Q: How do you test and match your tubes?
Q: Tell me about NOS / New tubes?
Q: I have some noisy tubes, do I need to replace them?
Q: I have a microphonic preamp tube, do I need to replace it?
Q: I’ve heard Groove Tubes are just repackaged Russian and Chinese tubes, are they worth the price?
Q: Do I need to re-bias the power tubes in my Class A amp?
Q: Can I swap a 12AX7 with a different type of tube?

Q: How do you test and match your tubes?

A: We extensively test and match (as appropriate) all of our vacuum tubes to ensure our customers will get the best quality tubes available!

TUBE TESTING
First, we visually inspect all tubes to ensure pins, internal components and bottle are manufactured properly.

EVERY preamp and power (output) tube is then tested on two testers, per the following.

  • B&K 747 or 650 Mutual Conductance (GM) Tube Tester: used to test preamp, power (output) & rectifier tubes for shorts, grid emission, mutual conductance (Gm) and life
  • VTV Dual Triode Characterizer: is a sophisticated, modern tube tester used to test preamp tubes for triode-to-triode balance, gain, microphonics (noise) and sound quality. It even has a built in speaker, so you can actually hear the noise level of the tube during testing.
  • Maxi-Matcher Digital Tube Tester: used to test power (output) tubes for transconductance, plate current and matching characteristics at real-world amplifier operating voltages (325V - 400V)
TUBE MATCHING
Unless otherwise stated, ALL of our matched power (output) tube duets (2-tubes) and quads (4-tubes) are matched to within 3mA or less plate voltage and 5% or less mutual conductance. Many dealers only match for mutual conductance. When shopping for matched output tubes, ensure that the dealer has tested and matched the tubes for both plate voltage AND mutual conductance!

TUBE LABELING
A label with the detailed test results will be attached to each new tube box. For matched tubes, an additional label will be attached to the tube set with the detailed matching characteristic. So, you can be confident you are getting top quality and well matched new production tubes!
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Q: Tell me about NOS / New tubes?

A: This is a somewhat complex issue, but here are a few thoughts on tubes.

US and western European manufactured tubes, from the 1950's to early 1970's, were definitely manufactured with more consistent processes and better quality than current production tubes (mostly manufactured in Russia, China and eastern Europe today). While new production tube manufacturers are getting better, they still have a ways to go. I'm a degreed engineer and I spent 15-years in the Silicon Valley high-technology industry (mostly building and designing telecom and network switches). In that capacity, I made numerous trips to Asia transferring our technology into manufacturing facilities in China, Hong Kong, Thailand and Singapore. On one of these business trips, I actually found the time to tour a Chinese tube factory and can tell you their manufacturing equipment and processes left a lot to be desired.

True new old stock (NOS) tubes will typically outlast new production tubes at a rate of 2x to 4x, depending on tube type. Tone is in the ear of the beholder and there are exceptions to everything, but in general, to my ears NOS tubes sound warmer, more airy and have more depth than current production tubes. The down side is NOS tubes are harder to find, are more expensive and there are some uneducated and even unscrupulous tube dealers out there selling "used" tubes as NOS.

To my ears, in general, new production tubes tend to sound brighter and some even have more sparkle than NOS tubes. These tubes are generally easy to find and are lower cost. If you are after a brighter, crisper sound and/or are on a more constrained budget, this may be the option for you.

As far as brands. I like NOS Mullard, RCA, GE and JAN output tubes and Mullard, RCA, GE, Brimar, Telefunken and Amperex preamp tubes. Sylvania preamp tubes have also been good, but inconsistent, as I've found a higher than normal percentage of microphonic NOS Sylvania's.

You can get some good new production tubes, the challenge is getting a good one. Since current production manufacturing processes and quality control are less consistent, you can get both good and bad tubes in the same batch. As an example, I recently received a lot (qty-30) 12AX7 tubes. On my tube testers, 16 of them tested really good (excellent gain, good triode-to-triode balance and low microphonics), 6 of them tested fair (good gain, decent balance, fair microphonics), and 8 tested poorly below my standards (poor gain, poor balance and/or high microphonics). The majority of tubes that don’t meet my standards are due to microphonics and some for excessively low gain.

I've had excellent test results (typically less than 2% fallout) with new production Groove Tubes, TAD, JJ, Tung-Sol (Russia) 5881 & 6V6, and Mullard (Russia) tubes and very good results (typically 5% to 10% fallout) with Electro-Harmonix Tubes. SED Winged-C (formerly branded Svetlana) and recent batches of Sovtek tubes have also been pretty good with an overall average 15% fallout. I've had a higher than acceptable fallout rate (25% to 45%!) with most Chinese branded tubes and therefore, do not carry these anymore. I do like and highly recommend the JJ 6V6, EL84/6BQ5, ECC83S, the Groove Tubes 6L6-GE, EL34-M, 12AX7-M, ECC83-S, the TAD 7025HighGrade, EL84-STR, 6L6GC-STR and 6L6WGC-STR, the Electro-Harmonix 12AX7 and 12AY7, the new Russian Tung-Sol and Mullard tubes (all of them), and the Sovtek 5881WXT tubes. I haven’t had much experience with Ruby tubes, so I don’t have an informed opinion of them.

As an added note, I WILL NOT SELL THESE POOR QUALITY “FALLOUT” TUBES. In my shop, these tubes are used for some limited testing, may be used in non-critical tube positions (like a tremolo oscillator) or are just thrown out.

Bottom line, you can get good tubes that are either NOS or new production.

