Question - 6DQ6 PLate Modulated

[Carl WA1KPD]

For various reasons I want to plate modulate a 6DQ6 rf amp circuit  (Run what you brung as Derb would say)

Looking at the specs I think I can run it at about 15 watts carrier input and plate modulate it?
From  http://www.nj7p.org/Tube4.php?tube=6dq6

Class A Amplifier
Plate Voltage ................................. 250 V
Grid No. 2 Voltage ............................ 150 V
Grid No. 1 Voltage ............................ -22.5 V
Plate Resistance (approx) ..................... 20K Ω
Transconductance .............................. 6600 µMho
Plate Current ................................. 75 mA
Grid No. 2 Current ............................ 2.4 mA


Would those of you who are far more technical then me agree with my assessment?

Thanks and 73

Carl /KPD

[kf6pqt]

Now, I'm still learning here, but for plate modulated fone, don't you want to instead be looking at the Class C specs?  Because, if I understand this correctly, the transmitter is all class C, up to through the plate modulated 6dq6. Then, any amplification after that has to be linear, yet getting the carrier out doesn't have to be.

Do I have his correct?

I was going to link to a datasheet showing the class C ratings, but that turned out to be a bit elusive, unless you sub for a -A or -B variant.

Thanks and 73,
Jason kf6pqt

[w4bfs]

essentialy correct, Jason ... this tv hortz output tube generally would not have a class C spec in the common literature, however ... a lot of these were empirically determined by experimenters and written up in the amateur handbooks of the 50's thru 70's or so ... as I recall, the 60's ARRL handbooks had a mobile rig using a 6dq6 for AM at 30 to 40 Watts so the 15W carrier would be conservative.  I think you might save some time by checking the handbooks or just experiment iffin you take the notion ... have fun ...73...John

[Carl WA1KPD]

as I recall, the 60's ARRL handbooks had a mobile rig using a 6dq6 for AM at 30 to 40 Watts

John and Jason

Thanks- I'll have to take a look for that as I have several of the Handbooks and mobile manuals from that era.

I felt the 15 was conservative, but thought I'd check.

73

Carl /KPD

[WD8BIL]

Hi Carl,

I built the Novice transmitter in the 66 handbook for a science project in High school. Worked good. It was a CW transmitter with 40 watts in it did 25+ out.
The 6DQ6 did a fine job for quite a while. I figured I'd be retubing every other week or so. Turned out the same tube lasted almost a full year.

[N3DRB The Derb]

Carl,

Got more than one? Run a pair. commonly used in CB amps on am. dont know what the bias should be, but run plenty of air across those mo fo's and figure 30% duty cycle on am with a plate voltage of 600.

A better way to go: Obtain the schizmatic of the Johnson Viking Challenger transmitter, which I owned and used for 2 years driving a SB 200 many years ago. sweep tube finals, driven by a sweep tube. screen modulated, 15 watts carrier on am, 100 watts cw. Best transmitter to run a amp with ever. you tune up the amp and tx on cw, rotate your loading control a smidge, drop the mode switch back to am, and you are tuned and ready to talk. and they are super cheap because no body wants em, collectors or amer's.  one went unsold on ebay a week or 2 ago for 24.95 with 50 bucks shipping.


Anyways, with some simple cap and rx substitutions in the speech amp, it sounds dynomite. I'd make me a copy or you can use that at a good starting point to making a plate modded version. Pair modded by a pair would be good choice. Gotta have a LOT of air though.

[KE6DF]

I think the Gonset G-76 uses a pair of them for the plate modulator -- triode connected.

It uses another tube for the final I think.

Schematic should be on BAMA

[ka3zlr]

Sweep Tubes like alot of Air on em...think lotsa CFM OM.

I'd like to watch an see how this transpires...

Something Different and It Can be done.

[WB2RJR]

Carl,

One thing to keep in mind is that sweep tubes in TVs ran in class C service. So the specs might not list class C service but will give you the rating as a horizontal amp. That is the same thing except in a TV it's only running at 15.75 kcs.

Things might have to be downrated a bit for higher frequencies.

Many RCA CTC color chassis used the 6JE6/6LQ6 in the 50s to mid 70s. That tube ran 420 volts on the plate at 210 mils with grid leak bias (about neg 85 volts). That's 88 watts input class C in continous duty. (try doing that with a 6146)

The 6DQ6 was used in B&W TVs. The Heathkit HX-11 used a 6CL6 pushing a 6DQ6. It was a CW transmitter rated at 50 watts input. For AM, plate modulated, I don't think I'd push that tube beyond 35-40 watts input.

350-400 volts on the plate at 90-110 mils ought to do just fine. It looks like about negative 18 volts on the grid but you might want to check the HX-11 print on BAMA.

