For those of you lucky enough to own one of Universal Audio's tube-based hardware preamps, channel strips, or compressors, you will eventually need to deal with replacing the tubes in these units. How is this done? What tubes do you need? Read on, my friends, and the answers to your questions will be revealed!

When should I replace the tubes in my Universal Audio hardware?
The answer to this question is quite simple. You should replace the tubes when they have failed. To be honest, there's quite a lot of hype and opinion about how often tubes need to be replaced. As a result, too many good tubes are replaced unnecessarily. The life of a tube can range from a few minutes to a few years, depending on the use, or imperfections during the tube manufacturing process. Basically, you should replace your tube when it goes “bad.”

Why do tubes go bad?
Just remember, everything has a lifetime, and tubes are no exception.

What are the symptoms of a bad tube?
The symptoms of bad tubes are easy to spot… they stay out late at night causing all sorts of trouble, never pay the rent on time, and never call their mother on Mother’s Day!

Ahem, but seriously. Most of the time the symptoms of a bad tube are clear enough: microphony (high pitched whistling), lack of volume, loss of bottom end, a ‘thinning’ of the sound, and loss of all high frequencies. Also, distortion in audio signals like intermittent pops, flickering, spikes, crackle, a thin sound, and spitting thermal noises can indicate a tube that is past its prime. If you have any of these issues it is likely you need to replace your preamp tubes. Other times the symptom is even clearer — there’s no output from the unit at all.

How do I replace the tubes in my Universal Audio analog hardware?
As easy as a tube replacement looks, we always recommend taking your unit to an authorized Universal Audio Service Center for repair, especially if the unit is still under warranty. Some units like the LA-610 and LA-2A require additional biasing after the tubes are replaced. To find the closest authorized Universal Audio Service Center, please contact Universal Audio Customer Support.

However, if you are handy with electronics, and decide to replace the tubes yourself, extra care should be taken. Vacuum tubes and supporting circuitry operate at very high voltages and can cause personal injury or property damage. When in doubt, use a bit of lyrical wisdom taken from a famous song written appropriately by The Tubes … “Don’t Touch Me There!”

Remember, safety first. Voltages in UA tube equipment can reach up to 350V and can be deadly! Always unplug the unit to be serviced and allow it to cool down for a few minutes before removing top cover screws and/or changing any tubes. Also, tube circuit capacitors can store these high voltages for long periods — even when the unit is powered down — so be careful not to touch adjacent circuitry with fingers or tools. 
12AT7 and ECC81 are equivalent to a 6072.
12AX7 tubes are known as ECC83S as well.

The tubes used in your Universal Audio hardware are common parts that can be purchased locally at most music stores or through online tube retailers. It is very common for tubes to have multiple part identifiers, but they are functionally identical. For example, the ECC81 and 12AT7 are equivalent to the 6072 tube. In the same manner, the ECC83S is equivalent to the more commonly known 12AX7. The variation in identifiers may be simply the country of origin or some other distinction. There's a helpful article on the subject over on Wikipedia.

You can also purchase these tubes directly from Universal Audio:

  • Universal Audio Part# 27-0001 / 12AX7 Raw Tube 
  • Universal Audio Part# 27-0002 / 12BH7 Raw Tube
  • Universal Audio Part# 27-0003 / 6AQ5 Raw Tube
  • Universal Audio Part# 27-0004 / 6072 Raw Tube
  • Universal Audio Part# 27-0006 / 12AT7 Raw Tube
  • Universal Audio Part# 27-0007 / EL84 Raw Tube
In the descriptions below, you will notice the cryptic reference to V1, V2, V3, and so on. Most electronic equipment is made up of tens, hundreds or even thousands of components, small and big. In a PCB (Printed Circuit Board), every component needs a reference name and number to clearly identify it. Thus, a standard way of referencing components was developed. An R represents a resistor; a C represents a capacitor; a Q for a transistor, and so on. For vacuum tubes, the letter V was designated. (V stands for valve everywhere but in North America.) The number in the abbreviation, say V1, represents the first vacuum tube, V2 for second vacuum tube, and so on.

Testing Your Tubes

You can test the tubes in your Universal Audio product using the following microphonics testing procedures. Repeat the steps as often as necessary to find the bad tube — it’s mostly just a process of elimination. Due to tube variances, extremely loud squeals, spikes, and/or unpleasant noises may suddenly appear during this procedure. These procedures are performed at your own risk.

