Re: [Amps] Filter-C

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

R. Measures wrote:
> RE:  Filter-C, how much is enough?  A friend of mine built a 
> 4-1000A/8166 g-g amplifier that ran c. 6500v on the anode from a FWB 
> rectifier/C-filter.  The only filter-C he had on hand that would handle 
> this much V was a 2uF unit.  To get the project going, he installed the 
> 2uF C, but he left extra space on the chassis so that he could add more 
> C when it became available - at the right price, of course.  To 
> initially check out the amplifier, he fired it up with the 2uF filter 
> C.  He fired up the amplifier on 80m.  I could hear no ripple in the 
> SSB signal.

It isn't likely that you would hear any 120Hz ripple on SSB, in among 
all the other voice-frequency components.

> Is this plausible?  Hmmmmm.  During a SSB voice transmission, indicated 
> anode-I is roughly 1/3 of the max anode-I rating, so in this case, the 
> average load would be 750mA / 3 = 250mA.  Thus, the equivalent RL on 
> the PS was 6500V / 0.25a = 26,000-ohms.  The rule of thumb for the 
> filter-C for a FWB is 70k / RL, or 70,000 / 26,000 = 2.7uF.  This led 
> me to conclude that this rule of thumb might be a bit conservative. 
> Eventually, a 4uF unit was added to the 2uF, bringing the total to 6uF. 
> No subsequent change in audio quality - or change the HV meter 
> indication  - was observed.

6500V @ 0.25A is an unusual combination of high voltage and low current, 
where you can get away with a very low filter C. Running the numbers 
through 'PSU Designer' (and estimating the transformer voltage and 
winding resistances) it seems like 2u would give about 7% ripple at 
120Hz. Increasing the capacitance by a factor of 3 will decrease the % 
ripple by the same factor.

As Angel has pointed out off-list, tetrodes are quite insensitive to 
anode voltage, so HV supply ripple tends not to get through to the RF 
output (remember how tetrodes needed to be plate *and* screen 
modulated?). That isn't true of triodes, so I'd guess that if Rich's 
friend had been using a triode-connected tube, Rich might just have been 
able to hear 7% of 120Hz ripple on a CW carrier.

But that really was a far-out case with a very high value of RL. Almost 
any other amplifier is going to run at both lower voltage and higher 
current, so RL will be much lower. That means almost everybody is going 
to need *much* more filter C than Rich's friend did.

Yer typical 8877 is operating at around 3kV and 0.8A peak, so RL is only 
3750 ohms. A pair of 3CX800s at say 2.2kV and 1.2A is only 1800 ohms. 
That's more than a factor 10 less than the case Rich quoted, so the 
filter C would need to be higher by about the same factor.

For these values of RL, the typical 25-30uF looks very reasonable. 
Rich's rule-of-thumb 70000/RL looks quite reasonable too.

Way out at the other extreme are the guys using two GU-84Bs in parallel, 
at 3kV / 1.2A *each*! Their power supplies see an RL of only 1.25k, so 
they're really struggling to find enough filter C to let them run 4k 
out... the poor dears :-)


--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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