6L6GC Class C AM Parameters

[N4LTA]

Anyone know the approximate grid current and grid voltage for a 6L6GC at 300 volts on the plate running plate modulated AM?

I am assuming about 8-9 ma and -70-80 volts or so.

The data sheets show class A and AB  only.

Pat
N4LTA

[W3GMS]

Pat,
According to the 1943 ARRL handbook the parameters are as follows for plate modulated class C service:

plate V=325
screen V=225
grid V= -45
plate current=90 ma
screen current=9 ma
grid current=3ma
driving power=.95W
output power=20W

Hope that helps...
Joe, W3GMS
 

[N4LTA]

That helps a bunch  - Thanks.  I didn't look that far back.


I think I'll try a 15K grid resistor and about  a 12 K screen resistor. My supply delivers about 100 Ma at 290 volts.

Maybe I'll get 15 watts carrier if I am lucky. I am using a Hammond 125ESE as a mod transformer which is good for 15 watts so I should have plenty of modulation power.

Pat
N4LTA

[W3GMS]

Good luck with the project.  Should be very easy to get it up and running. 
With the Hammond XMTR you should have some good audio.  Thats a nice level of drive to run into a linear amp for some dB improvement.
Regards,
Joe, W3GMS

[N4LTA]

I am trying to build a little transmitter with decent audio for the local net and to experiment with. I'll post a photo when I get it going.

I have three transmitters and two receivers in the mill. Finishing is my toughest problem.

Pat
N4LTA

[W3GMS]

Pat,
What are you going to use to drive the 6L6?  I have used them as single tube xtal oscillator transmitters, but you don't get that much power out of them as an oscillator.  You could use almost anything tube to drive the 6L6.   12BY7 or a 5763 would work well. 
Joe, W3GMS   

[w4bfs]

I am trying to build a little transmitter with decent audio for the local net and to experiment with. I'll post a photo when I get it going.

I have three transmitters and two receivers in the mill. Finishing is my toughest problem.

Pat
N4LTA

you have said a mouthful there Pat .... I seem to get a project about 80% done and something else more interesting (at least at the time) comes along to do

[w1vtp]

Pat,
What are you going to use to drive the 6L6?  I have used them as single tube xtal oscillator transmitters, but you don't get that much power out of them as an oscillator.  You could use almost anything tube to drive the 6L6.   12BY7 or a 5763 would work well.  
Joe, W3GMS  

Anything that would drive a 807, 6146 etc.  Eico uses a 6CL6. A 5763 would be OK if you can find one.  If you want to keep period, a 6AG7 would do the trick.  Is the driver to be a XTAL osc or take an input from a VFO?  There are some nice circuits out there in the 40's / 50's that would fit the bill. Come to think of it -- the 6L6 would make a nice AM transmitter to drive a linear. Something like Ken W2DTC's, HB 6146 or Joe N1VIV's HB 6146 -- the 6L6 would cut the mustard.  There are lots of nice mod irons out there in that power class that would fit the bill.  A 6L6 modded by a pair of 6L6's sounds like a great combo.   Or use a reverse HIFI piece of iron and use a 30 - 40 watt AF amp (for plenty of head room  ).

I think a 5763 would be way too fancy unless you have a  collection of 'em.  If you are going to use a VFO input, you might want to put a wirewound pot in the screen circuit of the 6AG7 to control the drive to the 6L6 to optimize the required drive current (3 MA) for best AM mod without driving the crap out of the 6L6.

HMMM

Al

[KK4RF]

Pat,
     I have a 6AG7-6L6 novice type transmitter from 1953 QST (Lew McCoy) that I built about 10 years ago for 80/40 CW. It is a 35 watt input xtal-control rig. I've been thinking of trying to audio modulate it for some time now. I'm curious what you plan to use as a modulator. I presume that you are using the 125ESE "backwards" connected to  an audio amp, then using a Heising-type circuit to modulate the 6L6. I looked up the 125 ESE transformer on Hammond's website. Looks like a neat project. Please give us more details when time permits. I would like to do something similar.
             ---Marty, KK4RF---

[N4LTA]

It is a little toy project.

I have layed out a 6CL6 crystal oscillator on a PC board for fun as well as a small PC Board power supply and an antenna relay board. It is going to be a sort of weird looking hybrid project with toroids for the inductors. I put a LC tuning network after the oscillator which will fancy it up a bit - so now the pi-net needs to switch for 80 and 40 meters.

I am getting close to finishing - just need to keep chugging. Got a offshore fishing priject for this weekend - so I am trying to get it finished.

When I do, I'll post a photo and then ask me what really, really, stupid thing I did last night. (it involves punching holes). I might admit to it.

The 125ESE is being used backwards. It is rated for 80 Ma DC current since it is a single ended transformer and 15 watts. Hammond is very conservative so I should  not need a modified Heising choke and cap. The modulator is a little 22 watt solid state amp from the late 60s (nostalgia again)

It was made by Southwest Technical Products as a kit and was featured in Popular Electronics. I built quite a few of these in high school and used one to modulate a school radio station transmitter (controlled carrier  - the power line type) They kept blowing out at full output because of RF on the power supply.
We didn't know why at the time and made a quick and dirty 6L6 PP modulator to replace them. The amp was called the Lil Tiger and was the smallest in a line up of amps using the "new" Motorola complimentary symetry transistors. They are still made and I reproduced the old board layout from a reprint of the original Poptronics article.

http://www.swtpc.com/mholley/PopularElectronics/Dec1967/PE_Dec1967.htm

Am I the only one reliving my childhood electronics projects?


