UPGRADE (June 2003):
Paul's T-Bar Mod for the Manley Variable Mu® Limiter/Compressor VERSION 4

PURPOSE: To provide a replacement for the obsolete 6386 tubes in earlier Manley Variable Mu® units or convert later 5670 units to have the same limiting characteristics of the original 6386 for smoother operation.  

METHOD: To install a converter board and surrounding modifications to enable usage of two 12BA6 or 6BA6 single pentodes wired in triode mode to take over the duties of a single envelope dual triode 6386 or 5670 (per channel).

UNITS AFFECTED: Serial numbers for units that will benefit from this update are (all). See the 

Version Identification page for more info.  

Location of that page online:          http://www.manleylabs.com/techpage/VARIABLEMUVERSIONS.html

INTRODUCTION: The Manley Variable Mu® has been in production for some years

now and has become an indispensable tool in studios since its inception. These units

operate on the same principle as several top-notch vintage tube limiters, and the original

Manley version indeed used the same 6386 input tube as the Fairchild, Gates, and others.

This tube is the one that actually performs the gain reduction; it, more than any other

component, determines the sonic signature of these units.

HISTORY: The 6386 dual triode was designed as a cascode RF/IF gain-controlled

amplifier, and as such, had many characteristics ideally suited for use as a gain-controlled

audio amplifier. However, it seems this tube was used more in commercial/industrial

applications rather than high volume consumer applications (like in radios and TV’s).

Also, it appears that none of these have been manufactured in recent years. The

combination of these two factors means that present supplies of 6386 tubes are quite

limited, expensive, and in many cases unusable due to poor matching, high noise, or

microphonics (which may not have been issues for the tube’s original intended use). Manley

decided in June of 1996 to begin using a type 5670 dual triode which were similar, importantly 

sharing the same pin-out as the 6386 and available in sufficient quantities to allow for testing and 

matching.

ENTER NEW TUBE TYPES: The 5670 equipped Variable Mu’s were well received,

and retained many of the characteristics that made the original version of the Variable Mu® popular. 

They did sound different than the original, especially when pushed past 6db or so of limiting. Even

though some people even preferred the new version with the 5670, it was felt that it would be 

worthwhile to give the users the ability to return to the traditional sound without resorting to use of an

esoteric (or unavailable) tube. We also wanted to furnish an alternative replacement for the 6386 in older

units. The goal was to find a tube that was 1) nearly identical to the 6386 in performance

2) manufactured (or being manufactured) in large quantities 3) able to be retrofitted into

existing units (in the field, if at all possible). This was a tall order, as other available

remote-cutoff dual triodes (such as the 6BC8 and 6ES8) are not close enough in

characteristics to be used as a direct replacement. Enter the 12BA6, which is a remote-cutoff

single pentode. Wired as a triode, this tube has characteristics very close to the

6386 as confirmed by a Tektronix 570 curve tracer, and by A-B listening tests. These

tubes were manufactured in large quantities for use as gain-controlled IF amplifiers in

common radio receivers. The hitch: the 6BA6 (or 12BA6) is a single-section tube; the 6386 is a dual!

ENTER THE ADAPTER BOARD: So why not use two 12BA6 tubes to replace one

6386? There is enough room inside the unit to accommodate two tubes; the pair will

directly replace a 6386 in an older unit or a 5670 in a newer unit with only minor

modifications. The other advantage to this approach is that the tubes can be tested and

installed as a matched pair. We have numerous examples of 6386’s where the two triode

sections inside do not match each other within a usable range and there is nothing we can do about it 

because the two triode halves are locked inside the bottle. So what we did

was to design a 1"x 2" circuit board that has two sockets for the 6BA6 or 12BA6 tubes 

("6" or "12" depending on the heater voltage) that plug in where the original tube went. 

