Retro Instruments 176 User manual

Retro Instruments, Inc.
User’s Manual
176
Tube Limiting Amplifier

Retro Instruments, Inc.
176
Tube Limiting Amplifier
User’s Manual
2008 Retro Instruments, Inc.
P.O. ox 5066
Modesto, CA 95352-5066 USA
[phone]

Manual Revisions
Revision Print Date
Initial Release December 29, 2008
Notices
2008 Retro Instruments, Inc.
All rights reserved. No part o this document may be reproduced, transmitted,
transcribed, stored in a retrieval system, or translated into any language in any orm by
any means without the written authorization o Retro Instruments, Inc.
Printed in U.S.A.
User Manual Part Number – 176001-01
XLR, RCA, Xcelite, UA are trademarks o their respective companies.
Retro attempts to provide in ormation that is accurate, complete and use ul. I you
require urther in ormation or ind discrepancies in the text within, please contact Retro
Instruments.
Retro Instruments, Inc.
P.O. Box 5066
Modesto, CA 95352-5066 USA
[phone]

Contents
Section 1 Description
1.1 Your Limiter
1.2 Applications
1.3 Speci ications
1.4 Sa ety Considerations
Section 2 Installation
2.1 Operating Environment
2.2 Power Connections
2.2.1 AC Line Voltage Selection
2.2.2 Fuse
2.2.3 Line Cord
2.3 Input Connection
2.4 Output Connection
2.5 Stereo Link
2.6 Grounding
Section 3 Operation
3.1 Power Switch, Indicators and Interlock
3.2 Ampli ier Active/Bypass Switch
3.3 Input Level Control
3.3 Output Level Control
3.4 Compression Ratio Control
3.5 Attack and Release Controls
3.6 Sidechain High Pass Filter
3.7 Asymmetry Switch
3.8 Meter Select Switch
3.9 Stereo Linked Operation
Section 4 Principles of Operation
4.1 Ampli ier Circuit Board
4.1.1 Input Trans ormer Stage
4.1.2 6BC8 Variable Gain Stage
4.1.3 Interstage Trans ormer
4.1.4 12AX7 and 12BH7 Ampli ier Stage
4.1.5 6AL5 Detector and Ratio Switch
4.1.6 Asymmetry Switch
4.1.7 Sidechain Highpass Filter
4.1.8 Output Attenuator

4.1.9 Test Facilities
4.2 Power Supply Circuit Board
4.2.1 Mains Input
4.2.2 Power Trans ormer
4.2.3 5Y3 High Voltage Recti ier
4.2.4 OB2 Gas Discharge Tube
4.2.5 Bleeder Resistor
Section 5 Alignment and Maintenance
5.1 Gain Reduction Meter Zero
5.2 Internal Balance Test Switch
5.3 Balancing the 6BC8 Gain Reduction Stage
5.4 Balancing the Ampli ier Stage
5.5 Noise and Distortion Tests
5.6 Frequency Response Tests
5.7 Hum and Microphonics
Section 6 Schematics and Drawings
6.1 Board Layouts and Schematics
6.2 Recall Sheet
Section 7 Support and Service
7.1 Service
7.2 Support
7.3 Replacement Parts
7.4 Warranty
7.5 Factory Service Instructions

Section 1 Description
This section provides a general description o the 176 Tube Limiting Ampli ier. Please
review this in ormation be ore installing or operating the 176.
1.1 Your Limiter
The 176 Tube Limiting Ampli ier is designed to provide the quintessential character o
vintage tube compression in the modern recording studio environment. It is modeled
a ter the superb UA 176 limiter and adds new eatures and conveniences. The Retro 176
uses high quality components that should provide many years o trouble- ree service.
Your limiter has been through a ull burn-in and testing procedure at the actory.
Ease o use is key with user- riendly ront panel controls and easily recallable settings.
No matter what settings you use, the great sonic traits o the 176 will enhance your
recordings.
1.2 Applications
The Retro 176 is designed or great versatility in many applications. Because o the way
real tube compression handles dynamics, the 176 enhances dimension, depth and detail.
Independent tracks such as vocal, bass, acoustic guitar, piano ill out nicely without harsh
restraint. The 176 is great or tracking, mixing and multibuss compression applications.
Possibly the best application or the 176 is use on the stereo buss, bu ing it to a glossy
inish. The Retro 176 adjusts tone in a way that equalization cannot.
1.3 Specifications
Frequency Response Flat within 0.5 dB rom 20-20,000 Hz.
Harmonic Distortion Below 1% rom 20-20,000 Hz at 0-15 dB Gain Reduction
Noise Level Greater than 76 dB below normal operating level
Gain Reduction 20 dB o available gain reduction
Maximum Gain 32 dB with input and output controls set maximum
Input Impedance 600 Ohms - loating trans ormer balanced
Output Impedance 600 Ohms - loating trans ormer balanced
Minimum Input Level –26 dBm@2:1 ratio –12 dBm@12:1 ratio
Normal Operating Level +4 dBm
Maximum Operating Level +20 dBm
Maximum Operating Temp. 55° C
Physical Dimensions Standard 19” rack mounted. 2U, 3.5” high, 9.5” deep
Power Requirements 40 Watts at 115/230 VAC, 50/60 Hz.

