MOSFET Follies - letting the Solid State Demon into your tube
amp

Copyright 2000 R.G. Keen. All rights reserved. No permission
for serving from web sites other than http://www.geofex.com.

Aaaacccckkkk!!! Why would anyone even think of letting solid
state parts creep into their virtuous tube amplifier?? That's consorting with
the devil, isn't it?

Maybe. What we want tubes for is how they sound as amplifiers.
There are some other things that tubes are and have been used for that just
maybe might be as well served by solid state parts, given that we do this
carefully and test the tone each time. At the first glimpse of the silicon
monster raising it's head, we can excise the tonal sins, recognizing that we've
gone too far. Maybe our musical souls would come through intact.

I guess it will come as a surprise to some people that there are
any things in a tube amp that can be done by a solid state part without
adding ugliness to the amp's tone - in fact, there will in all likelihood be
people who refuse to believe this at all, and will be morally offended at the
suggestion. If you're one of these, quit reading this now and save your time
and outrage. A mind is a terrible thing to close.

Where to NOT put MOSFETs in a tube amp.

In general, not anywhere that you count on for gain or
distortion. Since different tube amps have different places where distortion is
generated, accounting for the wide difference in sounds between amps, this can
be a tricky thing to determine. Furthermore, even followers can make a
difference in tone by providing enough drive to push a following tube stage
into positive grid operation. This can soften distortion or make it harsher,
depending on the particulars of the circuit.

What kinds of things might we do with silicon parts?

First, we'll limit ourselves to MOSFET parts. Bipolar
transistors do many things well, but are not in general good for tube amp use.
MOSFETs have characteristics very much like monster pentodes anyway, so we may
be able to do some good things.

Second, we'll reserve all the voltage gain stages for tubes.
This keeps the tube-iness where it can be heard. This leaves only followers and
power supply functions for the MOSFETs.

First application: Source Follower to replace Cathode
Follower.

Many amps use tubes for cathode followers to buffer the plate of
a triode for more drive capabilities. The most common is Marshall's use of a
cathode follower to drive their tone stack. The cathode follower is hooked up
with the plate to the local B+ supply, the grid to the plate of the driving
tube, and the cathode to ground through a resistor. The signal is taken
from the cathode through a capacitor. This connection works well and drives the
tone stack hard, but is hard on the tube, as the heater of the tube is usually
kept at ground potential and the cathode is some 100 to 200V higher than
ground. All indirectly heated tubes have a maximum voltage rating for the
insulation between the heater and cathode, most of the 300V. With any largish
signal, the Marshall cathode follower is very close to this.

However, if we use a MOSFET that can take the voltage, we can
literally replace the triode section in the follower with the MOSFET, connecting
drain for the plate and source for the cathode. The gate is connected to the
preceding plate through a 220 ohm gate-stopper resistor. No other changes are
needed! A 12AX7 triode will bias with the cathode about 1V more positive than
the preceding plate; a big MOSFET like an IRF820 will bias with the source about
2V more negative than the preceding plate - the 3V difference is utterly
inconsequential.

"But wait!" I hear you yell. "The MOSFET has a big gate-source
capacitance. Won't that suck all the high frequencies out of the signal?".

No, it won't. The big gate-source capacitor is there, OK, but
when the device is hooked up as a follower, the apparent capacitance seen by a
circuit driving the MOSFET gate is reduced to a very small level by the fact
that the source is following the gate almost perfectly, so the apparent
capacitance is reduced by the local feedback to an inconsequential level. Even
a high impedance 12AX7 plate is not affected in the audio range by this
capacitance. Even the big power, big capacitance MOSFETs do this one OK. I have
done this, and what's more, done the necessary measurements to find the effect
on the response of the composite stage - and there is no detectable change in
frequency response below 30kHz. None.

This squares with the empirical evidence of listening to it - it
sounds the same as when a triode is used as a cathode follower.

Zetex makes a TO-92 package MOSFET worth some consideration for
replacing 12AX7 followers. It is a 450V/0.07A device with the type number
ZVN0545A. You can find the full data sheet at ftp://www.zetex.com/data_books/thru.hole/zvn0545a.pdf.
The gate-source capacitance on this one is considerably smaller than the bigger
power MOSFETs, and it should perform very nicely indeed as a follower as long
as the power dissipation of the package (700mw)  is not exceeded.

The MOSFET does have some differences from a tube follower. The
MOSFET will still be linear when the preceeding plate has bottomed out; a
typical 12AX7 triode section just can't "saturate" in normal operation to much
less than 50V. When the driver tube is saturated, the MOSFET follower is still
happily linear. The MOSFET can also get within a few volts of the B+ line, which
is much better than the triode section it replaced could do - the triode could
only get within about 50V of the B+ level. So the MOSFET will be linear for a
wider range of signals than the tube it replaces. Some amps use the triode
cathode follower to generate distortion. If your amp is one of those, the
MOSFET will NOT generate distortion like the triode did. The MOSFET will
accurately reproduce whatever the amplifying tubes send it. In most amps, the
fact that there is a MOSFET there instead of a triode will not be audible. I
have personally done this mod, and I can detect no hint of harsh solid state
sound; no one I've asked for the opinion can either.

Source Follower Drive for Output Tubes

Another place MOSFETs can be used as followers is as follower
drivers for output tubes. When output tubes are overdriven, their grids are
often driven positive. If the coupling to the grids is by capacitor, this can
lead to grid blocking/ bias shift as the grid current skyrockets when the grids
go positive. This current builds up as a negative charge on the coupling
capacitors and causes the tube to effectively be biased more negative (towards
class B) than the static bias setting. So on overdriven notes, the output tubes
are pushed into a harsh-sounding crossover distortion. In addition, the usual
12AX7 or 12AT7 phase inverter plate impedance is so high that it can't pull the
output tube grid positive in the face of grid current, so the grid of the
output tube is hard-clipped at Vgk=0V. This clipping can be as harsh as any
solid state situation.

