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In Reply to: RE: EICO HF-89 restoration posted by mike1127 on August 28, 2010 at 15:20:07
Schematic is here http://sanacacio.net/hifi/hf89schematic.jpg
I heard one of these sometime back - nice amp, EL34 UL, 6SN7 driver
using direct coupling to the gain stage. Wow! Bold design! Someone back
then decided not to just copy the same old 7199 driver scheme.
One thing to remember, you must never ever operate that amp without those 12AX7s plugged in! The 6SN7 absolutely requires the 12AX7's presence to set its own bias current! If you forget to do that, you can roast the 6SN7. That's always the case with any direct coupled stage.
If you've never done anything like this before, it might take you
3 months to rebuild clean and lean with some periodic help here.
I do believe it's worth doing since that actually is a really nice amp.
I would completely disassemble it. Gut it. Fix whatever items on the
chassis metalwork need to be fixed (now, or never). Passivate all the rust. Remove the big electrolytics (easier to do using a mini torch once
all the parts are dismounted).
I'd start on the HV supply and bias supply first, get the new parts, wire up that module and test it out (using dummy loading resistors). Next, wire up the output stage. Test it. Ground out the EL34 grids, put some 8 ohm dummy loads on the speaer terminals, set your bias voltage, measure cathode current, burn it in. Next, move onto the driver stage.
You could modify this amp slightly during a rebuild too.. There's not
awhole lot you'd need to modify actually, which is nice. The big instant
obvious mod I'd immediately do is replace the 18K resistors on the 6SN7 LTP with a self-biased CCS. Huge mega improvement - that is not only easy, but cheap. Second, I might consider throwing in a small choke to cleanup the B+ to the 12AX7 plates (Point X in the PSU diagram). Since
the rest of the amp is push pull - it already has good PSRR. The choke
for th 12AX7 stage would squash out the last bit of ripple going into
that high-gain pre stage.
-- Jim
Follow Ups:
"replace the 18K resistors on the 6SN7 LTP with a self-biased CCS. Huge mega improvement - that is not only easy, but cheap. Second, I might consider throwing in a small choke to cleanup the B+ to the 12AX7 plates (Point X in the PSU diagram"
If you're going to give the LTP a CCS tail, which is an excellent idea, then you need also to make the plate load resistors of the 6SN7, R11 and R13, equal and precisely matched.
I agree with the idea of a choke to improve B+ for the 12AX7 stage, but I would also be inclined to use another choke straight after the voltage doubler caps and their 100k bleeder resistors (i.e. to feed the junction of R45 and R46), to help clean up the spiky B+ coming off the doubler. Although the PP amp stages do have good PSRR, you still don't want all that noise spreading itself around the PS.
Jim
The gain stage 12ax7 is already direct couple to the 6sn7. As a matter of fact,the only coupling caps used are the .1s to the outputs and the .22 or .25 decoupling caps.
I agree that the 7199 was a total POS. It was great in integrated amps driving 7189s or 7591s but not when you want to drive higher power pentodes. My one HF87 I left cathode bias but,I didn't keep the DC filaments that were derived from the cathode voltage of the el34s. Was that a waste or what? I just paralleled 4,5 and use pin 9,ac filaments. The hf87 does that have the bias tap on the PT.
Smell it,dust it,clean it,test it,measure it.If you miss doing this,vengeance shall follow.
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