Suppressor Modulation, 4E27A ??

[SM6OID]

Hi !

I thinking of buildning an AM transmitter (80 + 40 meters) with a suppressor modulated 4E27A.
The reason for using this concept is purely that modulation transformers are difficult to find here in Sweden. And the fact that I have a few 4E27A's.

My question is will this concept work well? I would like to have good audio quality. As far as I have understood this type of transmitter will be much more critical to adjust compared to a traditional plate modulated tx. So, what do you say?
Should I go for it or should I build a plate modulated instead?

[WD5JKO]

I think G3 modulation will be fine so long as you realize that any form of efficiency modulation will cut the RF carrier output by a factor of 4 from what you get from Plate modulated service. So getting about 50 watts carrier at 2 KV plate is all you get.

Jim
Wd5JKO

[w1vtp]

Wonder why not screen grid mod?  Here's an excerpt taken from Radio Handbook 16 edition:

"... However, suppressor-grid modulation has one sizable disadvantage, in addition to the fact that the pentode tubes are not nearly so widely used as beam tetrodes which of course do not have the suppressor element. This disadvantage is that the screen grid current to a suppressor-grid modulated amplifier is rather high.  The high screen current is a natural consequence of the rather high negative bias on the suppressor grid, which reduces the plate-voltage swing and plate current with a resulting increase in the screen current..."

Wonder if you would be better served by considering using screen-modulation?  We have lots of experience on this forum with that kind of modulation

73 es GL  Al VTP

[N3DRB The Derb]

heres all the info you will ever need on the 4e27. I have like 12 of them.

[KE6DF]

I think the 4E27A (5-125B) is quite a bit different than 4E27 without the A, isn't it?

The Navy has some shipboard transmitters in WWII that used an 803 with suppressor modulation. They must have had the choice of going with screen modulation, but chose suppressor for some reason. The screen grid on an 803 can dissipate 20 watts which apparently was enought to avoid problems there.

[N3DRB The Derb]

it handles more power due to holes in the base and fins on the plate, but  for that application i dont think theres more than a few watts difference

[SM6OID]

Hi !

Thank You All, for the information, I'm not 100% sure of what will happen, I would like to bulid a plate modulated tx, I guess that it all depends on if I can find the transformers that I need or not... 

Meanwhile, I have to build a power supply for my T-47/ART-13 to get on the air, using AM.
With a little bit of luck, maybe we will meet over the air... 

[k9qi]

They chose suppressor modulation for the simple fact that the suprressor is almost always biased below 0 volts.  In a 4e27, 5-125A or even 803-based transmitter, you can get extra power out of it by raising the screen above 0 volts by as much as 100 volts, but the audio power required to do this is always less than that required for a screen modulator where the screen is always drawing current / above 0 volts except for, perhaps, the last 5 - 10% of downward modulation.  With the 803, you need about 300 volts p-p audio swing, but it is essentially into a very high impedance.  And, you can limit the screen current by adding a dropping resistor in series with the screen much like protective and operating bias on the grid.  It will have a very minimal impact on distortion.  If you have a pair of these tubes, you might consider building a suppressor-modulated, high efficeincy Doherty transmitter - I've donw that and it works quite well.  It is essentially like a Continental 317C2/3 MW broadcast transmitter except that the suppressor, rather than teh screen grid, gets the audio.  Mine is on 75 meters and i use a crystal at 4xFc and a Johnson counter to generate the quadrature on frequency references to drive the 803s (in my case).  Distortion is about 5% at 95% modulaiton without feedback but with careful adjustment of bias on the suppressors.
- k9qi

[WU2D]

Not many tubes have a suppressor. The ones you mentioned and the 837 and 6AG7 come to mind.

I Suppressor Grid modulated a 6AG7 XTAL oscillator and it worked OK but you need a fairly high voltage negative supply to cut the tube off completely. The audio quality was OK. I think of this as a low power transmitter method for simple military sets. You have to be careful not to over dissipate the screen grid during adjustment because you are essentially removing its protection. I agree that screen grid modulation would be easier to manage.

Mike WU2D

[KM1H]

The US Navy was the one who had the 803 developed. They wanted a healthy MF to low HF CW TX that could pump out 150W for serious fleet work as an emergency or secondary unit and 50W on AM for chatting when in harbor and not requiring a seperate audio box and all from the same 2000V PS and in one cabinet.

In fact the Navy rarely used plate modulation on their big shipboard rigs. The available but scarce 811 deck for the TBK was mosly supplanted by an 813 grid modulator in a small box on the floor. The 500W on CW/RTTY was used even when at the pier to maintain fleet traffic.

The 803 is very underated, just sit one next to an 813 which is also underated. Ive run a pair on CW/SSB at 1200W out and not ever a complaint or color at 3000V during contest duty.

Carl

[k9qi]

I should proof what I write before submitting....

"In a 4e27, 5-125A or even 803-based transmitter, you can get extra power out of it by raising the screen above 0 volts by as much as 100 volts, but the audio power required to do this is always less than that required for a screen modulator where the screen is always drawing current / above 0 volts except for, perhaps, the last 5 - 10% of downward modulation."

That should read rasing the suppressor above 0 volts .....

