AM Conversion for CW TX
 

Not sure if this posting should be under Amateur Radio.
I have acquired a very well made Home Brew valve CW TX, with a valve PA, that I would like to convert to AM.
Mainly due to the lack of space in the cabinet, and the availability of cheap ready made Audio Modules, I am thinking of using one as the Modulator out put.
As they have been all designed with a low O/P impedance, and I want really want to modulate the Anode current of the PA bottle, I am open to suggestions for the best way to achieve this:dunce::idea: :shrug:
All suggestions welcome,

Ken, G6HZG:beer:

Re: AM Conversion for CW TX
 

Transformer coupled was the norm for Anode modulation but with the obvious dearth of such speciality transformers these days, mains transformers have been successfully used.

Something in the region of 240 - 12 at around 1A type of thing depending on how beefy the PA is.

Re: AM Conversion for CW TX
 

Terry, thanks for the reply, I knew I had to use a transformer, but the out put impedance of the Audio module will be about 8 ohms, as this is solid state I assume the 8 ohms is is a fairly critical factor, the PA current is about 60 mA at 450V.
So am I right in assuming, any transformer capable, of handling the power with roughly an 8 ohm winding is a possible candidate :shrug:

Thanks again,

Ken G6HZG, VMARS.

Re: AM Conversion for CW TX
 

While anode modulation will probably give the best signal, just to get you going, it may be easier to find components to set up either screen grid or cathode modulation.

The lack of "real" mod transformers that you see for sale means that it is probably worth having a go with other transformers; there may be people on here who could offer some guidelines?

B

Re: AM Conversion for CW TX
 

You can get 90% mod with screen grid mod, e.g. Heathkit DX40U. Series gate mod is
an alternative at a lower level.
A good quality valve output transformer can be used in reverse, epecially if it offers
separate 4, 8, or 15 ohm windings as these can be paralleled when driven by an audio amplifier - some of the modern IC types have a very high output. It's important to provide good rf decoupling for the modulator stage.

Re: AM Conversion for CW TX
 

Thanks for the comments, I didn't realise the DX40 used screen grid mod, I always thought screen grid mod, was a poor choice and reserved for the WS19 and its like.:shrug:
The valve line up of this TX is a 5763 in the PA, then two EF91`s I assume buffer and driver, and CV287 VFO, I am not familiar with that bottle, I recall CV being Ex Gov numbers, and its not a valve line up, I remember being common, in a home brew TX ,if any one recognises the line up from a construction project in the past please let me know.

Thanks again,

Ken G6HZG.

Re: AM Conversion for CW TX
 

Sorry just found out the CV 287 is a Voltage stabilizer:dunce:. So its EF91 VFO, EF91 Buffer and 5763 PA, makes a bit more sense now:idea:

Ken G6HZG.

Re: AM Conversion for CW TX
 

It's more the impedance transformation ratio rather than the actual winding resistance which is of importance, along with the power handling capability.


I know I've got a circuit here somewhere similar to what you are thinking about that uses a small mains transformer for the modulation transformer.

Re: AM Conversion for CW TX
 

Hi for anode / screen mod you need half rf power as audio so for 8w out you need 4w for 100 % audio anode/sccreen mod gives the best audio . Series gate is ok but close to the tx when signal is strong it plays with the agc in the radio and gives distorted audio because of the carrier going from a low level to a high level on mod peaks. Screen mod is ok but again is a low level mod system. For a high level ie a/s a small single end valve output tx is all that is required some thing like a single el84 tx wired so the audio goes into the 8ohm input and the anode and screen is fed via the other winding. This is used on the codar at5 tx with super sounding AM. Mick

Re: AM Conversion for CW TX
 

Just read 450v on the anode of a 5763 will damage it in around 10mins carrier fully modulated to around 100%.. i have done it around 300v is ok Mick

Re: AM Conversion for CW TX
 

Better measure the HT then, only quoted 450, because that's written on the power supply, I would in the future like to swap the 5763 for a 807.
Ken G6HZG.

