screen modulation again...

[N2DTS]

I like it hot, I have 3 motorcycles to ride.
A single section of a 6080 looks close, worth a try when radio season starts full time.

[DMOD]

Its coming up on radio season again and I was testing the equipment since its not been on for a while.
I hooked the QIX dx60 screen modulator circuit to the pair of 4x150's and gave it a tune up on 40 meters.
I can not find the diagram here, but its around someplace.
That design really works well, line input or D104, but I am pushing the tube (6de7) with screen current into a pair of 4x150 tubes at 200 to 250 watts out.
So I was thinking of changing the last Triode out with a 6080 triode section.
Or is there a better triode in a smaller package?
 

You might try the 6EW7.

Phil - AC0OB

[N2DTS]

That is the same as the 6DE7.

I tried making an adaptor to sub the 6DE7 with a 6080.
I got about .001% modulation out of that setup, the 1st stage of the 6DE7 has much more amplification then the 6080.

One nice thing about the 6080 is the 300 volt filament to cathode rating, along with the double plate dissipation rating.

I was running the modulator on 40 today, amazing that the little 6DE7 modulates a 200 watt carrier 100%.
800 watts PEP or more...
 
[/quote]

You might try the 6EW7.

Phil - AC0OB
[/quote]

[WD5JKO]

    I found this chart on the net. Might be useful.

Jim
Wd5JKO

[N2DTS]

Nice chart!
The modulator as it is now has a 12ax7 mic preamp (d104) and line level amp, then into the 6de7.
The best I can come up with is to sub each section with a tube that is close to the same specs.
The 6080 may be close enough to work ok for the 2nd section, only a test will tell.
I like the 300 volt filament to cathode voltage rating and the plate wattage.

I found something close to the 1st section but forget what it was.

 

[w4bfs]

hi Brett ... the 6080 has a u of 2 which means you will need a LARGE V swing to drive it as a cathode follower .... seems like a good job for the 6de7

[WD5JKO]

   Remember that cathode followers have a gain of just under 1. A good cathode follower for this application has high transconductance, and high emission capability. Having decent plate dissipation is useful also. The Mu, whether it is 2, 5, or 20 makes little difference except for biasing, the voltage gain is still just under 1. The Mu in the first stage is likely where we need some voltage gain, so low Mu there is not desirable.

   Brett, you might expand your list to individual tubes. Perhaps a triode connected sweep tube such as a 6DQ6 would do for the cathode follower, and something like a 12AT7 for the 1st tube (Mu = 70'ish). Using both 12AT7 sections together will double the transconductance. For Lower Mu on the first tube, use the 12AU7 (20'ish).

Jim
Wd5JKO

[N2DTS]

Sounds like fun, but how clean is a 6dq5/6dq6/etc as a cathode follower?

What would happen if I used two 6DE7's in parallel (last section)?

Yes, I could triode connect all sorts of tubes I suppose, but how clean would they be?
What about a 6B4, I have some of those....

I really like how the modulator works, I just want to increase the screen current capacity, the 6DE7 runs warm at 200 watts out.

[w4bfs]

   Remember that cathode followers have a gain of just under 1. A good cathode follower for this application has high transconductance, and high emission capability. Having decent plate dissipation is useful also. The Mu, whether it is 2, 5, or 20 makes little difference except for biasing, the voltage gain is still just under 1. The Mu in the first stage is likely where we need some voltage gain, so low Mu there is not desirable.

   Brett, you might expand your list to individual tubes. Perhaps a triode connected sweep tube such as a 6DQ6 would do for the cathode follower, and something like a 12AT7 for the 1st tube (Mu = 70'ish). Using both 12AT7 sections together will double the transconductance. For Lower Mu on the first tube, use the 12AU7 (20'ish).

Jim
Wd5JKO

gain can be approximated as:    gain = mu / (mu + 1) for a triode cathode follower so for a 6080 with a mu of 2 this results in an approximate gain of 0.67 which possibly is a considerable change from 0.9 to 1 .... this of course means that to get a 200 V swing out of the 6080 cathode follower you would have to drive it with a 300 V signal

[N2DTS]

The 6B4 looks like a good idea.
Separate filament transformer with a 1000 volt rating is better then a 200 or 300 volt filament to cathode rating.

The 6B4 has an amplifcation factor of 4.2, mid way between the 2 of the 6080 and the 6 of the 6DE7 2nd section.
Good for plenty of current.

12ax7 as mic and line amps, a 12au7 as the 3rd stage (just one triode) and a 6B4 as the cathode follower.

[N2DTS]

I think I will also build another quad of 4-125's to screen modulate.
They seem to screen modulate real well.
I will make it a no fan rf deck.

[DMOD]

Sounds like fun, but how clean is a 6dq5/6dq6/etc as a cathode follower?

What would happen if I used two 6DE7's in parallel (last section)?

Yes, I could triode connect all sorts of tubes I suppose, but how clean would they be?
What about a 6B4, I have some of those....

I really like how the modulator works, I just want to increase the screen current capacity, the 6DE7 runs warm at 200 watts out.

While the 6EW7 has the same pinout as the the 6DE7, the 6EW7 has a higher power rating, higher transconductance, and lower plate resistance because the plate areas are larger.

For either tube, you must not exceed the 200 volt Fil to Cathode differential unless you use a floating filament transformer.

When driving a single 6146 or 6DQ5 for example, the screen voltage needs to be set to about 60-70 volts quiescent for a 150 volt P-P signal on the screen.

