POWER TUBE MATCHING

Tubes and Tube Testing

Power tubes are matched by cathode current as a check of tube match. Each tube pair or quartet, (now days called quad), are biased by the old-school cross-over-distortion method. 

Fenders and most other amps are tested for cross-over distortion at our Standard of 440Hz. Marshall amps are tested at 1Khz as this frequency is specified by Marshall.

 Then I measure the Cathode Current and Plate Voltage for the tube. Idle Plate Dissipation, IPD, is then figured by multiplying these together, (P=IE as you may recall…)

 For fixed bias designs the IPD can not exceed 70% of the Max Plate Dissipation. For cathode biased amps the IPD can not exceed ther Max Plate Dissipation.

Tubes are then burned-in at max clean power for 1 hour

Minimum to assure reliability and stability. Tubes that will fail do fail during this test.

Before and after Burn-in all tubes in matched sets are checked for cathode current match.

Any set not meeting these requirements is rejected and the process starts all over again.

We charge 1 Hour bench time, currently $75 or $94 same day for this service.

Tubes sold and biased by TONECRAFT AMP REPAIR carry the manufacture’s warranty,

90 Days to 6 Months depending on the maker.

Class A, single-ended and push-pull cathode biased power tube amps are biased by IPD.

 PRE-AMP TUBES-

 Pre-amp tubes are tested for mutual conductance on a vintage Weston, or Hickok 800

Mutual Conductance tester. Each tube is marked with it’s mutual conductance ,

Also known as transconductance , with it’s figure in Gm, or uMhos. This is a figure which describes the tubes Current Gain and how we rate matched triode section, or “Balanced” Tubes. All preamp tubes carry the manufacture’s full warranty.

Typical tube warranty:

JJ- 90 Days

Ruby- 6 months

NOS- 30 days

We burn-in all tubes before shipment.