1.5 Safety Considerations
The Retro 176 is to be used in a metal equipment rack with adequate ventilation. The
vacuum tubes become hot. The AC mains power should be disconnected prior to
servicing. For sa ety, the ront door o the 176 is equipped with an interlock switch that
interrupts the AC mains power when the door is opened. This switch should not be
de eated. Only trained technicians should open the ront door to gain access to the
internal adjustments. Tubes should not be removed or inserted with the power on. An
accidental breakage o the glass envelope can expose high voltage on the plate o the
tube.
This product is grounded through the AC Power Cord. For sa ety, do not li t or remove
the ground rom the AC Power Source.

Section 2 Installation
2.1 Operating Environment
Mount the 176 in a standard 19” equipment rack. Always install the bottom rack screws
irst to support the unit. Please allow room above the top ventilation holes and above the
tubes. Additionally, there are ventilation holes on the sides to draw in cool air. Be aware
o the heat sources in your rack to allow proper cooling. Heat will shorten the li e o
your equipment.
Audio equipment is sensitive to magnetic ields caused by nearby power supply
trans ormers. I you experience 60 Hz. hum in any o your gear, try unplugging adjacent
gear irst.
I you purchased a matched pair with the link panel, mount the panel between the two
units and connect the RCA cables to the Couple jack on each 176.
2.2 Power Connections
2.21 AC Line Voltage Selection
AC Mains Voltage is applied to the IEC standard AC receptacle on the rear o the unit.
Set the Red AC Voltage Selector switch on the rear o the unit to 115 or 230 Volts or
your mains voltage. You can use a small lat screwdriver.
Power Fuse
Voltage Selector
2.2.2 Fuse
The use is a 1A ast blow 3AG type or 115 or 230 Volt Mains Voltage. Disconnect the
power cord be ore changing the use.

2.2.3 Line Cord
You can use a compatible IEC power cable or 100-120 Volts or 200-240 Volts per your
country’s speci ication.
USA Europe Australia
2.3 Input Connection
Audio Input is a standard emale XLR connector. It is trans ormer balanced and loating,
meaning there is no path to ground rom pin 2 and 3. Unbalanced sources connect the
center conductor to pin 2 and the shield to pin 3.
2.4 Output Connection
Audio Output is a standard Male XLR connector. It is trans ormer balanced and loating,
meaning there is no path to ground rom pin 2 and 3. Unbalanced loads connect the
center conductor to pin 2 and the shield to pin 3.

2.5 Stereo Link
An RCA jack on the rear panel allows strapping o multiple 176 limiters. An optional
stereo link panel is available to couple two units.
Coupling Jack
2.6 Grounding
The 176 is primarily grounded through the AC Mains. For sa ety, do not disable the AC
ground. The chassis is also grounded through the rack ears to the rack. The XLR
connectors are grounded on pin 1. Pin 1 is only used to drain the shield o the audio
cable on one end o the cable. The exception to the rule is a microphone cable, where pin
1 is used to carry ground to the microphone case.

