[BC] Re: RCA 5T, and 5U-2

[Lamar Owen]

On Friday 13 May 2005 20:21, DHultsman5 at aol.com wrote:
> In a message dated 5/13/05 12:45:40 PM Central Daylight Time,
> broadcast-request at radiolists.net writes:
>> The 5T was put in to replace the 5F; it was a
>> 'junkyard dog' 5T that John lovingly restored from absolute ruin.  Put many
>> of his own vacuum variables, glass windows, tubes, etc, in the beast, and
>> brought it up from ruin in two days.  He showed me how to properly align
>> the 3rd harmonic resonator on the beast; that's how the 5T got away with a
>> single 5762 unlike the 5R which had a pair.  With the 3rd harmonic
>> resonator out of alignment, the box would do only 3300 watts; the 3rd

> I had an RCA 5 U2, three cabinet 5 kW. with one 5762 and the cathode and
> plate resonators.  I never could get this one to work properly. And I could
> make the 5400 I needed for night but no head room for modulation.  Even had
> RCA in to work on it. So we just continued to clip our negatives for
> loudness and distortion.

To get high positives out of a 5T or its younger brother 5U2 is a real 
exercise and requires a fresh 5762 (and 3CX3000F1's) and nearly 1.5A plate 
current.  Filament voltage must be set pretty high, and thus tube life is 
pretty short.  But the third harmonic resonators must be aligned; alignment 
is best done with a scope, and the idea is to adjust until the two bulges at 
the top and bottom of the waveform are equal in amplitude and a flat as 
possible.  It is not easy to do.

John showed me how to do it by ear; unfortunately it has been a really long 
time since I did an alignment, so I can't really describe the sound, but 
there was a definite change in the sound of the transmitter (I'm talking 
local acoustics, not air sound).  Watching plate voltage and current helped 
somewhat, but you weren't tuning in the conventional sense.  

The 5T's manual had a procedure, but if followed literally could produce an 
arcing vacuum variable at high positive modulation.

When the resonator was 'right' the plate ammeter would read in the 1.4A range, 
even though the current peak was just a little over an amp, thanks to the 
squarewave drive and way the ammeter was fed.

There is a kit to convert to a 3CX2500F7 (I think it was an F7, might have 
been a different one) from the 5762.  The 2500 supposedly was more rugged, 
and it certainly was and is easier to get and less expensive.  Required a 
socket change and a new filament transformer, IIRC.

> The earlier 5H with two 5762's, that my competition had would really talk.
> The 5762 triode was rated at 1.0 Amps and typically you needed 1.25 Amp to
> make power.The only way it would work half rite was for the resonators to
> be working.

WISE's 5R outperformed WKJV's 5T by a long margin, but the 5T was more 
'fun'.  :-)  When the DX25U went in, I did a simple test with the DX25U at 
5KW, and the DX25U was easily heard in areas the 5T was not.  Of course, at 
25KW WKJV will really talk now....

The 5T's trapezoid told the tale, though.  Looked like the Bent Pyramid.  
(Look it up in a book on Ancient Egypt).

I fortunately had a spare pair of modulators that were new; popping new mods 
in improved the modulation many fold, but the PA popped (a 5762 that pops due 
to this sort of over current issue typically melts a hole in the plate 
structure itself, rendering itself unrebuildable, and the overcurrent was set 
to 1.5A (using magnetic overcurrents, IIRC)) not long afterward.  And since 
the DX25U was there, and the Gates One that was there for the nighttime was 
the backup (the Gates One was louder in the fringes than the 5T!) they told 
me to just unhook the 5T and let it lie.  It's still there, as far as I know.
-- 
Lamar Owen
Director of Information Technology
Pisgah Astronomical Research Institute
1 PARI Drive
Rosman, NC  28772
(828)862-5554
www.pari.edu