When driving the Knight T-150 with parallel 6146's, you only need about 35 volts quiescent on the screens to get the same power out.

For the 4-125A, I would set the initial screen voltage to about 150V.

With those screen voltages, you might have to use a tetrode or pentode shunt modulator configuration if can't find a suitable triode for a cathode follower circuit.

What does your current schematic look like?  

Phil - AC0OB

[N2DTS]

I mis read, the 1st section is the same, the 2nd section has 3 more watts of dissipation?

I will have to try the 6EW7.

The design would be the basic DX60, WA1QIX design.

I like the idea of a big triode with a separate filament transformer.
 

[N2DTS]

I ordered a bunch, cheap on ebay...

[DMOD]

Another idea: How about a 4-65 driving your 4-125's, as in a "stripped-down" Continental 317?   

Phil - AC0OB

[N2DTS]

Very cool, but I do not have a 4-65.
A bit overkill for 4-125's or even the 3X 4-400 rig.
Would be really cool though, a quad of 4-125's modulated by a 4-65.
I think a 6b4 would be better though, cleaner?

Fun stuff.

[KD6VXI]

Phil, 

That Continental schematic has flown around in my head for a couple years.   I've been collecting parts to replicate a miniature version.

Single 4cx5ka.  Instead of tubes series mod on the screen,  pulse width modulated using hv mosfets.

I have tube,  socket,  3A CCS Dahl and the fil xformer and a choke,  for good measure.

Ought to be good for legal limit am.   

--Shane
KD6VXI

[N2DTS]

Got a 6EW7 in the mail today.
Its the biggest 9 pin tube I have ever seen!
Will not fit in a socket with a shield so I had to replace the socket.

Works great, I did not see any plates glowing at 200 watts carrier.

200 watts seems to be about the maximum power out of a pair of 4X150's at about 1800 volts.
I can increase the carrier power but the peak power does not seem to increase past about 800 watts.
No problem with the modulator though.

I am accumulating parts for the bigger modulator and the new quad of 4-125's.

[DMOD]

Got a 6EW7 in the mail today.
Its the biggest 9 pin tube I have ever seen!
Will not fit in a socket with a shield so I had to replace the socket.

Works great, I did not see any plates glowing at 200 watts carrier.

200 watts seems to be about the maximum power out of a pair of 4X150's at about 1800 volts.
I can increase the carrier power but the peak power does not seem to increase past about 800 watts.
No problem with the modulator though.

I am accumulating parts for the bigger modulator and the new quad of 4-125's.

I take a 4" aviation sheet metal shears and slice the base shield into 4 petals and then cut the rest of the metal away.

Some of the later RCA 6EW7's had the smaller 9-T6-1/2 Bulbs, while the earlier 6EW7's had the larger 9-T9 bulbs.

Glad to see the 6EW7's are working out for you.

Phil - AC0OB

[DMOD]

Phil, 

That Continental schematic has flown around in my head for a couple years.   I've been collecting parts to replicate a miniature version.

Single 4cx5ka.  Instead of tubes series mod on the screen,  pulse width modulated using hv mosfets.

I have tube,  socket,  3A CCS Dahl and the fil xformer and a choke,  for good measure.

Ought to be good for legal limit am.   

--Shane
KD6VXI

You could probably use a 3-500Z "switcher" with the proper filtering for PWM.

Owing to the relative simplicity of the 807/4-65 (or equivalent) driver configuration for modulating the screen of the 4cx5ka, that's the way I would go.

Phil - AC0OB

[KD6VXI]

Thanks for the input,  Phil.

I was thinking of mosfet switches because I have them from another scrapped project....   A series mod low power rig from here.

Not against using a tube,  just never done it.   Pdm is new to me.

I have a 4-125 here,  for a single 5k, that might be just the ticket.   The   use a single fet for the switch?

--Shane
KD6VXI

[N2DTS]

That is not going to have much in common with the Continental transmitter...

What is the screen voltage and current of the 4cx5000?
Maybe you could modulate it with the DX60 modulator (with a 6EW7 in it).
Be good for about 2kw carrier.

I myself wonder how far you can take the DX60 screen modulator.
It works fine at 200 watts out.
Not bad for a pair of 9 pin tubes as a speech amp and modulator, and it likely could do 300 watts, say a three 4x150's..

[N2DTS]

Looked it up, the 4cx5000 is good for 3500 watts carrier with screen modulation, with only 350 volts on the screen (7500 on the plate).

Might need to go bigger then a 6EW7 though...

[KD6VXI]

The switch fets I have on hand are 1.5 kv.   Same ones as described in a homebrew series modulator elsewhere on the site.

Since I have them on hand,  that was my reasoning for use.   I could just use the series mod,  I'm sure.   But,  pdm / pwm has my attention at the moment.  With judicious use of clip leads.

My screen supply is a seperate 1.4 kv DC supply.

I'll have to check into the pdm switch tube designs.   I know the t be has to be floated......   Guess it's time to start a new thread.

--Shane
KD6VXI

I also have a 4PR1000 on hand.    Not

[N2DTS]

Started making holes for the BIG modulator:


https://n2dts.smugmug.com/Ham-radio/i-n6PwTmt/A

12AX7 mic and line preamps, 12AU7 (one triode), 6B4.

Separate filament transformer for the 6B4.

Waiting for a front panel, I seem to have used up most of my good parts building all sorts of stuff...