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I am considering building a couple of monoblocks from scratch but I was wondering if I could get an oppinion as to which design you experts consider to be favorable. The 2 designs I am considering the most are the HF-60 and the D mk3. I am going to build a solid state power section for either with uf diodes with more filter caps.
Any thoughts as to which is a more desirable curcuit sonically and power wise ????
Any other designs you guys would consider for this ????Thanks
Rodney
Follow Ups:
Why limit yourself with a 40 year old design when there are plenty of more interesting ones available.
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
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
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
I prefer the Mullard "520" circuit of the Eico over the anemic stock driver circuit of the Dynaco.The Acrosound transformer of the HF-60 is superb also.
There are several really nice after market driver boards for the MkIII's that are vastly superior to the original Dynaco circuit and the MkIII Transformers are also much more readily found.
--
Al G
Born To Tinker!
I did notice the differences in the 2 different pre stage curcuits.
Is this difference the "Mullard phase split section " that is raved about ???
I want to put kt88's in the output of the hf-60. I plan on custom EDCOR transformers as well.Thanks
Rodney
the Poseidon/VTA/Curcio/Triode/etc boards are a worthwhile upgrade for the Mark IIIs...a note on the HF-60 schematic - it drives the EL34s pretty hard...
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