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 Post subject: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Mon 06, 2020 2:10 pm 
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Hello to all. I am restoring a Knight T-60 transmitter and happened to notice the below pictured orb-like component hanging off the plate of the 6DQ6 PA tube. I swear, it was not there a few month's ago (lol).

I have worked on dozens of different rigs over the years and must admit, I have never seen this type of component. It was not stock to the T-60, nor in it's schematic, to my knowledge. It appears to be haphazardly added....perhaps some type of vacuum cap?

It is vacuum sealed glass, about 1/2" in diameter and one inch in length. Two fine leads, one tied to the 6DQ6 plate, the other n/c. There is a sealed 'center post', I am not sure if that is a terminal.

Any help on this one would be appreciated.

Thanks.
Todd
ka8gef

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Last edited by KA8GEF on Apr Mon 06, 2020 2:33 pm, edited 1 time in total.

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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Mon 06, 2020 2:30 pm 
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Maybe a neon light bulb to monitor tuning or just the presence of high voltage?


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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Mon 06, 2020 2:35 pm 
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Todd,

I would just remove it. Best guess is that it was an attempt to add a neutralizing circuit and that is a small capacitor. Check carefully to make sure that nothing else was modified in the final and driver circuit during this attempt.

The T60 is a neat little set that works well when properly built. It also sounds quite good on AM with its controlled carrier setup, especially when the listener isn't OCD about watching the S meter because controlled carrier rigs don't result in a steady S meter reading but if the listener has a properly working AGC system it will be fine. The T-60, like other controlled carrier AM rigs, is very compatible with older SSB linear amps which typically don't have the power supply or cooling system robustness to work with traditional constant carrier AM rigs but most of these amps can run at or near their SSB rating with a controlled carrier rig without stress to the amplifier.

Rodger WQ9E


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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Mon 06, 2020 3:24 pm 
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It's the guts of an old fluorescent lamp starter. If so, then it contains mercury vapor, which might glow when HV is present and that's the only reason I can think of that someone would have put it there. It serves no useful function as far as normal operation of the transmitter, and should be removed.

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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Mon 06, 2020 3:31 pm 
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Nice vintage Ham station.


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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Mon 06, 2020 6:24 pm 
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Location: Colorado Springs, CO
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I am also in the process of restoring a T-60. Everything is working except the output meter which has an open coil. Am in the process of rewinding the coil. A real pain in the arse.
Bill - K5MIL


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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Fri 10, 2020 1:24 pm 
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Thanks to everyone for your input. That component was some type of gas filled tube. I am not sure if it was once part of a 'starter' as suggested or a large neon 'bulb', of a type that I have never seen before. The 'starter' component makes more sense, as this does not appear to be an indicator bulb, it is too dim.

Either way, it was clearly added/used as an indicator of HV, perhaps even as a crude plate tuning indicator.

Bill...good luck in rewinding the meter core and I hope it works FB for you. I am thinking that the final resistance of that coil may not be too critical, as the meter circuit designed around it could easily be 'adjusted' to compensate, it only indicates relative RF, not plate or cathode current- as you probably know.

If you have not already done this- I would check those meter circuit resistors, especially the 3.3.K that connects directly to the SO-239 pin. This resistor is in line for 'sampling' of the RF signal and mine was well over 6K ohm. Obviously this resistor takes a beating...

BTW, nearly all of those 1/2w resistors in my T-60 speech amp and modulator circuits were found to be over 2X in value.

Todd
ka8gef

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Last edited by KA8GEF on Apr Sat 11, 2020 1:59 pm, edited 1 time in total.

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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Fri 10, 2020 6:44 pm 
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Got the coil re-wound and it works fine. I said before that everything is working but, I am only getting a little over 25 watts out (have changed tubes, no joy) and at certain settings of the drive tune and final tune it goes into oscillation. Got to figure that out.

Bill - K5MIL


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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Mon 27, 2020 2:21 am 
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KA8GEF wrote:

BTW, nearly all of those 1/2w resistors in my T-60 speech amp and modulator circuits were found to be over 2X in value.

Todd
ka8gef



Aw, that's nothing- I had a PA grid leak resistor in my T-60 that was
supposed to be 47K and it measured 193K. That's over 4X normal.
Talk about drifting up! Hi! Finding and fixing that returned the T-60
to normal operation.

BTW I use my old T-60 quite often on 40m and 80m. Keying sounds
pretty good.

David

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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Mon 27, 2020 2:41 am 
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Bill Harris wrote:
Got the coil re-wound and it works fine. I said before that everything is working but, I am only getting a little over 25 watts out (have changed tubes, no joy) and at certain settings of the drive tune and final tune it goes into oscillation. Got to figure that out.

Bill - K5MIL


Bill- My T-60 at times also gets a little squirrelly like yours. Certain antenna
loads seem to set it off if it's in Transmit position rather than Standby. OTOH
it is very well behaved most of the time with most loads. (Key up in Transmit
position)

BTW I should mention that I have a TR switch hanging off of the output SO-239
connector with a UHF 'tee' connector. That could be contributing to that
occasional problem.

If you haven't already checked, it would be a good idea to look at the PA grid
leak resistor. Mine had drifted way high over the years. That abnormally high
resistance was causing a relaxation type LF parasitic oscillation. Tuning on
the drive cap was real touchy, and I found I had to detune it slightly from max
output to kill the oscillation. Replacing R6 with the correct 47K totally fixed it.

