8950 / 6LF6 / 6MH5 / 6LB6 / Etc

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KD6VXI:
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

Pete, WA2CWA:
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.

W2XR:
Quote from: Pete, WA2CWA on October 22, 2009, 11:00:13 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

Pete, WA2CWA:
Got to have some pain, if one wants the gain :D

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.

KD6VXI:
Quote from: Pete, WA2CWA on October 22, 2009, 11:39:14 PM

Got to have some pain, if one wants the gain :D

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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