Pat
N4LTA

[KX5JT]

I am trying to build a little transmitter with decent audio for the local net and to experiment with. I'll post a photo when I get it going.

I have three transmitters and two receivers in the mill. Finishing is my toughest problem.

Pat
N4LTA

you have said a mouthful there Pat .... I seem to get a project about 80% done and something else more interesting (at least at the time) comes along to do

I call this PCD.  Project Creep Disease.  Other projects tend to creep in before the last one is finished.

John KX5JT

[KK4RF]

Pat,
     Thanks for the thumbnail sketch of the project. Will be watching this project updates. ---Marty, KK4RF---

[W3GMS]

Pat,
Sounds like a fun creative project! 

I certainly remember the Southwest Technical Products line of audio gear.  I have downstairs (somewhere) one of their high power amps.  I use to use it as a driver for a pair of 833's.  It worked out vey well and if I remember correctly it was at least 60W of power.  The form factor was like a shoe box on its side with a silver panel.

Good luck on your latest project.

Regards,
Joe, W3GMS   

[N4LTA]

I finally got it all put together late last night. Having some problem in that I am getting very little RF to the pi net.

The grid tunes up close to design at -30 volts with full drive and I get about 90 ma at 275 volts (25.5 watts input) with full drive on the 6L6 and the pi net has a dip but not very sharp. The loading cap gives max current at minimum capacity. The is at 3720 mHz.

I only see 20 volts pk or so RF at the 6L6 plate and 3 pk or so volts at the tuning cap. I am using a 1000pf coupling cap and it probably should be a 5000 pf or so but that is not whole the problem.

Screen voltage was about 210 volts, screen current about  5.4 mA.  Grid current was about 2 mA at max drive.

The pi- net is calculated for 1600 ohm plate load roughly - about 8 uH, with a  100-300 pf variable at the tune and a 3 gang 425 pf cap at the load paralleled with an extra 470 pf in parallel - prox 500- 1700 pF

It was late and I decided to go to bed and work on it tonite.  Probably something stupid.

The 6L6GC is a shiny new guitar amp Tesla model - maybe I need to install a real 6L6. I was hoping for 12-15 watts out.

Pat
N4LTA

[k4kyv]

you have said a mouthful there Pat .... I seem to get a project about 80% done and something else more interesting (at least at the time) comes along to do

I call this PCD.  Project Creep Disease.  Other projects tend to creep in before the last one is finished.

John KX5JT

And your workbench gets cluttered to the point you can't find anything and don't have room to work on any of the projects. So it all sits there collecting dust, because every time you decide to tackle one of the projects you get discouraged just looking at the mess.

[N4LTA]

Every year or so - I HAVE to clean up the workbench - good stuff starts falling off and breaking - yesterday a brand new 100K 50 watt power resistor fell off the pile and broke.

It will take 2 days to get it slightly uncluttered.

I hate to think how many tubes have met their demise from gravitational accelleration and instant decelleration at  concrete floor below. I probably need to put a rug under it.

I really need to get rid of some stuff but its all good stuff that no one but me wants.

Pat
N4LTA

[KM1H]

Whats with so many having problems getting RF out of simple circuits recently?

One thing that appears common is using computer programs or "calculations". What happened with the old way....cut and try?  Then when you get the best power transfer revise the formula

Here is a pretty decent design suite that Ive found to work well.

http://www.ifwtech.co.uk/g3sek/

Carl
KM1H

[N4LTA]

Don't think this is a pi-net problem. I haven't looked very far at this point.

I do use a program  - but one that I wrote years ago in basic and it has worked well for me for a long time.
My pi-net inductor is wound on a toroid (I use this method often) and the value confirmed with a bridge at 8.1 uH.

Plate current is 90 ma and the voltage is 277

For class C operation - the load resistance is  277/(.09 x 2) or about 1530 ohms

My program gives me C1 = 246pf, C2 =1067pf and L = 7.9uh for a Q of 10

I have a 200pf variable with a 100pf in parallel and a three gang 425 pf with a 470 pf in parallel for starters.

That is where I quit last night after a quick test.

The scope showed only 30 volts pk AC on the 6L6 plate  with AC coupling to the scope- that seems like a problem to me

[N4LTA]

Just got home from work and fired up the transmitter. Got plenty of AC at the tube plate - (It was late last night)

One main problem is that my Bird 43 is dead - worked fine the last time I used it? Now - not a wiggle.

Used the scope, peaked up the Pi - net changed the blocking cap from the .001 to a .005uF .

Pi net tunes up as expected now - BUT 7.5 watts out - about 5 or 6 less than hoped for.

Update - Got it tweaked to about 13 watts - a metal 6L6s worked best along with a little tweaking on the Pi - net coil to raise the Q.

I am about where I expected to be. Tried all the 6L6s that I have. 6L6 metal was best and a  Svetlana 6550 was about the same as the metal tube. The Russian and Czech  6L6s were a couple of watts worse.

I don't have a decent USA made 6L6GC.

Time to try the modulator. I need to open up the Bird and see what has come loose.



Pat
N4LTA

[ke7trp]

On the Bird, Its most likely the contact at the slug. Look in there and clean it with an eraser or cloth and cleaner. You might need to put some tension on it.  Mine JUST failed today.  After cleaning, Its back to working.

C

[N4LTA]

That is what was wrong with mine - I took it apart and tested the meter  - it was ok so I cleaned the cavity and slug contact and now it is fine. Confirmed my 12-13 watts out on the little rig that I had calculated with the scope.