6386 Vacuum Tube Limiter - Operating Manual
· Classic feedback compression with the same gain control
tube as the Fairchild 670
· Modern class A, solid-state gain make-up stage for wide
bandwidth and low distortion
· Soft-knee design with smooth transition from compression
· Fast attack (0.5ms) and release times ideal for tight
program compression or tracking
· Low noise and superior transient response
· Three sidechain operating modes: Fast, Presets and Manual
· Two completely independent channels with stereo linking
· Tubes calibrated for precise stereo balance
· Hard bypass switches for each channel
· Sidechain inserts for frequency-dependent compression
· Transformerless balanced output stage capable of delivering
· Soft-start warm-up with outputs bypassed until all voltages
· Gold-plated relay contacts, I/O connectors and tube
· Fully regulated high voltage (250V), filament and bipolar
· Custom toroidal power transformer with shield for minimum
· Polypropylene capacitors and metal film resistors
Thank you for purchasing the Pendulum 6386, a modern two channel
tube limiter designed to have the compression characteristics
of a vintage unit, but with a higher level of sonic accuracy.
Rather than trying to recreate the past, we decided to taken
an entirely different approach. The 6386 uses the same gain control
(remote cutoff triode) tube as the vintage Fairchild limiters
to provide the classic feedback response unique to the characteristics
of this tube. Subsequent amplification is accomplished with a
Class A, balanced transformerless solid-state line amp, rather
than the push-pull transformer output stage used on vintage variable-mu
units. The result is a limiter that possesses the 'dynamic signature'
of a classic tube compressor, but with a more accurate, less
colored signal path.
The soft-knee design used in the 6386 offers effortless dynamic
control with a smooth transition from compression to limiting.
In addition, both channels can be operated independently, or
linked with the flip of a switch. When linked, the threshold
and attack/release parameters are controlled entirely by channel
1, assuring precise stereo balance over the entire range of gain
For maximum versatility, the 6386 incorporates a solid-state
sidechain to provide a wide range of control with three modes
of processing: In the FAST mode, the limiter works extremely
fast (0.5ms attack time), and is remarkably free of the 'pumping'
artifacts normally encountered with slow variable-mu compressors.
It's the ideal choice for invisible dynamics processing, either
for tracking or tight program compression.
The PRESET mode consists of six classic attack/release time
settings (identical to the Fairchild 670), that are optimized
for program compression, including two settings with program-dependent
The MANUAL mode offers total control over the attack and release
times, which is particularly useful for creative compression
effects or for processing bass guitar.
Most important, the hard bypass switches on the front panel
conveniently allow a quick determination of exactly what the
6386 is, or in most cases, isn't doing to your signal.
The unit was carefully packed at the factory to protect against
damage in transit. Nevertheless, be sure to inspect the unit
and shipping carton for any signs of damage that may have occurred
during shipment. If there is any damage, notify us immediately
for further instructions. It's also a good idea to save the carton
and packing materials should ` you ever need to return the unit
for repair. The shipping carton should contain the following
items: the 6386 Vacuum Tube Limiter, a 3 prong IEC power cord,
and this operating manual.
The 6386 uses two EIA-standard rack spaces, and can be
mounted in any standard 19 inch (483mm) equipment rack. If the
6386 is mounted in a mobile rack or road case, it is important
that the rear of the chassis is supported to prevent possible
damage from mechanical shock and vibration. Excessive shock and
vibration can cause damage or premature failure of the vacuum
tubes, or cause them to shaken loose from their sockets. Please
avoid rough handling.
For proper operation, it is very important that adequate
ventilation is provided. Vacuum tubes produce a significant amount
of heat that must be removed from inside the chassis. The side
panel vent holes and top panel slit vents should never be blocked
in any way. Never mount the 6386 below a rack unit with
a depth greater than 7 inches (178mm). Do not mount the 6386
near other heat-producing equipment such as power amplifiers
or other vacuum tube products. If possible, leave open at least
one rack space above the unit, and use a rack spacer with a ventilation
grille. Never operate the 6386 inside a road case where
the side panels are cushioned in foam.
Preventing Ground Loop Hum
One of the reasons the 6386 sounds so good is that it
uses an active-balanced output stage rather than an output transformer.
However, it does not benefit from the galvanic isolation provided
by an output transformer. For this reason, a few precautions
are necessary to insure hum-free operation:
· Isolate the front panel from the rack rails. Use plastic
shoulder washers to prevent electrical contact between the rack
ears of the 6386 and the metal rails of the equipment
· Isolate the 6386 from units mounted above
or below it in the rack. Make sure the front panels are not in
electrical contact and that the top or bottom cover screws of
the 6386 are not touching those of any other units.
