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Title: Elmac AF-67 Modulator Transformer Post by: KE4LRL on March 29, 2009, 06:57:04 PM Does any one know the modulator impedance ratio (values) of the stock transformer? I am rebuilding one for reduced output.
Title: Re: Elmac AF-67 Modulator Transformer Post by: KM1H on March 30, 2009, 10:59:52 AM You can get the primary value from the modulator tubes specs sheets, Handbooks, Mobile Manuals and the like. Then calculate the secondary using the finals voltage and current.
Typical values for PP 6L6's is in the 4000-6000 Ohm range, A 6146 with 500V @ 100ma is 5000 Ohms. Juggle values a bit with the 6L6's and a 1:1 ratio will do it fine, otherwise you will need a multitap job. Carl KM1H Title: Re: Elmac AF-67 Modulator Transformer Post by: KE4LRL on March 30, 2009, 08:15:06 PM Carl
Thanks for the input. I think the EL-67 is 10K mod to 5k PA based on the circuit configuration voltages and currents. However, I understand (from other positings) that the mod transformer is miss-matched. I am considering replacing the 6146 with a 2E24 or 2E26 and I was looking for the actual turn’s ratio so that I may get an idea of what to expect. I will pull the mod transformer and measure the ratio. Title: Re: Elmac AF-67 Modulator Transformer Post by: flintstone mop on March 31, 2009, 09:55:17 AM If you have space and don't mind an outboard "mess", then consider Mr Tron's using a power transformer as a mod transformer. It will amaze you.
I have not heard that there was a mismatch in the original unit. There's a comm shop near New york or Jersey that specializes in repair of Elmac and he has spare, expensive replacements. Fred Title: Re: Elmac AF-67 Modulator Transformer Post by: Carl WA1KPD on March 31, 2009, 11:31:50 AM I heard somewhere taht the mod transfomer from a TCS-12 works nicely in those.
YMMV and I have not tried this. Carl /KPD Title: Re: Elmac AF-67 Modulator Transformer Post by: flintstone mop on April 01, 2009, 02:55:04 PM Right on the money, Carl.
He might be able to find a junker (parts unit) on ebay The stock iron would barely make 100% pos and I think there were ways to get around that problem. Question: Why would you want to use a 2E26 for the final? There are other ways to reduce RF output in the Elmac. Fred Title: Re: Elmac AF-67 Modulator Transformer Post by: WZ1M on April 27, 2009, 07:16:54 AM I have an original modulation transformer for the AF-67. This transformer has been tested for shorts, opens and also hipot tested at 2.5kv. It also has been dipped and baked and new leads put on it. If interested, email me off list.
Regards, Gary...WZ1M Title: Re: Elmac AF-67 Modulator Transformer Post by: WD5JKO on April 29, 2009, 03:55:07 PM If I were to look at re-doing something like a Ranger or AF-67 where I needed a modulation transformer, while also wanting lower output to use as an Rf driver for an amplifier, I would redesign the the modulator like I did in my recent Gonset G-50 post. (on page 2 now here at AM-FONE). This idea would do away with the interstage transformer, and make the 6L6 output tubes parallel class A. All you need is a push pull Hi fi output transformer (don't need the secondary). The result will be much cleaner IMHO. A push pull modulator if balanced will cancel out the second harmonic, but the third can be strong, and harsh. With Class A, parallel, the harmonic spectra is less harsh as you overload than a P-P design. Also since we don't rely on a modulation transformation the usual way, the leakage inductance primary to secondary is not an issue. This technique will reproduce a clean square wave up to several Khz (5 in my case). This modification to the Heising idea gets away from the saturated Heising reactor dilemma so long as the class A modulator plate current matches the RF tube plate + G2 current. Also the parallel R-C on the modulated B+ to the 6146 allows for > 100% upward modulation at reduced carrier output. Go look at that post again, it is worth the trip. My 2 cents, Jim WD5JKO |