Microphonics Test for the Universal Audio 6176, 2-610 Grayface, 2-610 Silverface, and M610

  1. Remove the top cover of the unit. Turn on the unit and let it warm up for about 5 minutes or so. Replace the cover lid, but do not install the screws just yet.
  2. Set the Input Select switch to MIC 500 Ω, Gain switch to +10, EQ switches flat, EQ toggles to the middle. With no input to the unit, send the output signal from the unit to any channel on a mixer. Set the faders on the mixer to about halfway up. Do not send output to the main monitors. Use headphones only, but do not put on the headphones yet.
  3. Slowly turn the level knob all the way up. If you get a squealing sound before the output reaches full level, that’s a symptom of a bad tube.
  4. If the unit reaches full output as it should, then put on the headphones — but just partially! Remove the cover lid, and with the tip of a fingernail, gently tap on top of the tubes (be careful!). Listen for any pops, spikes, rattling, or noises other than a normal hiss. Any noise present in the audio path is a sign of a bad tube.
  5. If you find a bad tube, replace that tube only. Then, repeat the microphonics testing process to make sure everything works properly. If you get more problem results, test any remaining tubes.

Microphonics Test for the 710 Twin-Finity and 4-710d Twin-Finity Preamplifiers

  1. Remove the top cover of the unit. Turn on the unit and let it warm up for about 5 minutes or so. Replace the cover lid, but do not install the screws just yet.
  2. Set the Input Select switch to MIC, 15dB PAD switch to OFF, BLEND control all the way clockwise to TUBE, LOW CUT switch to OFF, and PHASE switch to INø. With no input to the unit, send the output from the unit to any channel on a mixer. Set the LEVEL pot to ‘5’ and the faders on the mixer to about halfway up. Do not send output to the main monitors. Use headphones only, but do not put on the headphones yet.
  3. Slowly turn the GAIN pot all the way up. A squealing sound before the output reaches full level is a symptom of a bad tube.
  4. If the unit reaches full output as it should, then put on the headphones — but just partially! With the tip of a fingernail, gently tap on top of the tubes (be careful!) and listen for any pops, spikes, rattling, or noises other than normal hiss. Any noise present in the audio path is a sign of a bad tube.
  5. Tube replacement for the 4-710d and 710 requires additional calibration due to their tube-to-transitor topologies. With this in mind, we recommend that tube replacement for the 4-710d or 710 should only be performed at a qualified Universal Audio Service Center. To find a service center near you, please contact UA Customer Support.

Microphonics Test for SOLO/610 Preamplifier

  1. Remove the top cover of the unit. Turn on the unit and let it warm up for about 5 minutes or so. Replace the cover lid, but do not install the screws just yet.
  2. Select toggles ON for MIC (up) and Lo-Z (up). 48V, Phase and Pad switches off (down). With no input to the unit, send the output from the unit to any channel on a mixer. Set the faders on the mixer to about halfway up. Do not send output to the main monitors. Use headphones only, but do not put on the headphones yet.
  3. Set Gain and Level knobs to about 7. Put on the headphones partially! With the tip of a fingernail gently tap on top of the tubes, one at a time (be careful!), and listen for any pops, spikes, or rattling or noises other than normal hiss. Any abnormality in the audio path is a sign of a bad tube.
  4. If you find a bad tube, replace that tube only. Then, repeat the microphonics testing process to make sure everything works properly. If you get more problem results, test any remaining tubes.

Microphonics Test for the LA-610, LA-610 Signature Edition, and LA-610 MkII

  1. Set the Gain switch to 0, Select switch to MIC 500, 15dB pad toggle switch off (down), Phase toggle switch in (down), +48V off, Low and High EQs flat, MODE switch to BYP, Peak Reduction knob fully down, and Gain knob fully up.
  2. With no input to the unit, send the output from the unit to any channel on a mixer. Set the faders on the mixer to about halfway up. Do not send output to the main monitors. Use headphones only, but do not put on the headphones yet.
  3. Set the level to about 6. Put on the headphones partially! Gently tap on top of the tubes from V1 through V3 one at a time (be careful!), and listen for any pops, spikes, rattling, or noises other than normal hiss. Any abnormality in the audio path is a sign of a bad tube.
  4. If you find a bad tube, replace that tube only. Then, repeat the microphonics testing process to make sure everything works properly. If you get more problem results, test any remaining tubes.
  5. If you own the LA-610 MkII and suddenly it does not compress the audio signal, try replacing the EL84 (V5) and the 12AX7 (V3) tubes. These tubes are part of the compressor side chain and failure of one or more of these tubes can cause this “no compression” problem.  LA-610/LA-610SE users would replace the 12AX7 (V4) and the 6AQ5 (V5).
  6. The LA-610 series requires biasing if tube V5 needs to be replaced. Read on in the "Replacing and Seating Tubes" section below for more information on how to do this part of the procedure.

