Matching pre-amp tubes

[Steve Blackdog]

I have a damaged pre-amp tube - one of a matched pair of Sovtek 12AX7s. I am wondering whether there is any actual benefit of matching pairs of pre-amp tubes.

what would the adverse impact be if I replaced the single 12ax7 with a single new Sovtek? Indeed what’s the problem with two completely different pre-amp tubes?

cheers


Steve

[xtian]

Which amp?

The only position where matched triodes in an 12A?7 tube is valuable is the phase inverter. But then again, we're guitarists, not hi-fi purists, so no worries.

[Steve Blackdog]

Which amp?

The only position where matched triodes in an 12A?7 tube is valuable is the phase inverter. But then again, we're guitarists, not hi-fi purists, so no worries.

It is a Blackstar HT60 soloist. Schematic below.

[pdf64]

It is a Blackstar HT60 soloist. Schematic below.

I think there must be more to it than that.

The only position where matched triodes in an 12A?7 tube is valuable is the phase inverter...

Bear in mind that a cathodyne only uses one triode section, and that balances itself (dunno which type this amp uses).

[Enzo]

MAtched PAIR??? Power tubes like 6L6s work together, so there is a rationale for matching them. But two preamp tubes? What are they matching to what? Like in my car, I want my four tires to match each other. But how would I match say my water pump with my alternator?

In your amp, one tube handles maybe the input stages while another runs the reverb. What purpose would be served by "matching" them?

Now if you really meant ONE 12AX7 with matched triodes inside, even then, the two triodes are always doing different jobs, so matching means nothing.

And keep in mind that Fender or anyone else making amps does not use "matched" tubes. 12AX7s come in boxes of 100, and the assembler grabs the next two in th box.

I'd agree that the paired phase inverter circuit - I don't know if yours uses that - is the only place a triode matched 12AX7 make any sense. But even then, the triodes are only part of a circuit, so unless all the other parts are matched/balanced/whatever it won't matter.

Preamp tube matching is a scientific process designed to extract an extra couple of dollars from your wallet for each tube.

[Chuck H]

MAtched PAIR??? Power tubes like 6L6s work together, so there is a rationale for matching them. But two preamp tubes? What are they matching to what? Like in my car, I want my four tires to match each other. But how would I match say my water pump with my alternator?

In your amp, one tube handles maybe the input stages while another runs the reverb. What purpose would be served by "matching" them?

Now if you really meant ONE 12AX7 with matched triodes inside, even then, the two triodes are always doing different jobs, so matching means nothing.

And keep in mind that Fender or anyone else making amps does not use "matched" tubes. 12AX7s come in boxes of 100, and the assembler grabs the next two in th box.

I'd agree that the paired phase inverter circuit - I don't know if yours uses that - is the only place a triode matched 12AX7 make any sense. But even then, the triodes are only part of a circuit, so unless all the other parts are matched/balanced/whatever it won't matter.

Preamp tube matching is a scientific process designed to extract an extra couple of dollars from your wallet for each tube.

This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^

But...

You didn't say this was a guitar amplifier. If it were, for example, a stereo tube amplifier then it might make sense to use matched preamp tubes. ie: each stereo channel has a matched tube for a critical function to balance output. But even this is sort of audiofool stuff because such amps typically have variable resistors in circuit or employ enough NFB to more than make up any differential. A matched "pair" of preamp tubes for a guitar amp makes no sense at all. A single preamp tube with matched triodes for a long tailed pair phase inverter makes a little sense, but, again as noted, only if the rest of the circuit is balanced. Which it never is. The blunt designs actually sort of negate the possibility for guitar amp purposes where distortion and clipping are part of the picture. Further, who says a matched output from a phase inverter is better? For some tonal preferences it may be worse. In fact that's likely since the result would be greater cancellation of 2nd order harmonics. Bah! Snake oil. I do a little "tube rolling" when I build and the two tubes in an amp that make the biggest difference are the first triode (so, actually half a tube) and the long tailed pair inverter. I don't look for balance, per se'. I just go with whichever tube sounds best in there.
There may be an imbalance for all I know. I've never taken enough notes. But I do know that most 12**7 tubes have some differential between the triodes and invariably one from my stash will sound best with a given amplifier. In this case the level of balance OR a specific degree of imbalance is likely responsible. Though I couldn't tell you which.

Enzo put it best by stating that all the triodes in the amp are serving a different function that ABSOLUTELY does not require and cannot be improved with balanced triodes for any reason.

