[AMRadio] 860 tubes used as modulator output tubes


Larry Szendrei ne1s at securespeed.us
Tue Jul 29 07:41:28 EDT 2014


Hi Bob,

Sounds like a neat project! No personal experience - I'm not lucky (or 
rich) enough to have a pair of good 890s.

My 1938 edition of the RCA Air-Cooled Transmitting Tube manual has the 
conditions for a single-ended class-B RF linear amplifier in a tetrode 
connection. The DC voltage will be the same for a pair in class B audio 
service:

Plate volts: 3000 (2000)
DC Plate current: 60 (43) mA (will be double for a pair of tubes, of course)
Screen volts: 300
Grid bias: -50V

Looking at the curves it doesn't look like a class-B high-mu triode 
connection would would work out unless you  put some positive bias 
(below 300V, you'd have to experiment with a variable supply) on the 
screen grids to get the idling current you want. The supply would have 
to deliver screen current to both tubes. And you'd probably have to dive 
the p!$$ out of them with audio.

Be careful, and treat them gently - there's not a lot of these bottles 
left, fewer that are serviceable, and they're not making any more ;-)

73,
-Larry/NE1S

On 7/29/2014 12:49 AM, Bob Deuel wrote:
> I desire to use a pair of 860 tubes as modulator output tubes in a special
> homebrew transmitter project.  I am aware that tetrodes are not the best
> choice for use in a modulator and especially a lowly inefficient 860, but
> that is the tube that is most physically fitting for this particular
> project.  I am also aware that using the 860 will require an added screen
> and bias supply and a substitution of a triode or its sister 852 triode
> would be better choice but those are not options and the extra power
> supplies are no problem.  I cannot find any information on anyone using a
> pair of 860's in a modulator and obviously no manufacturer's published
> operating specifications as an AB modulator.  Have any of you used PP 860's
> in a modulator either as tetrodes or  triode connected tetrodes?  Any
> thoughts and information on suggested operating perimeters would be helpful.
>
>
> Thanks,
>
> Bob, K2GLO
>
>



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