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Registered Member #60260
Joined: Fri May 27 2016, 06:35PM
Location:
Posts: 14
Recently I acquired three Eimac 8252w's and although they are meant for high power pulsed operation, I would like to try to build a audio amplifier with them.
I have been thinking about trying an OTL design, but the inefficiency hold me back.
For most tube output transformers, I have not seen a voltage rating. Since these Eimacs can work up to 20 kV, I thought automotive ignition coils might be a good and relatively cheap option. However, I know these are also meant for pulsed application and not for continuous signals. Even more, I know nothing about their frequency response, which might very well be awful.
Does anyone have any experience with this or suggestions for a better option?
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
I don't have any experience using ignition coils for audio stuff or CW waveforms, but the "HEI" coils with an E core *might* have better characteristics than the old style with a straight I core and oil-filled can. Some of the cheap HEI ones available online appear to have the iron laminations welded, like a MOT, which might not be desirable. I'm also not sure whether they have an air gap hidden on the central core leg, but I guess it's likely.
There's a common type available in Australia that features an epoxy-potted construction, E core and an isolated HV secondary, complete with dual output connections isolated from the core/primary. They were used on Holden Commodores and probably several other models or makes, throughout the 90's and early 2000's.
As seen here, three coils are used with a V6 motor and can often be found for free at a scrap yard or very cheap at a used parts dealer.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Molenaar
Audio transformers should display good response at both ends of the audible frequency spectrum. In order to to so, they need to have both superior coupling between windings as well as low interwinding capacitance. Since these 2 parameters work against each other in the design and manufacturing process, audio transformers have evolved into the realm of the black arts, where useful guidance is scarce. An ignition coil will definitely only deliver poor audio quality, due to low coupling btwn. windings. There will be other issues as well. Cheers, Finn Hammer
Modern replacement “one size fits all†audio output formers such as those from Hammond can provide a good impedance match due to a bunch of taps on both sides. I’d think it unlikely to come by that fluke, and either your audio valve or transformer would get hot.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
I think you are trying to make nightwear out of chainmail. Both audio output transformers and ignition coils are iron cored transformers. That is where the similarity ends.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
GrantX wrote ...
... "HEI" coils with an E core *might* have better characteristics than the old style with a straight I core and oil-filled can. Some of the cheap HEI ones available online appear to have the iron laminations welded, like a MOT, which might not be desirable. I'm also not sure whether they have an air gap hidden on the central core leg, but I guess it's likely.
yes they have a gap in the very middle of the EE core. every one ive seen. In general, output from HV coils is very dependent on input pulse drive.
Registered Member #60260
Joined: Fri May 27 2016, 06:35PM
Location:
Posts: 14
The Hammond transformers are only tested up to 2kVrms or it is not specified. The 8252w's can go up to 20 kV, so that would suggest an ignition coil might be more suitable.
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Ignition coil as tube amplifier output transformer
