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Author Topic: 8950 / 6LF6 / 6MH5 / 6LB6 / Etc  (Read 7093 times)
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KD6VXI
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« on: October 22, 2009, 09:36:26 PM »

Found a little sweep tube amp from back in the day, along with a plate modulated CB radio, one of the Johnsons.  Figured I'd get it up and running, just in case we EVER get any type of dx again around here...  I have an HR2510 dedicated radio for 10 meters, but a all tube rig would be MUCHO MACHO!!!

So, here's the question:  The amp has a driver in it, and I wish to dispense of such trivial nonsense.  I have 35 to 50 watts PEP of drive out of the little radio (it isn't stock, by a long shot, sweep tube final, etc), so drivers are not necessary. 

Anyone know the input impedance of one of the sweep tubes common to the CB linears in grounded grids / cathode driven service?  My plan is to put the pair of 8950s in service.

The circuit is straightforward, pair of tubes driven by 1, I'm eliminating the driver.  SWR input is horrible as stock, I have it fairly low, but would like to design an optimized input, not this cluge I have now.

The 8950 datasheet says NOTHING about it, other than to be determined.  8950s are VERY similiar to the 6LF/6KD6 tube, pin 6 is also connected to the cathode in the 8950, as it was designed for RF service (according to the GE datasheet)...  So, I'm driving pins 2 and 6, 800 on the plates with a 10 volt zener on the cathode.... 

--Shane
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Pete, WA2CWA
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« Reply #1 on: October 22, 2009, 11:00:13 PM »

Why not take a look at the Swan 700CX schematic , has 8950's in it, and just reverse engineer the transmitter circuitry into your box. Most likely, you'll also need a resistive pad between your Johnson and the linear.
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« Reply #2 on: October 22, 2009, 11:21:47 PM »

Most likely, you'll also need a resistive pad between your Johnson and the linear.

Gee, Pete; having a resistive pad between his "Johnson" and the linear could end up being a little on the painful side, don't you think?

73,

Bruce
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« Reply #3 on: October 22, 2009, 11:39:14 PM »

Got to have some pain, if one wants the gain Cheesy

And, on a "serious" note, I've never seen a CB linear that was worth the time and effort to convert over for use on a ham band unless you stripped it down to bare bones first and then rebuilt it using some sound technical logic and components.
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KD6VXI
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« Reply #4 on: October 23, 2009, 12:05:00 AM »

Got to have some pain, if one wants the gain Cheesy

And, on a "serious" note, I've never seen a CB linear that was worth the time and effort to convert over for use on a ham band unless you stripped it down to bare bones first and then rebuilt it using some sound technical logic and components.

Which is what I'm doing now.  The tubes are ORIGINAL PAL marked 8950s.  Still output full Pout on 11 meters into the dummy load...  BUT, the input is horrendous!  Definately designed for tube radios.

I'm also pulling the driver tube OUT, as it's flat as a pancake...  (1 watt in, 3/4 out, through the driver and pair of 8950s...  Put it into the 8950s with the kluged input I built today, I get 10 out..  Not bad, IMO).  Since I pulled the driver out, I figured I had enough to drive a pair of them to full, or near full Pout.  The original driver was a 6JB6.  Not a hearty tube, by any means.  I'm still finding it semi-amazing that it has full Pout on the factory tubes!

This was a repair from about a decade ago, and the customer never answered the phone when I gave him the cost of the new tube...  No labor, just the new tube, and I think he pretty much "donated it"...  Never heard back after a year.

I've heard that the pair would like about 35 watts PEP in, so I was going to lower the output on the Johnson.  As it is now, approximately 50 input gives me 350 output, so I think this input is working to some extent....  Unfortunately, the L is screwed all the way in, so I need to wind a couple more turns on it tomorrow.....  That's saturated Pout, as well, definately not linear at that point.  Might need to play with the bias, too.

Thanks for the ideas on the Swan, I'll check it out.  I also thought of the DrifTronix 1011, IIRC, it had an 8950 in it.

Funny, the datasheet states the 8950 is an RF tube...  Wish I could find a pair of M2057s for it.  A pair of them, ran at 200 watts PEP would last LONG time.

If this works out, I'll do it grid driven next.  I <<think>> I have enough Lambda supplies to do it...  Funny, each bias supply would be worth close to 10 times or more the original price on the amp!

