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The first real High Fidelity amplifier my dad ever owned was purchased around 1962 from an audio shop across the street from the campus of Oklahoma University. It was a Heathkit W5M which had already been assembled by someone else who enjoyed putting kits together. He sold it in 1972 to a local fairground to use as a PA amplifier. The fairgrounds were torn down only a couple of years later and someone who was helping with that ended up with the amplifier. |
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While visiting a local thrift store a few years ago I noticed a 1950s tube amplifier. Upon closer inspection I found that it was a Heathkit. I also noticed that it had RCA 5881s in the sockets which were clearly marked "KT66." My dad had always said that his Heath had 5881s in it. I asked the owner where the amp came from and found that he had acquired the amplifier when the fairgrounds were torn down in the 1970s. He had stored it since that time and never used it and didn't know if it even worked. I paid his price and took the jewel home. I replaced the capacitors and found that this 14dBW Williamson style amp sounds wonderful. |
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I have since acquired another identical amplifier which still had the original Tung-Sol 12AU7As and genuine GEC KT66s. I had no extra pairs of KT66s and only mismatched sets of 30 watt 6L6GCs so, after replacing capacitors in the second amp, I installed a matched quad of Sovtek 6L6WXT+ tubes and have used these as the main amplifiers in my HiFi system. I had planned to possibly use these as the mid-range part of a tri-amp setup but found them to be very grossly overpowered for that application. They are more than adequate for my low-frequency section. These amplifiers use a split load inverter direct coupled to the input stage, and have a driver stage before the cathode biased ultra-linear connected output tubes. B+ is 520 volts and surges to 630 volts momentarily at startup! The plates and screens of the output valves run at 500+ volts and this amp will destroy any valve not designed to take this kind of voltage. I have not even tried to use any of my 19 to 23 watt 6L6 family valves. I have no idea how the 5881s held up for ten years of use but it is a testimony to their excellent quality and durability. I now have a matched set of Sylvania tall envelope 30 watt 6L6GCs as an alternate set. The Williamson circuit design is a proven circuit that can still hold his own against competing designs. There is room for improvement in a few areas. I have improved the power supply greatly and details of this modification will be presented in the future. Many people feel that the 6CG7 sounds superior to the 12AU7 and the sockets can easily be rewired to accomodate. The 6CG7 has the same plate size and structure as the 6SN7 and sounds very similar. It does draw twice the heater current of a 12AU7 and this must be considered if you are using KT66s. With the first Heath W5M above I also got the matching Control amp / Pre-amplifier which has a variety of phono curve settings adjustable through two selector switches on the front panel. It has no power supply of its own and was intended to run off of power supplied by the power amp. It is a Heath WA-P2 preamp. Here is the schematic. |
