EBERGER@B.PSC.EDU (04/08/91)
My friend got some tubes to try out in his curcio pre-preamplifier.
He ordered the board from Old Colony and stuffed it with his own
parts, using WIMA MKP10 (polypropylene) .47uF coupling caps. We tried
both 741 and MC34081 op-amp ICs in the regulator section. The Motorola FET
op-amps did produce clearer sound. Using the 741 the sound was overly warm
and veiled. Source was a 'nude' Supex SDX1000 moving coil cartridge (.2mV)
litz wire from the cartridge pins to the test circuit input. Load was a
modified Audio Research SP-6 with 'India' 12ax7s. Modified Acoustat X
speakers/amps finished the component chain.
Tubes tried:
-------------
General Electric 6DJ8 (made in USA)
Amperex 6922 (made in USA)
Mullard 6DJ8 (made in England)
Westinghouse 6DJ8 (made in Japan)
RCA 6922 (only said RCA EK 6922 on tube)
The curcio prepreamplifier circuit is a fairly simple parallel tube gain
stage, with an op-amp regulated power supply, and filament regulator. Our
evaluation of the above tubes is thus sorely lacking in how they might
perform in a more exotic circuit.(like a current-sourced differential cascode
topology with a 'White' Cascode follower stage and global feedback....)
We also didn't go through the trouble to clean the tube pins, which could
possibly greatly change the results of our listening tests. LPs used were
Fresh Aire II, and The Motels' Little Robbers.
Some basic notes:
------------------
- None of the above tubes had great sounding bass.
Perhaps the WIMA coupling caps weren't large enough, or just not
good for bass.... Seeing that we could not use bass performance as an
evaluation criteria, we focused on what was left, the midrange and treble
performance, and how musically involving the sound was.
- Noise level was never as low as a good solid-state pre-preamp, and
microphonics were observed with most if not all the tubes.
If you plan to build one, think about isolating the tubes from external
vibration, as well as possible, especially power transformer vibration.
A higher output moving coil output than .2mV is reccomended.
The Evaluation:
---------------
- The General Electric tubes were the most boring sounding tube of the
group. I was simply disinterested in the music when they were used.
They did nothing severely wrong, but were veiled throughout the frequency
ranges of interest.
- The Amperex tubes were a little better than the General Electric, but
they weren't as 'consistent' in sound quality across the frequency ranges,
and this could be easily mismatched with various components making
the system sound weird.
- The Mullard tubes were the most visually pleasing tubes of the group.
The quality of the lettering/logo on the tubes is excellent. The sound
quality however, was not much better than the Amperex, though more
consistent across the spectrum. Perhaps with better coupling caps, these
would sound more 'accurate' than the above tubes, but the musical magic
is not quite there.
The differences in the above 3 tubes were almost totally obscured when using
the 741 op-amp in the regulator circuit... I'd go for the looks of the Mullards!
...On to my favorites
- The Westinghouse tubes were much lighter sounding than the above, but had
clarity in the upper-midrange and treble regions that resolves more detail,
and brought out some of the musical sound I knew was on the records. With
the 741 Op-amp regulator the highs were kind of wispy, though I still
preferred this sound to the above tubes. I want my music!
- The RCA tubes were a surprise, even with the 741 regulator circuit.
These had the upper-midrange and treble clarity of the Westinghouse, but
without the wispy highs, and their clarity reached lower down into the
midrange. With the MC34081 regulator they became even better.
I found my music! Now where do I find more of these tubes...
It is time to investigate better coupling caps, tweak the power supply a bit
more, and 'finish the design' of this pre-preamp.
-Ed Berger
eberger@b.psc.edu