>>Carl, KM1H wrote (about 4CX250Bs and the like):
>>
>>
>>>BUT what is the IMD if one runs say 1700V, proper drive and
>>stabilized
>>>screens?
>>
>>Worse than at 2000V for the same "attempted" power output; or
>>alternatively less output for the same general IMD levels - see below.
>
>That was not what I asked Ian and I am sure that you know that.
It looks like GW4FRX can make some more measurements of IMD to answer
your question more specifically, Carl. What he proposes to do is:
1. Set up 2*4CX250R with Va 1500V, Vg2 350V and adjust Vg2 to give ZSAC
133mA, all as specified in the EIMAC datasheet.
2. Adjust drive, tuning and loading to reproduce as closely as possible
the two-tone operating conditions and output power in the EIMAC
datasheet. The output power will be adjusted to the datasheet value of
262W PEP per tube, and IMD levels will be reproduced as closely as
possible (see note below).
3. Increase Va to 2000V - touching nothing else - and measure the new
output power and change in IMD levels.
4. Re-resonate the output tuned circuit if required - but still
touching nothing else - and again measure the output power and change in
IMD levels.
5. Adjust both tuning and loading to GW4FRX's own satisfaction, and see
what we get.
Notes:
1. The tests will be done at 144MHz in a carefully neutralized W1SL
push-pull PA. EIMAC did not specify a test frequency on the datasheet,
but it might have been as low as 2MHz (see description of their IMD test
setup in 'Care & Feeding'). We'll have to assume that the tank-circuit
efficincies of the two test rigs are the same.
2. It may be difficult to reproduce the original EIMAC IMD levels
because they also depend on the power supply characteristics, which
EIMAC did not specify. GW4FRX has found that stabilized screen supplies
tend to give better IMD than EIMAC found. (This is possibly because
EIMAC were using simple potential-divider screen supplies with an
impedance of several thousand ohms, which represented the state of the
art back in the 1950s when the measurements were made.)
3-4. It may not be a fair test to simply jack up the anode volts without
re-resonating the PA. We'll see what difference it makes.
5. The tubes will be significantly under-loaded at the higher anode
voltage, so it's quite possible that the IMD will get worse in tests 3
or 4. But if the loading is adjusted correctly, staying well on the
heavy side, there should be a significant increase in power and the IMD
could also improve.
What do you think, Carl - will those proposed tests answer your
questions adequately?
Thinking further about the underlying question, which I interpret as
"What happens to the IMD when you increase the anode volts on a tetrode,
leaving the screen volts the same?", the true answer is: it all depends.
Specifically, it depends whether you're allowed to re-adjust the
loading, because the loading control can make the IMD levels whatever
you like.
I have to agree with Carl that if 4CX250Rs are operated under the 2000V
conditions shown in the EIMAC datasheet, they will sound absolutely
rotten! That's because IMO the operating conditions do not give a good
balance between power output and IMD: by loading for 470W PEP output
per tube, they're being far too greedy. The IMD values quoted (3rd-order
at -23dB below either tone and 5ths at -27dB) are simply not acceptable
by today's amateur-band standards, especially not from a big VHF signal.
The reason for our misundertanding is that I had assumed that the tubes
would be operated much more reasonably, and loaded for a power output
more like 250-300W PEP per tube. Under these conditions, and with good
power supplies, the IMD can be much better as the data on my web-site
show.
(Some corrections: the web-site figures are for a pair of 4CX250Rs [not
350As] at 500W PEP output, with 2000V on the anodes, 350-360V on the
screens and about 4500 hours on the heaters. For easier comparison of
IMD levels, I will be converting the numbers to dB below either tone
rather than dB below PEP.)
Another aspect of IMD levels that the common measurements of 3rds and
5ths fails to point out, is how quickly the higher-order IMD falls away.
In many respects this is even more important than the close-in IMD
because the higher orders cover far more frequency space. You expect
some problems if you move in close to a very strong station; but you
also expect the signal to disappear TOTALLY as you tune away. For that
reason, there's a lot of advantage in optimizing the loading for lower
IMD levels far out from the main signal.
Moving on to a different aspect, I said:
>>There was an interesting posting in r.r.a.homebrew several weeks ago,
>>from one of the actual engineers who generate the numbers for the
>>datasheets of new amplifier devices. The "proper" figures reflect his
>>personal judgement about the best mix of several different orders of
>>IMD (within other device operating constraints of course). He happened
>>to work with transistors, but I'm sure that's how tube datasheets are
>>written too.
Carl replied:
>I'm not so sure. Most tube design engineers were raised in an era where
>professional integrity still counted.
It has absolutely nothing to do with integrity. It's about engineering
judgement, nothing else.
The operating conditions for any device can adjusted to give a literally
infinite number of combinations of output power, IMD levels and
efficiency. The device engineer HAS to use his professional judgement to
decide which particular combinations will be of most interest to the
user. These will be the operating conditions that get published in the
datasheets - but they're not the only "correct" ones.
If your judgement is that you want better IMD and are prepared to accept
less output power, you're absolutely entitled to work out a different
set of operating conditions, and those will be equally "correct".
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
www.ifwtech.demon.co.uk/g3sek
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