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Re: [Amps] QRO 2500DX MARK lll 4CX800 / triodes vs tetrodes

To: "'Jim Thomson'" <>
Subject: Re: [Amps] QRO 2500DX MARK lll 4CX800 / triodes vs tetrodes
From: "Leigh Turner" <>
Date: Thu, 4 Apr 2019 22:10:56 +1030
List-post: <>
All good points you make here Jim on which I concur with.  I agree SDR rigs
are not yet prime time and certainly not well suited to out of the box
appliance operator hams. They are more for technically savvy hams who can
take say a current generation OEM Orion Mk2 board from Apache-Labs and
successfully integrate it into a homebrew SDR exciter equivalent of a
turn-key ANAN 7000/8000DLE rig.  However the earlier generation and now low
cost ANAN 100 and 200 rigs are also amenable to Pure Signal operation with
the appropriate software upgrades like PowerSDR mRX PS v3.4.9 that supports
this neat adaptive pre-distortion system IMD cancellation mode.


I like simple input circuitry swamped grid-driven AB1 tetrodes because of
their high power gain; you only need to tickle the grid with a few watts
drive to comfortably get over the 400W limit here in VK or around 10 Watts
to achieve well over 1 kW. This modest drive level of course matches very
well with a 500 mW SDR exciter in combination with a very linear Class-A
output driver stage to get up to circa 10 W PEP.


The high-mu G-G triodes like the 8877 or 3CX3000 are good choices when you
already have 100W+ of drive available from a good high performance
transceiver rig having a (rare to find) low IMD clean final PA stage. The
bigger tubes need an IPA to achieve enough drive really boils down
to swings and roundabouts and the old adage of different horses for
different courses argument.





-----Original Message-----
From: Jim Thomson [] 
Sent: Thursday, 4 April 2019 1:43 AM
To: Leigh Turner
Subject: Re: [Amps] QRO 2500DX MARK lll 4CX800


Subject: RE: [Amps] QRO 2500DX MARK lll 4CX800



< Of course any of these amps when married up with a modern SDR exciter

< running Pure Signal IMD cancellation feedback produce truly

< performance.


< In the Tx tetrode tube space today there are very few choices to be


< Leigh



##  The only modern SDR xcvr for sale on the market right now, 

with...Pure signal... is the ANAN 7000 + 8000  xcvrs,

which, imo, are not ready for prime time.  Esp judging by the 37 K postings 

of various issues..on their yahoo group, that I belong to.

Pure signal will really take ahold when Icom, Yaesu, Kenwood start using 

it in their future products.


##  Here's my take on tubes Leigh.   Why mess with a tetrode in the 1st 

place ?    Tetrodes offer ZERO advantages over a high mu GG triode.   

Heck, you don't require a socket on some triodes, nor regulated screen
supplies, regulated

bias supplies, screen and anode current protection,  +  compensation on the
input for stray 

tube C, + neutralization etc, etc.


##  A  dead simple, single Chinese  8877 GG triode amp makes more sense. 

2 x 3-500Z is also dead simple..and instant on, no grid current

protection required...perhaps a simple fast 3ag grid fuse at most.


##  From a HB perspective, why bother with a complicated  mess, like the typ

tetrode ?   A simple 3CX-3000F7 requires no socket, instant on,

rebuildable, 5-8 kW pep out...or CCS cxr, and  -59db IMD-3.   Most hams 

will die before the tube does.   Moot point, cuz your kids can have it 

rebuilt, use it.then pass it on to their kids.


Jim   VE7RF 




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