70V tube system for home use- line matching transformers? - diyAudio
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Old 30th April 2013, 01:12 AM   #1
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Default 70V tube system for home use- line matching transformers?

I know only a little about tubes, or 70V systems. I'm interested in learning about tube amplifiers.

I have an old tube amp designed to be used as a 70V PA distribution system. I'm wondering how best to use it for home use. I'd like to use it for either hifi or as a guitar amp.

It's about 200W (uses two 805's in push-pull). Turns ratio of the OPT appears to be about 13.5. The OPT by itself weighs about 30 pounds (Kenyon). Made by Schulmerich in the 60's for use with church carillons.

Looking at the Edcor site it appears the only appropriate transformers are only rated down to 35Hz. Do you know of alternatives which would support better base extension, down to 20Hz or so for hifi use?

Also, I'm confused about the differences between the various Edcor line matching options. Why the distinction between device end, speaker end, or bidirectional? Since they are about the same price, wouldn't bidirectional be more versatile? Or is there some drawback to the bidirectional type?
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Old 30th April 2013, 03:45 AM   #2
DUG is offline DUG  Canada
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A 70V system at 200W would be a 24.5 ohm load.

If you loaded it with 8 ohms and did not run it past 20W the output transformer would work to 20 Hz.

The tubes would be dissipating a lot more power though...same current but less voltage swing.

I would look for a different output transformer...budget permitting

Or don't worry about 20Hz and find a transformer for 24R:8R @ 200W

Or build a 24 ohm cabinet...3 * 8R
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Old 30th April 2013, 11:16 AM   #3
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Hi DUG- thanks for responding. Yea, the 20Hz idea is not necessary, just checking my alternatives.

The 200W figure was just an approx to give folks an idea of the size. Using a 32 ohm load I actually measured 230W. It will put more power out before clipping- haven't tested it that hard yet. Although one can think of ways to combine loads, practically and most commonly for home use in order to make use of this system it needs to run a single 4,8,or 16 ohm speaker load.

This system has a fairly steep high frequency rolloff when used with a 32 ohm resistive load. I'm thinking since it was designed to use a line matching transformer maybe that type load will flatten the freq response out some. Or maybe not, depending on how it was designed. Too bad the company won't help with providing any info.

So my question is really about my options for a transformer.
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Old 30th April 2013, 01:18 PM   #4
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I tested it today with a 200 ohm load, and that flattens the response reasonably. I calculate the line matching transformer would present about 182 ohms.

So, I am still wanting suggestions about transformer options, particularly the difference between a bidirectional or speaker side line matching transformer.
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Old 30th April 2013, 02:12 PM   #5
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Ooops.. I did the math wrong.... for an assumed 805 push-pull output impedance of 6600, and a OPT turns ratio of about 13.5, the line transformer would need to present a load of 6600/(13.5^2) = 36 ohms? Oh boy, I'm bad at math and also guessing how this all goes together.
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Old 30th April 2013, 03:11 PM   #6
JMFahey is offline JMFahey  Argentina
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1) you need a new transformer anyway, either a new OPT or a 24:8 matching one, so going straight to a good OPT is the simpler/better choice.

2) forget 20 Hz, we are talking tubes here, OPT design is a compromise; going down 1 octave (which you'll hardly use anyway) at least doubles cost.
May quadruple because it will be a custom job instead of an off the shelf type.

3) instead of that, try to get a properly interlaced , Hi Fi type; the one you have clearly is more of an "industrial" type.

Think greasy overalls instead of a tuxedo and you'll get the point
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Old 30th April 2013, 03:22 PM   #7
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OK, thanks. The existing OPT has other needed windings, so the matching transformer would be necessary, I think.

I'm looking at an Edcor EM3600. The EA300 would be nice if I had extra funds- but they are $120 each!

BTW- do you know of the famous (deceased) guitarist John Fahey? I really like his stuff.
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Old 30th April 2013, 04:12 PM   #8
JMFahey is offline JMFahey  Argentina
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Yes, I do.
A great Acoustic Guitar pioneer but his big contribution was tracking , finding and reviving all that ancient (think Civil War and earlier ) *real* Folkloric Music (not to be confused with what today's usually called "Folk").
We all originally come from around Galway, Ireland, but his Grandpa took a different ship than mine

In fact mine first sailed 5 or 6 times to the Boston area, going back to Ireland after he made some cash (very common in those days), until somebody told him "go to Argentina, incredibly good opportunities there* ... and he stayed
Thanks for reminding him
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Old 30th April 2013, 11:11 PM   #9
Enzo is offline Enzo  United States
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A very aggressive style of picking, he had. I used to play a couple of his tunes on my old radio show.
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