... on the floor laughing my freaking ass off ...

Basic Tube Facts

  • • tube matching occurs at a single test point
  • • tube match point is seldom operational point
  • • tube response is non-linear as all datasheets show. Responses are curves
  • • response curves are often design parameter and not actual
  • • tubes vary a few percent unit to unit when new and drift as they age
        [The Mullard Story on UBoob is a fascinating insight into thermionic valve complexity]
  • • tubes begin to 'wear out', i.e. change, from the moment they are turned on
  • • tube seals are not 100% and old tubes take considerable time to de-gas


The image below is for 2x Gold Lion KT88 matched quads. It's highly suspect that 8 tubes would match Gm to zero parts in 7400. [All were labeled with Gm of 7400.] Measured after about a year at around 300 hours on a Hickok, they are close, but not identical.

The image below is for 5x JAN6829 measured in a stock M-125 driver board. While they are close, they do vary within a trained listener's detectability.

Power Output

If I had a dollar for everyone whose told me that putting KT120 vs KT88 in an amp results in more power, I could replace tubes annually! Power is determined by the power transformer. FULL STOP!! The power transformer supplies the current and the tubes regulate it. See KT88 6550 KT120 KT150 Tube Interchangeability at Blueglow Electronics on UBoob. [The origin of this myth is likely from replacing worn out 6550/KT88 w KT120.]

Assuming the circuit is properly designed and the tubes are functional, changing tubes may change the gain and/or frequency response. If the SPL is louder at the same input level, almost all perceive it as better.

6550/KT88 vs KT120

  • • The KT120 draws about 15% more heater current, so depending on the heater supply design this may be a benefit or a detriment.
  • • The KT120 datasheet shows curve non-linearities not present in the KT88. Depending on the amp design and the program, this could be a positive or a negative.

Plate Curves

6550 KT88 KT120
Click thumbnails for a larger image.
ESC or Click Upper Right 'X' to close.

The plate curves have lines for I/V values of 100mA/200V to 250mA/300V and 80mA/300V to 220mA/400V. Note that these lines are merely illustrative as they exceed the tube SOA.
One can clearly see the kinks in the KT120 plate curve vs lines drawn at the same I/V slope, relative to the KT88 & 6550. Naturally this alone will make the KT120 sound different and vary with load depending if the kink is rising or falling relative to the load and V/I curve. "Better" is another matter entirely depending on the actual operation point. Whether the designer is better off using 2x 6550/KT88 or 1x KT120 likely to be subjective. Bias will add yet another parameter and shift the operation point along the plate curve.

Most tubes do not exhibit kinks in their datasheet plate curves. However, they have different parameters which consequently affect the sound. As all things audio, performance is Design AND System Dependent. What makes one system dulcet may make another dreadful. IOW, six nines [99.9999%] of all online audio advice relative to your system is highly suspect.

gambling in Monte Carlo

Simulation software often has Monte Carlo analysis. This emulates the circuit at part tolerance extremes for comparison to nominal values. Well designed circuits are relatively immune, but some require close tolerance parts in some areas to function to design.

One reads online of audiophools swapping tubes willy-nilly in their amplifiers searching for audio nirvana. It's largely a fools errand. With the M-125 and its four power tubes, there are 24 ways to arrange in one amplifier. Each will vary slightly.
Ditto 4x 12AU7 in both amps:
{v1L,v2L,v1R,v2R} {v1L,v2L,v2R,v1R} {v1L,v1R,v2L,v2R} {v1L,v1R,v2R,v2L}
{v1L,v2R,v2L,v1R} {v1L,v2R,v1R,v2L} {v2L,v1L,v1R,v2R} {v2L,v1L,v2R,v1R}
{v2L,v1R,v1L,v2R} {v2L,v1R,v2R,v1L} {v2L,v2R,v1L,v1R} {v2L,v2R,v1R,v1L}
{v1R,v1L,v2L,v2R} {v1R,v1L,v2R,v2L} {v1R,v2L,v1L,v2R} {v1R,v2L,v2R,v1L}
{v1R,v2R,v1L,v2L} {v1R,v2R,v2L,v1L} {v2R,v1L,v2L,v1R} {v2R,v1L,v1R,v2L}
{v2R,v2L,v1L,v1R} {v2R,v2L,v1R,v1L} {v2R,v1R,v1L,v2L} {v2R,v1R,v2L,v1L}

Combinations of eight tubes in two amps? 40,230!!!

If you love the way your amps sounds, indelibly identify the tubes so they may be returned to the correct location.