Section 3 Operation
3.1 Power Switch, Indicators and Interlock
Upon initial installation, apply power. The red indicator and meter should light
immediately. I not, check power connections. Make sure the ront door is securely
closed. Check that the voltage selector is set to the proper voltage. Finally, check the
use. I the use is blown, the likely cause is the use itsel or a damaged 5Y3 recti ier
tube.
A ter a minute the meter should read near zero in the GR setting. The 176 is now ready
to use.
3.2 Amplifier Active/ ypass Switch
This switch lets you easily evaluate what the 176 is doing. It provides a hard-wired
bypass o the 176 or instant A/B comparisons.
3.3 Input Level Control
The Retro 176 has a ixed threshold. The gain reduction is adjusted by the Input Level
control. The input is optimized or standard +4 dBm line levels to provide 0-20 dB gain
reduction.
3.3 Output Level Control
The Output Level control matches the Output Level o the 176 to the Input Level or
provides less or more ampli ier gain as the user requires.
3.4 Compression Ratio Control
The ratio can be adjusted in our steps rom 2:1 to 12:1. Lower ratios will put a
compression signature over a wider dynamic range. Higher compression ratios are best
to hold the signal level down. The ratio control also changes the threshold. The
threshold axis point is at 10 dB gain reduction. This results in similar gain reduction at

the various ratio settings when doing approximately 10 dB o gain reduction, however at
less amounts o gain reduction the lower ratio settings will start compression sooner than
higher ratios.
3.5 Attack and Release Controls
The Attack and Release controls determine how quickly the dynamic range is adjusted by
the compression. The e ect o these controls can be heard as well as observed in the
meter action. The Retro 176 has extended adjustment o the attack and release
characteristics.
Slow Attack lets more impact through. Fast Attack holds down peaks and sounds more
stressed.
The Release control set aster adds energy and density. Slower settings give gentler
control and retain more o the original source dynamics.
The Attack knob has a pull switch that disables the Retro 176 program-controlled time
constant. The original UA 175B and 176 have a single time constant. By pulling the
Attack knob OUT you can have that mode. The program-controlled time-constant (with
the knob IN) makes the Retro 176 gain reduction more natural, better controlling
dynamics with less distortion.
3.6 Sidechain High Pass Filter
The Sidechain HPF is a very e ective tool to eliminate pumping on low requency
material. It allows the compression to work more e ectively controlling dynamics in the
midrange requencies. Bass requencies will pass through with little or no control, so it
really depends on your application. The Sidechain HPF is ideal or vocals. It is disabled
by a switch at the minimum setting.
3.7 Asymmetry Switch
Many voices and instruments are highly asymmetrical wave orms. The Asymmetry
Switch selects whether the detector is ull-wave, detecting both positive and negative
sides o the wave orm or hal -wave, which detects only one side.
With the Asymmetry switch set to + or −, you generally hear the compression open up.
You will notice the most e ect on signals that look spiky on your editor screen.
Generally the spikes are acing one direction, up or down. The other direction may be
smooth. The Asymmetry Switch determines which edge o the wave the compression
will ride on.

3.8 Meter Select Switch
The Meter Select switch provides metering o the Input and Output levels and the Gain
Reduction. The Input and Output meters are re erenced to a standard operating level o
+4 dBm. It is simple to check your input and output levels or easy setup and A/B
comparisons with the Bypass switch. The Input Meter is ed directly rom the Input
XLR. The Output Meter is ed directly rom the 176 Output Control be ore the Ampli ier
Bypass switch.
Normally the meter is kept in the GR position.
3.9 Stereo Linked Operation
Stereo linked operation is generally only desired when the stereo image must not wander.
For example, when dialogue is slightly panned and you are using heavy amounts o
compression. I the 176’s are unlinked, the panning would tend to move to the center.
For contemporary music recording, we pre er the 176’s unlinked. The unlinked stereo
pair actually enhances the stereo cues and create a wide soundstage. Instead o being
heavy-handed on the mono mix, they have better dexterity to reward the dynamics o the
independent le t and right channels.

