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Old 19th February 2008, 11:58 AM   #1
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Default Replacement for single-winding OPT?

All,

Trying to bring back life to a mono PPP amplifier with 2 x EL34 (circuit attached). The output transformer is missing but had a one-side winding, no secondary, according to a previous employee at the factory (Carad, local Belgian brand, bankrupt ca. 1974). Yes, the man is 65 years old.

Are there any OPT available that could be used as a replacement? Can I use an OPT with separate primary and secondary windings?

According to the classification habits they had, the 'full' or primary winding should have had an impedance of 1000 Ohms, winding it back to 200 Ohms for the 100V output, etc.

Of course I wouldn't need a 100V output, 8 Ohms would already be appropriate.

Thanks for any light that may be shed,

Jacques
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Old 19th February 2008, 06:49 PM   #2
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Hi Jacques,

Looking at the circuit supplied, it appears to be a type of balanced bridge output stage using dual independent HV supplies like a Circlotron. The single winding output transformer is also known as an autotransformer. I think a transformer with a seperate secondary would be OK if you can fine one with a 1000 ohm primary. I really don't think a secondary winding makes much difference. A balanced autotransformer was just more expedient. You will need a push-pull type with a center tap (not single ended) as it appears to go to ground.

In a pinch, a transformer with a 2000 ohm primary might be used by connecting an 8 ohm load across the 16 ohm secondary to reflect a 1000 ohm impedance back to the circuit. Hammond makes one with a 1900 ohm primary. (1650T) The down side of this is mostly cost as well as a possible loss of extreme top end response because of the added winding capacitance. This may, or may not, be very noticeable.

Victor
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Old 19th February 2008, 07:29 PM   #3
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Thank you, Victor.

The output also provides feedback (via an octal plug, pin 4) to the preamp, as in the attached circuit fragment. The feedback is taken from the 16 Ohm tap, to make things more complicated.

Jacques
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Old 19th February 2008, 08:06 PM   #4
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Since the preamp is not balanced and doesn't require balanced FB, then ground the secondary common as usual and take FB from the 16 ohm connection, assuming the transformer supplies 16 ohms. If not, you'll have to use the 8 ohm tap and reduce the 27.0K resistor accordingly.

Edit: Btw, If you go this route it's a 50-50 chance you get the phasing right the first time. If it oscillates, reverse the primary leads.
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