PW Series Modulated Transmitter

[ka1tdq]

I was thinking of a new tube project that would have low power and clean audio. The attached schematic uses four tubes. The RF section consists of a 5763 driving a 7984 Compactron, which is series modulated by a 6L6. A 12AU7 is the audio driver.

I calculated 62 mA of plate current on the 7984 for 15 watts carrier output.

I've never built a series modulated rig before and I borrowed pieces of schematics here and there. Any glaring errors in the schematic?

Jon

** I did just think of something else. With series modulation, the power output is cut in half because of the class A modulator. If I want 15 watts output, I'll need to calculate for 30 watts output. That puts plate current at 122 mA and brings plate impedance down to about 1450 ohms.

[DMOD]

I am not sure the 6G6 will handle the needed series current.

May I suggest a 6DQ5 for both the Final and the modulator if you want all tube rig? They are tough tubes and I have more than 8000 hours on one 6DQ5 final in a series modulated rig.

A better series modulated circuit IMHO is to use a HV power mosfet as the modulator since it has a lower series resistance.

Phil

[ka1tdq]

I would use something like a 6DQ5, but it has the plate connection on the top of the tube. I want to have all the tubes exposed on the chassis for cosmetic reasons. The 7984 has the plate connection on the bottom pins.

I could parallel 6L6's or use a single KT-88 as the modulator.

Jon

[w4bfs]

hi Jon .... the limiting factor in the series modulator is its plate dissipation .... say with 900V available from the hv ps the modulator should drop 60% quiescently or so to be able to reach 100% modulation .... so this is about 540V and at 33 Ma this is 18W which is reasonablle for a hv 6L6 family tube ... the remainder of 360V tmes the 33Ma  gives 12W input to the rf amp and at 70% efficiency gives around 8 W rf out which is a good drive level for a rf amp ... making the Q point variable will let you shift output power and mod index if you wish

by the way I thot you had gone solid state for yer transmitters .... guess the soft glow of hollow state gotcha ...it has me 

[ka1tdq]

Yeah, I still have my 24-FET transmitter that I haven't finished testing yet. I've been thinking of a new project and I like exposed vacuum tubes without the danger of high voltage. I think I can build it to have a "wow, cool..." factor too.

I'm mostly concerned with the 'floating' 7984 above ground at high voltage. Anything look funky with the way I drew the schematic on that?

Jon

[w4bfs]

i apologize ....I didn't look at ur skizmatic before I spoke .... I looked at it and didn't see any show stoppers .... just a few things .... you could operate the 6l6 with screen and plate tied together or triode connected .... helps keep the screen out of trouble and is good at 400 V .... biggest thing is getting some isolation between grid and plate circuits on the rf final ... I imagine that neutralization and grid resistance willl be needed as well as a dividing plate between them .... maybe you'll get lucky and not need them ... if you could find a GE mastr pro low band tx you could see the 7984 applied AIRC

[DMOD]

Here is a suggestion:

Page 2 is the Final.


Phil

[AG5UM]

What a Fun project!  Keep us posted on your progress. There are so many cool overlooked tubes like Compactrons,
Home-brew is great since you can use what you want, not just standard run-of-the-mill stuff.
Fun project!

[ka1tdq]

Phil, that's... that's awesome!

I wanted to use tubes that were still in production for the audio section, and both the 12AX7 and KT88 are.

Thanks!

Jon

[K8DI]

Here is a suggestion:

Page 2 is the Final.


Phil

Sorry for going off subject...Phil, what do you use to draw schematics?

Ed

[DMOD]

Phil, that's... that's awesome!

I wanted to use tubes that were still in production for the audio section, and both the 12AX7 and KT88 are.

Thanks!

Jon

I looked at the Svetlana KT88 tube curves and they wanted a bit more control grid bias and a bit more screen grid current so those two values were modified and I added your "OP" switch at the cathode.

One thing about cathode modulated transmitters using tetrodes is they need solid screen voltages, low modulation stage series resistances, and plenty of RF drive.

CM transmitters have great audio.

Hope this helps.

Phil - AC0OB

[DMOD]

Here is a suggestion:

Page 2 is the Final.


Phil

Sorry for going off subject...Phil, what do you use to draw schematics?

Ed

I use LTSpice 'cause it's free and only takes about 20 minutes to learn. I have other CAD packages but I use LTSpice mostly.

[ka1tdq]

I have most of the major components already and I just ordered a new Hammond 750 vct transformer. That'll be mounted on top of the chassis, so I wanted a transformer that was 'pretty.'

There's a hamfest coming up here the first week in January that I hope to pick up some other miscellaneous stuff for it.

I'm also going to use a Hammond chassis enclosure for it. I can't order the one I need until I get all the big parts together and figure out what size I need.

On a side note, I need a PW rig right now. I bought one of those LDG feedpoint autotuners rated at 100 watts. It's installed on my delta loop (my only antenna), and running 2000 watts AM is out of the question with it.

Thanks again for the schematics!

Jon

[DMOD]

I have most of the major components already and I just ordered a new Hammond 750 vct transformer. That'll be mounted on top of the chassis, so I wanted a transformer that was 'pretty.'...

Thanks again for the schematics!

Jon

Hi Jon, I take you're PS will be a Full Wave Bridge design?

