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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== | ||
It's common to find a Type W plugin missing its three type [[8416]] electron tubes. | It's common to find a Type W plugin missing its three type [[8416]] electron tubes. The 8416 is the industrial version of the [[12DJ8]]. 8416 and 12DJ8 are crazy-expensive when you can find them at all. | ||
It is possible to modify a Type W to use [[6DJ8]] (the 6 V version of 12DJ8), which is expensive but readily available. | |||
Studying datasheets, I found that under the conditions in Type W, the [[12AT7]] has almost the same g<sub>m</sub> and r<sub>p</sub> as the 6DJ8. The 12AT7 is cheap and abundant. | Studying datasheets, I found that under the conditions in Type W, the [[12AT7]] has almost the same g<sub>m</sub> and r<sub>p</sub> as the 6DJ8. The 12AT7 is cheap and abundant. | ||
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You can use any type but you can't mix them. | You can use any type but you can't mix them. | ||
Adjust the series resistor and current supply for the type you use. | Adjust the series resistor and current supply for the type you use (the numbers below are for 6DJ8). | ||
The 6DJ8's heater is half the voltage (6.3 V) and twice the current (365 mA). | |||
The 6DJ8's heater is half the voltage (6.3 V) and twice the current (365 mA). | |||
It requires series resistance to drop the extra 19 V, and a power supply for the extra current. | It requires series resistance to drop the extra 19 V, and a power supply for the extra current. | ||
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but some mainframes elevate that rail to +100 V which will put an uncomfortable 200 V between heater and cathode on V124. Besides, Tek used DC to avoid hum. | but some mainframes elevate that rail to +100 V which will put an uncomfortable 200 V between heater and cathode on V124. Besides, Tek used DC to avoid hum. | ||
:A) Add 100 Ω, 5 W in series with 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 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. | ||
:The current regulator is just an LM317 with a resistor between OUT and ADJUST, and | :The current regulator is just an LM317 with a resistor between OUT and ADJUST, and output from ADJUST. | ||
:Two ¼ W resistors in parallel, one 18 Ω, the other 12 Ω, gives 180 mA, raising the total to 360 mA. (Or use two 15 Ω and a 100 Ω.) | :Two ¼ W resistors in parallel, one 18 Ω, the other 12 Ω, gives 180 mA, raising the total to 360 mA. (Or use two 15 Ω and a 100 Ω.) | ||
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. | |||
===2. 12AT7=== | ===2. 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. | ||
:B) Tek used V214 pin 9 as a tie point. It must float when a 12AT7 is used, because it's the heater center-tap. | :B) Tek used V214 pin 9 as a tie point. It must float when a 12AT7 is used, because it's the heater center-tap. | ||
:C) V114B and V214B grids must be biased 3 V positive for successful operation of the Q154-12AT7 cascode. Details below. | |||
===3. Other types — not tested === | |||
===3. | |||
: 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. | ||
=== | ===4. Tube types that do not work === | ||
* 12AU7 | |||
* 12AV7 | |||
* 12AX7 | |||
* 12AY7 | |||
* 12BH7 | |||
=== | ===Heater-Cathode breakdown=== | ||
Most types exceed their V<sub>hk</sub> spec, including the 8416 itself, assuming it's the same as 6DJ8. If Tek can ignore it so can we, I guess. | |||
===Effect | ===Effect of a heater-cathode short on V124=== | ||
*R126 will smoke. | * R126 will smoke. | ||
*Cathode may arc to grid but I don't see it burning anything. | * Cathode may arc to grid but I don't see it burning anything. | ||
*Stuff downstream might get hot. | * Stuff downstream might get hot. | ||
===Transformer power supply for 6DJ8=== | ===Transformer power supply for 6DJ8=== | ||
