EVJ III
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To give the EVJ two channels and a circuit based on a Matchless
Hotbox, another preamp tube is added.
Above Schematic uses a DPDT Toggle switch to swap channels
Above Schematic uses separate inputs to swap channels
Coupling Capacitors with a value from .01uf - .033uf are wired between the
stages and block the high voltage DC but allow the AC signal to pass through to the next stage. The Coupling capacitor
combined with the Grid leak resistor form a high-pass filter, a high-pass filter is an easier path for any frequency
above the cut-off frequency, while any frequency below the cut-off frequency will be attenuated. If we were to double
the size of the Coupling capacitor or the Grid leak resistor, we would halve the cut-off frequency (reduce it by an octave)
allowing more bass frequencies through. By increasing the value of the Coupling capacitor from .022 to .033 we lower
the cut-off frequency by half an octave allowing more bass through.
The Cathode bypass capacitors are wired in Parallel with the Cathode
resistors, they effect the low frequency cut-off point and will reduce or eliminate the effects of cathode current feedback.
When the anode current increases, the voltage across the cathode resistor also increases, and it falls when the anode current falls.
Cathode current feedback is caused when the cathode voltage attempts to follow the grid voltage, it reduces gain & distortion
and increases headroom.
A capacitor blocks DC but allows an easy path for AC, any AC signals on the cathode are
bypassed to ground, so AC current does not flow in the cathode resistor helping to hold the cathode voltage constant and prevent
cathode feedback. A capacitor allows greater current flow at high frequencies than it does at low frequencies.
If the Cathode bypass capacitor value is small (partially bypassed), (Matchless Hotbox uses .22uf),
then only high treble frequencies will be boosted. The stage will have maximum gain at high frequencies and minimum gain at
low frequencies, producing a treble boost.
If the Cathode bypass capacitor value is large enough it will be fully bypassed and
will have maximum gain at all frequencies. Doubling the value of the cathode bypass capacitor halves the cut-off frequency.
Every frequency above the cut-off will be boosted giving control over the gain between lower and upper frequencies.
The EVJ cathode bypass capacitors C3 & C4 are 22uf boosting almost all bass frequencies, which can sound dark.
Lowering the value rolls off bass frequencies which cuts low end mud and tightens a flabby bass. 1uf and 5uf are used in some
high end amps, .68uf are used in some Marshall amps. The Matchless Hotbox uses .1uf at C3 & .22uf at C4
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The EVJ ver 1 & 2, uses two 68k resistors for R1 & R2, to form a
Voltage Divider that halves the signal and sets the input impedance to 136K
(R1 and R2 in series), a heavy load for a guitar pickup. As in the mod kit the Pull down resistor R1, is changed to 1M and
repositioned so as to remove the voltage divider, although just replacing R1 with a 1M value will hugely reduce the effect of the
voltage divider and increase the input impedance to 1M. R2 the Grid stopper resistor is replaced with a 39k resistor.
The Plate load resistor R4 is changed to Carbon Composition for increased gain, using 220k
resistor, the same value as a Matchless hotbox.
This value will produce more gain but less harmonic distortion than a 100k plate load resistor. The link to the
12AX7 Tube Covers more about the Plate Load and Cathode resistors.
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The 6.3 volt heater supply is tapped into at pins 5 and 9 of the preamp tube socket.
R13 is beefed up with a 3.9k/2w resistor, and jumper JP3 is replace with a 4.7k/2w resistor.
The high voltage B+ is tapped into from the leg of this resistor.
To Tap into the Signal path I remove R5 the Grid stopper resistor, and R15 the Grid leak resistor and mount these resistors on the
switches but I could have left R5 & R15 where they were and cut the trace between C2 and R5 and then taken the output from the
leg of C2 and feed the signal back in at the leg of R5.
Grid leak resistors provide the ground (zero) voltage reference to the grid, while preventing the audio signal from being
shunted to ground. Grid leak resistors set the input(load) impedance of the stage, Larger values maximize power transfer, they
increase the load impedance, increasing signal voltage and reducing current drawn by the load. However grid leak current from
the tube, will cause a larger voltage drop across a larger value Grid leak resistor. (this may mess with the bias)
They are typically 240K - 1M.