[vintagekiki]

About preamp tube matching & comparison

https://bmamps.com/v01/home/techie-corner/valve-tube-comparison-tests/
Tube Comparison Tests

https://tubedepot.com/pages/tube-comparison-tool
Tube Comparison Tool

https://www.thetubestore.com/matched-sections-service
Preamp Tube Matching

https://www.thetubestore.com/gain-factor
Preamp Tube Gain Factor

https://www.thetubestore.com/12ax7-tube-reviews
12AX7 / 7025 Tube Reviews

https://www.thetubestore.com/12ax7-tube-reviews-chart
12AX7 Tube Comparison

http://www.guitarsite.com/news/features/12ax7-comparison-of-current-made-tubes/

https://thetoneking.com/a-comparison-of-current-production-12ax7-tubes/ ¹⁾

[Mick Bailey]

I have proven to myself without any doubt that tube matching is only valid at the point they are matched. Afterwards the tubes go their own merry way in use and even after an initial burn-in the tubes (or sections if a multiple triode etc) will exhibit some form of drift over time and use. It may be slight, but it's there. I've worked on many Blackstars and never needed to match preamp tubes, or have the triode halves balanced. With my Hi-fi builds I've noticed that even moderately mismatched preamp triodes make no difference to sound balance. A theoretical imbalance doesn't translate into a real-word effect, especially where human hearing and room acoustics come into play.

One particular commercial amp I had in had a control to shift the balance of the PI and the owner had it set fully rotated over in one direction to give the maximum asymmetric clipping and it really did sound best at this setting when turned up loud. I've never bought into the 'Matched Triode PI' thing anyhow. Did any manufacturer other than Mojo-Boutique-Custom builders ever match triodes? To me they sit on the shelf right next to directional cables.

[Steve Blackdog]

Thanks for the replies. Enzo’s answer sums it up for me. That’s why it confused me that so many sellers sell guitar amp specific tube sets with matched pre-amp tubes.

There’s plainly a huge market selling snake oil to the gullible!

[vintagekiki]

snake oil is snake oil
but
matched is matched

[TomCarlos]

Steve.... Thanks for posting this question.

Recently, a guy was insisting that an amp I am selling must have all preamp tubes from the same manufacturer and preamp must be matched. When I heard that, I shook my head. Oh those conspiracy theories to drive down a sale price. At that point in the conversation, I suggested he go buy a new amp and ensure himself a Matcha-Palooza!!

There are some posts I have read where the tech insisted that certain "brands" work better in V1, other brands better for the Reverb, and other brands for the PI. I don't know about that. Again, that is getting too techie for me to investigate.

[Mick Bailey]

Maybe certain brands do work better in some positions; the old E.I. long-plate ECC83s were always too microphonic in certain positions in high gain amps, and some tubes stand up better as a cathode-follower, or in SRPP configuration where the heater/cathode voltage can be excessive. When you look at 12AX7s you get a lot of differences in internal assemblies - short plate, long plate, long plate spiral etc.

[J M Fahey]

There is ONE common use (and itīs not a Guitar amp) where both triode halves NEED to be matched , and thatīs in a PRECISION measuring instrument, the VTVM (Vacuum Tube Multimeter), such as may have been used by Leo Fender himself
when designing or repairing amplifiers, go figure.

The chicken and egg problem appears because you must first build the tube and then measure it.
Unless by extremely sheer chance, halves will NOT be matched , and what will you do? ... crack glass open and pull halves to repack those matching?

And even so, suppose they match incredibly close, say within 5% .... thatīs not enough, measuring on the 100V scale you may have 5V error. Unacceptable.

So *all* VTVM include a Balancing/Matching potentiometer (Zero Adjust).

But then it is justified.
On an amplifier?

5% error means , what? 0.1dB? Completely inaudible.

Basic VTVM schematic, notice te Balancing pot. There is also a full scale pot to adjust calibration.

[vintagekiki]

Matching pre-amp tubes has a reason in HiFi pre-amps where one half of the tube is in one channel and the other half of the same tube is in the other channel.
Pot balance helps as a static balance, but in dynamic mode, the differences may be slightly noticeable.
When matching pre-amp tubes, the anode current is measured for several values of the input signal.

[Mick Bailey]

I mentioned in post #8 - I've never needed to match preamp tubes in hifi amps and none of my customers have ever commented that they heard an imbalance. I would be rare to find someone that had matched hearing between L and R, especially as a person ages. Rooms are not symmetrical, listening positions aren't either. Only an inexpensive amp will have different triode halves in different channels - the accepted method with any decent quality build is is to use separate tubes for each channel. Obviously you don't want a severe mismatch, but within reasonable limits (tube from same production batch and same brand/type) the difference is not an issue. Of course, there are people who claim to hear the difference in a mains cable or fuse - the psychoacoustic nuts. They will hear a difference in anything.