[Amps] 4CX250 IMD

[Steve Thompson]

jeremy-ca wrote:
> 
> ----- Original Message ----- From: "Steve Thompson" <g8gsq at eltac.co.uk>
> To: <amps at 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.
OK - it was a figure of speech. Please substitute 'at 470W' for 
'with massive overdrive'.

>They are simply showing the degradation when the 
> tube is run at its TYPICAL rating of 2KV.

and at much higher output power. The IMD goes bad because the tube 
is being pushed to deliver too much power, not because the HV is 
higher.

STC data, 'FRX's and my measurements are all at 2kV or close. We 
all agree on the linearity at 250-300W pep output. Eimac quote 
figures at 470W pep - the IMD is going to be much worse at that power.

  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.

Why - I see nothing to suggest it's a typo. 470W pep (two equal 
tones) is 235W average if there's no IMD. At 470W pep from a 
single '250 there's going to be considerable flat-topping, as 
evidenced by the IMD levels, so the average output power will be 
higher, maybe 260-270W. That's entirely reasonable for 490W dc input.

I see no reason to assume that, in 1961, Eimac couldn't have read 
pep accurately without any bother, or to assume that they wouldn't 
have properly regulated supplies in their test systems.
> 
> When run at only 1500V the IMD is acceptable. Im again making the 
> assumption that the screen is VR tube regulated in Eimacs1961 test.

When run at 1500V and 260W pep the IMD is acceptable. What would 
have happened if they had run more power at 1500V?

> 
> 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.

Only if you try to push the output power too high. Under all 
normal circumstances higher voltage will give you higher output 
for given linearity or better linearity for a given output.

IMD figures need to be tied to power levels (and any other factors 
which have a significant effect).

Steve