· Connect the 3 prong IEC power cord to the single-point
star grounded electrical source for your
The idea here is to make sure the 6386 seeks ground
at only one point. For safety reasons, do not lift the ground
at the IEC power cord. Keep in mind that in a properly grounded
hookup, the 6386 does not hum. Please, take the time to
do this right, and you will be rewarded with hum-free operation.
Call us if you have any questions. Please note that pin 2
The 6386 is equipped with a 3-prong IEC power connector and detachable
cord. Never operate the 6386 with the ground on the power
cord defeated. Unless otherwise stated, this unit operates from
120V/60 Hz at 30W. The ac fuse is accessible from the rear panel
and is rated at 1A/250V (3AG) SLO-BLO. To check or replace the
fuse, make sure the unit is unplugged. Operation at 240V/50Hz
is available as an option. Please contact us for more information.
Other than changing the tubes, the user should not attempt to
service the 6386 beyond that described in this manual.
Never remove the covers or attempt to replace the tubes until
the unit has been disconnected from the ac power source, and
all circuits inside have been allowed to discharge for a period
of at least 30 minutes. The vacuum tubes become very hot once
the unit has been turned on, and they should not be touched until
they have cooled to room temperature. To reduce the risk of fire
or electrical shock, do not expose to rain or moisture, or operate
it where it is exposed to water. Since potentially lethal voltages
are present inside the unit, it should only be opened by qualified
service personnel. Refer all servicing, or any questions about
servicing, to Pendulum Audio, Inc.
While the operation of the 6386 may appear to be rather
straightforward, there are a few features which may differ from
what you're accustomed to seeing on other vacuum tube compressors.
You may find it useful to refer to the Condensed
Operating Instructions to quickly identify the operation
of the front panel controls. However, we suggest you read through
this section to take advantage of all its features, and to make
sure you are operating the 6386 in the way most appropriate
for the type of dynamics processing you're doing.
Please refer to the rear panel layout (see the Condensed Operating
Instructions) for the location of the inputs, outputs, and ac
power. Make all connections to the 6386 before applying
On the right hand side of the rear panel are the transformer-balanced
inputs for channels 1 and 2. The female XLR connectors on top
are 3 pin balanced inputs with Pin 1 = ground, Pin 2 = positive
phase input (+), and Pin 3 = negative phase input (-).Use them
for connecting the 6386 to +4dBu transformer-balanced or active-balanced
sources. If you encounter ground loop hum when connecting to
balanced sources, lift the shield from Pin 1 on the input end
of the cable.
The two 1/4" jacks below the XLR connectors are unbalanced
input connectors, wired in parallel to the XLR inputs, with Tip
= Pin 2, Ring = Pin 3, and Sleeve = Pin 1. Use them for connecting
the 6386 to unbalanced sources. Do not attempt to use the XLR
and 1/4" inputs simultaneously.
To the left of the input jacks are the active-balanced output
jacks for channels 1 and 2. The male XLR connectors on top are
3 pin balanced outputs, with Pin 1= ground, Pin 2 = positive
phase output (+), and Pin 3 = negative phase output (-). Connect
these outputs to XLR- or TRS-balanced line-level inputs. If you
encounter ground loop hum when connecting to balanced inputs,
lift the shield from Pin 1 on the input end of the cable.
The two 1/4" jacks below the XLR connectors are unbalanced
output connectors, wired in parallel to the XLR outputs, with
Tip = Pin 2, Sleeve = Pin 1. Use them for connecting the 6386
to unbalanced inputs or mixer inserts.
On the left side of the rear panel is the IEC input socket. Connect
to a 120V/60Hz receptacle with the 3 prong IEC power cable supplied
with the ES-8. For safety reasons, do not lift the ground
on the power plug by using a 3-to-2 ground lift adapter.
Turn on the power to the unit using the ac power switch located
above the power inlet socket. Although it might seem a bit awkward
to put the ac power switch on the rear panel of the 6386, there
is a very good reason for it. By keeping the ac away from the
front panel, hum induced by the ac line into the front panel
circuitry can be virtually eliminated.
If necessary, replace the 1A/250v 3AG SLO-BLO fuse only with
the same type and rating.