Also, I, too, can attest to the fact that beam tetrodes like the 813 with the seperate beam-forming elements brought out seperately, or modified 1625's (sawing open the base to seperate the beam-forming element from the cathode) don't modulate worth beans.  I tried it just to prove it; first with a 5763 at a few watts and then with the modified 1625 and 813.  I'll find the transfer function curves and post them; very non-linear modulation and very tough to get past about 60% negative mod at all.  Plus, the screen is not designed to take the heat -it is just like running the tube with little to no plate voltage but full screen potential applied; the screen becomes the plate and, in the 1625, they become night lights quickly.

The 803 screen is pretty hefty and, if you use a dropping resistor from the plate supply, it is easily kept from over dissipation and destruction.  Also, the 4E27 and 5-125A's suppressor modulate quite well.  But, you don't get a lot of power out of them at carrier; it is efficency modulation; even if a different element compared to the screen or control grid.

Current project is a pair of 4-1000A's in a Sainton high efficiency Doherty arrangement for 75 meters.

[F6BGV]

Thank You All, for the information, I'm not 100% sure of what will happen, I would like to bulid a plate modulated tx, I guess that it all depends on if I can find the transformers that I need or not...  

Any first-rate and cheap transformer for ham radio, can be order to : www.abltransfo.fr

Sorry for delay...

Best 73 !
Jean-Pierre

[VE3LYX]

PT215 (VT104) is purpose built for suppressor grid modulation. Seems to me an 837 also likes it. Go for it. You have nothing to lose and everything to gain. Not to mention the fun. And you can switch it over to any other mod system if you dont like in a heartbeat. (how long could it possibly take to ground a suppressor grid. )
I have been working on a suppressor grid modulated transmitter based on the Lanc Bomber T1154 but with one PT215 tube instead of two. I still find NOS tubes around. Medium priced. $12 TO $60. VE3XFE has a dozen nos ones at his QTH.
I have avoided plate mod because I have to build two radios to get one (One RF radio and One AF radio essentially) Doesn't interest me. I always hear how its more efficient. I fail to get the point? Whose counting? Frankly Scarlet.......... Have a one tuber with passive modulation working, Now that's efficient.
A moderate power GG linear can bring any good exciter up to snuff and can be built for peanuts. (I have about $100 in mine. ) Truth be known probably for less then  a plate modulator.
don

[WU2D]

Of all forms of efficiency modulation, suppressor is the easiest to set up and the most forgiving, so it was used in several military transmitters including the GRC-9. Basically you set the SG for zero volts and tune the class C final for normal output, then increase the negative bias until you are at half plate current which will be around 1/4 power out.  Here is a PDF that shows exactly how to handle the 4E27 tube. This tube is made for the job.

An RF detector sampling the output fed back inverse to an early audio voltage stage will linearize the AM.

[Opcom]

Thank You All, for the information, I'm not 100% sure of what will happen, I would like to bulid a plate modulated tx, I guess that it all depends on if I can find the transformers that I need or not...  

Any first-rate and cheap transformer for ham radio, can be order to : www.abltransfo.fr

Sorry for delay...

Best 73 !
Jean-Pierre

for some of us:
http://translate.google.com/translate?hl=en&ie=UTF8&prev=_t&sl=fr&tl=en&u=http://www.abltransfo.fr/

Be sure, no matter what, to look at the neon sign transformer!

[VE3LYX]

Thanks for bringing this up. My suppressor grid project has been sitting dormant too long for a really minor issue. It is hitting the bench today. Time I got off my butt and got it done.
don
Ever get that, "Now where was I?" feeling?

[k7pp]

The Navy has some shipboard transmitters in WWII that used an 803 with suppressor modulation. They must have had the choice of going with screen modulation, but chose suppressor for some reason. The screen grid on an 803 can dissipate 20 watts which apparently was enought to avoid problems there.

Yes,  the Navy had a model TCM-2 that used a pair of suppressor grid modulated 803's but the transmitter was only able to operate
25 watts on AM mode.   
Stacked on top of the power supply it stood over 6 feet.

 

[kb3ouk]

If the components can take it, that pair of 803s should do 100 watts suppressor modulated with 1500 volts on the plate. I've been thinking about finding a GU-81M and running it suppressor modulated. With 2500 volts on the plate, the Polish datasheet I found says that tube should do 230 watts running as a suppressor modulated final (same sheet says that tube will do 400 watts plate modulated at a maximum of 2000 volts). If that number sounds high, the CW rating for that tube is 900 watts out at 2500 volts, so  the 230 watts for an efficiency modulation mode sounds about right. Seems like that tube was made for suppressor modulation, the screen dissipation is 84 watts (in suppressor modulated service, it's drawing 70 watts on the screen, 500 volts at 140 mA).

[VE3LYX]

Thanks to your thread my suppressor grid mod rig came of the shelf. Osc and PA are up and running and most of the modulator is done. The circuit I have is designed for a EM mic with an option for carbon. I may just redo it to one of my own circuits I like to use with my own favourite dynamic mic. I will keep the carbon option as I like them and their sound. Should be going in a day or so I think. I look at some of the original circuitry and say why would they do it that way but I duplicated it first anyway just to see. Not too impressed though at the input circuit for the mic. Seems to me it should have had another cap. Why would you throw 8.2 K resistance in front of the audio input? I can see biasing the grid of the mod tube but I normally feed the audio input direct through a decent size capacitor. This circuit doesn't have that but I will try that tomorrow. What would be a typical EM mic from WWII?

don

[Opcom]

For those like me who like to see into the crevices.