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EVJ Mod Kit
WARNING Safety first!
Building your own tube amp can be a rewarding experience and it is nice to know how to bias your amp.
Working inside a tube amplifier can be dangerous, tube amps have lethal voltages that can kill you and a little common sense can
go a long way when working around dangerous voltages. If you aren't prepared to take the time to learn and apply the right
precautions to keep yourself safe, don't work on your own amp. You can seriously injure or get yourself killed.
To Measure the
voltage in the Filter Capacitors when the amp
is switched on, and check for residual voltage when the amp is switched off. Set
the Multimeter to DC, you can test the Multimeter on a battery. Put
your left hand in your pocket, Connect the (-) lead to ground with an alligator clip. Use
the other (+) Multimeter lead, to Probe the positive side of high voltage filter caps.
These are accessed through the legs of resistors R10, R12 & R13, the legs of these
resistors are connected to the positive side of high voltage filter caps.
Checking the charge left in the first two filter caps by probing the legs of R10 and
in the Second & third filter caps by probing the legs of R13 |
The filter capacitors can retain a charge after the
amp is shut off. Many amplifiers incorporate "bleeder"
resistors, which will drain the capacitor automatically, after the amp is turned
off. Don't depend on this resistor, always measure the voltage in the capacitors
before working on the amp. R11 is the 220k Bleeder resistor on the EVJ and R26 is
the bleeder resistor EVJ Hot Rod, These will bleed the charge from the filter caps and
stop a charge building up when the amp is switched off.
To Discharge the capacitors and make sure your amp is safe to work on you could use a cable
consisting of alligator clips on the ends of a 22k or higher 3 watt resistor. Connect one end to ground, then the other end to the
positive side of the high voltage filter caps. Do not leave this resistor in place when the amp is switched on.
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Checking the charge left in the first two filter caps of the EVJ Hot Rod,
30 mins after the amp was switched off, by probing the legs of R15.
This high residual voltage is because the first two filter caps are isolated
from R26 the bleeder resistor when the stand-by switch is open.
Don't depend on the bleeder resistor to do it's job
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Once the filter capacitors are discharged the circuit board can be removed,
mark where the wires are removed from, you could put tape on the wire and write on the tape. The schematic shows the
high voltage secondary wires coming from the power transformer are connected to the T1 & T2 lugs, It shows the
output transformer connected to the T3 and T4 lugs and the 6.3 volt heater supply is connected to the T5 and T6 lugs.
The schematic shows ground connections as triangles, resistors as rectangles and capacitors as two lines, one of the
lines is bent if the capacitor is polarized.
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I found this drawing on the net, it shows the high and low input jacks of a Fender Champ
With the fender low input, the two 68k resistors 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.
The fender low input jack is used on the EVJ ver 2.
With the fender high input, the two 68k resistors are connected in parallel
to form a 34k grid stopper resistor. With the input impedance set to 1M, it is a much easier load for a guitar pickup.
The fender high input jack is very similar to the EVJ ver 3 and identical to the boxed roc ver.
The low input jack can work for high output humbuckers or active pickups, but it can kill the tone for single coils or lower output
humbuckers. |
The Pull down resistor R1 is changed to 1M and repositioned to go directly across the
input jack, removing the Voltage Divider and increasing the input impedance to 1M.
The lifted leg of R1 could be soldered to the leg of R2 or removed from the board and placed across the input jack itself.
R2 The mod kit uses a 39k Grid stopper resistor, when combined with the capacitance
between the control grid and cathode it forms a low pass filter. This low pass filter has a cut-off frequency well above the
audio range, filtering out radio frequencies that could cause parasitic oscillation in the stage, while leaving audio frequencies intact.
The Plate load resistors R3 & R4 are upgraded to 110k Carbon Composition for increased harmonics,
Carbon comp resistors have their resistance vary with the voltage across them, making the signal gain non-linear,
adding harmonic distortion. The link to the 12AX7
Tube Covers more about the Plate Load and Cathode resistors.
The Grid leak resistors R5 are usually up to 1 meg in value.
They set the input impedance of the stage and provide the ground (zero) voltage reference to the grid,
while preventing the audio signal from being shunted to ground.
