Modulation transformer bodge? - UK Vintage Radio Repair and Restoration Discussion Forum

Alf · 2nd Mar 2008, 2:22 am

For some time, I've toyed about with the idea of making an A.M. top band transmitter using valves. The problem being, lack of a suitable modulation transformer. A question!. Could an ordinary sound output transformer, it's secondary coupled to the secondary of another sound out transformer who's primary winding is in the anode circuit of the P.A. valve work? Obviously, impedance matching would be a factor. Has anyone tried this?

kalee20 · 2nd Mar 2008, 8:52 pm

In principle, yes it would work.

For matching, the impedance of the modulated stage is (more or less) the quiescent HT voltage applied to it, divided by the quiescent current taken by it (and this is anode plus screen current, if you are modulating anode and screen grid). And, the transformer driving the modulated stage needs to be designed to carry DC without saturating. If you are contemplating using a single-ended output transformer in reverse, then this will have been taken care of in its design.

Obviously it's not as elegant as a 'proper' modulation transformer, but it'll get you on the air!

Sean Williams · 2nd Mar 2008, 9:10 pm

Or use a choke for heising modulation!

you could put a wanted ad out on the forum.....

Aub · 3rd Mar 2008, 12:30 pm

Or you could use an HT transformer. I've done this on many occasions with good results.

73's

Aub

Ian - G4JQT · 3rd Mar 2008, 1:11 pm

The Codar AT5 (160/80m AM/CW valve tx) uses Heising modulation. It uses the same choke for modulation as it uses in the PSU HT smoothing.

I can't remember the choke type/value off hand, but a Google search may bring something up. I don't think it particularly critical, but as Kalee20 says, it needs not to saturate when passing DC. The modulation on the AT5 sound reasonably good, but a good AF sinewave into the mic socket shows its limitations!

If the AT5 details I have contain choke info, I can dig them out if you need them.

A "bigger than normal" mains or audio transformer used as a choke (just use one side of the windings) can be successful, just try whatever you have and be prepared to experiment - in best amateur radio style!

(By "bigger than normal" I mean bigger than you might expect for the transformer power rating as the extra size will help minimize the chance of saturation when handling predominantly DC.)

Ian

2nd Mar 2008, 2:22 am	#1
Alf Hexode Join Date: May 2005 Location: Wigan, Greater Manchester, UK. Posts: 479	Modulation transformer bodge? For some time, I've toyed about with the idea of making an A.M. top band transmitter using valves. The problem being, lack of a suitable modulation transformer. A question!. Could an ordinary sound output transformer, it's secondary coupled to the secondary of another sound out transformer who's primary winding is in the anode circuit of the P.A. valve work? Obviously, impedance matching would be a factor. Has anyone tried this?

2nd Mar 2008, 8:52 pm	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: Modulation transformer bodge? In principle, yes it would work. For matching, the impedance of the modulated stage is (more or less) the quiescent HT voltage applied to it, divided by the quiescent current taken by it (and this is anode plus screen current, if you are modulating anode and screen grid). And, the transformer driving the modulated stage needs to be designed to carry DC without saturating. If you are contemplating using a single-ended output transformer in reverse, then this will have been taken care of in its design. Obviously it's not as elegant as a 'proper' modulation transformer, but it'll get you on the air!

2nd Mar 2008, 9:10 pm	#3
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: Modulation transformer bodge? Or use a choke for heising modulation! you could put a wanted ad out on the forum..... __________________ Engineers make things work and have spare bits when finished

3rd Mar 2008, 12:30 pm	#4
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,039	Re: Modulation transformer bodge? Or you could use an HT transformer. I've done this on many occasions with good results. 73's Aub

3rd Mar 2008, 1:11 pm	#5
Ian - G4JQT Octode Join Date: Jun 2004 Location: Reading/Fakenham, UK. Posts: 1,326	Re: Modulation transformer bodge? The Codar AT5 (160/80m AM/CW valve tx) uses Heising modulation. It uses the same choke for modulation as it uses in the PSU HT smoothing. I can't remember the choke type/value off hand, but a Google search may bring something up. I don't think it particularly critical, but as Kalee20 says, it needs not to saturate when passing DC. The modulation on the AT5 sound reasonably good, but a good AF sinewave into the mic socket shows its limitations! If the AT5 details I have contain choke info, I can dig them out if you need them. A "bigger than normal" mains or audio transformer used as a choke (just use one side of the windings) can be successful, just try whatever you have and be prepared to experiment - in best amateur radio style! (By "bigger than normal" I mean bigger than you might expect for the transformer power rating as the extra size will help minimize the chance of saturation when handling predominantly DC.) Ian