The Grid stopper resistor, when combined with the capacitance between the control grid and cathode forms a low pass filter
that filters out radio frequencies that could cause parasitic oscillation in the stage, while leaving audio frequencies intact.
They are best placed close to the tube socket.
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| After C2, the signal leaves the PCB and goes through the red wire of the shielded cable
to the centre pin of a DPDT toggle switch. From there the signal can go up, between the top two pins of the switch and out the 10k grid
resistor, to the next DPDT toggle switch. Or the signal can go down, out at the red wire through a voltage divider, R19/R20, and into
the second preamp tube at pin 2. The signal comes out of the second preamp tube at the cathode, pin 8 on the tube socket through
the green wire, through a coupling capacitor and into a volume pot. Then out at the centre pin of the volume pot
and back to the DPDT toggle switch and out the 10k grid resistor. From the next DPDT toggle switch, the signal can go through a
tone stack or through the white wire back to the PCB.
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I have posted a 20min YouTube Video on the
Matchless Hotbox Page about sorting the power supply on a Matchless Hotbox.
The second tube is wired up the same as a Matchless Hotbox, as a DC Coupled Cathode Follower
We have already tapped into the 6.3 volt heater supply, connect these white wires to pins 5 and 9
of the tube socket and tie pin 4 and pin 5 together.
We have already tapped into the high voltage B+, the blue wire connects to pins 6 and across R23 to pin 9.
The signal comes into the tube at the grid, pin 2. The signal comes out of the tube at the cathode, pin 8.
The control grid has a big influence on how much current flows in the
12AX7 Tube,
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As the voltage at the grid goes more negative the current through the tube decreases until
no current flows and the anode current is cut-off. The grid voltage may continue to swing more negative, the tube will remain in
cut-off and the output waveform will be clipped. Depending on the circuit a 12AX7 will cut-off anode current when the grid is around
- 4 volts, the grid may go to - 8 or - 12 volts, but current stopped flowing in the tube at - 4 volts, and the output waveform
started clipping. Biasing a tube closer to cut-off Distortion results in less power dissipation and is called cool biasing.
As the voltage at the grid goes more positive the current through the tube increases until maximum current flows.
Grid current Clipping occurs when we try to drive the grid beyond 0v. Electrons being drawn from the cathode get attracted to the grid
rather than the anode causing a forward grid current into the grid. This current causes a voltage drop across the resistance that
is in series with the grid, making it hard to drive the grid beyond 0v. It is the grid signal that is being clipped, while the
tube amplifies what appears on its grid normally. The grid voltage may continue to swing more positive, no more current can flow
and the output waveform will be clipped. Biasing a tube closer to Grid current Clipping results in more power dissipation and is
called warm biasing.
To set the bias point of the grid to - 2 volts relative to the cathode. the Control grid is held at 0 volts via the grid leak resistor
and 2 volts is applied at the cathode via the cathode resistor (Cathode Biased).
An AC sine wave swings both positive and negative relative to the bias point, so if the signal is 3 volts, on the up-going signal the
voltage at the grid will be 1 volt and on the down-going signal, the voltage at the grid will be at - 5 volts. 4 volts peak to peak
is enough to swing a 12AX7 from cut-off Clipping to Grid current Clipping.
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The signal voltage at the R19/R20 voltage divider would normally feed the power tube.
The power tube would need much more voltage than a preamp tube to swing the tube from cut-off to saturation.
Starting with a voltage divider that halved the signal 470k/470k, I added resistors in parallel with R20 using alligator clips,
while I played the amp. This is also how I chose the resistors for the R6/R7 voltage divider. Voltage Divider R19 470k,
R20 91k could also be swapped for a pot.
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EVJ III Parts List
DPDT Toggle Switch
Tube Socket
Filter Stage
Metal Oxide, R13 3.9k/2w, 4.7k/2w
Filter Cap 22uF/400v
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Coupling Cap C13 0.022uf Mallory 150
Cathode Cap C14 1uf
Plate Load Resistor R23 100k Carbon Comp
Cathode Resistor R21 1.6k Metal Film
Cathode Resistor R22 62k Carbon Comp
Cathode Resistor R9 1.6k Metal Film
Voltage Divider R19 470k, R20 91k, could instead use a pot
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