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 EVJ Coupling capacitor
from .022 to .033 we lower the cut-off frequency by half an octave allowing more bass through.
To improve tone the EVJ mod kit uses a K42Y-2 Paper in oil, Glass sealed in a metal body,
aluminium foil and oil saturated paper for the Coupling capacitor
On a fender Champ the pot wiper (centre pin) to the pot ground leg form a variable
Grid leak resistor, at full volume the grid leak resistance will be 1 meg. Turn it down so the grid leak resistance is 500k
and we double the cut-off frequency reducing the bass. Turn it down so the grid leak resistance is 250k and we double
the cut-off frequency again and reduce the bass further, and so on. Giving a treble boost at low volume.
R6 1M, & R7 1M, form a Voltage Divider
that halves the signal voltage at there junction.
R7 is wired in Parallel
with the 1Meg volume pot, turning the pot into a 500k pot. Removing R7 or lowering the value of R6
will increases the voltage at the Voltage Divider junction (giving you more gain). Higher resistor values in the
signal path will result in more highs being attenuated so lowering the value of R6 will
also increase brightness. The mod kit reduces R6 to 240K, increasing the brightness & gain.
R10 the Dropper resistor controls the B+ Voltage, with a Value of 220r it runs the tube hot,
exceeding maximum dissipation resulting in shorter tube life.
The mod kit supplies a 560 ohm resistor to lower the B+ Plate Voltage From 340 Volts to around 310 Volts.
The EVJ Hotrod page covers Biasing with a YouTube video.
R14 the power tube Cathode resistor is 220 ohms and sets the tube bias to hot.
The mod kit supplies a 270 ohm resistor to cool the bias increasing tube life.
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The screen grid (pin9 of the EL84) is red hot when
the amp is cranked and tube life suffers.
To limit current flow & protect the screen grid, a 1.5K Screen grid
resistor is added. Then the trace is cut so all current here goes through this
resistor
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 Bitmo kit uses 3.3uf or removes them altogether and Fromel uses a 2.2uf. The mod kit
supplies values 1uf/2.2uf/3.3uf
With a large value bypass capacitor all frequencies are boosted giving a full dose of bass.
Use a smaller value bypass capacitor and just the mid and high frequencies are boosted.
Use smaller again and just the high frequencies are boosted.
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The board shows a resistor
to ground with two caps wired to a switch, to choose between two
bypass capacitors or no bypass capacitor.
To adjust the preamp bias a three position toggle switch could also be installed.
To bias the preamp cooler, closer to cut-off clipping use a 2.4k cathode resistor.
For a centre biased stage for maximum headroom use a 10k resistor in parallel to get a 1.9k cathode resistor.
To bias the preamp warmer, closer to grid-limited clipping use a 3.6k resistor in parallel to get a 1.45k.
All sound slightly different.
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The Power tube Cathode bypass capacitor C5 is 22uf/25v.
The mod kit increases the value to 220uf/25v to Stabilize the Bias, Smooth out the top and give a fuller sound.
A Brightness cap from the leg of the pot to the wiper (C10),
or any resistor in the signal path (C12), will provide an easy path for high frequencies above the cut-off frequency,
while lower frequencies are attenuated by the resistor, the higher the value of the resistor the more
attenuation. As you increase the value of this cap you will allow more frequencies
an easier path. Doubling the value of the brightness capacitor halves the cut-off
frequency. The mod kit supplies values 82pf, 250pf,
The Power supply filter caps act as electron storage tanks which fill in the gaps between the pulses
giving you smooth, low ripple, DC. If the caps do not provide sufficient filtering, you end up with a 100 Hz
C9 to 47uf/47uf or just C6 to 120uf, providing better filtering of the power supply,
Reducing ripple Current and noise and increasing bass response.
The Diodes convert the AC power from the transformer to DC. The Stock Value is 1N4007
The mod kit supplies Ultra-fast UF4008 Diodes to eliminate any switching noise.
WARNING Pay Attention to polarity on Cathode Filter Capacitors.
