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Old 30th June 2009, 05:23 PM   #1
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Default Finally have some big chokes, how would you use them?

Well, I finally managed to find three identical broken microwaves, so now I've got some big chokes to play with for a stereo amp!

I was thinking SE with Super-6BG6GA's in them (i.e. 7027A specs), for which I have the tubes and OPT's.

How would you use the chokes? Is the DRD really the way to go? It certainly looks appealing, a DC coupled driver without needing a separate rail for it. Opinions? Or is blocking distortion not really a primary concern with an easy to drive final?
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Old 1st July 2009, 01:21 AM   #2
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I don't know if salvaged microwave chokes would be good enough for use in an amp, other than in its power supply.
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Old 1st July 2009, 01:29 AM   #3
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I have a MOT which I had always hoped to use as a power supply filter choke. It is currently doing duty as a paperweight, and a rather effective one at that.
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Old 1st July 2009, 02:13 AM   #4
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OK, thanks for the warnings about the MOT's, duly noted, but as there are plenty of examples on this forum of their successful application, please don't let that hijack what I wanted to discuss.

Anything about Free Lunch? DRD? Parafeed? Anything on A2?
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Old 1st July 2009, 08:10 PM   #5
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I used a MOT as a plate choke for a version of the RH807 amp in parafeed configuration and using Toroidals as outputs. Unfortunately to my taste the experiment didn't work so well. It played and there was nothing blatantly obvious wrong - but it just sounded not quite right. A friend postulated that there was phase shift in the lower registers. This would figure as the primary inductance of the toroidals is not stella and neither is the inductance of the MOT. Place them in parallel and that marginal performance is pushing phase shift into the bass region. The killer sting in the tail is that the RH807 has very low output impedance so it was getting the best out of the inductance that was available. Try the experiment with a higher impedance output stage and you can expect horrible results.

I have to agree that they are probably only good for Power supply duty. If you want to try parafeed - remember that parafeed transformers are more expensive than normal output transformers so you might as well go with normal SE.

As for DRD - sledgehammer and nut.

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Old 2nd July 2009, 03:46 PM   #6
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Many thanks for your comments; you have dissuaded me from trying parafeed at all.

However, I would still think that the MOT's are at least good for the Free Lunch design. I mean, as I understand it, if the finals are biased into A1, then the choke only needs to feed the driver, which is in the 5-10 mA region, so I would think even an MOT would be unsaturated by DC and the inductance would be huge, no?
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Old 3rd July 2009, 12:00 AM   #7
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Have you been able to find both ends of the secondary? On my MOT, one end was welded to the brackets riveted into the core. That end was tucked underneath the primary winding. Rather than run the thing hot, I unwound the primary so I could access that end of the secondary. It's a pity, since I threw away a lot of turns. I'm not quite sure how I would have done it differently. Maybe I could have cut the winding just where it was attached to the bracket, and then pulled the free end back out?
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Old 3rd July 2009, 02:31 AM   #8
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Well, on one of my other MOT's, which was as you described, I just cut off the primary winding with a hacksaw to expose where the one end of the secondary was bonded to the core. Why are you trying so hard to conserve the primary windings? Cut first, ask questions later.

For these MOT's that I have 3 identical of, no such issue, the bonding screw is plainly visible and not near anything else. A DIY dream.

Anybody wanna talk about amp topology and not MOT's?
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