Of special importance in using two single element tubes to replace a dual element tube is the ability

to match and select very well matched tubes not only for each channel's two phase-halves, but also for

both channels. This factor alone, especially when considering that there are really no decent 6386 tubes 

left, allows the "T-Bar Mod" units' performance to easily exceed the older units. There are four major 

versions of the stereo Variable Mu® that have been built since 1994, and consequently three different 

versions of the adapter boards. Refer to the Identifying the Different Versions of the Variable Mu®  

page for specific details as to which board you'll need.

Location of that page online:          http://www.manleylabs.com/techpage/VARIABLEMUVERSIONS.html

“T-BAR” INSTALLATION FOR VARI-MU, VERSION 4, USING REV A2 ADAPTER BOARDS (REVISED 11/05)

1. Remove the 220 ohm resistors located between the 5670 sockets and the input transformers.

2. Refer to the photo on the right. Most Version 4 units use the plastic 5670 tube socket shown in this photo.  This is the correct socket.
Earlier units use a ceramic socket with a shield, as shown in the photo to the right.  This will have to be replaced with the plastic socket. Bend the pins of the new socket in such a  way that the socket sits flush with the pc board.

3. The photo to the right shows the original heater wiring for the 5670
This photo shows the proper wiring for the mod. The wires are simply switched from one side of the 8 ohm resistors to the other. The resistors can be removed, or left in place; they are no longer in the circuit.

4. Replace the 33K- 2 watt resistors next to the “meter cal” trimpots (rear corners of the unit) with 22K-2 watt resistors.

5. Install a wire from the “GND” pads on the adapter boards to the junction of the two 1K resistors located next to the input transformers.

6. Remove the 5670 tubes, and insert the adapter board into the socket. Use a plastic washer between the screw head and the adapter board. Use the 8-32 self-threading screw to secure the adapter board to the existing socket (see below).

7. Replace the meter scales with ones labeled “6BA6/ 12BA6”.

8. Install a matched set of 12BA6 tubes.

9. NOTE: There will be some overall gain loss with the mod as installed

compared with the 5670. If this not acceptable, replace the two 3k57

resistors going to pins 4 & 6 of the 5670 socket with two 6k8 resistors

(supplied with the kit), and jumper the two 3k3 resistors (R2 and R3)

on the adapter board.

CALIBRATION

Follow the standard calibration procedure as found in the Owner's Manual with the following exceptions:

1. Set the front panel “balance” adjustment to the middle of its range.

2. Set the trimpots located on each adapter board for a reading of about 0 VDC at the front panel jacks. Use the front panel adjustment to set the balance at exactly 0 VDC.

3. Adjust the “threshold” controls for 6db of limiting. Adjust the “meter cal” trimpots for a reading of “6” on the scale. 

4. If the meters will not calibrate, or they require turning the “meter cal” trimpots fully clockwise (CW), use a different set of 12BA6 tubes, or compromise a bit on the meter calibration. Running the unit with the trimpots fully CW may cause failure of the A+ power supply.

SOUND COMPARISON: In direct A-B comparisons, the 6BA6/12BA6 version sounds nearly

identical to the 6386. Comparing to the 5670, the 6BA6/12BA6 sounds less "squashed" as the

amount of limiting is increased; consequently, it is possible to obtain more limiting with

fewer artifacts than before. Performance at faster attack times is enhanced as the 6BA6/12BA6 tubes 

can be better matched than before with the dual-triode tubes.

COST AND TURNAROUND TIME: The conversion kit installation can be done by Paul Fargo in 

our Arizona Service Center for $250+ shipping , and includes all necessary parts and labor plus 

re-testing and calibration. This usually takes 2-5 days depending on existing workload. It is

recommended to schedule in advance with Paul for fastest turnaround. We can also supply just the kit 

with installation instructions for $150 plus shipping (and sales tax as necessary) for installation in 

the field by a qualified technician. 

For further info to have us mod your unit or to order this kit, please contact Paul in our service department. This is best done by using the form on the Service Request Page so he will have all the information at hand he will need in trying to help you.

Location of this page online: 

http://www.manleylabs.com/techpage/TBAR.html