I love these old EL84 push-pull amps. They tend to need a lot of TLC (tender, loving care) but like an old cat, have mellowed into the best place they can be.
re: replacing OPTs, you face two separate issues. The simplest one is getting the transformers' windings in the right polarity so the feedback is negative. There are lots of different ways to tackle this, depending on what instrumentation you have, or could get use of temporarily. Scope, signal generator, some modern version of either?
The trickier issue is also related to feedback, but only related at frequency extremes. For the amplifier to be stable (not oscillate) with feedback around it, it has stay within an envelope of gain X timing errors around the feedback loop. Trickier to measure, but still quite do-able. Many folk will tackle the polarity issue first, then just try things to see if it's a good enough replacement. Others will start with more invasive changes, to reduce sensitivity to the loop feedback, and live with that. Still others will want to optimize amplifier performance for that OPT, sometimes further on down the road.
Your call. Limited by test equipment and burning desire.
All good fortune,
Chris
re: replacing OPTs, you face two separate issues. The simplest one is getting the transformers' windings in the right polarity so the feedback is negative. There are lots of different ways to tackle this, depending on what instrumentation you have, or could get use of temporarily. Scope, signal generator, some modern version of either?
The trickier issue is also related to feedback, but only related at frequency extremes. For the amplifier to be stable (not oscillate) with feedback around it, it has stay within an envelope of gain X timing errors around the feedback loop. Trickier to measure, but still quite do-able. Many folk will tackle the polarity issue first, then just try things to see if it's a good enough replacement. Others will start with more invasive changes, to reduce sensitivity to the loop feedback, and live with that. Still others will want to optimize amplifier performance for that OPT, sometimes further on down the road.
Your call. Limited by test equipment and burning desire.
All good fortune,
Chris