Vintage Guitar - October 2017 - open - 39

ultralinear aspirations, the 4/25, says owner
Mike Tamposi, absolutely roars while still
providing plenty to ponder in a fussy and
unusual circuit.
In addition to organs, Sydney S. Bird &
Sons, of Poole, Dorset, did make several
nifty guitar amps between around 1959
and '65. Quality-wise, they were more on
par with Watkins and Selmer than Vox and
Marshall, but several carried interesting
features. This Golden Eagle 4/25 seems
to have been the flagship. And while the
bones were more robust than some of the
company's other offerings, its bells and
whistles were mostly sub-par. Tamposi
said both of his 4/25s are finicky.
"They remind me of my Selmers, but with
printed circuit boards instead of standard
point-to-point style wiring," he said. "And
while Bird used cheap caps and resistors, the
transformers look like Drakes. It's supposed
to have an echo effect, but I can't get it to
work on either amp."
His latter observation seems common,
likely due to the fiddly nature of the "echo"
circuit, which itself relies on the fiddly nature
of the reverb circuit. The spring pan uses a
crystal gramophone cartridge at each end
to transmit and receive signal. These units
sound interesting at best, though not as lush
as the types used by Fender and Ampeg, and
they're prone to breakage. Bird's Repetitive
Echo effect used a tube-driven network to
tap the spring at different points to produce
a pre-set pseudo echo, with (it would seem)
short-lived success.
Otherwise, from preamp to output
stage, the Golden Eagle 4/25 follows
familiar British circuit topology derived
from Mullard's applications manuals of
the day, siblings of which are seen in some
Vox and Selmer peers. Each channel has
its own EF86 pentode preamp tube (a
third drives the reverb), followed by a
Volume potentiometer linked to a simple
treble-bleed Tone pot. Skipping around the
befuddling complement of effects - and
making its way from the upper preamp
chassis to the lower output-stage chassis
via a cumbersome umbilical cord - the
signal from each channel hits a sturdy
long-tailed-pair phase inverter populated by an ECC83 (a.k.a. 12AX7). From
there, it's on to the EL34 output tubes
which are independently cathode-biased
with individual 470-ohm resistors and
50-microfarad bypass caps. All in all, the
configuration makes Bird's 25-watt rating
fairly likely, even possibly conservative,
though the same setup in a fixed-bias
output stage fed with higher voltages might

(Top) A trio of EF86 pentode preamp tubes (one for each channel, one for reverb) in the upper
chassis gets things started in grand British style. (Bottom) The 4/25's lower chassis houses
its preamp's EF86 and ECC83 tubes, two EL34 power tubes, transformer, and more.

generate in excess of 50 watts. Rectification comes from another British standard,
a GZ34 tube, and Bird included a hefty
choke in the power-filtering line to help
keep the tone punchy and the lows beefy.
This amp's lower chassis has pencilwritten legends for KT66 tubes in the output
positions, though other Golden Eagle 4/25s
carried the same EL34s; the schematic we've
seen also calls for EL34s, and not only is
Tamposi's other example loaded with them,
he's fairly certain all of the tubes in this
amp are the original Mullards. So, perhaps

October 2017

39

VINTAGE GUITAR

KT66s were a design whim that never got
off the ground?
All of this 25-plus watts is pumped into
a hulking 15" Wharfedale W15/EG speaker
adorned with a spec label that cooperatively
reads "10-15 ohms." Relatively robust for its
day, it might still have balked - or simply
folded - at the full force of the cranked 4/25,
but Tamposi doesn't take any such chances
when he wants to set loose the Golden Eagle.
"This sucker has a lot of gain," he said.
"So, I run it through a Marshall 4x12 and
shake the farm!"


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