That's the info I needed, Steve! -30 db IMD at 300+ watts.
Yes, as you said, the 8560's [4CX-250's] will be used as class A voltage
drivers putting out barely a watt or two, and 30 volts of swing. This will
help bring them up into the -50 db+ IMD range, I'm hoping.
I ordered some OPA-695 RF op amps yesterday for the 5mW to 250mW
amplification, so I'm on the way. Newark has them in stock for about $2.88
each.
For the general list members:
In the meantime, I was thinking of the possibility of a grounded grid chain
for my other project, a 10M-40M amplifier. Since grid driven is a little
more squirrelly and GG is inherently more stable, I was thinking of a mostly
all GG amp. Say the same 250mW op amp drove a class A 6550 [ grid driven]
that put out 2 watts. This drove a class A GG 3-500Z that put out only 35W.
This drove a GG 8877 that put out ~1500W. Question... what numbers do
you feel the 6550 to 3-500Z chain would do for IMD considering it is
running at about 10X LESS than it's rated output? But, the 8877 final is
running at about 60% of it's ratings... If I put negative feedback around
it, could it be brought up to a reasonable IMD match to the driver tubes if
I add more low level gain to work with? It would appear the effort would
be lost on the 8877 final... Or maybe I need a 3CX-3000A7 in the final to be
able to derate it too.
My point is, can a GG DRIVER chain running at 10 times less than rated power
compete with a conventional grid driven DRIVER chain with NFB, giving -50db
to -55 db+ IMD figures?
Another question: If I put 10db of negative feedback around the GG final and
GG driver, I can make up for that gain loss by increasing the gain 10 db in
the low level op amp stages I can still get 1500W out, right? Just want to
be double sure.
73,
Tom, K1JJ
--------------------------------------
>>
>> Does anyone know what the IMD figures are for 8560A's or how this tube
>> compares against the standard 4CX-350 for IMD? These tubes were once
>> popular in commercial quarter kilowatt FM rigs.
> The STC data for 4CX250B gives typical values for class AB1:
> 2kV anode, 350V screen, 100mA Iq. 2 tone Po 325W pep, IMD3 -30dBc, IMD5
> -50dBc.
>
> Same IMD at 1500V anode, 225W pep out.
>
> The power levels mean it's not that useful as a guide to what they will
> do at much lower level. -30dBc IMD3 is probably somewhere around 1dB
> gain compression, and the IMD might be somewhat higher than you would
> extrapolate from a compression point prediction. Back the power off 10dB
> and you will probably see more than 20dB reduction in the IMD relative
> to the tones.
>
> If you can find data where someone has used them in a class A TV tx, you
> will get better figures for what they will do as drivers/predrivers in
> your system.
>
> Steve
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