They may catastrophically fail if voltage is reversed. The capacitor will short circuit following
the loss of the dielectric material, and with sufficient current, the electrolyte will rapidly heat up and either leak or
cause the capacitor to burst. Polarity is very clearly indicated on the case. A bar across the side of the capacitor usually
with an arrow being used to indicate the negative terminal. The negative terminal lead of an electrolytic is shorter than the
positive lead also.
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Modified Ver2 Board,
R1 & R2 have been replaced with Metal film resistors these are the quietest so they
are put at the input before the first stage. R4, R6, R7, R8 & R9 have been replaced with carbon composition resistors.
The carbon composition resistors can suffer from noise and drift but these are the warmest sounding.
R4 the second stage plate resistor benefits the most from using carbon composition.
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Stock Hot Rod board
The EVJ is a Cathode biased amp using EL84 power tubes with a
Maximum Plate Dissipation of 12 watts.
Exceeding this value can dramatically shorten the life of the power tubes and possibly cause major problems
if one should fail while in operation. No two power tubes are exactly the same. Some may move more current
than others. Whenever you change the tubes you should check the bias.
To Bias the EL84 Power tube we need to find the Plate current, the Cathode current will include
both the Screen current and the Plate current. So if we subtract the Screen Current from the Cathode
current we will get the Plate current.
To get the Cathode Current measure the voltage drop across the cathode resistor and divide this by the
value of the cathode resistor. Ground the (-) lead of the Multimeter, Use the other lead (1 hand only)
to Probe pin3 on the EL84 tube socket or the leg of R14. This will give you 9.5 volts(approx) across the
cathode resistor with a value of 220 ohms on the stock EVJ
Cathode Voltage (9.5 volts) / Cathode
Resistor Value (220 ohms) = Cathode current (.0431amps
or 43.1mA).
To get the screen current measure the voltage drop
across the screen resistor.
Current = Voltage / Resistance
Ohms Law
Calculator
So if we drop 4.2 volts across the 1K screen resistor, 4.2 volts / 1000 ohms = Screen current
(.0042 Amps or 4.2mA).
Subtract the Screen current from the Cathode current to get the Plate current.
Cathode current (43.1mA) - Screen grid Current (4.2mA) = Plate Current 38.9mAb.
To find the plate dissipation of the power tube, multiply the voltage drop between the Plate (pin7)
and the Cathode (pin3), by the current that flows between the plate and cathode. Plate Voltage (330 volts)
on stock EVJ - Cathode Voltage (9.5 volts) x Plate Current (38.9mA) = Plate Dissipation (12.45 Watts)
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Getting the Cathode Voltage by probing pin 3 of the EL84. |
Getting the Plate Voltage by probing pin 7 of the EL84. |
Most EL84/ 6BQ5 tube Manufactures Recommend a Maximum Design centre value
(should not be exceeded under normal operating conditions)
300 Plate Volts and 12 Watts Plate dissipation, while allowing for 10% +/- for variation in supply voltage.
The best way to Reduce power dissipation is to reduce the Plate Voltage.
The Dropper resistor controls the B+ Voltage, with a Value of 220r
it runs the tube hot. Increase the value to lower the B+ Plate Voltage.
To further Reduce power dissipation lower the Cathode current by increasing the value of the Cathode resistor
from 220 ohms to 270 ohms.
Cooling the power tube improving tone and increasing tube life.
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EVJ Mod Kit Parts List
Cathode Caps (2)1uf-3.3uf/50v
Power tube Cathode Cap 220uf
Filter Cap 120uf/420v.
Coupling Caps .033uf K42Y-2 Paper in Oil
Brightness Cap 250pf Ceramic,
Diodes (4)Ultra-Fast UF4007
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R1 1M Metal Film Pull Down resistor
R2 39K Vishay Dale resistor Grid Stopper
R15 3.6K Carbon Comp Grid Stopper resistor
R5 470K Metal Film Grid Leak resistor
R6 240k Voltage Divider resistor
(2)110k Allen Bradley Carbon Comp Plate Load resistors
560r 3 Watt Metal Film Dropper resistor
270r 2 Watt Metal Oxide Cathode resistor
1.5k 2 Watt Metal Oxide Screen grid resistor
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