To prolong tube life, the 6386 goes through a soft-start
sequence for gently applying power to the tubes and stabilizing
the circuit before engaging the outputs. When the power switch
is turned on, the outputs remain in BYPASS and the dc voltage
on the tube filaments is ramped up to 12.6Vdc. Next, the high
voltage supply is slowly increased to 250V and the circuit is
allowed to stabilize for about 2 minutes. Finally, the IN/BYPASS
switches are activated and the blue 'on' led on the front panel
is illuminated. For best results, please allow the 6386 to
warm up for 10 minutes or longer before use. TO PROLONG TUBE
LIFE, TURN OFF THE ES-8 WHEN NOT IN USE.
6386 Front Panel Controls
Please refer to the front panel layout (see the Condensed
Operating Instructions) for the location of all switches and
controls discussed below. The operation of both channels is identical,
unless the two channels are linked. All toggle and rotary switch
contacts are gold-plated for high reliability. All panel rotary
controls are long-life conductive plastic potentiometers.
Input: Off to Max
The INPUT control is actually a passive balanced attenuator positioned
between the input connector and the input transformer. It controls
the input level to the input transformer, and consequently, the
input level to the 6386 gain control tube. In normal operation
the INPUT control should be set fully clockwise to MAX, since
the 6386 is designed to operate with +4dBu input levels. On
occasion, it will be necessary to reduce the input level to the
6386 to prevent overload of the input transformer or the tube
gain stage. Monitor the input level with the VU meter to determine
if input attenuation is required. Unlike most other compressors
of this type, the input control is located in the primary circuit
of the input transformer, and does not affect the balance of
the gain control stage.
Threshold: Off to Max
The THRESHOLD control determines how much gain reduction is applied
to the source material. Since the 6386 is a 'soft-knee' feedback
compressor, it does not have a strictly-defined 'threshold',
like VCA-based units that uses feedforward detection. A maximum
of 12dB of gain reduction is available from the 6386 gain control
tube. The range of the THRESHOLD control is off to -20dB, which
permits a steady-state input level as low as -8dBu to achieve
the full 12 dB of gain reduction.
Output: Off to Max
Use the OUTPUT control to increase the signal level after compression.
Up to 12dB above the level of the input signal is possible. The
unity-gain setting (0dB) is 10:00. The OUTPUT controls on the
6386 is are 20% audio taper potentiometers, which permits finer
adjustment of output level in the critical 0-12dB range.
Meter: Input/Gain Reduction/Output
The illuminated ANSI VU meters are electronically isolated from
the signal path, and can be switched to measure input, output
or gain reduction. Keep in mind that a VU meter is a mechanical
device, designed in accordance with a with a well-accepted ballistic
standard, to indicate an average loudness level. On the other
hand, the led meters on your mixer or digital recorder are reading
a peak program level, and faithfully register all those short
transient spikes that add little to the perceived loudness of
the program material. The ratio of the peak to average levels
can be 20dB or greater depending on the source (e.g. drums).
So, if you're wondering why the led meters on your recorder are
flashing near zero, but the input and output levels on the VU
meters of the ES-8 are hovering at or below -10, you're simply
seeing the difference between the peak and average program level.
When the METER switch is in the INPUT position, the VU meter
indicates the signal level at the XLR and 1/4" input connectors.
The meter is calibrated to 0dB = +4dBu (1.23vrms). Use this setting
to monitor the average program level into the input transformer
and compression tube of the 6386 To minimize control voltage
artifacts in the signal path (especially when using the FAST
mode), the input level should be in the range of -5 to 0VU.
When the METER switch is in the GAIN REDUCTION position, the
VU meter indicates the amount of gain reduction applied to the
input source. Keep in mind that the meter is indicating an average
gain reduction, and does not reflect how the 6386 is responding
to peaks faster than the meter's response time. When the peak
to average ratio is high (e.g. drums), trust your ears to be
the ultimate judge.
A screwdriver - adjustable trim control, located below the METER
switch, is used for zeroing the meter when it is set to read
GAIN REDUCTION. Use a small, flat-bladed screwdriver, and adjust
the meter to indicate 0VU with the THRESHOLD control set to OFF.
Gain Reduction is measured directly from sidechain control voltage,
so very little drift of the meter should occur. Let the unit
warm up for at least 20 minutes before making any adjustment.
When the METER switch is in the OUTPUT position, the VU meter
indicates the signal level at the XLR balanced output connectors.
The meter is calibrated to 0dB = +4dBu (1.23vrms). Use this setting
to monitor the average program level sent to a tape machine,
mixer input or channel insert.
The 6386 incorporates a solid-state sidechain circuit to provide
a wide range of dynamic control. For maximum versatility, there
are three distinctly different modes of operation.