--Shane
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w4bfs
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« Reply #5 on: October 23, 2009, 08:14:49 AM »

Anyone know the input impedance of one of the sweep tubes common to the CB linears in grounded grids / cathode driven service?  My plan is to put the pair of 8950s in service.

input Z (grounded grid) = 1/ Gm

example: a 4-1000 has Gm = 10000 x 10E-6 mho

so: 10000 x 10E-6 mo = 1 x 10E-2 mho = .01 mho --> Z = 1/.01 mho = 100 ohms

hope this helps
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Beefus
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« Reply #6 on: October 23, 2009, 09:54:24 AM »

Putting a converted tube-type CB on 10 is a fun project, I think you'll enjoy it. My only observation is that 350W PEP is a bit overkill for 10 meter AM work when that band is truly "open." There is no QRM to speak of in the AM segment of the band, and atmospheric noise and static levels are quite low. During the last sunspot cycle, I had almost daily QSO's from W6-land with WB9GKZ in Green Bay, WI that featured 40 over S9, full-quieting signals on both ends. We experimented by lowering our power outputs down, from 50 watts to 3. No difference in signal strength. The ionosphere does all the work.
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« Reply #7 on: October 23, 2009, 12:11:20 PM »



It might be that the sweep tubes are worth more than making the amp work... see what they go for on ebay??

Just a thought...

           _-_-WBear2GCR
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KD6VXI
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« Reply #8 on: October 23, 2009, 12:11:37 PM »

Putting a converted tube-type CB on 10 is a fun project, I think you'll enjoy it. My only observation is that 350W PEP is a bit overkill for 10 meter AM work when that band is truly "open." There is no QRM to speak of in the AM segment of the band, and atmospheric noise and static levels are quite low. During the last sunspot cycle, I had almost daily QSO's from W6-land with WB9GKZ in Green Bay, WI that featured 40 over S9, full-quieting signals on both ends. We experimented by lowering our power outputs down, from 50 watts to 3. No difference in signal strength. The ionosphere does all the work.

I aggree wholeheartedly.  I traveled for years with my grandparents and one of the 25 watt 10 meter rigs in the truck (they pulled a 5th wheel and traveled the last 20 years of Grandpas life)...  25 to 30 watts was the word of the day.

I made international contacts with that thing on the East Coast, and on the West, I could talk pretty much to anyone I could hear.  Slopbucket of course.

My plan on the linear is just to get it up and operational, and when I NEEDED it, be able to use it to drive the SB220.  The Johnson will drive my Harris RF-103 nearly to full output (only needs 65 watts), but the Heath is another story.

On another note, anyone tried converting one of these to another band?  Shouldn't be too hard, just wondering if I'd just be reinventing the wheel, getting a Messenger on 75 Smiley

BTW, my math led me to 62 ohms on the cathode, it had 16,000 mho on the datasheet listed.  SO, off to build a Pi net.  I'd also like to be able to drive this thing with the multimode radios here...  I've got about 3 or 4 of them.  The L works, but I'd rather play with pI.

And thanks to the gentleman that posted the formula....  I for the life of me, could not find or remember it.


--Shane
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KD6VXI
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« Reply #9 on: October 23, 2009, 12:13:07 PM »



It might be that the sweep tubes are worth more than making the amp work... see what they go for on ebay??

Just a thought...

           _-_-WBear2GCR

I thought of that...  If I decide this isn't worth it, after I'm done, I'll build a monoband tuna out of the linear and sell the tubes.

I'm sure a PAL collector (if they exist) would LOVE to have darn near NOS 8950s, labeled with the PAL logo on them....  If anything, they look neat on the mantel next to the 4-400s and 4-1000 dud.

--Shane
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KC4VWU
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« Reply #10 on: October 23, 2009, 02:58:18 PM »

Shane, you must have a boatload of 8950's. That's the very reason why I gave up on the Swans and such. I can easily get a good pair or three 4-400's for the price many dealers want for those sweep tubes.

I was thinking of building a multiband sweep tube amp just to get a little extra "ooomph" for the PW tx's I like to play with. But after a lot of reading and thought, I've pretty much gave up on that idea. I was thinking of using the cheaper, higher heater voltage tubes such as 31LQ6, 31JS6C, 35LR6, 42KN6, etc; there are quite a few more. BTW, not to be advertising here but since we're on the subject, I have quite a few NOS sweeps that I would like to sell; mostly the higher heater voltage ones.

If your really determined though, have you ever given consideration to the 6KN6? You could buy approximately 6 NOS ones to one 8950.

