[Boatanchors] Re: [AMSWAP] 812 vs 811 Transmitting Tubes

[D. Chester]

> Looking at the data sheets, the 811 and 812 look to be nearly identical.
>
> Can you replace 811's with 812's in an amp such as a 30L-1 or any other 
> amps that use 811's?
>
> Any comments?

> Dave N7RK

811(A)'s are very high mu tubes, whereas 812's are medium mu.

The 812's would not work unless you added some negative grid bias.  In
linear amplifier service, 811's (and 811A's) usually operate at zero bias,
except when they are run near the maximum rated plate voltage.  Even then,
the bias required for the 812's would be much higher, probably attainable
only by modifying the bias supply.  The zero-bias characteristic is
precisely the reason they are so popular in linear amps and class B
modulators.

812's are normally used in class-C finals and driver stages, although both
tube types can be used for either purpose if biased and driven correctly.

But a major factor to consider is that 812's are getting to be much rarer
and harder to find, than 811's.  I just talked to someone the other night on
75 who is running a Millen plate modulated transmitter, with 811's in the
final because he can't find any 812's.  In class-C plate modulated service,
the lower mu 812's are likely to exhibit better modulation linearity than
would 811's.

If I had a linear or class-B modulator, some good 812's on hand but no
811's, I would try to swap the 812's for a good set of 811's, rather than
modify the rig.  You would probably have to modify the rig back to original
next time the tubes had to be replaced.  Plus you would be doing someone who
is in need of  a pair of 812's a big favour.

It would seem to me to be a big waste to use a pair of 812's in class-B
linear or modulator service.

811's and 812's can be directly replaced with 811A's and 812A's with no
changes except perhaps re-neutralisation of the rf final.  But the "-A"
versions may not necessarily be replaced with the "non-A" versions unless
the power level is cut back, sinece non-A's have lower plate dissipation and
plate voltage ratings.

Don k4kyv


_______________________________________________________________

This message was typed using the DVORAK keyboard layout.

http://www.mwbrooks.com/dvorak/
http://gigliwood.com/abcd/