Re: AM Conversion for CW TX
 

Is the PA cathode keyed? If so, you could plug a modulator in there. This could be a smallish valve output transformer, with the high impedance winding in the cathode PA , and the low impedance winding fed by the speaker output of the solid state amp. A little bit of experimentation will be required. You could even try a power transformer, say 240 input and 12v output, connected in the same way. Obviously, if you convert to an 807, with greater input power, you'll need more audio and maybe a larger mod. transformer.

73

Aub, G4KQL

Re: AM Conversion for CW TX
 

You don't need a proper modulation transformer.
You could use choke anode mod where you just use the high impedance side of a valve audio transformer (or any choke that is suitable) where you leave the secondary (speaker side disconnected) you connect the supply line for the 5763 to the anode of the modulator valve (say EL84 single ended audio amp), the audio is developed across the transformer audio load and you will achieve high but not 100% modulation. To achieve fuller mod. if you really need you will need a resistor and parallel capacitor on the supply line to the 5763 (lovely valves BTW) and some audio fed to the screen of the 5763 also. It is called Heising Choke modulation and here is more https://www.w8ji.com/Heising%20modulation.htm
PS: My personal experience of Cathode and control grid modulation have not been great if you want any decent audio quality.

Re: AM Conversion for CW TX
 

Hi Folks, the design of mod transformers is a bit of a black art as there is a high standing DC in one of the windings as well as lots of RF to damage insulation within the transformer; plenty of decoupling as mentioned as well as plenty of fusing and go ahead and try a cheap mains transformer

Ed

Re: AM Conversion for CW TX
 

Thanks, yes I remember choke Mod, its just that I had the idea of using a solid state audio amp as the modulator, and was concerned about a impedance mismatch and or HT getting into the out put IC.
G4KQL I will have a look at the Key Jack socket and see if its in the Cathode, worth a experiment, I think.
Ken G6HZG.

Re: AM Conversion for CW TX
 

I've seen previous comments that the AT5 sounds good; does anyone have a cct diagram of the arrangement?

Does anyone remember the "bean can" transformers used in early Pye Rangers? About the same size and shape as a small can of beans and oil-filled, they were designed to use a pair of EL90's in a push-pull modulator driving a QQV03-10 RF PA. There were lots of them around in the 60's; not sure how many may be sitting forgotten at the bottom of junk boxes. In the larger Pye Vanguards, there was a mod transformer with the AF coming from 4(?) NKT404's (OC35) for a QQV03-20 PA. That transformer also has a separate winding (low impedance) to allow the modulator to be used as a Public Address system via some form of speaker. I wonder if AF could be put in through that winding?

B

Re: AM Conversion for CW TX
 

The AT5 used a centre tap auto transformer or choke arrangement, it was actually very clever. I think it used a phase reversal idea to save on higher currents in the auto transformer? Years since I have seen a cct. diagram.

https://www.mrwebman.com/fx1/files/m...t5/AT5MODS.htm

I never owned one, so have no idea as to what modulation depth it could achieve though.

I would avoid mains transformers as mod transformers as the freq. response would be pretty awful.
Proper modulation transformers are complex beasts with twisted wire turns on big one's to improve the freq. response, and expensive to make.

Re: AM Conversion for CW TX
 

I have two thoughts, firstly using a valve output transformer in reverse is a good idea, they are designed to have a standing current on the HT side. The other is to find an old Pye Cambridge PMR and use the modulation transformer from that, these had a transistor mod amp modulating a 10 watt valve output stage

Peter F4VSA

Re: AM Conversion for CW TX
 

Another option would be a "100V Line" transformer as used in public-address loudspeakers: as designed thye have a multiply-tapped primary for connection to the 100V line, and the secondary usually has taps for 4 and 8 Ohm speakers.

In the past I've used a beefy one of these [they're available in ratings up to 20 or 30 Watts of audio] as a push-pull output transformer for a pair of 6V6.

See here: https://ozvalveamps.org/optrans.htm for some such usage. One of these transformers could be used in the 'centre tapped autotransformer' approach as adopted by the Codar AT5, or you could just stick audio from a little amplifier into he 4- or 8-Ohm winding and then mix-and-match the taps on the 100V side to get the right amount of modulation.

Re: AM Conversion for CW TX
 

The Pye Westminster W30AM used a p-p transistor modulator and transformer for the
rf output valve, (the fast heat version).