HomeBrew Modulator

[wd8pcu]

Hi Everyone.

I am building a Classic Class AB2  Modulator to use on 15 and 10 meters and it will use a
pair of 5881's .
Does anyone happen to have any that are priced reasonably or would perhaps be
willing to trade?
I am designing it to Plate Modulate the Johnson Viking Adventurer .
Its circuitry allows full carrier control via suppressed carrier.
Works real real good and is simple.
It is a real Iron Horse weighing out minus the power supply at close to 70 pounds.
Save those Ole Novice CW Transmitters Guy's.
73 All,
Dan  de  WD8PCU

[W3SLK]

Why not use a pair of 6L6's? Essentially the same tube and fairly common. Let me know if you can't find any 6L6's.

[WA3VJB]

Yeah the guitar heads have soaked up nearly all the real 5881.
Same for the 7581.
Same for the 6L6, except maybe the metal variants.

A friend of mine who builds guitar amps swears by some of the new production, new stock stuff from whatever Yuogoslavia is being called .

[WBear2GCR]

My thought exactly.

Why not some 6CA7/EL34 if you need a thin bottle, or 6550s if you have room?
The Russian ones are ok.
Electro Harmonix imports a range of them.
I've used their 6550s, they're pretty good.
6L6 is ok fine too...
Prices are not bad.

Plus ur local guitar shop probably carries them as do the mail order houses, but you can order direct, which is probably best if you want EH tubes... plus you can ask them for matched sets and for the bias point or Gm you want... if you want.

             _-_-bear

[n3lrx]

I believe I have a few 6L6's if you need some. Can't guarantee their working condition, but their free if someones willing to try to put them to good use. I've also got a couple of pairs of 1625's which I think have never been used.

[David, K3TUE]

This may interest you.

http://www.thetubestore.com/earfenam.html

Finally, 5881 meant a special high-quality tube in the past, but it no longer means much today. True NOS 5881s are scarce and costly. [...] The exception is the re-issue Tung-Sol 5881. It is built to the original 5881 specs and is certainly high-quality.

http://www.thetubestore.com/tungsol5881.html

The Tung-Sol name and trademark have been acquired by the New Sensor Corporation. [...] New Sensor is an American corporation and since taking ownership of this factory has been steadily improving the quality of current production tubes.

[Tom WA3KLR]

If you are looking to use beam power pentodes in that power class, an over-looked unpopular tube is the 2E26.  You can get them for $1 at hamfests.  A pair will do 50 Watts out AB1 with 700 Volts plate supply, and 54 Watts out AB2 with 500 Volts plate supply.

I would be sure to put grid and plate supressor resistors at the grid pin and the plate cap, like 120 Ohms at the plate and 1k? at the grid (and maybe the screen also, between socket pin and screen bypass cap.) to keep it from oscillating at some radio frequency.

[The Slab Bacon]

Tom,
        2E26s work great for audio. I have a pair of them as modders in my screen mod 4-1000 rig. Even though it only needs about 15w of audio to mod it, I felt like why waste a perfectly good pair of audio tubes when I had a 5gal bucket of used 2E26s laying around.  I have had no problems with parasitic oscilations. I run them AB2 with 500v on the plates.
       
                                                   The Slab Bacon
                                     

[Steve W8TOW]

I am surprised that none of the other guitar players commented...well,
5881's are quite inexpensive these days.
Typically can be had for $11 each...(try Elderly Instruments, here in Lansing MI)
They stock them and work FB....5881's are standard items in Fender Amps still...
so ya can walk into any guitar store and find them...
I use them in my HB speech amp on my T-200 rig...gl es 73 steve
8tow

[WU2D]

If you have the room how about the 1625 and the good old 807. I made a dandy AB2 modulator with a pair of 1625's driven by 12SN7's.

Mike WU2D

[The Slab Bacon]

807s and 1625s are really nothing more than a 6L6G with a plate cap. Grid and plate curves are identical. Plate cap was added to help prevent zorching pver at higher voltages and have a little shorter path for rf use.

                                                             The Slab Bacon

[K9FH]

You may want to try out the modulator on a different transmitter for 10 meters.

I think the normal mode of operation for the Adventurer was to double the frequency in the final amplifier for 10 meter operation. Efficiency is rather poor.  Maybe add a muffin fan to cool the 807 ?  

[wa1knx]

re tom klr
    right on bro, 2e26 nice lil bottle! 1625 another bargain. audiophools
heard of the 807, not the 1625 that I know of, same pube. nice
for modulater service

[Bacon, WA3WDR]

If there is room, push-pull parallel is an option.  Twice the available plate current.

I sometimes think of an unbalanced push-pull modulator, with two tubes in parallel on the positive side, and one tube on the negative side, and 2X the drive to the negative-side tube for the same AC plate current as two tubes.  A single tube can handle the negative side, because the negative peaks are not as heavy.  Just a little trick with the phase splitter...  The unbalanced modulator bias would cancel some of the modulated B+ current if no modulation inductor was used.

[The Slab Bacon]

Bacon,
          I never thought of that, but its a pretty kool idea. 2 tubes on one side, and one on the other side in shove-yank!! Big positive peaks, somewhat "built in" negative limiting, whatz not to like!!
If you are running the tubes somewhere near AB1, they dont draw any grid current, they're only looking for a sniff of voltage on the grids. So you wouldnt have to have any special allowances for the driver (or fuzz inverter), Whatz not to like!! You could balance the idling current somewhat by biassing each side seporately (like a broadcash rig) AND WATCH THE MONKEY SCHWING!! I may give this scheem a serious consideration for the next hot rod tx that I build!!  Thanks!!

                                                                                      The Slab Bacon

[WA1GFZ]

Frank,
Two tubes on one side would only slightly improve things near the saturation voltage of the tube.
You would also have to reduce the primary turns on one side of the mod transformer which would cause a weird dc offset. gfz

[The Slab Bacon]

Frank,
Two tubes on one side would only slightly improve things near the saturation voltage of the tube.
You would also have to reduce the primary turns on one side of the mod transformer which would cause a weird dc offset. gfz

Hmmmm.........  One of those "multimatch" transfomas with all of the gazillion taps for impedance matching would prolly do the trick here. Just use a tramsfoma with a higher power rating than you need and select the primary tap that gets you where you need to be. It could still be good food for thought. I still cant believe that someone hasent tried it before.
                                                                          The Slab Bacon

[Bacon, WA3WDR]

Actually, it would be better to use a lower tap on the mod transformer for the positive tube plates, and juggle the grid drive differently, but it can be done in several ways.

Parallel operation is also an answer to the problem of insufficient peak current capability when no grid current is desired (AB1, etc).

If you put the parallel positive tubes on a lower tap, the 2X bias DC goes through fewer turns, so you lose the beneficial amp-turn unbalance that I mentioned earlier.  (Not a big deal.)  But, you will get better efficiency on the negative tube because it will run out of voltage swing around 100%, instead of maybe 200% (which you won't reach in the negative direction).

Also, there are ways of equalizing any leakage-inductance imbalances that may result from the odd tap connections.  Most amateur modulation transformers have these imbalances even with normal connections.