I see nothing wrong with experimenting, that's what we're supposed to do.

On the Messenger I conversion, I have several here that maybe someday I'll get to play with and I've contemplated that (band change) before. A neighbor, Mike KF4QKR, has converted one to a 75M transmitter and has been using it for several weeks now. He has posted about it here a week or so back.

Phil
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K4TLJ
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« Reply #11 on: October 23, 2009, 05:21:54 PM »

Some not all 6LB6 are in fact 8950 with a six volt heater. You have to examine the tube elements and base connections. I have a box of various 6LB6 and there are differences. The two Westinghouse labeled tubes are identical to 8950 tubes. The GE labeled tubes are missing the curved shield around the plate lead and some do not have the double base pin connections for the screen.
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Terry
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KD6VXI
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« Reply #12 on: October 25, 2009, 09:56:37 PM »

Some not all 6LB6 are in fact 8950 with a six volt heater. You have to examine the tube elements and base connections. I have a box of various 6LB6 and there are differences. The two Westinghouse labeled tubes are identical to 8950 tubes. The GE labeled tubes are missing the curved shield around the plate lead and some do not have the double base pin connections for the screen.

I have an actual GE Datasheet to the 8950, and no, it's not identical to ANY other sweep tube, physically.  Electrically, yes.  Physically, this tube has pin 6 tied with pin 2 to the cathode, and was sold, marketed and designed as an RF amplifier tube to 30 mhz.  I was REALLY surprised at this, as I was led to believe the M2057 was the "RF" version of that tube...  Guess it was more a "ruggedized" version...  In any shape, it was made for Maco.

As to do I have a boatload?  Nope, just these two tubes.  BUT, the amplifier is pretty clean, cosmetically, and has original tubes in it.  I DON'T want to kill them, hence me not "fudging" around to find what the impedance on input is.  I've already cherried them once, and my next move was down to here to ask.

Had fun this weekend.  CQWW /mobile.  3 contacts.  Turkey, Japan and Canada..  Next year, I'll be legal limit on 20 and 40 mobile.

--Shane
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W3RSW
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« Reply #13 on: October 26, 2009, 12:47:55 PM »

Pete said,
Quote
And, on a "serious" note, I've never seen a CB linear that was worth the time and effort to convert over for use on a ham band unless you stripped it down to bare bones first and then rebuilt it using some sound technical logic and components.

Well!
I'm insulted.  Grin

A certain unnamed ham and "()" built a beautiful CB amp way back in the day. A 6JB6 driving a trio of 6LF6's, all four in GG.  Plate tranny from a Link xmitter, meters by Motorola, the finest variables from ARC-5's, ... I mean , you name it we had quality schtufff... 

I'm sure it got burnt up by "ignerant lak of tunin'up" knowledge, but it 'shore was purty. '

We were so worried that we labeled it as the "10 Meter Intensifier."

"()" still has a wierd growth wiping out a fingernail on left had from sticking hand behind rig to pinch final coil for resonance while energized. Um.. yeah, first Darwin moment 'fer shure.'
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RICK  *W3RSW*
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« Reply #14 on: October 26, 2009, 05:52:38 PM »

Some not all 6LB6 are in fact 8950 with a six volt heater. You have to examine the tube elements and base connections. I have a box of various 6LB6 and there are differences. The two Westinghouse labeled tubes are identical to 8950 tubes. The GE labeled tubes are missing the curved shield around the plate lead and some do not have the double base pin connections for the screen.

I have an actual GE Datasheet to the 8950, and no, it's not identical to ANY other sweep tube, physically.  Electrically, yes.  Physically, this tube has pin 6 tied with pin 2 to the cathode, and was sold, marketed and designed as an RF amplifier tube to 30 mhz.  I was REALLY surprised at this, as I was led to believe the M2057 was the "RF" version of that tube...  Guess it was more a "ruggedized" version...  In any shape, it was made for Maco.

As to do I have a boatload?  Nope, just these two tubes.  BUT, the amplifier is pretty clean, cosmetically, and has original tubes in it.  I DON'T want to kill them, hence me not "fudging" around to find what the impedance on input is.  I've already cherried them once, and my next move was down to here to ask.

Had fun this weekend.  CQWW /mobile.  3 contacts.  Turkey, Japan and Canada..  Next year, I'll be legal limit on 20 and 40 mobile.

--Shane


Shane a M2057 is an 8908 with a 12 volt filament and a compactron base.

73 N8QPC
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