Modulation Question

[sndtubes]

The guy that built the amp said he tried to get it to work on the higher bands but it just didn't have much output.  I don't think the problem is arcing in the bandswitch.  The tank circuit bandswitch is a pretty hefty one.  The tank coil might be too big to work on 20 and higher.  Possibly need to switch in a different inductor rather than trying different taps on the existing one??? 

[K1JJ]

Yes, bandswitch tap arcing will be obvious if it happens. I mentioned it only so you will be ready with a solution if it happens once your rig is operating.

Yes, the tank coil may be too large and needing a smaller unit for the higher bands. If there is low power output, most likely the L/C tank ratio is off and the coil L/D ratio is too high.  A good coil is about as long as its diameter or maybe X2 longer than diameter.  You will need to calculate the correct tank L/C values for each band, then adjust the coil in or out a few turns (or stretch/compress it) until you find the best efficiency for a given band.

There are good calculators on the web for tanks.


Another possible problem is not having enough drive on a higher band. Be sure you can adjust the input tuning to the amp  near to 1:1 swr and you are able to get the correct grid and screen currents so that the tube can produce power. After that, the tank is the challenge. The tank values can be very critical on the higher bands above 20M.

T

[sndtubes]

Yes, bandswitch tap arcing will be obvious if it happens. I mentioned it only so you will be ready with a solution if it happens once your rig is operating.

Yes, the tank coil may be too large and needing a smaller unit for the higher bands. If there is low power output, most likely the L/C tank ratio is off and the coil L/D ratio is too high.  A good coil is about as long as its diameter or maybe X2 longer than diameter.  You will need to calculate the correct tank L/C values for each band, then adjust the coil in or out a few turns (or stretch/compress it) until you find the best efficiency for a given band.

There are good calculators on the web for tanks.


Another possible problem is not having enough drive on a higher band. Be sure you can adjust the input tuning to the amp  near to 1:1 swr and you are able to get the correct grid and screen currents so that the tube can produce power. After that, the tank is the challenge. The tank values can be very critical on the higher bands above 20M.

T

Do you think I'd be better off switching over to a variable roller inductor for the tank coil?  I hate to disturb the already excellent performance on 160/80/40.  My friend said he had a difficult time achieving the correct Q for the tank circuit on 160.

I really don't know the reason he couldn't get it to work on the higher bands.  I'm figuring that you guys are right about the L/C ratios in the tank circuit, but I think my friend is sharp enough to know that.  He may have not tried too hard to make it work since his primary interest is on the lower bands.  There really isn't much activity yet on the higher bands so, there was no pressing reason to make it work at the time. 

I'm probably not going to try to tackle this until this winter as I have a lot on my plate at the moment. 

[WA1GFZ]

How about a picture of the tank circuit.
Sounds like adding a small inductor for the upper bands would be the way to go but it may force you to modify the main inductor or taps. Carl's idea of the tapped tank coil will work on all bands. My old 4-1000A was configured that way and had good efficiency on all bands. We actually added 160 after I sold it to a friend. The connection to the plate tune is critical for stability It needs to be a short wide lead

[K1JJ]

If you make the 10M coil as calculated and then place the Tune cap dead center it will require 4X the C to resonate. 

Could someone elaborate on this technique?  "Place the tune cap dead center" << What does this mean when using a pi network? Maybe someone cud draw it out.

I also have a linear that requires minimum C on 20M - no prayer of getting it on 15 or 10M without a trick like this.

T

[The Slab Bacon]

Ya know, thatz a trick I never even thought of    

"Tapping down" the plate tuning cap. Instead of hooking it directly to the plate lead of the tank, move it a few turns down toward the "cold" end of the coil. HUH!!

The only thing is that in a bandswitched amp or final, it could pose some switching
and/or lead dress issues. I also wonder how it will effect the Q of the plate tank

[ke7trp]

This amp claims full output on 10 meters with 813s.  1k out on 10.

C

[The Slab Bacon]

This amp claims full output on 10 meters with 813s.  1k out on 10.

C

Look at dem final tank values........................

Iz dey bird watts or dosey watts     

[WA1GFZ]

PEP output will be about 2X dissipation if you want it to last. Efficiency around 65%. Frank I tapped the coil on my 4-1000A back in about 1976 still works great today. BTW I once ran 5500 volts on 10 meters. and it was nice and stable.

[K1JJ]

BTW I once ran 5500 volts on 10 meters. and it was nice and stable.

Big difference in stability using a 4X1 on 10M in grounded grid linear vs: grid driven, no swamping, plate modulated...

T

[W9GT]

Ya know, thatz a trick I never even thought of    

"Tapping down" the plate tuning cap. Instead of hooking it directly to the plate lead of the tank, move it a few turns down toward the "cold" end of the coil. HUH!!

The only thing is that in a bandswitched amp or final, it could pose some switching
and/or lead dress issues. I also wonder how it will effect the Q of the plate tank

Just trying to get this straight..Seems like Tom Vu had the same question.  You are saying just take the plate tuning cap lead off of the plate end of the Pi net tank coil and move it to the center of the 10 Meter portion of the tank coil and leave the plate coupling cap connected to the plate end of the coil?  This technique might be good for other amps with multiple tubes and high total output capacity???

73,  Jack, W9GT

   

[K1JJ]

From what I understand, I think you got it correct, Jack.


I'm wondering if a second independent bandswitch is needed to pull this off or can it be cleverly configured to be tapped all the time for say, 10M, and still work on the other bands with the main bandswitch??  The 10M coil is on the "hot" side near C1 anyway.... 

A schematic of a commercial bandswitched rig using this technique would help.


T

[w4bfs]

Tom ....look at early 60's ARRL handbook at the single band 813 rack mount amp for the 10 mtr scheme .... guess this is a dual pi/l output .... gnarly