..well, I'd like only to add some info about the last item.
FET's Do need quite a high idling current, depending on your linearity
demands: for really low IMD performance you need also currents in the 500 to
1A range. Of course, compared with the 20 or 30 Amps full power consumption
it looks small! But still, it's about 10 % of full dissipation!
BTW, at least with FET's, larger currents offer an improvement of IMD
products at low drive levels, rather than at full drive.
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Manfred Mornhinweg
Sent: Tuesday, March 17, 2009 3:22 AM
To: amps@contesting.com
Subject: Re: [Amps] 8122
Hi Peter,
> The 6146 (QE05/40) which is a beam power tetrode has 20W allowable
> dissipation in CCS service.
Wasn't it 25W, or was that only for ICAS?
> According to my Valvo tube manual idle current at 750V is 12mA, that is
9W,
> almost 50% of dissipation.
> The 813 (QB2/250), also a beam power tetrode with 100W dissipation is
> specified at 25mA/2KV for AB1, that is 50W.
> Burle data for the 8122 is 100mA idle current at 2KV which is 50% of its
> 400W maximum dissipation.
Interesting. I hadn't done such a comparison, and indeed the idle
dissipation is high on all of them!
But in checking data sheets, I found some interesting facts. One is
differences between manufacturers. I saw an RCA datasheet for the 6146A
stating an idle current of 25mA per tube, and an General Electric
datasheet for the same tube type specifying only 11.5mA per tube.
Also, I found data that support the feeling I had (and still have) that
the 8122 for some reason was thought to require a comparatively high
idle current: The GE 6146A datasheet states an idle current of 11.5mA
and a maximum signal current of 110mA, a ratio of almost 1:10. The RCA
813 datasheet states 50mA idle current and 255-290mA maximum signal
current, a ratio of 1:5 to almost 1:6. The RCA 6146B datasheet states 25
and 125 mA, still a ratio of 1:5. But the RCA 8122 datasheet states
100mA idle current and 335mA maximum signal current, a ratio of only a
bit over 1:3! And yet, the plate curves of the 8122 actually look better
in the low current range than those of the 6146, for example, which
should mean that a comparatively lower idle current is sufficient.
Unless my tired eyes did some nonsense, all the data above are for ICAS
AB1 service.
Interesting, but quite historic in this time and age... I would love
more discussion about solid state amps on this forum! With Mosfets
definitely we would NOT bias them to half the rated dissipation when
idling!!!
Manfred.
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