My recommendation is, if at all possible, ONLY purchase tubes (NOS or new production) from dealers who tests each tube themselves. Remember, buying tubes from a dealer who hasn’t tested them, is a gamble at best. Will you get one of the good ones or one of the bad ones?
I myself, test EVERY tube that comes through here on two testers. As an example of what to look for, here's what I test for:
  • B&K 747 or 650 Gm Tube Tester: to test preamp, power (output) & rectifier tubes for shorts, grid emission, mutual conductance (Gm) and life
    •
  • VTV Dual Triode Characterizer: to test preamp tubes for triode-to-triode balance, gain, microphonics (noise) and sound quality
    •
  • Maxi-Matcher Digital Tube Tester: to test power (output) tubes for transconductance, plate current and matching characteristics at near operating voltages (325V – 400V)
    •

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Q: I have some noisy tubes, do I need to replace them?

A: When you say noisy, do you mean a crackling type noise? If so, you should check the tube sockets for loose clamps that are not gripping the tube pins properly. First thing, UNPLUG THE AMP and remove the tube(s), being careful to replace them in the same socket they were removed from. These sockets have a metal clamp that is designed to grip the tube pins. When tubes are removed and replaced the metal clamps lose some of their tension and don't grip tight enough to make a good connection.

If you find loose clamps, you can retention the socket clamps by using a pointed instrument like a dental pick or very small flat head screwdriver. The idea is to gently squeeze the clamps together until they will provide better contact with the tube pin. Be careful not to close the socket clamps so much that the tube will not fit back into the socket.

It is also a good idea to spray a bit of electronics contact cleaner (Caig DeOxit D5 or equivalent) to enhance electrical contact and preserve the metal parts. Especially if you notice any carbon buildup (light colored powdery substance) on the tube pins or sockets, as this condition can also cause crackling.

If that doesn't help, or if you are hearing a ringing type noise, then the tubes are most likely microphonic and need to be replaced.

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Q: I have a microphonic preamp tube, do I need to replace it?

A: Possibly. If the noisy tube is in a gain stage or reverb position, then yes, it should be replaced. However, before you spend money on a new tube, you may have another option. Does you amp have tremolo or vibrato? If so, it most likely has a 12AX7 tube that is used as an oscillator for the tremolo/vibrato circuit. This is the least critical tube position in your amp and one where a microphonic tube will have no effect on the tone.

So, assuming both tubes are the same type (usually 12AX7’s), try swapping the noisy preamp tube with the tube in the tremolo/vibrato position. You may resolve your noisy tube problem and it didn’t cost you anything!

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Q: I’ve heard Groove Tubes are just repackaged Russian and Chinese tubes, are they worth the price?

A: Yes, Groove Tubes (GT) sources their tubes from factories in Russia, China, Slovakia and Serbia. However, GT adds another level of selection and computer-based testing into their process. This further weeds out the lower quality tubes due to the inconsistencies in the new production tube manufacturing processes.

My in-house test data has shown GT to have one of the lowest fallout rates of tubes that don’t meet my standards. GT’s, JJ’s, TAD’s, and Russian Tung-Sol 5881 & 6V6’s, and Mullard tubes have been running around 2% fallout rate, EH tubes around 5% to 10% and Sovtek and SED-C tubes around 15%. I’ve suffered through fallout rates of between 25% to 45% for most Chinese branded tubes … I don’t carry these brands anymore!

Lower fallout rates, translates into lower overall costs for me, and lower prices for you.
As an added note, I WILL NOT SELL THESE POOR QUALITY “FALLOUT” TUBES. In my shop, these tubes are used for some limited testing, may be used in non-critical tube positions (like a tremolo oscillator) or are just thrown out.

For more information on new production tubes, refer to “Tell me about NOS / New tubes?” Q&A.

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Q: Do I need to re-bias the power tubes in my Class A amp?

A: In general, if the amp is running in true Class A operation, it is most likely Cathode Biased (sometimes referred to a "Self-Bias”). Every Class A amp, that I'm personally familiar with, is biased with Cathode Bias. With this design, there is no bias control. The bias is set by the relationship of the grid to the cathode, which in turn is set by the Cathode Resistor. The power tube(s) in a Class A / Cathode Bias amp are generally self-biased and are adjusted as part of the power circuit design.

There is typically no need to service the bias in a Class A / Cathode Bias amp, unless the Cathode Capacitor or Cathode Resistor fails. In this scenario, you'll notice a significant loss or increase in gain, or a hum in the amp.

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Q: Can I swap a 12AX7 with a different type of tube?

A: As far as changing dual triode type tubes, it is usually safe to swap to a different type of dual triode tube in most amp circuits. The family of 9-pin, small bottle, dual triode tubes, typically used in guitar amplifiers, are the 12AX7, 5751, 12AT7, 12AU7 and 12AY7. These tubes have different degrees of gain and can be interchanged to raise or lower the gain and change the sound of most amps.

Here is a quick "gain factor” comparison, from highest gain to lowest gain dual triode tube types.

12AX7 / 12AX7A / ECC83 / 7025 / CV4004 = 100% Gain Factor
5751 = 70% Gain Factor
12AT7 / ECC81 / CV4014 = 60% Gain Factor
12AY7 / 6072A = 44% Gain Factor
12AU7 / 12AU7A / 5963 = 20% Gain Factor

The 5751 is just slightly lower gain than the 12AX7 and typically has less high frequency response. So, it can be used to clean up a harsh sounding or excessively bright amp.

The 12AT7 is commonly used to replace a 12AX7, where the player is looking to lower the gain and improve note definition and clarity.

The 12AY7 has even lower gain and was very popular in the primary driver position in many 1950's Fender amps.

The 12AU7 has a considerably lower gain than the 12AX7, and is sometimes better for use with distortion pedals or applications where very low gain is desired.

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