There is a blow by blow of my struggle with my T-60 here:
https://www.antiqueradios.com/forums/viewtopic.php?f=5&t=180838&start=40

It's kind of long at 4 pages but still interesting to reread 8 years later.
W9BRD had some especially interesting comments. If you track down
his writing mentioned in my thread, he has some pretty insightful comments
about the 'RF illiterate' designs seen in that era, such as T-60 and HT-40 etc.
Also some good comments on beam power tubes in general and how to
mitigate against 'negative resistance' LF parasitic oscillations often seen
in these valve types. 807's were notorious for this BTW...

If you have access to a spectrum analyzer or a good scope, you may be able to
catch the beastie in action and determine whether you are seeing VHF or LF
parasitics.

BTW the design of the grid tuning circuit is such that your peak setting on each
band should be near zero setting on the dial. That means cap nearly full unmeshed.
I think Allied Radio was taking pains to make sure you couldn't peak it on the 2nd
or higher harmonic.

I get 35 to 40 watts out with my T-60 on 40 and 80 meters. I have a bunch of
6DQ6B's. Selecting for a better tube got me slightly more output but it wasn't
a huge difference, however.

David

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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Mon 27, 2020 2:58 am 
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Bill Harris wrote:
Got the coil re-wound and it works fine. I said before that everything is working but, I am only getting a little over 25 watts out (have changed tubes, no joy) and at certain settings of the drive tune and final tune it goes into oscillation. Got to figure that out.

Bill - K5MIL


If it is an LF parasitic, you might get rid of it by installing ~100ohm carbon comp right at the
6DQ6B screen terminal prior to any screen bypass cap- i.e. no bypass cap directly on
the screen pin, it goes to the outside of the resistor. See W9BRD's notes on negative
resistance parasitic oscillations in beam power tubes- interesting stuff:

http://dpnwritings.nfshost.com/ej/beam_power_tube_parasitics/

It is IMO well worth reading all the way to the bottom of this article. Bonus for T-60
owners dealing with instabilities- the critical information we need shows up in the
last two paragraphs.

David


P.S. Oops! One inaccuracy to correct: the two paragraphs I am referring to appear at
the end of W9BRD's post in my thread:

https://www.antiqueradios.com/forums/viewtopic.php?f=5&t=180838&start=40

Quote:
Returning to the T-60, two more things:

Because the T-60's 6DQ6 final is not neutralized and because the set's mechanical design is so RF-illiterate, the final may operate as a locked oscillator--locked, that is, to the signal from the crystal oscillator. (If you can pull out the crystal and still see RF power output, there's your proof. Wrote George Grammer, W1DF, in his review of the T-60 in May 1962 QST: "The 7-Mc. oscillation was only observed when the amplifier was lightly loaded and crystal was pulled out, but on 3.5 Mc. the crystal had a hard time getting control under any condition of final-amplifier loading.") This is the cause of the hysteresis (output peak occurring at different TUNE/LOAD settings when tuning upward and tuning downward) and output-power jumps users of the T-60 (and the HT-40, BTW) experience. Allied's later modifications included additional shielding and adding a 27-k resistor across the 6HF8 pentode's plate RF choke, and these may help, but I'd go with the fix described by Grammer: Solder a 22-k resistor across L2, and solder a 4.7-k resistor across L1.

If the 100-ohm 6DQ6-screen stopper resistor doesn't do away with the set's low-frequency parasitics, leave that fix in place--really--and then also try adding 220- or 470-ohm resistors in series with one or both of the 5-mH chokes associated with the 6HF8 pentode driver. The 6HF8 pentode is a high-transconductance tube, and installing same-value RF chokes in the grid and plate of such a tube sets the stage for tuned-plate, tuned-grid oscillation. A few hundred ohms in series with either or both chokes should sufficiently reduce their Q such that oscillation cannot occur. The effect of those resistors won't be much at dc, and therefore they should not affect the oscillator/drivers' ability to oscillate and drive. :-) Adding just such resistors cured just such an LF oscillation in the 12HL7 driver stage in a homemade transmitter of mine some years back.

Best regards,

Dave
amateur radio W9BRD
radio pages: web-search for "David Newkirk" and "Enjoying Radio"

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Last edited by Clutter on Apr Tue 28, 2020 5:28 pm, edited 4 times in total.

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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Mon 27, 2020 2:31 pm 
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Questions...(for David, et al)...

In Service Bulletin 83 Y 294, there is a statement indicating that a 27K resistor (noted as R31) should be placed across the choke RFC2 (5mh). This would be from pin 9 (6HF8 multiplier section plate) of V1 to lug #1 of TP5. Again, in effect this is directly across that 5mh choke.

My T-60 does not have that resistor installed across the choke, however I wonder if later RFC2's shipped were wound around a 27K resistor. I cannot tell by looking at the choke and would prefer to not risk disconnecting/damaging the delicate choke lead to measure resistance. I would guess that this choke is simply wound around a core.

Or...is it possible that this 27k resistor was actually eliminated in later (vs. early) units?

Thoughts?

Thanks.

Todd
ka8gef

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 Post subject: Re: Knight T-60 orb-like 'ufo' ?
PostPosted: Apr Mon 27, 2020 4:04 pm 
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My understanding is the 27K resistor was not in early T-60's, and was added in
later revisions. I checked mine and it had the resistor. If you don't see the
resistor in yours, it would probably be a good idea to add it. Not aware of
any chokes wound around resistors in this case. BTW I have copies here
somewhere of the Allied Radio addendum notes that came with later kits.
It describes the addition of the 27K resistor.

David

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