Support Report: Replacing Tubes in Your UA Hardware
Arfel Hernandez
Replacing tubes in UA hardware is a simple task. Knowing the symptoms for when the tube needs replacement is a procedure that requires sophisticated frequency analysis and complicated mathematical equations. On the other hand, a pair of “analog ears” will do just fine. Always trust your ears!
Oftentimes the symptom of a bad tube is clear enough: distortion in audio signal like intermittent pops, flickering, spikes, crackle, thin sound and spitting thermal noises. Other times the symptom is even clearer: no output from unit. And very rarely the unit just does not turn on, or blows the fuse on powering up. Everything has a lifetime, and tubes are not the exception.
Tube lifetime can range from a few minutes (trust me, I have seen tubes go bad in less than 3 minutes) to a few years, depending on the use the tube is subject to or imperfections during tube manufacturing process.
Although replacing the tubes looks like, and is, an easy task, extra care should be taken because vacuum tubes operate at very HIGH VOLTAGES and can cause personal injury or property damage.
Extra care should be taken because vacuum tubes operate at very HIGH VOLTAGES and can cause personal injury or property damage.
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. In addition, make sure the tubes are properly seated (see figures 1 and 2), a miss-seated tube can cause several problems to the audio signal once the unit is warmed up.
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| Figure 1. Correct way to insert tube into socket. |
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| Figure 2. Incorrect way to insert the tube into socket. Must be avoided. |
A Primer on Tube Varieties
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 (see figure 3). In the same manner, the ECC83S is equivalent to the more commonly known 12AX7 (see figure 4). The variation in identifiers may be simply the country of origin or some other distinction. Some tubes come with a “JAN” (Joint Army Navy) identifier, which is a more robust, military-grade tube. These are highly desirable in terms of reliability and longevity. The NOS (New Old Stock) identifier is also common in tube-sales circles, and indicates a tube of older manufacturing origin, usually U.S. or British. This also often indicates greater reliability and longevity compared to modern tubes.
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| Figure 3. 12AT7 and ECC81 are equivalent to a 6072. (Manufacturers, from left: GE, Amperex, GE again, J&J) |
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| Figure 4. 12AX7 tubes are known as ECC83S as well. (Manufacturers, from left: J&J, Sovtek? and Electro-Harmonix) |
Replacing Tubes: 6176, SOLO610, 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: SOLO/610 and 2610S tube locations are swapped: V1 is the 6072/12AT7 tube and V2 is the 12AX7/ECC83S tube respectively (refer to figures 5 through 9, below, for allocating tube sockets in each specific unit).
Important: 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. If you specifically wish your unit to be modified for use with the opposite tube configuration, the work MUST be done at Universal Audio.
What is V1 or V2 anyway?
Most electronic equipment is made up of tens, hundreds or even thousands of components, small and big. These components need a common place to be mounted and connected to each other. The place they “get together and do their job” is called a PCB or Printed Circuit Board. A PCB "is used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive substrate (usually a glass epoxy).” In a PCB, 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 North America.) The number in the abbreviation, say R1, represents resistor 1, R2 for resistor 2 and so on. V1 and/or V2 stand for valve (vacuum tube) at locations 1 and 2, respectively.
MODELS
M610
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| Figure 5. M610 model tube locations |
2610 (greyface)
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| Figure 6. 2610 model tube locations |
2610S (silverface)
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| Figure 7. 2610S (grayface) model tube locations |
6176
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| Figure 8. 6176 model with tubes labeled |
SOLO/610
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| Figure 9. SOLO 610 model with tubes labeled |
Microphonics Test: 6176, 2-610 Grayface, 2-610S Silverface and M610
Note: Due to tube variances, extremely loud squeals, spikes and/or unpleasant noises may suddenly appear during this procedure. DO IT AT YOUR OWN RISK!
1. Turn on unit and let it warm up for about 5 minutes or so. Place on cover lid but do not install screws yet.
2. Set the Input Select switch to MIC 500 Ω, gain switch to +10, EQ switches flat, EQ toggles to middle. With no input to the unit, send the output from the unit to any channel on a mixer. Set faders on mixer to about halfway up. DO NOT send output to main monitors. Use headphones only, but DO NOT put on headphones yet.
3. Slowly turn on the level knob all the way up. A squealing sound before the output reaches full level is a symptom of a bad tube. At this point, repeat replacement tube procedures and start test over. If unit reaches full output (as should be the case) then …
4. Put on headphones partially! Gently tap on top of the tubes (BE CAREFUL!) and listen for any pops, spikes, or rattling or noises other than normal hiss. Any noise present in the audio path is a sign of a bad tube. Replace one tube only and repeat entire process. If same results, try the other tube and repeat.
