60100 Watt

This workbench gadget allows you to connect a light bulb in SERIES with
your amp. It will allow you to determine if your amp is drawing excessive
current due to a shorted PT or filter caps, etc., without blowing fuses or
smoking valuable components such as a PT. A dead short on the primary
side of the PT will cause the bulb to glow at full brightness. Partial shorts
or a short on the secondary side of the PT will show some increased level
of brightness. A properly working amp will cause the bulb to glow fairly
bright when first turned on, but will fade to a dimmer glow as the amp
warms up.
Once you have confidence that there is no high current drain from your
amp, you should remove this gadget. Voltage measurements will be very
low and probably meaningless while your amp is plugged into this gadget.

(Original drawing)

LINE
Radio Shack
Cat # 273-1511
12.6 VAC @ 3 A
With center tap

A
B
C
D

A = full line voltage

B = 6 volts down
C = 12 volts down
D = off

NEUTRAL

This gadget uses a bucking transformer to reduce AC line voltage. Its useful
for reducing todays 120-125 VAC line voltage by approximately 6 or 12 volts
which will also reduce your HT and filament voltages by an equal percentage.
(Tip. If you find that the output voltage actually increases, you have the
transformer phased for a boost operation. Simply reverse the primary leads to
correct this.)

++

25F
25V

++

100K

LTP PHASE INVERTER

25F
25V

Bypass cap
Power tube #1
cathode

56K
470K

47K

22K

15K

10K
470K

330
Bias resistor
DPDT /w
Center OFF
100K

Bypass cap
Power tube #2
cathode

4.7K

820

UP = full power
Center to power
Down power
(power estimates
based on using 330)

330
Bias resistor

Marshall 18 Watt Power Dampening Switch

Da Geezers Bypass Cap attenuator

(can be adapted to other LTP phase inverters)

(Bias resistor value will be 2X the value

of a single bias resistor)

470
To Bias

470

Cathode/Fixed Bias Switch

Pentode/Triode Switch

Standard Jacks
Switchcraft #11
Return
3PDT push/push

Stereo Jack
Switchcraft #12B
Guitar

Amp

tip
ring
Send

RX

- +
9v

RX =

9 - VLED
ILED

Return
T

Amp

Guitar
T

T
G

3PDT Switch
operation is up/down

+
9V

Send

Additional failsafe resistor

from
bias tap

220K
470

27K

Bias
220K

15K

Bias

10K
+

from
HT winding

47K
+

Deluxe/Deluxe
Reberb use 10K

+
25K

Typical Fender AB763

Typical Marshall

No dc current flows through grid or grid

resistors, therefore no dc voltage is
dropped across grid resistors

3
4

Current flows through bias

supply divider network to
develop adjustable bias voltage

1500

220K

BIAS

470
10K
+

27K

220K
8

1500
5

Where does the current flow?

B+

BIAS
Bias voltage

.047F

15K

56K

Range
47K
Bias
Range

Bias circuit and full wave bridge

Separate bias winding and full wave bridge

(from Marshall JCM900 and some old Ampegs)

(nice when you have a separate bias winding)

To Bias
Fixed

BIAS MODE
Cathode

Figure 1. Use this DPDT switch with any bias supply.

BIAS

To Bias

BIAS MODE
Cathode

Fixed
1

COM

BIAS

Figure 2. Use this SPDT switch with a bias supply that gets its AC supply from one end
of the PT HT winding ***AND*** uses a high value resistor between the PT and the
bias rectifier/filter circuit. (See Fender Princeton Reverb for example.)

Hi/Lo Input jack switching is often misunderstood. The operation is usually straightforward, but the
actual circuit drawing is often confusing, especially to the casual observer. Hopefully the following
illustrations will demystify the circuit operation.
These first two circuits represent the typical Hi/Lo jacks found in most Fender and Marshall amps. Many
other amp manufacturers use this circuit as well.
Using the Lo Input

Using the Hi Input

Switch open

LO

Switch closed

68K

LO

68K

68K
68K

HI

HI
1M

1M

Switch closed

Switch open

The LO jack delivers the signal to a 2:1 voltage divider made

up of the two 68K resistors. The 1meg is shorted out by the
switch contacts on the HI jack. The signal taps off the
junction of the two 68Ks. Half the signal is dropped across
each 68K, therefore only 50% of the signal is applied to the
tube.

