----- Original Message -----
From: "Steve Thompson" <g8gsq@eltac.co.uk>
To: <amps@contesting.com>
Sent: Saturday, September 29, 2007 3:45 AM
Subject: Re: [Amps] 4CX250 IMD
>
>>>>>> At Ep of 1500 in AB1 3rd/5th is -30/35. However at 2000V it slips to
>>>>>> -23/27.
>>>>>
>>>>> At what power and efficiency?
>>>>>
>>>>> Steve
>>>> 250/245 ma and 262/470 W PEP Note they specify PEP. There is also a
>>>> "worst case" 3rd order line entry of -29/21dB.
>>>
>>> Single tube or pair?
>>>
>>> Thanks, Steve
>> That rating is for AB1 SSB service, 2KV@ 250ma is the listed typical
>> operation for one tube as with all in the straight 4CX250W family.
> Pushing a 4CX250 to 470W pep output is absurd and it's no surprise
> that the IMD is awful. It's totally unrepresentative of proper use
> of the tube in linear operation.
>
> Collating all the info. we've been through, looking at pep from a
> single tube and IMD3 ref one tone:
>
> STC data says -30 at 285W
> Eimac data says -30 at 262W
> My amps measured -30 or better at 250-300W
> GW4FRX measured -30 at 250W
> GW4FRX measured -22 at 250W with screen supply having 150 ohms
> source resistance.
> Carl measured -22ish with a gas tube screen supply, source
> resistance probably >300 ohms
> Eimac data says -21 with massive overdrive
>
> There's no inconsistency here, and no reason to doubt anyone's
> measurements. It's clear what the tube's linear capability is, and
> it can do better if backed off and biassed/loaded for optimum.
>
> Steve
The tube is being tested in AB1, ZERO grid current, therefore Eimac is not
overdriving it. They are simply showing the degradation when the tube is run
at its TYPICAL rating of 2KV. I have no idea how they computed the PEP; the
spec sheet is dated 1961 and the procedure may have been different. If you
read back to my prior post I stated 2KV @ 245ma which is only 490W INPUT.
Let us both agree that the PEP is a typo.
When run at only 1500V the IMD is acceptable. Im again making the assumption
that the screen is VR tube regulated in Eimacs1961 test.
OTOH, I know of no commercial ham amp that runs any 4CX250 at 1500V and many
hams push them above 2KV which just makes matters worse.
Carl
KM1H
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|