Microphonics Test: SOLO/610
Note: Due to tube variances, extremely loud squeals, spikes and/or unpleasant noises may suddenly appear during this procedure. DO IT AT YOUR OWN RISK!
1. Allow unit to warm up for about 5 minutes or so. Place on cover lid but do not install screws yet.
2. Switch select toggles to 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 faders on mixer to about halfway up. DO NOT send output to main monitors. Use headphones only, but DO NOT put on headphones yet.
3. Set gain and level knobs to about 7. Put on headphones partially! 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. Replace one tube only and repeat entire process. If same results try the other tube and repeat.
Replacing Tubes: LA-610 and LA-610 Signature Edition
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 (at 4691 and above) use a 12AT7 (6072s equivalent). The 6005W tube at location V5 requires more than just a drop-and-play type of change. It needs biasing.
Note: 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. If you specifically wish your unit to be modified for use with the opposite tube configuration, the work MUST be done at Universal Audio.
LA-610
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| Figure 10. LA-610 model with tubes labeled |
Biasing Procedure: LA-610
Note: The following instructions are for the next skill-set level, and require using a waveform (or function) generator and oscilloscope.
1. Set 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 EQs flat, MODE switch to BYP and level, gain and peak reduction knobs fully up.
2. With the waveform generator, send a 1kHz sine waveform signal of about 700 mV (or -10 dB) to the unit’s line input. Connect the output of the unit to an oscilloscope. The output should read about 8.5 VAC (peak-to-peak). Adjust oscilloscope’s VOLT/DIV and TIME/DIV for proper viewing (normal settings are 2 VOLTS/DIV and about 0.5 mS/DIV).
3. Switch mode to COMP. Turn trim pot at location R89 (see figure 11) until output in the oscilloscope reads about 1.2VAC (Pk-Pk). NOTE: Due to component tolerances, expect a +/- 5% deviation in readings; also, due to photocells charging/discharging time, allow a minute or so for output to settle before adjusting R89.
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| Figure 11. R89 is located between the “black box" and the 6005W tube. |
Microphonics Test: LA-610
Note: Due to tube variances, extremely loud squeals, spikes and/or unpleasant noises may suddenly appear during this procedure. DO IT AT YOUR OWN RISK!
1. Set controls as follows. 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 faders on mixer to about halfway up. DO NOT send output to main monitors. Use headphones only, but DO NOT put on headphones yet.
3. Set the level to about 6. Put on 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, or rattling or noises other than normal hiss. Any abnormality in the audio path is a sign of a bad tube. Replace one tube only and repeat entire process. If same results try another tube and repeat.
Replacing Tubes: LA-2A
The tubes for the LA-2A are clearly labeled on the back of the unit (see figure 12). 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 once replaced. No microphonic or noise test is required.
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| Figure 12. Back view of LA-2A with tube locations clearly labeled. |
Biasing Procedure: LA-2A
Note: The following instructions are for the next skill-set level, and require using a waveform (or function) generator and oscilloscope.
1. Loosen up R3 lock nut (see figure 13) and turn R3 pot all the way down. R37 should be fully CW and locked. Set COMP/LIM switch to COMP (down) Peak Reduction knob fully CW.
2. With the waveform generator, send a 1kHz sine waveform signal of about 4.5VAC (Pk-Pk) to the unit’s input. Connect the output of the unit to an oscilloscope and adjust the gain knob until the output reads about 25VAC (peak-to-peak). Adjust oscilloscope’s VOLT/DIV and TIME/DIV for proper viewing.
3. Slowly turn R3 CW until output decreases to about 1VAC (pk-pk) on oscilloscope. NOTE: Due to component tolerances expect a +/- 5% deviation in readings; also, due to photocells charging/discharging time, allow a minute or so for output to settle before adjusting R3.
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| Figure 13. R3 lock nut location. |
Final thought
As easy as a tube replacement looks, you should always consider taking your unit to an authorized service center for repair, especially if the unit is still under warranty. Doing this will allow you to get tubes directly from UA.
Should you have further questions during the tube replacement process, do not hesitate to contact UA customer support for help.
Acknowledgements
Based on test procedures written exclusively for Universal Audio by Gary Hull. (Used with permission).
Reference
1. Printed Circuit Board. Modified January 3, 2008.
Accessed January 4, 2008.
Useful sites
Great variety of tests tones in WAV format.