Sweep tubes got a bad rap in the 70s because they were pushed to their max limits by SSB transciever manufacturers who were in a "max wattage input war"

In the 70s I could buy one for $1.80 whereas a 6146 would cost me $8-9.00. The larger color TV sweep tubes have become "rare" and are now overpriced. You can get around that by looking for the ones with "unusal" filament values. Three 40KD6s in series and you can run that right off your AC line.

The 6DQ6 is still a good value today. I've had 2 transmitters with them as finals.

Best of luck on your project.

Marty WB2RJR

[Carl WA1KPD]

I'd like to watch an see how this transpires...
Something Different and It Can be done.

It is different and I hope to have a full report on what I have done in the next week

Marty,
Thanks for your comments. They clarified things for me.
I have a whole box of odd TV tubes and have often thought it would be fun to series em up for a xmtr.
Really odd filiment values. But that is another project

73
Carl /KPD

[KI4YAN]

Even better...use 6BQ6/6CU6 as modulator tubes. Think 6V6, only with a 20W plate.

Yep, you can push the hell out of that 12W plate rating...I've had a pair on my desk running 28W Pd each at 340V plate as an audio amp for well over a year now. and at a buck-fifty or less each, I can afford to stock up.
Having the 80 or so 6CU6 from an old TV shop, I torture tested one in class C, and pushed 65W input through one before it finally gave out. the 6DQ6, like you are talking about, is simply a bigger plate version.

Heh, run a quad of 6DQ6 triode strapped, two per side of a PP transformer, and emulate a push pull quad of 300B's...I tell you, the plate curves are damn near identical! That ought to make a verr gud modulator, indeed.

[ka3zlr]

All good info here,

 I think what i'd do as an experiment grab one of those old D + A amps or an old Interceptor, of course on the interceptor you'd have to clean up the tank circuit for 80 or 40 or where ever yer gona operate, and apply a reverse Audio Tranny with a small reactor go modified Heising ....simple quick and half the work is done...Hi-Fi with Low Cost...

A small plate modulated Transmitter..then run that into yer fav RF amp..

just an idea...

73 jack.

[Carl WA1KPD]

Even better...use 6BQ6/6CU6 as modulator tubes.

I have a 10 watt mod tranny for this project. I was gong to use a single 6L6 for the modulator. DOne it before with good results and the xfrmr is set up for sngle toob feed - no center tap

I open to ideas of a smaller TV tube that might work well and give me 8 watts of audio!

Thx

Carl /KPD

[Carl WA1KPD]

All good info here,

 I think what i'd do as an experiment grab one of those old D + A amps
73 jack.

Jack,
What I am doing is just about as twisted, only lower power. Film at 11.....

[AB2EZ]

Carl

The Heath DX-20 (basically the same as the HX-11, which was a cosmetic update of the DX-20) used a 6DQ6.

I've done a number of experiments plate-modulating a DX-20... which I reported a while back on this board.

To get really good linearity... it is important the modulate the screen voltage from a source of audio that has a source impedance of less than 5000 ohms, and which produces a modulation of the screen voltage that is about 20% of the modulation of the plate voltage.

The way I did this is described here:

http://amfone.net/Amforum/index.php?topic=13378.msg100735#msg100735

You don't need to do this in as elaborate a way as I described in the post... but the key is to place (approximately) a 5k ohm resistor from screen to ground (to create the equivalent of a low impedance source of screen voltage modulation, as seen by the screen of the tube). The resistor that runs from the bottom of the plate choke (i.e., the modulated B+) to the screen should be approximately 25k ohms (10 watts or greater, wire wound is fine).

Make sure that the screen rf bypass capacitor is .001 uF (not .01uF, as is usually selected for Novice class CW transmitters). Likewise, make sure that the total rf bypass capacitance from the modulated B+ to ground is .001 uF.

It sounded really good on the air.

[Steve - WB3HUZ]

Although the 6DQ6 is not listed, you can get some idea of the Class C rating of other (in some cases similar) sweep tubes. Derating by 2/3 or so for plate modulation would be advisable.

[WU2D]

The Sonar Marine Radiotelephones used 6DQ6's in the final and they plate modulated them with Bipolar transistors! I had one of these and I seem to remember a 6AQ5 was in there too.

Mike WU2D

[Carl WA1KPD]

Project finished and thanks for the help. A 6DQ6 plate modulated by a 6DQ6 AND band switched to 160... the T-160?

Art Collins is turning in his grave but, hey- I had fun

http://home.comcast.net/~chnord/T-160/T-160.htm

 

[N3DRB The Derb]

The KPD-160? Nice job Carl.  Now swing the munky.