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My intention with the output stage of this project amplifier was to use
a completely conventional output stage; with my odd and somewhat suspect
output transformers to worry about I wanted to be able to eliminate any
other output stage variables I could. This wasn't the time to experiment
with something wonderous and new in terms of stage topology or valve
types! So I had a fair idea that EL84's or something similar would be the
go, striking an appropriate balance between the available chassis sockets,
B+ and heater supplies available, and physical size, and the power I
thought I might drag out of my odd output transformers.
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EL84 valve data was readily available at Nostalgia
and Triode Electronics. The
EL84 (a.k.a. 6BQ5) is a relatively common medium power output pentode,
with many many application examples. In Class AB1 a push-pull pair
is capable of in excess of 10Wrms into an appropriate transformer coupled
speaker load. Anode power dissipation is 12W. Design anode voltage maximum
is 300V although it is not at all uncommon to see it used well over this
rating, to 350V or so is not unusual, and it appears to survive such abuse
well enough! These Pdiss and Va ratings suggest a
quiescent bias anode-cathode current to around 40mA or so.
At a Va of 300V, (the highest for which manufacturer ratings
are given, since that's the design max for the valve), an Raa
of the appropriate output transformer of 8 kOhms is given, making the
primary impedance of the transformer I proposed to use substantially too
low. I appreciated that that would result in a lower output power
realisable in my amp than would otherwise be possible.
I had decided that I would at least try to maximise the albeit low
power that at least I could achieve. The speakers I use are poor quality
and low efficiency, and the power available from this amp would be low in
any event.
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I felt I could not afford the "luxury" of sacrificing more, and
looked wistfully but merely passingly at the "triode mode" data also
available for the EL84. I would have to pass up the better sonics but lower
power of the triode mode connection.
Leak
Stereo 20
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I had to search a bit to find an example or two of the output stage I
had in mind; EL84 Class AB1 push-pull in ultralinear mode, with cathode
(self) bias. Many examples existed for triode mode or pentode mode, and
using grid bias. But the web provides such an accessable and immense
resource for such a purpose. The output stage of the Leak
Stereo 20 and Leak
TL12Plus3 at Kiewa were
interesting, and included most of the circuit elements and component
values (near enough) that I would eventually use.
Then I came across website references to the Heathkit UA-1 at several
websites; Heath Hi-Fi
(or Heath Hi-Fi), SDS Labs
(Sheldon Stokes), and "Tom80".
The Heathkit SA2 also used much the same circuit, again seen at a number
of websites including Heath
Hi-Fi (or Heath Hi-Fi),
and "Tom80".
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Leak
TL12
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Then followed an extremely useful and informative note on the Heathkit
UA1from Sheldon Stokes;
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"The amp's -3dB points are at 8Hz and 65kHz at full power. Plus
the square-wave response at 20kHz (5 Watts) is very clean; it looks like
there isn't a transformer there at all. After all the most critical parts
of all tube amp's are the output transformers."[Notwithstanding
Sheldon's comments, I got pretty much the same frequency response
measurements with my el-cheepo transformers, which confused me
considerably, but left me satisfied enough!]
Sheldon continues; "The amp was originally designed with two
feedback schemes. The first is the traditional voltage feedback scheme,
where some of the output signal is taken from the 16 Ohm tap on the output
transformer, and fed back into the cathode of the input pentode. The amp
sounds amazing in this mode.
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The second scheme uses some current feedback as well as voltage feedback.
With this second scheme, the current from the speaker is sensed via a small
value resistor and fed back into the cathode of the input stage. The voltage
feedback from the output of the amp is reduced by about half to keep the total
feedback about the same. This may have worked well with old speakers, but
doesn't sound very good with my Mission bookshelf speakers. And it sounds just
awful with the Quad ESL's. You can flip a switch on the front of the amp to
select either the great sounding traditional all voltage feedback, or the bad
sounding current/voltage feedback scheme. When I rebuilt these amps, I removed
that feature completely from the amp, and used the voltage feedback
scheme."
I constructed my derivative version without this second type of
current/voltage feedback scheme, and on the strength of Sheldon's adverse
comment decided simply to leave it that way, with a conventional feedback scheme
only!
"The UA-1 and UA-2 amps are essentially identical. The only
differences between the UA-1 and UA-2 are that Heath increased the value of the
cathode resistor on the output tubes because they were running them WAY too hot,
and burning them out quickly. The UA-1 had a 130 Ohm resistor in the cathode at
first. They changed that pretty quickly though, and the schematic here reflects
that change. They also reduced the value of the screen grid resistor on the
input stage to 680 kOhms. I have tried both the 1 MOhm and 680 kOhm resistor and
there is no difference that I can hear or measure."
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I decided to leave the bias I had on the EL84's, which was roughly the
same as having one 130 Ohm per pair, and suffer the consequence of reduced
valve life. The 1 MOhm to the screen on the 6AN8A pentode was easy enough
to change, and since at least it didn't appear to have a detrimental
effect I decided to go with the Heath mod and change down to the 680 kOhm
value. Presumably this would pull the screen voltage up somewhat,
presumably to improve the pentode's linearity, I guess.
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Adjustable
Bias
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My nifty little 120 Ohm
preset's. I wonder where I "liberated" these from! Damn handy
when I needed them though!
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EL84
Bias Detail
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I then looked at one or two ways of doing the cathode biasing; that
shown above was interesting although in the end I didn't use it.
Rummaging through my parts collection I discovered four (one for each
output valve for the two channels) nifty little 120 Ohm wirewound
presets. Bingo! On the strength of this discovery I decided to
use discrete (unlinked) bias arrangements for each valve, selecting a
series resistor that I hoped would put these pots at about 60 Ohms, the
middle of their range, at a nominal cathode current of 35~40mA. With
a bit of fiddling the resistor value turned out to be 270 Ohms. I
added a 1 Ohm resistor in series for test purposes (the voltage across it
in mV is the current through it in mA), to make set-up easy. I found
it relatively easy to set the bias of each valve for the same cathode
current by these means, although I guess I'll have to recheck the bias
setting regularly as the valves change over time.
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