Testing for the LA-2A Classic Leveling Amplifier

The LA-2A takes a line level signal, so a microphonics test isn’t necessary. However, the same themes hold true; if you come across a lack of compression or intermittent compression, distortion, a lack of volume, loss of the bottom end, a ‘thinning’ of the sound, or a loss of all high frequencies, a tube probably needs to be replaced. 

  1. There are two tubes in the amplifier section, and two tubes in the compression section. These tubes are easy to access directly from the back panel — no need to unscrew or pop the lid on the LA-2A.
  2. For compression problems, check the tube at the 6AQ5 marking, and the tube at the 12AX7 marking on the center-left side of the unit. Start by replacing the 12AX7 tube. If problems persist, then move onto the 6AQ5.
  3. The LA-2A requires biasing if the V5 tube needs to be replaced. Read the "Replacing and Seating Tubes" section below for more information on how to do this part of the procedure.
  4. For output, noise, or distortion problems, check the 12BH7 tube and the 12AX7 tube just below the T4 module. Be careful, because the 12BH7 tube gets very hot! Care should be taken when touching or removing this tube. The 12BH7 tube will be more likely to cause a lack of output. The 12AX7 will be more likely cause noise, crackle, or hum issues when it fails.

Replacing and Seating Tubes

When replacing the tubes, make sure the tubes are properly seated. A misseated tube can cause several problems to the audio signal once the unit is warmed up.

This is the correct way to insert a tube into the socket.
This is the incorrect way to insert the tube into the
socket. Must be avoided. Notice how the pins are bent.

Replacing Tubes: 4-710 and 710 Twin-Finity

Tube replacement for the 4-710d and 710 requires additional calibration due to their transitor-to-tube topologies. With this in mind, we recommend that tube replacement for the 4-710d or 710 should only be performed at a qualified Universal Audio Service Center. To find a service center near you, please contact UA Customer Support.

Replacing Tubes: 6176 Channel Strip

These units require changing one 12AX7/ECC83S tube at location V1 and one 6072/12AT7 tube at location V2.

Although the 12AT7 tube and the 6072 are interchangeable, there is a small variation in the power circuitry that may reveal subtle differences in gain or distortion levels at full gain. We recommend that if your unit was originally equipped with 6072s, you continue to use that tube. Likewise, if your unit is equipped with 12AT7s, it is ideal to continue using that tube.

6176 Vintage Channel Strip with tubes labeled.

Replacing Tubes: LA-610 MkII Channel Strip

This unit requires one 12AX7 tube at locations V1 and V3, and one 12AT7 tube at locations V2 and V4. In addition, one EL84 tube is required at location V5.

LA-610 MkII Classic Tube Recording Channel with tubes labeled.

Replacing Tubes: LA-610 and LA-610 Signature Edition Channel Strip

This unit requires one 12AX7/ECC83S tube at locations V2, V3 and V4. In addition, one 6072 tube is needed at location V1. In older units (serial numbers below 4691), 6072 tubes are used. Newer serial numbers (4691 and above) use a 12AT7 (6072s equivalent). In addition, a 6005W tube is required at location V5.

Although the 12AT7 tube and the 6072 are interchangeable, there is a small variation in the power circuitry that may reveal subtle differences in gain or distortion levels at full gain. We recommend that if your unit was originally equipped with 6072s, you continue to use that tube. Likewise, if your unit is equipped with 12AT7s, it is ideal to continue using that tube.

The LA-610 series requires biasing if the V5 tube needs to be replaced. We recommend contacting Universal Audio Customer Support and having this replacement done at an authorized Universal Audio service center, but if you want to try it yourself, read the biasing instructions below.

LA-610 / LA-610SE model with tubes labeled.

Biasing Procedure: LA-610

The following instructions are only for advanced customers, and require using a waveform (or function) generator and oscilloscope. (Note: "p-p" refers to peak-to-peak values.)

