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Old 27th August 2011, 06:05 AM   #1
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Default Identifying an ultralinear OPT

Recently I unearthed a pair of output transformers that came from an old Philips reel-to-reel. It's a single-ended design (one EL84 tube per channel) but there seems to be a centre-tap on the primary of each OPT, connected to B+. What gives? Is this so-called 'ultra-linear' operation? Can I use these in my planned RH84 build?
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File Type: gif el34 output tranny.gif (13.1 KB, 112 views)

Last edited by fyzygy; 27th August 2011 at 06:17 AM.
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Old 27th August 2011, 06:40 AM   #2
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I think this circuit is inteded to use the OPT as a part of hum filtering, not as UL. Same circuit is often used in old tube radios.
I think the phase of the remaining hum component at +Ub is inverted at the OPT and feed to screen grid.

This extra tap may be used as a UL tap, why not ?
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Old 27th August 2011, 08:44 AM   #3
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check if the speakers are 800 ohm
in that case it is a special kind of choke
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Old 27th August 2011, 09:49 AM   #4
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I'll have to dig this circuit diagram out of my 1950's cupboard.......my version uses the output transformer switched into as a 50Khz HF bias oscillator when in record mode. True it looks like an UL tap when it isn't quite...Those of us with reel to reel machines may remember HF bias is superimposed on the recorded signal to prevent transfer distortion caused by the non linear characteristics of the iron head as the signal goes through the zero part of the BH curve. However....so many designs are out there.

richy
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Old 27th August 2011, 11:36 PM   #5
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The transformers do seem large and heavy, relative to the output power of the amplifier. The full schematic is attached, perhaps that will reveal what's going on in the original circuit. In any case, will I be able to use these transformers as "regular" output transformers in a single-ended design, also using EL34s? That is, simply ignore the centre tap?
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File Type: gif schematic.gif (84.0 KB, 51 views)
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Old 28th August 2011, 05:12 AM   #6
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There is 6 Henries inductance between points (c) and (d) on each transformer. So, my guess is it's part of the power supply filtering. Can anyone confirm, based on the schematic above?
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