Tech Forum
for guitar amplifiers and their parts.
Second, if you want to redesign
the output stage of a tube amp, your
best bet is MOSFETs ( www.mouser
.com). They act more like tubes than
bipolar transistors, and don't suffer as
much loss of current gain at high
voltages. In fact, you will need to
suppress their current gain with a
"cathode" resistor and/or a negative
feedback loop in order to reduce their
performance to resemble a vacuum
tube. I've never seen a vacuum tube
that would go from zero to four amps
for two or three volts of grid signal,
but MOSFETs do. Then, there is the
matter of designing DC bias supplies,
limiting the drive voltage for the
phase that shuts off each MOSFET,
and adjusting for minimum distortion.
There is no way to just plunk a
drawing in your lap because every
tube design is peculiar to the available
voltage and the output transformer.
On the other hand, tube amps are
simpler than you might think. After
all, most of them were designed
about 50 years ago. My favorite tube
book is Tube Audio Design by Bruce
Rozenblit. He covers the basics in
about 60 pages, then does another
60 pages of examples. If you go
through that book like it is a school
text book, you will have all the
concepts and math required to "fake"
a vacuum tube.
Third, if you just want to get the
signal to an external transistor power
amp, you can simply pick a point
before the phase splitters, or off the
cathodes of the phase splitters, and
build a capacitively-coupled output
jack for each channel. That's how I
use a transistor amp to troubleshoot
tube amps. I use the volume control
on the transistor amp to keep from
blowing my ears out when I drive
the preamp stages to their limits,
and I can hear noise and distortion
easier than I can see them on my
oscilloscope. Meanwhile, the power
tubes are either unplugged or driving
into a dummy resistor. Never leave a
tube output unloaded. They are
current drivers and if you don't load
the output current, they react with
JULY 2005
inductive voltage spikes that often
cause smoke to be released.
Chuck Larson
Largo, FL
#2 My first reaction to this question
was something on the order of "why
would you want to?" But after
thinking a moment, I recalled that
there are actually some solid-state
devices made to directly replace
certain tubes. Mostly these are
rectifiers, and replace rather hard-to-find or expensive and unreliable
rectifier tubes. But there were some
made at various times that were
intended to replace amplifier tubes,
as well. These boasted higher output
and better efficiency over their tube
counterparts. Some have even built
custom replacements using MOSFET
devices.
That said, let's get to practicality.
If your goal is to own a working H.H.
Scott or Fisher amp, check out
www.tubesandmore.com which is
the website of Antique Electronic
Supply in Tempe, AZ (480-820-
5411). They won't help you replace
your tubes with transistors, but they
do stock your 6BQ5 tube, and may be
able to help you out with a cross for
the others. There's hardly a tube
found in old amps they don't have
and they usually aren't all that
expensive. You might consider
replacing your tubes with tubes, and
keep the amps as original.
Frankly, the point of owning an
older tube device is the tubes. If you
replace the tubes in your old amp
with solid-state, all you'll have is an
old amp. The time and expense of
modification might well exceed the
cost of tube replacement. On the
other hand, if you want a solid-state
amp, consider this option: your H. H.
Scott and Fisher have collector value,
even without a complete working set
of tubes! You can sell them (on eBay,
for example) and partly or wholly
fund a new solid-state amp that will be
higher power and more reliable,
perhaps more to your liking. A recent
eBay auction for a Fisher tube
receiver closed at $379.00, not bad
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