Section 4 Principles of Operation
4.1 Amplifier Circuit oard
4.1.1 Input Transformer Stage
The Input rom the Female XLR Input connector eeds the bypass side o the Bypass
switch and the input pad and input trans ormer. The input pad provides 20 dB o loss and
trans orms the 600 Ohm input to the proper 120 Ohm trans ormer impedance. The input
trans ormer eeds the grids o the 6BC8 variable-mu gain reduction stage through the
Input Level control in a balanced 200K Ohm arrangement.
4.1.2 6 C8 Variable Gain Stage
The 6BC8 gain is determined by a negative bias applied to the grids through the Input
Level control. This control voltage is derived rom the detector and timing circuitry. The
6BC8 stage is powered by a regulated power supply using the OB2. This circuit is
balanced with VR2 across the cathodes and VR3 across the plates.
4.1.3 Interstage Transformer
The interstage trans ormer separates the gain reduction stage rom the ampli ier stage. It
provides a distinctive load or the variable gain stage to work into. Alternatively, the
interstage trans ormer can be bypassed which changes the tone and character o the
compression.
4.1.4 12AX7 and 12 H7 Amplifier Stage
The ampli ier stage is a ully push-pull circuit. The irst stage, a 12AX7 and the second
stage, a 12BH7 are closely tied through a dual balanced negative- eedback loop. This
provides gain and balance stability and provides low distortion and a low virtual driving
impedance or the output trans ormer. VR4 trims the balance o this ampli ier and serves
as a simple way to veri y correct operation.
4.1.5 6AL5 Detector and Ratio Switch
The output trans ormer has our sets o input taps to accommodate the our ratio settings.
Each set o taps is brought out to the ratio switch. The ratio switch then eeds the 6AL5
ull wave recti ier through the Sidechain Highpass ilter network and Asymmetry mode
switching. Bias or setting the compression threshold and ratio is derived rom a resistive
ladder also on the ratio switch. The 6AL5 detector output eeds the attack/release
controls and timing capacitors be ore driving the gain reduction stage.

4.1.6 Asymmetry Switch
The asymmetry switch raises the bias and threshold o the negative or positive side o the
detector depending on the position o the switch.
4.1.7 Sidechain Highpass Filter
The Sidechain HPF applies additional resistance across the detector input coupling
capacitors e ectively creating a irst order highpass ilter. The ilter is disabled through a
switch when the ilter control is set at minimum.
4.1.8 Output Attenuator
The Output Level is implemented immediately a ter the output trans ormer. The Output
control is integrated into a T-Pad that provides proper loading to the output trans ormer
and attenuates the output to the required user setting.
4.1.9 Test Facilities
The Retro 176 has an integral balance testing system. The static test injects a common-
mode 60 Hz signal into the push-pull ampli ier. The dynamic test injects a dc voltage
with 60 Hz component to observe balance during gain reduction. It allows the technician
a way to balance the ampli ier without test equipment. See the Aignment and
Maintenance section or procedures.
4.3 Power Supply Circuit oard
4.3.1 Mains Input
The AC Power enters the 176 through the IEC power receptacle. It travels to the use,
the door interlock switch and the ront panel User Power switch. Finally it reaches the
Voltage Selector switch and the Power Trans ormer.
4.3.2 Power Transformer
The Power Trans ormer provides several voltages that are used in the 176. It has
additional shielding and low losses to operate cool and e iciently. It accepts 120 or 240
Volts at 50/60 Hertz. It provides 5 Volts and 600 Volts to the high voltage recti ier. It
also has a center-tapped 6.3 Volt 3 Amp output or the ampli ier tube ilaments.
4.3.3 5Y3 High Voltage Rectifier
The 5Y3 is a commonly available ull-wave power recti ier. The original design used a
5AR4 that can also be used and provides a slower turn-on to allow the ilaments to warm
up. A common ailure is arcing in the tube, which can blow the power use.

4.3.4 0 2 Gas Discharge Tube
The 0B2 is a cold-cathode, glow discharge tube. In combination with a 10K Ohm
resistor it provides a regulated 108 Volts to the 6BC8 gain reduction stage. It will glow
orange or purple depending on the variety. This tube can ail two ways; it stops glowing
and the gain reduction meter rises to +3, or the light dances in the tube causing a rumble
or squeal in the audio signal. With normal gain reduction this tube will dim slightly.
This is normal.
4.3.5 leeder Resistor
There is a 270K Ohm bleeder resistor across the irst power supply capacitor. In the
event o a choke ailure, the input iltering capacitor will discharge. Under normal
conditions the power supply will not carry residual voltage seconds a ter power is
interrupted. Always check the power supply capacitors or stored voltage when
per orming repairs.