The solution to the class B bias shift and grid current clipping
is to drive the output tube grids direct coupled from a cathode follower. The
follower is set up so that its cathode sits at the normal negative bias voltage
of the output tube, maybe -10V to -40V depending on the tube type. This means
that the cathode resistor is tied to a more negative voltage of something more
than twice the bias voltage, so it can pull the output tube grid negative. The
grid of the source follower is connected where the output tube grid used to be,
to the bias voltage source through large resistors and through capacitors to
the phase inverter.  The plate is connected to a postive voltage high
enough to allow full signal operation at the cathode and low enough not to
damage the tube.

The phase inverter cannot drive the grid of the cathode follower
positive with respect to its own cathode, as its cathode follows the grid up,
and we have supplied enough power supply voltage to the follower so it doesn't
run out of volts. The cathode can drive the grid of the output tube positive and
has a low enough impedance to pull the grid positive in the face of grid
current. The grid current is "eaten" by the follower, so it does not cause bias
shift into class B crossover distortion nor grid current limiting distortion.
The output tube can now be driven into plate saturation, which is much softer
than grid current limiting.

Cathode follower drivers for output tubes are hardly ever used,
largely for economic reasons. It's expensive to put an extra tube in to do
that. However, we can once again use power MOSFETs to do the job. Again, the
MOSFET drops right in, drain for plate, gate for grid, source for cathode. An
IRF 820 has both enough voltage rating and more than enough current and power
ratings to do the job. And the IRF820 is about $2.00 each, doesn't need heater
current, and won't wear out like a tube will.

A couple of amp builders have written up their experience with
this mod at the Ampage discussion board. The results were uniformly good - no
hint of  "solid state sickness" in the sound, just a sweetness and an
increase in apparent amplifier power.

Although it's a little harder, you can also do this for cathode
biased amps. You have to not only generate a negative voltage for the sources
to work from, but also include a "MOSFET bias" for each of the MOSFETs. Here's
why: the mosfets we're using have a turn-on voltage of between 1 and 5 volts
before they start conducting any thing at all. If we just popped them into the
cathode biased setup, the Vgs would effectively be subtracted from the voltage
on the cathode resistor. While the inherent feedback in the cathode biased setup
would correct some of this, we'd like to have no change in the output tube
biasing at all from the MOSFET use. All you need is a couple of pots to bias the
MOSFET gates a little positive. Five volts will be enough, and this can be had
from B+ through a 5V zener. The gates are connected to the wiper of a bias pot
across this 5V supply and this can pull the gate just enough positive to
counteract the loss in voltage with Vgs.

A MOSFET Standby Switch

A MOSFET makes an almost ideal switch for DC currents. Instead
of using a mechanical switch that can is both expensive and probably not
rated to handle the voltages involved in switching B+ off, we can instead take
note that at the cathodes of the output tubes, we could just open the cathodes,
or even better, insert a high resistance. The high resistance lets the tubes
self-bias to the edge of being completely off, and the switch doesn't need to
be floating at B+. Even better, we use a power MOSFET to be the
switch.

The recipe is simple. Where your output tubes' cathode circuits
connect to ground, break that connection. Insert a 10K 3-5W resistor to ground.
Across that resistor connect the drain-source of a high voltage MOSFET rated
for at least 100V more than your B+ a the first filter cap. This may require 700
to 1kV MOSFETs - which are readily available! Place a 220 ohm 1/4W resistor in
series with the gate *right at the gate lead* and then tie the 220 ohm resistor
though 10K to a +12V source. The "standby switch" is a small toggle switch that
shorts the junction of the 220 and the 10K resistor to ground. When that
happens, the MOSFET is switched off, and the 10K appears in series with the
tubes' cathodes. The tubes self-bias off -dramatically off, but with a tiny
trickle of current still flowing. They stay hot and ready, and the trickle
keeps the tubes from being damaged by the held-off condition. The 12V source is
a zener fed by a large resistor from B+ that feeds a fraction of a milliamp to
the zener.

When the standby switch is open, the gate of the MOSFET is
pulled to +12V by the 220 ohm and the 10K resistor. This turns it fully on, and
it saturates to a few ohms. This shorts out the 10K resistor and lets the
output stage operate normally. The MOSFET looks like a real resistor, and so
does not cause any tonal aberrations.

A MOSFET B+ Reducer

Sometimes you just want the B+ voltage on an amp to be smaller
than it is. This can often happen when you're working with an existing set of
transformers and want to use different output tubes or limit dissipation on the
tubes. A common way to do this is to use a power Zener in series with the
center tap of the power transformer. The zener voltage is directly subtracted
from the resulting B+. It's fairly common to want to reduce the B+ by about
50V.

However, a 50V zener will need to be rated at 10-20 watts to do
this job. These can be expensive. You can do much the same thing by using a
power MOSFET with a low power zener to make an "amplified zener". The power
zener is replaced by a low power zener that sets the voltage on the gate of a
MOSFET. The MOSFET conducts the vast majority of the current, an so is the
device that absorbs the waste heat. Notice that you MUST use a power MOSFET
that has a large enough power rating - which isn't hard at all, as 75W MOSFETs
cost about $2.00 - and you MUST provide the MOSFET a suitable heat sink to
remove the heat.

A MOSFET Phase Inverter

Coming...