The HI jack delivers ALL the signal to the tube. The signal
enters the HI jack and first sees a 1 Meg resistor to ground.
Since the LO jack switch is closed, the two 68Ks are parallel
for an effective resistance of 34K and the signal travels
through the paralleled 68Ks to the tube. There is no voltage
divider so 100% of the signal arrives at the tube.

The following circuits represent special case switching. The first shows the Marshall 18 Watt parallel
tube switching circuit and the other shows a Marshall JCM-800 high gain cascade switching circuit.
Marshall 18 Watt Normal Channel Inputs

Marshall JCM-800 2204 Channel Inputs

LO

V1A

V1B

LO
1M

HI

HI
1M

The LO jack delivers the signal to V1A only. The HI jack

delivers the signal directly to V1B and also to V1A through the
closed switch of the LO jack. The parallel tubes give a fatter
sound with a slight gain increase.

V1A

VOL

V1B

The LO jack delivers the signal directly to V1B for a single

gain stage. The HI jack delivers the signal to V1A and then
to V1B through the closed switch of the LO jack. The
cascaded tubes give a high gain sound.

There are two variations of the input switching jacks for this amp. Type A is the classic circuit that has
been used in many Fender and Marshall amps. Type B uses a slightly different circuit to accomplish the
same functionality. The Hi input operation is slightly different for the two type circuits. However, the
difference is so slight that it can be practically ignored. You would need precision lab equipment to even
measure the slightly different signal levels applied to the tube grid. When comparing the Lo input
operation, it can be seen that the two type circuits become identical, although achieved through a
slightly different approach.
Hopefully, the summary below will explain the functionality of both types and also point out the slight
differences.

Type A Using the HI input

J1
HI

Type B Using the HI input

J1
LO

1M

68K

Switch open

1M

68K

Switch closed
68K

68K

J2
LO

J2
HI

Switch closed

Switch open

The HI jack delivers ALL the signal to the tube. The signal
enters the HI jack and first sees a 1 Meg resistor to ground.
Since the LO jack switch is closed, the two 68Ks are parallel
for an effective resistance of 34K and the signal travels
through the paralleled 68Ks to the tube. There is no voltage
divider so 100% of the signal is applied to the tube.

The HI jack delivers almost ALL the signal to the tube. The
signal enters the HI jack and is applied to a voltage divider
consisting of both 68Ks and a 1M through the closed switch
on J1. 6% of the signal is dropped (lost) across the first 68K.
The other 94% signal that is dropped across the second
68K and 1M is applied to the tube.

Type A Using the Lo input

J1
HI

Type B Using the Lo input

J1
LO

1M

68K

Switch closed

1M

68K

Switch open
68K

68K

J2
LO

J2
HI

Switch open

Switch closed

The LO jack delivers the signal to a 2:1 voltage divider made

up of the two 68K resistors. The 1meg is shorted out by the
switch contacts on the HI jack. The signal taps off the
junction of the two 68Ks. Half the signal is dropped across
each 68K, therefore only 50% (-6db) of the signal is applied
to the tube.

The LO jack delivers the signal to a 2:1 voltage divider made

up of the two 68K resistors. The 1meg is removed from the
circuit by the switch contacts on the HI jack. The signal taps
off the junction of the two 68Ks. Half the signal is dropped
across each 68K, therefore only 50% (-6db) of the signal is
applied to the tube.

SS/Tube Rectifier Switching

6
8
2
4

5 VAC

Using a SPDT Center Off switch allows for SS STBY TUBE function.

Hybrid SS/Tube Bridge Full Wave Rectifier

6
8
2
4

5 VAC

This full wave bridge circuit retains the characteristics of a tube rectifier.
Note there is no center tap on the HT winding.