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One AliExpress candidate is YHDC. If we reverse a 110 V primary, we need at least a 20 VA rating to stay within spec for copper loss. That means a big transformer, and hum unless you shield. | One AliExpress candidate is YHDC. If we reverse a 110 V primary, we need at least a 20 VA rating to stay within spec for copper loss. That means a big transformer, and hum unless you shield. | ||
Specify | Specify 110 V | 110 V primary and 15+15 secondary. Convenient available space in the plugin is 65 mm × 50 mm × 50 mm.\ | ||
Convenient available space in the plugin is 65 mm × 50 mm × 50 mm. | |||
Here are YHDC 20 VA transformers: | Here are YHDC 20 VA transformers: | ||
* PE5424E-M is PCB mount, | * PE5424E-M is PCB mount, 57 mm × 48 mm × 44 mm. Not a toroid. | ||
The following don't fit: | The following don't fit: | ||
*PE5424K-M PCB mount plus holes, | * PE5424K-M PCB mount plus holes, 75 mm × 49 mm × 44 mm. | ||
*LKB5424-L bolt-down with wire leads, | * LKB5424-L bolt-down with wire leads, 75 mm × 43 mm × 50 mm. | ||
*PU3917B low-profile bolt-down, | * PU3917B low-profile bolt-down, 68 mm × 55 mm × 33 mm. | ||
*PTC25 25 VA PCB with hole toroid | * PTC25 25 VA PCB with hole toroid – 60 mm × 60 mm × 38 mm. | ||
===Switching power supply for 6DJ8=== | |||
Several stores sell this. Just search for "sanmim" or "sanmin" and grovel through the hundreds of hits for | The Sanmim SM-PLG06A-24 is a 6 W model, 51 mm × 24 mm × 19 mm, cost $3. | ||
I bought mine at "Your Cee". | Several stores sell this. Just search for "sanmim" or "sanmin" and grovel through the hundreds of hits for 24 V and 5 W or 6 W. I bought mine at "Your Cee". | ||
>>> SUCCESS! | >>> SUCCESS! | ||
Mounted in a steel box, with simple filtering on input ( | Mounted in a steel box, with simple filtering on input (1 kΩ) and output lines (0.33 μF), it causes barely perceptible trace thickening at 1 mV/div. | ||
=== | ===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. | ||
Q154's V<sub>CE</sub> is the tube's grid bias. The 6DJ8 operates at –2.5V, which gives Q154 2 V. The 12AT7 is only –0.5 V, so Q154 starves. | |||
We must give the grids a positive offset. | We must give the grids a positive offset. | ||
I tried a resistive divider | I tried a resistive divider, 100 kΩ/100 nF to 10 MΩ to +350 V. This fails due to grid current from captured electrons, a normal phenomenon in low-bias tubes like the 12AT7. | ||
This fails due to grid current from captured electrons, a normal phenomenon in low-bias tubes like the 12AT7. | |||
Batteries work. One CR2032 in series with R152/R252. | Batteries work. One CR2032 in series with R152/R252. Minus side to Q154 emitter, plus side to the resistor. | ||
Out of RF paranoia I put a cap across each battery. | |||
100 nF axial-lead MLCC fits in the small space. I<sub>g</sub> varies from 10 μA to 70 μA depending on the tube. | |||
Since current is drawn only while the instrument is running, the batteries should last for their shelf life. | |||
I put them in insulated wire-lead holders anyway, to make replacement easy and to keep them from shorting. | |||
The batteries dangle below the ceramic strips to minimize stray capacitance. | |||
I unsoldered 5 terminals (silver-bearing solder!) and levered parts and wires out of the way to reach R152/R252. | |||
It's easier than it sounds. | |||
The batteries dangle below the ceramic strips to minimize stray capacitance. | |||
I unsoldered 5 terminals (silver-bearing solder!) and levered parts and wires out of the way to reach R152/R252. | |||
The grid bias mod is necessary for the 12AT7, but it's compatible with all tube types. | The grid bias mod is necessary for the 12AT7, but it's compatible with all tube types. | ||
[[Category:Instrument repair reports]] | [[Category:Instrument repair reports]] | ||