In Reply to: Tube amp for klipschorns posted by Spudman on August 15, 2002 at 18:33:59:
Try to find a pair of Eico HF-20 mono integrated amps. Without question the best bang for the buck in this situation. They can be used as is to start and lend themselves handily to any type of modifications you would like. I have several mod suggestions available for them that I can send you if need be.
Interesting! I once had an HF-20 I converted to a 50 Watt amp with EL-34's. The original power transformer is marginal, but I replaced it with a large potted Chicago and replaced the 5U4 with a GZ-34. A near perfect match for a Dyna MK II (also with GZ-34) but slightly less low frequency output at clipping, due to the better Dyna X-former. I later got a second MK II (actually a MK III I back converted to get EL-34's)
There were a few versions of power transformer on the HF20. The later models with the slotted lam stack are the best and most reliable ones. I'm curios as to how you got 50 watts out of an output transformer that has too high of a primary impedance to be able to do that even with EL-34's. What plate voltage were you running? What class were you operating in. A 430 volt plate will give you approx 25 watts with a pair of el-34's and the original output transformer.
I really cannot remember the plate voltage, but it must have been up around 475-500. Using the GZ-34 rectifier helped. I did this many years ago, around 1970-71 and used the Dyna MK II output circuit design with a single cathode resistor grafted onto the double dual triode input and phase splitter. I had to add a small transformer for the bias supply and used a silicon diode. It was set up for the same 1.56 volts as the Dyna. I don't recall any changes to the input circuit or phase splitter, but I did fiddle the feedback path. I no longer have either the amplifier or the paperwork, so I'm testing my memory over 30 years. I did have to add a small capacitor to the output transformer primary to kill parasitic oscillations.
The power transformer I added was the same size can as the output and was around 200 ma. I want to say it was a 390-0-390 volt model, but am not sure, the voltages do seem high even for an RC filter. I bought it at Allied Radio's Cicero, Illinois store in 1959 and installed it that fall. It got me 30 W/ch with 5881's with nice blue glows. The transformer was so big it overhung the chassis. It also caused the capacitor to fail, so I replaced it with a 500 volt one and added some resistance to the filter circuit. Seems to me I read somewhere back in those days that the HF-20 and HF-50 used the same output transformer. Although the part numbers differ. But the can is the same size.
My HF-20 was originally built in 1956 (I stayed up all night to finish it and woke my parents at 6 AM with music! Funny, they didn't seem to share my enthusiasm).
The power output was measured with an H-P model 400 audio voltmeter (I borrowed that one, but I now own one),an oscilloscope and an H-P distortion analyzer. All borrowed from where I was working. So I am very sure of the power output figures. Distortion just below clipping was 0.5% at 48 W/ch, and my MK II was 0.3% at 52. I called it the Dyneico and used it for two years. The only noticeable difference between them was the plot of clipping versus frequency showed a little more rolloff at the low end on the Eico transformer than the Dyna. But with my KLH 17's it was a moot point anyway. Same story on the top end, the Eico rolled off a little sooner than the Dyna. But in use, they made a good pair until I could get another MK II, which as I said earlier was converted back to a MK II configuration, even to removing the filter choke and going back to the original RC filter. EL-34's lasted very well in the MK II, but the higher voltages of a MK III (or the use of silicon rectifiers in a MK II)made short lived tubes. You could get 60 W/pair, but not for very long.
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