BTW, just the SA, Cathode Modulator, and Final require 195 mA by themselves.

The Power Output is calculated to be 45-53 Watts at an efficiency of ~ 55%.

I upped the SG current at the Final because GE preferred 8.5 mA and added a 5W power control pot at the cathode of the KT88's. Added bias protection zener.


Phil

[ka1tdq]

Yes, I'll use a full wave bridge with the center tap at 350 volts.

I won't run the rig at that power though and will back it off a bit.

Jon

[N4LTA]

Phil

Can you give some information on a mosfet series modulator. Could it be done for a larger tube such as an 811A or a 572B? Or even a pair?

Thanks

Pat
N4LTA

[DMOD]

Phil

Can you give some information on a mosfet series modulator. Could it be done for a larger tube such as an 811A or a 572B? Or even a pair?

Thanks

Pat
N4LTA

Hi Pat,

Normally we would use 1 HV Mosfet for plate voltages up to 750V but in the case of say a 2000V plate voltage on a 572B we need a cascode arrangement which complicates things a bit but here is a suggested circuit.

The neat thing about using a Mosfet is it doesn't take much gate audio p-p voltage to modulate a final stage because of the Mosfet's low Rds.

So here is a suggested circuit:


Phil - AC0OB

[N4LTA]

Thanks Phil

I have some 4D32s that could work well at lower voltage.

Also thanks for your past help with voltage regulators and screen regulators. I am firing up one of your circuits today. A screen regulator for a couple of SRS 551 tubes.

Pat
N4LTA

[w4bfs]

Phil

Can you give some information on a mosfet series modulator. Could it be done for a larger tube such as an 811A or a 572B? Or even a pair?

Thanks

Pat
N4LTA

Hi Pat,

Normally we would use 1 HV Mosfet for plate voltages up to 750V but in the case of say a 2000V plate voltage on a 572B we need a cascode arrangement with complicates things a bit but here is a suggested circuit.

The neat thing about using a Mosfet is it doesn't take much gate audio p-p voltage to modulate a final stage because of the Mosfet's low Rds.

So here is a suggested circuit:


Phil - AC0OB

hi Pat and Phil .... hope the holidays have been merciful to youall .... I have a question about your cathode modulator ... the skizmatic shows a nonconnection from the bottom of rv1 and coc ....is that correct .... and what kind of diode is d1 ...thanks  73  John

[DMOD]

RV1 is a 2.5 Meg pot. that sets the Mosfet bias in a feedback bias arrangement.
This and the 10 ohm metering resistor provide feedback to reduce distortion.

In LTSpice anytime a solid line intersects or is orthogonal to another line it is connected.

The only time a cross-over or a non-connection is shown is when there is a dotted or dashed-line or a jumper is shown.

D1 is a IN4151, a later version of the 1N4148.

[w4bfs]

ok Phil thanks for the info .... so the .7V or so drop of d1 will enable the drain of M1 to be at half potential V of M2's drain ... at least I assume that is the goal .... in this fashion will create the higher Voltage series pass element ....clever, if it works .... have you tested this with several parts of different production runs .... how about temperature changes

[DMOD]

Thanks Phil

I have some 4D32s that could work well at lower voltage.

Also thanks for your past help with voltage regulators and screen regulators. I am firing up one of your circuits today. A screen regulator for a couple of SRS 551 tubes.

Pat
N4LTA

4D32 tubes are very rugged tubes.

Here is a genral Mosfet modulator schematic for a final with up to 750 volts on the plate. I have used this as a modulator for dual 6146's, 6DQ5, and dual 807's in a friend's Globe Chief.

Depending on your tube or tubes, the only changes you may have to make are the L1 current rating and the fuse's rating.



Phil - AC0OB

[DMOD]

ok Phil thanks for the info .... so the .7V or so drop of d1 will enable the drain of M1 to be at half potential V of M2's drain ... at least I assume that is the goal .... in this fashion will create the higher Voltage series pass element ....clever, if it works .... have you tested this with several parts of different production runs .... how about temperature changes

I have used the HV version with an 813 at 1800+ volts plate-cathode voltage. (Future ER article).

Yep D1 drops about 0.4V to compensate for the Vds drop of M1 due to its Vds at the expected drain current.

I am not producing this commercially as it is for the benefit of the ARS community who wants to experiment. My original article in ER 359 gives the circuit theory, etc. for a 40 Watt CM transmitter using a 6DQ5 Final in which I had converted my Heising Modulated transmitter (ER 353) to CM.

The only drift I have found is due to tube element heating after warm-up.


Phil - AC0OB

[w9jsw]

Hi Phil,

How stout would the PS supply need to be here for the HV version. Say 1800V at 350ma be enough? I have an 8000 laying around that would be perfect for this I think (after the 813 rig is done, of course...have to stay on track).

John

[KK4YY]

Back on Oct 16, 2016 in another forum, Nico PA0NVD(SK) posted a cathode modulation scheme using a MOSFET that he built and tested. His design references the PA tube grid to ground, not to the PA tube cathode. This has the effect of modulating the grid along with the cathode. I don't know if this method has had any further development, but it does look very promising.

https://antiqueradios.com/forums/viewtopic.php?f=12&t=309046