling current in tetrodes. As long as you can cool the tube, let it conduct to whatever it takes for IMD. I've made amplifiers from ~3 to 200 MHz, for 30 years now, as work required it. Early on I worked with another engineer to develop a line circuit made from copper plumbing, that delivered 1.5 kW at 100 MHz from an 8877. It was for FM broadcast so linearity wasn't an issue. Then a cavity circuit for 4CX3500A, and one using 4CX7500A, class C. Those all had minimal quiescent anode current and hundreds were built. Then for 6 years I built or maintained a handful of industrial heaters using Eimac triodes up to 50 kW continuous. They didn't have linearity specs, being power oscillators.

When I moved into Scientific amplifiers, first was a 100 kW tetrode amp running class A (yes), that ran 40 amps of idling current with a push pull pair of Thales TH555A tetrodes. It was almost straight line linear. Being pulsed, the dissipation was much lower on average basis. Around 2000 I rebuilt a plasma products amplifier into a linear amp for testing some particle accelerator structures. It was an 8877 at couple kW and 5 MHz. Most recently a large tetrode TH628 in cavity circuit with 10 Amps of quiescent current. Linearity is good, although not measured the same as IMD, instead determinded by a transfer curve of Pin vs Pout. And they go into feedback circuits so linearity isn't top priority as with SSB. I have a pair of these cavities running right now, at 2.9 MW total power, 340 kW of average power. They are single frequency, have extremely stable gain and phase (< 1 % and 1 deg phase shift with power change or across a long pulse. Efficiency is in the mid 60s, gain is 14 dB.

The earlier FM class C tetrodes were grid driven common cathode amps. I tested the 4CX3500A in both common grid and common cathode, was the first user when the bottle came out from Eimac. Its written up in the 7th or 8th NAB Handbook. The class A TH555A tetrode was grid driven by a 300 watt ENI solid state amp. Most recently have been using grounded grid and screen grid (for DC they are biased, but RF grounded) and getting 14 dB of gain and good efficiency out of both tetrodes, a TH781 driving a pair of TH628. We have triodes from old RCA running 3 megawatts,complete grounded grid, cathode self bias. Not very efficient.

I have built with triodes and tetrodes. Both have their pluses and minuses. Grounded Grid, grounded cathode. I'm happy to have tried a variety of circuits, HF to VHF, components to cavities, and not gotten too biased (no pun!) to where I only will do it one way. Enjoy building amplifiers and don't make it a pain for yourself or your peers.

Over past
Message: 1
Date: Thu, 30 Oct 2014 00:50:04 +0100
From: "Peter Voelpel" <>
To: <>
Subject: Re: [Amps] Alpha 8410 using 4CX1500B - why?
Funny, most TV transmitters in Europe with combined sound and vision
amplification have been with tetrodes in their final because of their high
I never saw a commercial communication HF amplifier made in Europe with
triodes and all were designed for ISB.



Message: 2
Date: Thu, 30 Oct 2014 10:51:07 +1030
From: "Leigh Turner" <>
To: <>
Subject: Re: [Amps] Alpha 8410 using 4CX1500B - why?

Indeed Peter.

And high plate bias current and associated low efficiency is part and parcel
of achieving high linearity and low intermod from AB1 tetrodes.

Low plate idle current and low IMD are mutually exclusive objectives. There
are no free lunches here :-(

At least in an SSB linear amplifier one can significantly offset the impact
of high idle current with a competently implemented EBS system. When EBS is
done properly there are no audible or spectral artefacts stemming from the
tri-state transitional bias switching.

An alternate possibility along the lines of what Manfred was advocating may
also prove useful. I quite like simple analog linearization systems that
extract improved intermodulation distortion characteristics from the
imperfect PA using a pair of log-amps for detectors, one on the input and
one on output of the RF power amp (via a signal sniffing directional
coupler) feeding a comparator then the error signal drives a VCA on the
tube's bias feed. I much prefer this hardware based approach over doing the
PA linearization in the current fad using SDR pre-distortion correcting



Message: 3
Date: Wed, 29 Oct 2014 21:20:06 -0400
From: "Carl" <>
To: "Peter Voelpel" <>,        <>
Subject: Re: [Amps] Alpha 8410 using 4CX1500B - why?
And in North America the emphasis has been on both types as both have their
uses in broadcasting and in communications where the latter mostly use grid
driven with low power driver stages and large stage gains....unlike hams who
are happy with 10-13dB.

For ham amps the tetrode makes little sense with its poor AB1 (closer to A)
efficiency for acceptable IMD and having to burn most of the drive in a
resistor. Cheap cheap Russian surplus was the only reason and then it dried
up for the most part.

Pretty soon we will all be using whatever the Chinese give us. I cant see
Europe doing any new development and Eimac is basically a replacement


## agreed.  Most commercial ham amps will use 225 ma idle PER TUBE.
That?s a whopping 675 ma for the three hole amps.... which is looney tunes
imo.    Its done to get reasonable imd.   You folks can keep the 4CX-800As.

## The L4B runs just fine with 100 ma idle @ 2500 vdc.....and 40 ma idle,
when on the 1900 vdc position.   2 x 3-500Zs.   For CW use, you can rune the
idle down to 10-20ma on any GG tube.

## dunno why alpha continues to use these stupid tube line ups over the last 40 years. They already use a single chinese 8877 on their latest auto tune amp. They shoulda
just made a manual tune version of it, and be done with it.

Jim   VE7RF

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