In the FAST mode, the gain control tube operates with very fast
attack and release times (0.5ms and 50ms respectively). This
mode is remarkably free of pumping artifacts even at high compression
levels. It's the ideal choice for tracking or tight program compression.
Use it whenever you don't want to hear the 6386 working, particularly
on vocal tracks.
The PRESETS mode consists of six attack/release settings which
are ideal for program compression. These presets are identical
to those found on the classic Fairchild 670 Limiter. The first
four presets offer a selection of fixed attack and release times,
with the release times indicated on the dial markings. Presets
5 and 6 are program-dependent release times, where there is initially
a quick release, followed by a longer decay time to zero gain
reduction. Think of these settings as a 'gated' release time,
where the compressor operates more rapidly at the average program
level, but takes much longer to return back to zero gain reduction.
In other words, the compressor does not immediately 'suck back
to zero' when there is a brief pause in the program (e.g. between
words in a vocal track).
The MANUAL mode offers total control over the attack and release
times. The range of the attack and release controls is 1ms to
100ms and 0.1s to 2s respectively. Manual control is particularly
useful for creative compression effects or for processing bass
guitar. Quite often, a greater level of compression can be applied
to program material if a very long attack time and a short release
time are used. In this instance, the compressor is responding
to the average program level, does not 'over-compress' on short
peaks, and recovers quickly. In other words, it doesn't 'pump'
On the rear panel are two TRS 1/4" jacks (one for each channel)
for inserting an outboard EQ into the sidechain detector circuit.
This is useful for frequency-dependent compression or de-essing.
The Tip is Send, and Ring is Return.
When this switch is set to SPLIT, the two channels of the 6386
operate independently. In LINK, the sidechain circuits for the
two channels are coupled for stereo operation. The THRESHOLD,
dynamics processing (fast, preset and manual modes) and IN/BYPASS
functions are controlled entirely by Channel 1. The INPUT and
OUTPUT controls for the two channels should be set to the same
Although linking the two channels will keep the stereo image
centered, it may tend to collapse the stereo spread a bit too
much on some program material. In this case, run the two channels
of the 6386 SPLIT, and set all the controls for both channels
to the same values. Listen carefully to verify that the degree
of image shift that occurs is acceptable.
These are 'hard' bypass switches for comparing the processed
sound directly with the original source. IN connects the output
of the 6386 line stage directly to the output connectors. BYPASS
connects the input source directly to the output. When linked,
and channel 2 is set to IN, the IN/BYPASS switch for channel
1 conveniently controls both channels at once.
There are three adjustments to the 6386 that are only accessible
by removing the top cover:
· Calibration of the meter when measuring gain reduction
· Stereo balance of the gain control tubes in LINK
· Push-pull AC and DC balance of each gain control
tube for control voltage rejection.
These adjustments are required only if one of the tubes are
replaced, or if they drift out of calibration with age. Since
these adjustments must be made with the ac power on, and potentially
lethal voltages are present inside the chassis, we recommend
that they be made only by qualified service personnel who are
familiar with working around high voltage tube circuitry.
For your safety, we strongly recommend that you contact us
for servicing. On request, detailed instructions for performing
these procedures will be provided to qualified service personnel.
The 6386 is available with transformer-balanced outputs using
high-quality Jensen transformers. However, we do not recommend
this unless absolutely necessary, since the transformers will
color the sound. If at all possible, try to make it work without
them. See the section entitled 'Preventing Ground Loop Hum' for
Replacing the Tubes
TTO PROLONG TUBE LIFE, TURN THE 6386 LIMITER OFF WHEN NOT
IN USE! All vacuum tubes have a limited life due to reduced
electron emission from the oxide coating on the cathode and/or
a buildup of impurity gases is the bulb. The gain control tubes
in the 6386 are expected to give many years of service. If you
notice the sound quality deteriorating - higher distortion, muddiness,
or microphonic behavior - it's time to change the tubes. We recommend
changing only the tube that is defective. If you are uncomfortable
with replacing the tubes yourself, please have it done by qualified
service personnel. Replacement tubes are available directly from
us. They are rather expensive, but still available in limited
1. Unplug the 6386 and wait at least 30 minutes for the high
voltage in the unit to discharge and for the tubes to cool to
2. Remove the top cover by removing the nine #6-32 Phillips-head
DO NOT remove the bottom cover.
3. Note the position of the two tubes (V1 and V2) in the porcelain
The gain control tubes are 6386 GE (Five Star) miniature tubes
NOTE: THERE ARE NO SUBSTITUTES FOR THIS TUBE!
