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Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback

To: Steve Thompson <>
Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
From: Gary Schafer <>
Date: Wed, 21 Dec 2005 11:54:59 -0500
List-post: <>

Steve Thompson wrote:
> 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

Those figures are too good for a 4cx250B. They are probably referencing 
to pep.

Looking in the (older version) "care and feeding of power grid tubes" 
they show the 4CX250B at 348 watts pep output to have 3rd order products 
at -25 db and 5th order at -25db. This is consistent with what I have 
seen in other tube manuals on the tube spec.

The 4cx350A,F show 3rd order at -29db and 5th order at -30db.

The 4CX1000A has a 3rd order level of only -23bd and 5th order -26db.
This suggests why Collins ran it in GG in the 30S1 amplifier.

This is referenced to the level of one tone of a two tone signal and not 
pep level.

The care and feeding book goes on to say that it is not correct to 
reference distortion levels to pep. If you were to add the levels of the 
two tone signals together (giving you pep) then you would also need to 
add the levels of the two 3rd order product together too. This would 
again give you the same ratio that you get by referencing one of the 3rd 
order products to one of the two tone levels.

They also say that IM product do not necessarily drop as drive is 
reduced. It can actually be greater at lower power levels than at higher 
power levels. IM products drop way down at one point midway in the power 
range and then go back up again. They show a graph that is derived 
mathematically and say that most tubes follow the graph pretty closely 
to the predicted IM performance ratios of drive to bias amount.

It looks like there is around a 10db reduction on average as drive level 
is reduced to low levels and at some points drops much more than that 
but only for a narrow range of drive level.

Someone mentioned a few days ago that IM products drop 9 db for a 3 db 
reduction in power. I believe this is only true when one of the drive 
signals (one tone of two tones) is dropped in level and the other 
remains the same. But if both are reduced together and kept at the same 
level with respect to each other then the 9 db reduction does not come 
into play.

I think that the new version of the care and feeding of tubes is online 
somewhere. I haven't looked at it in a long time. It may be worth 
looking at for this project.

Gary  K4FMX

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