[HomeBrew] 833 grouned grid?

Sat, 02 Feb 2002 16:18:16

Hi Eric!
I don't totally agree with the replies that suggested that the 833's rather 
modest amplification factor of 35 makes it unsuitable for cathode driven 
service.
Such a tube would require more drive power than a tube that had a higher 
amplification factor, but most of that "extra" drive power would not be 
wasted, but be "fed through", to appear as added output power. In
fact, from what I've read, low mu tubes, at one time, were considered by 
some, to be desirable, and low mu tubes such as the 304TL, with a mu of only 
12, were sought after, for GG PA service! This was due to an interpretation 
of the FCC rules, that, as I recall, they later shot down.
To explain, if one were running a 304TL in class "C", at 1 KW input, (This 
was back in the old days, when the power limit was 1 kw dc input to the 
final amp) the RF output at a plate conversion efficiency of 78%, would be 
780 watts. As I recall, the required drive power in GG, was appx. 300 watts. 
Only appx. 30 watts of this would be dissipated as "grid loss"- the rest 
would appear as feed thru power. The net result would be a power output in 
excess of 1kw, for a plate input of just 1kw!!
The drive power requirements for such a class AB2 linear amp, would be much 
lower, and the plate conversion efficiency would be lower, for "AB2" 
operation, resulting in less power output, than for class "C" service, but 
the idea would be similar.
The FCC later stated that the driver plate input power had to be added
to the final amplifiers' plate input, to determine the actual total DC power 
input to the final, and this practice lost its' appeal.
Another problem with the use of a low mu triode, is that a relatively high 
negative voltage, on the control grid would be required to maintain a safe 
level of idling plate current, and this supply would have to be well 
regulated, if such an amp is to be used in linear service. However, that 
wouldn't be THAT difficult to deal with.
My main concern, however, is the rather high value of plate to filament 
capacitance, that the 833 has @ 8.5 pf. Apparently the 833 has some plate 
support hardware, that is electrically connected to the filament. Such was 
the case with 810's, that I once tried to use in GG service. They required 
no drive at all, as they oscillated VERY strongly, on their own!
You could neutralize them, but that's ackward, especially if you plan to 
bandswitch the amp. Such was the case with my 810 amp, so I decided to 
abandon the venture.
Thus, in the end, I agree, but for a different reason, that these big 
bottles are probably best suited to grid driven service!
Incidentally, my TT-5 RCA transmitting tube manual states that the 833 may 
be used at full input, up to 30 mhz., with natural cooling, and up to 20 
mhz. with forced air cooling! Apparently someone made a typo!! It probably 
should have stated "full input to 20, with natural cooling, and full input 
to 30 mhz, with forced air cooling!"
Anyway, that should erase any doubts about the high freq. capabilities of 
this bottle.
I hope to hear about your results with those big triodes!
                  Good luck & 73!   Ron   n4gjv

>From: "Eric Hughes" <[email protected]>
>To: <[email protected]>
>Subject: [HomeBrew] 833 grouned grid?
>Date: Tue, 22 Jan 2002 19:09:41 -0500
>
>
>Hello all,
>Has anyone on the list built or used the 833 tube in
>a grounded grid amp? I cant find any information on
>the data sheets or circuits in old issies of CQ, QST and
>73. I need some advise from someone who has experience using this type of
>triode in GG service. I have
>plenty of tubes and a 30 amp filiament transformer. My idea was to use two
>tubes in grounded grid to simplify the construction.  But, I am worried 
>that
>the tube will not be happy without the -70 volts bias specified in the data
>sheets? I dont want to destroy the tubes on first power
>up. Will GG operation be possible with the 833?
>Thanks, Eric
>
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