ONLY THE EXACT REPLACEMENT WILL WORK!
WATCH OUT FOR COUNTERFEITS!
4. Remove the tube and replace it with the same type.
5. Reinstall the top cover and screws.
There is only one variety of the 6386 tube available. It is
a GE five star military tube, with the tube number '6386' etched
into the glass envelope. If '6386' is screen-printed on the glass
but the number is not etched into the glass, it is probably a
counterfeit. The most common counterfeit is a 5670, which is
also a military tube that has the same pin configuration. However,
is not an exact replacement and will not work properly. It is
easy to tell the difference by holding the original tube and
the replacement side-by-side and comparing the internal structure.
Note: In most cases, tube rebalancing, gain calibration and
L/R balancing will be required. Although this not a difficult
process, it should be performed only by qualified service personnel
that are familiar with servicing vacuum tube equipment. Potentially
lethal voltages are present inside the unit. Detailed instructions
for performing these procedures will be provided on request.
Circuit Type: remote cutoff triode gain control tube
with balanced class A solid-state
line stage and transformerless output
Input Sensitivity: +4dBu
Input Impedance: 10k (nominal) with 1:1 input transformer
Freq. Response: -1.0dB 15Hz and 75kHz with 600 output load
Noise: less than -85dB below +4dBu output level with Output
control at unity gain (12:00)
Distortion: less than 0.08% THD+N, 20Hz to 10kHz
Output: +27dBu balanced into 600 load
Max. Gain Reduction: 12dB
Threshold: up to 12dB of gain reduction at -8dBu input level
Ratio: 1:1 to limiting, program-dependent
Output: off to +15dB
Modes: fast, preset or manual operation
Fast: 0.5ms attack, 50ms release
Presets: 1ms/0.1s, 1ms/0.3s, 2ms/1.0s, 4ms/2s,
2ms/1 to 4s and 1ms/0.5 to 20s (program dependent)
Manual: attack time varaible from 1.0ms to 100ms
release time varaible from 0.1s to 2s
Meter: selection of input level, gain reduction, or output
input and output levels are referenced to +4dBu = 0VU
In/Bypass: hard bypass of input signal to output connector
Link/Split: links channel 1 and channel 2 for stereo operation
Polarity: input and output XLR connectors are pin 2 hot
Vacuum Tubes: (2) 6386, GE 5-Star
Power: 120Vac, 30W (240Vac optional)
Power Supplies: +250Vdc, +12.6Vdc and ±18Vdc, fully
with soft-start warm-up and output bypass
Dimensions: 2U enclosure, 19" x 3.5" x 12.5"
(48.2 x 8.8 x 31.8 cm)
Weight: 14lbs (6.4kg)
Pendulum Audio, Inc. warrants to the first purchaser of a
new Pendulum 6386 Tube Limiter that the unit is free of manufacturing
defects in materials and workmanship for a period of one (1)
year from the date of purchase. Pendulum Audio, Inc.'s sole obligation
under this warranty shall be to provide, without charge, parts
and labor necessary to remedy defects, if any, which appear within
one (1) year from the date of purchase. All warranties expressed
or implied made by Pendulum Audio, Inc., including warranties
of merchantability and fitness, are limited to the period of
this warranty. Pendulum Audio, Inc. is not responsible for indirect,
incidental or consequential damages arising from the use or failure
of this product, including injury to persons or property.
This warranty does not cover damage due to: misuse, abuse,
modification, accident or negligence. The warranty does not apply
if the unit is repaired or altered by persons unauthorized by
Pendulum Audio, Inc. in such a manner as to injure, in Pendulum's
sole judgment, the performance, stability or reliability of the
unit. The warranty does not apply if the unit is connected, installed
or used otherwise than in accordance with the instructions furnished
by Pendulum Audio, Inc. There is no warranty on vacuum tubes.
If the equipment requires warranty repair, return authorization
must be obtained from Pendulum Audio, Inc. prior to shipment.
Equipment should not be shipped to Pendulum Audio, Inc. until
return authorization and the proper shipping address is obtained
from us. The equipment (with all its components parts and connecting
cables) must be suitably packaged, including a note with the
owner's name, address, telephone number and a description of
the reason for return. The owner pays two-way shipping (we recommend
UPS or FedEx), and we suggest that the shipment be insured for
its full value.
This limited warranty is in lieu of all other warranties,
expressed or implied, and no representative or person is authorized
to represent or assume for us any liability in connection with
the sale of our products than set forth herein. This limited
warranty gives you specific legal rights, and you may also have
other rights which vary from state to state.