Guitar Input
Guitar Output

V3A - Trem Oscillator

V4A - Low Pass Modulator

V3B Trem
Phase Inverter

V4B - Hi Pass Modulator

This unique tremolo circuit is found in several of the old Fender 6G_ amplifiers and also in the Revibe units available from
Hoffman or Weber. The sound is much richer than other typical tremolo circuits.
The Oscillator V3A is a standard Phase Shift Oscillator that operates at a low frequency range of approximately 2-10 Hz. The
oscillator output is applied to the grid of V4A through the Intensity control. The tremolo signal from the Intensity control is also
sent to the grid of phase inverter/amplifier V3B. The 180 out of phase tremolo signal is applied to the grid of V4B. These two
tremolo signals will control the gain of the modulator tubes by varying the bias at the slow oscillator frequency.
The input guitar signal is split and also applied to the grids of the modulator tubes. However, the guitar signal passes through a
low pass filter (blue path) to get to V4A and passes through a high pass filter (green path) to get to V4B. So, the V4A amplifies
only the low frequency components of the guitar signal and the gain is varied/modulated by the Tremolo oscillator signal.
Likewise, V4B amplifies only the high frequency components of the guitar signal and the gain is varied/modulated by the
Tremolo oscillator signal that is 180 out of phase with the Tremolo signal applied to V4A.
The modulated high frequency guitar signal is recombined with the low frequency guitar signal in the two 470K mixing
resistors. The out of phase Tremolo signals are also recombined in these mixing resistors, but since they are equal amplitude
and 180 out of phase, the Tremolo signals cancel each other, leaving only the guitar signal. Since the Tremolo signals cancel
each other, you will not hear the Tremolo signal breathing when no guitar signal ia applied.

Guitar Input

Trem Oscillator/Driver

Phase Splitter

Modulator

Guitar
Output

The Oscillator is a standard Phase Shift Oscillator that operates at a low frequency range of approximately 2-20 Hz. The
oscillator output is coupled through a cathode follower driver and then applied to the input of a phase splitter through the
Intensity control. The phase splitter produces two identical outputs that are 180 out of phase with respect to each other.
Each output is coupled to the grid of a modulator tube and will control the gain of that tube by modulating the bias at the
slow oscillator frequency.
The input guitar signal is split and also applied to the grids of the modulator tubes. However, the guitar signal passes
through a high pass filter to get to the top tube and passes through a low pass filter to get to the bottom tube. So, the top
tube amplifies only the high frequency components of the guitar signal and the gain is varied/modulated by the Tremolo
oscillator signal. Likewise, the bottom tube amplifies only the low frequency components of the guitar signal and the gain
is varied/modulated by the Tremolo oscillator signal that is 180 out of phase with the top Tremolo signal.
The modulated high frequency guitar signal is recombined with the low frequency guitar signal in the two 470K mixing
resistors. The out of phase Tremolo signals are also recombined in these mixing resistors, but since they are equal
amplitude and 180 out of phase, the Tremolo signals cancel each other, leaving only the guitar signal. Since the
Tremolo signals cancel each other, you will not hear the Tremolo signal breathing when no guitar signal ia applied.

0vdc

-10vdc
20Vpp Tremolo signal superimposed on -40Vdc fixed bias
voltage applied to pin 5 of a 6L6 tube.
-20vdc

-30vdc
+10v
peak

-40vdc

bias point = -40vdc

-10v
peak

-50vdc
Time = .2sec
Freq = 5Hz

-60vdc
This graph shows that a 20Vpp Tremolo signal superimposed on -40Vdc fixed bias
voltage will cause the resultant bias voltage to vary between -30vdc and -50vdc. You
could simulate the tremolo effect simply by rhythmically adjusting the bias pot between
-30 and -40vdc. Changing the bias will affect the gain of the 6L6 thus varying the
loudness of the instrument signal.

SPEAKER CAB #1
4 OHM

Special Y cable used to connect an amplifiers 8 output to

two 4 speaker cabs. This Y cable connects the speaker cabs
in series providing the correct load match for the amp.
Connect RED wires to plug tips.

AMP
8 OHM

Make this splice connection inside the plug shell

SPEAKER CAB #2
4 OHM

Typical Fender Mains Wiring

Improved Fender Mains Wiring

NEUTRAL

.047F 630v

Com

AC
CORD

LINE

FUSE

to PT
Primary