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In Reply to: Re: dynaco mk3 vs eico hf-60 posted by Bold Eagle on May 11, 2007 at 10:41:29:
i had read that about the EL34 lifespan in the HF60. One thing I did forget to say is I did want to put KT88s in the hf60 design in place of the el34's and I am going to have custom transformers from EDCORE built. Can you tell me what changes will need to be made in the HF60 schematic to accommondate the kt88's.
One reason (maybe misguided)I was leening toward the HF60 was the Mullard phase split in the design. Like I said maybe misguided but from what I read this is favorable ??????thanks
Rodney
Follow Ups:
I'm not sure what changes would be needed in the HF60 schematic for KT88's, but it's a good move.Personally, I like the Dyna circuit. The phase splitter is self balancing and the accuracy of the split is only dependent on the matching of the matched pair resistors in the Mk II/Mk III schematic. The outputs in the Dyna are in a common cathode circuit and are also self balancing if the tubes are a matched pair. They ought to be in any circuit, anyway!
The voltage amplifier and phase splitter/driver circuits are fine if the distortion is low and the amp can be driven to rated output and has decent dynamic headroom.
I've owned the HF60, and once built a 50 watt amp based on a large, potted Chicago transformer. It used a Dyna output circuit and a Mullard voltage amplifier and phase splitter/driver circuit. It measured quite well and sounded exactly like my Dyna Mk II. The Dyna had better measurements at full power at the frequency extremes because of a better output transformer (less core saturation at low frequencies, and lower leakage inductance at high frequencies)so the power bandwidth was more extended. So much for the supposed differences in the voltage amplifier and phase splitter!
People develop strong biases toward certain circuits (I have my own) both positive and negative. I can accept that. What I can't accept is "circuit evangelism" where everything outside their favorite is torn down.
I do feel the best Dyna is the Mk II. (although I would use the GZ34 and put in a silicon diode to replace the selenium rectifier) I think it represents the optimum use of the transformers. It seems to me the Mk III is a "hot rodded" version, brought out to counter higher power ratings from competition. You can use KT88's in the Mk II for a richer sound, although I like the EL34's. The differences in the Mk II and Mk III are the 5U4 Vs GZ34/5AR4 rectifier, a slightly different cathode resistor in the output circuit, and an LC, instead of RC filter in the power supply. Decades ago, I back converted a MK III to a Mk II to have a matched pair.
Jeez! Now you've got me thinking about a project.
Jerry
Jerry, have you tried the new JJ KT77 in place of EL34's?
Sorry, but I no longer have any tube gear. My last tube amp left home in 2003. So, no, I haven't tried those.Jerry
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