  1. Set the controls as follows: Gain switch to -10, Select switch to Line, 15dB pad toggle switch off (down), Phase toggle switch in (down), +48V off, Low and High EQ’s flat, MODE switch to BYP. Make sure the Level, Gain and Peak Reduction knobs are fully up.
  2. With the waveform generator, send a 1KHz sine waveform signal of about 700mVp-p (or a -10dB tone) to the unit’s line input.  Connect the output of the unit to an oscilloscope. The output should read about 8.5Vp-p (+9dBu). Adjust the oscilloscope’s VOLT/DIV and TIME/DIV for proper viewing (normal settings are 2 VOLTS/DIV and about .5mS/DIV).
  3. Switch mode to COMP.  Output should drop about 0.55Vp-p to 1Vp-p (or -7dBu to -12dBu), depending on the current state of the bias trim pot. Wait a minute or so for the output to settle before adjusting the R89 trim pot to allow for the photocell's charging/discharging time. 
  4. Turn the bias trim pot located at R89 until the output in the oscilloscope reads about 0.9Vp-p (or -8dBu). Due to component tolerances, a +/- 5% deviation in readings is acceptable. 

Replacing Tubes: SOLO/610, 2-610 Grayface, 2-610S Silverface, and M610

These units require changing one 12AX7/ECC83S tube at location V1 and one 6072/12AT7 tube at location V2. It’s that simple! No further calibration is needed, except a simple test for microphonics.

Note that the SOLO/610 and 2610S tube locations are swapped: V1 is the 6072/12AT7 tube and V2 is the 12AX7/ECC83S tube respectively (refer below for allocating tube sockets in each specific unit).

For the M610, reference designators are printed underneath the tube socket and are not visible. The 12AX7 tube is next to H1 header and the 6072 tube is next to the H4 header.

Although the 12AT7 tube and the 6072 are interchangeable, there is a small variation in the power circuitry that may reveal subtle differences in gain or distortion levels at full gain. We recommend that if your unit was originally equipped with 6072s, you continue to use that tube. Likewise, if your unit is equipped with 12AT7s, it is ideal to continue using that tube.

SOLO/610 Tube Preamplifier with tubes labeled.
2-610 (Grayface) model tube locations.
2-610S (Silverface) model tube locations.
M610 model tube locations.

Replacing Tubes: LA-2A

The tubes for the LA-2A are clearly labeled on the back of the unit. Replacing the 12AX7/ECC83S and the 12BH7 tubes should be a direct swap and does not require opening the unit. 

The 6AQ5 (or 6005W) tube needs biasing after replacement. We recommend contacting Universal Audio Customer Support and having this replacement done at an authorized Universal Audio service center, but if you want to try it yourself, read the biasing instructions below.

Back view of LA-2A with tube locations labeled. Loosen up the R3 Lock Nut
before adjusting the bias to avoid internal damage.

Biasing Procedure: LA-2A

The following instructions are only for advanced customers, and require using a waveform (or function) generator and oscilloscope. (Note: "p-p" refers to peak-to-peak values.)

  1. Loosen up the R3 lock nut and turn the R3 pot all the way down, fully counterclockwise. R37 should be set fully clockwise and locked.  Set the COMP/LIM switch in the down position to COMP.
  2. With the waveform generator, send a 1KHz sine waveform signal of about 4.5Vp-p (+6dBu) to the unit’s line input.  Connect the output of the unit to an oscilloscope and turn the gain and peak reduction knobs fully on. Output will be clipped, almost a square waveform.  Adjust the oscilloscope’s VOLT/DIV and TIME/DIV for proper viewing. Wait a minute or so for the output to settle before adjusting R3 to allow for the photocell's charging/discharging time.
  3. Slowly turn R3 clockwise until output decreases to about 8Vp-p (+11.5dBu) on the oscilloscope. The waveform shouldn't be clipped anymore. Lightly tighten the locking nut at R37, making sure not to touch the pot nut. Use caution! Over-tightening the locking nut can turn the internal pot and short the internal wiring. Due to component tolerances, a  +/- 5% deviation in readings is acceptable. 

Replacing Tubes: 2-LA-2

These units require changing four 12AX7 tubes at locations V3/V4/V8/V9, two 12BH7 tubes at locations V5/V10, and two EL84 tubes at locations V2/V7.

Locations V2 and V7 are not marked on the main board (these are location designators listed in the schematics). The EL84 – V2/V7 sockets are located to the left of the T4 cells at locations T7/T9. 

2-LA-2 model tube locations.