Section 5 Alignment and Maintenance
Balance Test Switch
VR1 VR2 VR3 VR4
Meter Zero Cathode Bal. Plate Bal. Amp. Bal.
All adjustments should be performed by a qualified technician.
5.1 Gain Reduction Meter Zero
Use a small Xcelite Greenie or precision adjustment tool to make internal adjustments.
The Gain Reduction Meter can be adjusted to zero to compensate or normal aging o
components over time. With no input applied and the release control set to 50, adjust
VR1 so that the meter sits exactly at the 0 dB mark. The Release control can have a
slight e ect on the zeroing. The asymmetry switch should have no e ect on the meter
zero. I it does, the 6AL5 tube may be at ault.
5.2 Internal alance Test Switch
The Internal Balance Test Switch has three settings. For normal operation, it must be in
the center position. The Top position (static test) is used to set VR2 and VR4. The
Bottom position (dynamic test) is used to set VR3. The Meter Zero VR1 may have to be
trimmed a ter balancing.
5.3 alancing the 6 C8 Gain Reduction Stage
Poor balance will be apparent on compression attack, causing a ripple sound or a thump.
Set the Meter Selector to Output. Set Interstage Switch to IN. Set Release to 100 ( ast).
Set Output Level to 100 ( ull). Set Balance Test Switch to Up (Static Test).
Adjust VR2 or minimum indication on the VU meter.
Set Balance Test Switch to the Bottom position.

Adjust VR3 or minimum indication on the VU meter. Sometimes this will not be a ull
dip. Only an extremely matched 6BC8 will dip ully.
Set Balance Test Switch to Up (Static Test). Adjust VR2 or minimum indication on the
VU meter. The meter should dip to −20 dB at the null. Continue to the next step to
balance the ampli ier stage.
5.4 alancing the Amplifier Stage
Only per orm this adjustment a ter balancing the 6BC8 stage. Continue by setting the
Interstage Switch to OUT. Adjust VR4 or minimum indication on the VU meter. The
meter should indicate −10 dB or less. I it does not, suspect the 12AX7 and 12BH7. An
unbalanced ampli ier can cause thumping on attack in the Interstage OUT operating
mode.
Upon completion o balancing, BE CERTAIN TO PLACE THE TEST SWITCH BACK
IN THE CENTER POSITION or you will hear hum modulating the audio signal.
5.5 Noise and Distortion Tests
To check the noise o the 176, send a 1 KHz tone into the 176 and adjust or 3 dB o
compression. Compression Ratio should be set to 12:1. Adjust the Output Level or a 0
dB re erence on your analyzer. Remove the input signal and decrease the Input Level to
0. Read the residual noise on the analyzer. It should be less than –76 dB. I it is not,
remove the 6BC8 and see i the noise remains. I the noise vanishes, suspect the 6BC8
tube or 0B2 tube. I the noise remains, suspect the 12AX7 or 12BH7.
Distortion can be measured by eeding +4 dBm signal into the 176. Adjust the Input
Level or the desired amount o gain reduction, then measure the distortion at the output.
Higher amounts o gain reduction increase the distortion slightly. Faster Attack and
Release times cause increased distortion at low requencies.
5.6 Frequency Response Tests
Frequency response tests should be per ormed below the threshold o gain reduction to
keep the compression rom adjusting the ampli ier gain. Frequency response should be
lat within +/ − 0.5 dB rom 20-20,000 Hz at all settings.
5.7 Hum and Microphonics
Hum in the 176 is generally due to tube degradation or ailure. Many tubes don’t make it
out o the gate, so remember that when replacing them. Tubes used in the 176
ampli ication are dual-section. I they are not matched well or one side ails be ore the
other, the 60 Hertz AC ilament voltage can induce hum into the ampli ier.
Noise tests will show that the 176 has e ects o microphonics like all tube equipment.
Microphonics are caused by vibration in the tubes. Sometimes tubes can whistle, ring,

squeal or make other space-like sounds. Mostly, they will make sound when you tap
them. A healthy tube may make a tink or bonk sound or nothing at all. An excessively
microphonic tube will make spastic sounds when tapped which can help isolate an
intermittently noisy tube. The 6BC8 will always exhibit some microphonics because it
is the irst stage o ampli ication and has a relatively tiny signal passing through it.

Section 6 Schematics and Drawings
6.1 oard Layouts and Schematics