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Re: dynaco mk3 vs eico hf-60

Posted by Bold Eagle (A) on May 11, 2007 at 10:41:29

In Reply to: dynaco mk3 vs eico hf-60 posted by rcb on May 11, 2007 at 08:12:28:

kstagger picked up on a very important point. The HF60 is trying to squeeze 60 watts out of a pair of EL34's. At that level, tube life is quite short and drift is fairly rapid. Further, the Acrosound TO-330 transformer is no better than the Dyna. They were both one company at one time and the designs are very close.

There is nothing wrong with the Dyna Mk III design if you use KT88's. The design started with KT88's and shifted to 6550's when the KT88's became unavailable. You're right about the diode bias supply instead of the selenium rectifier.

My personal preference is EL34's in a Dyna Mk II, with a GZ34 rectifier in place of the 5U4. That combo is good for a solid 52 watts and uses the same transformers and PC board as the Mk III. The main difference is the choke filtered supply on the Mk III which raises the plate voltage to give a marginal 60 Watts. the Mk II used an RC filter in the P/S and had a lower plate voltage and had a different cathode resistor for the outputs. The Mk II gave excellent tube life with EL34's. You can also use KT88's in the Mk II which gives a warmer sound, but a less glorious midrange. My belief is that the Mk II was the original design and was an optimum use of the transformers. Then they found themselves 10W short of the competition and "hot rodded" the design to match the competition. I once back converted a Mk III to a Mk II to have a pair, as I preferred the sound and lost less than 0.5 dB in output as the Mk III only measured 56 Watts.

I also owned an HF60, but preferred the Mk II.

Jerry

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