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W/Repairs: Difference between revisions
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==Adapting the Type W plug-in to use 6DJ8 or 12AT7 tubes instead of 8416== | ==Adapting the Type W plug-in to use 6DJ8 or 12AT7 tubes instead of 8416== | ||
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The instrument should be calibrated after either mod. | The instrument should be calibrated after either mod. | ||
=== | ===6DJ8/6922/ECC88/E88CC/7DJ8/PCC88=== | ||
NOTE: These types are often advertised as identical, but their heater current is different. | NOTE: These types are often advertised as identical, but their heater current is different. | ||
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:A) Add 100 Ω, 5 W in series with R290. I put it on a three-point terminal strip bolted to the top of R290. | :A) Add 100 Ω, 5 W in series with R290. I put it on a three-point terminal strip bolted to the top of R290. | ||
:B) Add an isolated current source across V124-V214-V114. | :B) Add an isolated current source across V124-V214-V114. | ||
:Start with either a 6 V - 24 V step-up transformer (details below), rectifier and reservoir, or a shielded switching supply powered by +350 V. | :Start with either a 6 V - 24 V step-up transformer (details below), rectifier and reservoir, or a shielded switching supply powered by +350 V. | ||
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The supply must handle 100 V backfeed if a tube burns out. Just put a diode in series. That's why I regulate current instead of voltage. | The supply must handle 100 V backfeed if a tube burns out. Just put a diode in series. That's why I regulate current instead of voltage. | ||
=== | ===12AT7=== | ||
:A) Add 1.2 kΩ, 2 W in parallel with V124-V214-V114. This bypasses 30 mA around the tubes, reducing the heater current from 180 mA to 150 mA. | :A) Add 1.2 kΩ, 2 W in parallel with V124-V214-V114. This bypasses 30 mA around the tubes, reducing the heater current from 180 mA to 150 mA. | ||
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:C) V114B and V214B grids must be biased 3 V positive for successful operation of the Q154-12AT7 cascode. Details below. | :C) V114B and V214B grids must be biased 3 V positive for successful operation of the Q154-12AT7 cascode. Details below. | ||
=== | ===Other types — not tested === | ||
: 12AZ7 requires supplement and grid bias. Curves == 12AT7. Meets V<sub>hk</sub>. | : 12AZ7 requires supplement and grid bias. Curves == 12AT7. Meets V<sub>hk</sub>. | ||
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: 6BQ7/6BZ7/6BS8/8BQ7A require series R + supplement + grid bias. | : 6BQ7/6BZ7/6BS8/8BQ7A require series R + supplement + grid bias. | ||
=== | ===Tube types that do not work === | ||
* 12AU7 | * 12AU7 | ||
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===Grid bias increase for 12AT7=== | ===Grid bias increase for 12AT7=== | ||
Applied to S/N 2369 and 7111 | Applied to S/N 2369 and 7111. | ||
12AT7 is fine as-is in V124 and V114A/V214A, but V114B/V214B need an additional mod. | 12AT7 is fine as-is in V124 and V114A/V214A, but V114B/V214B need an additional mod. | ||