W/Repairs: Difference between revisions

==ADAPTING THE TYPE W PLUGIN TO USE 6DJ8 or 12AT7 INSTEAD OF 8416==

The 8416 is the industrial version of the 12DJ8.  8416 and 12DJ8 are crazy-expensive when you can find them at all.<br />

It's possible to modify Type W to use 6DJ8 (the 6V version of 12DJ8), which is expensive but readily available.

Studying datasheets, I found that under the conditions in Type W, 12AT7 has almost the same gm and rp as 6DJ8.<br />

I have three Type W's.  I modded serial number 7465 to use 6DJ8, and 7111 and 2369 to use 12AT7.<br />

In each case, I calibrated before the mod using my three precious 8416's, and again after the mod.<br />

Two instruments failed on bandwidth. (2369 spec is 23MHz, actual is 21MHz.  7111 spec is 26MHz, actual is 25MHz.)<br />

But they did no better with 8416's; performance is equal with 6DJ8 and slightly better with 12AT7.

The two mods are about equal in difficulty.<br />

===1. 6DJ8/6922/ECC88/E88CC/7DJ8/PCC88===

NOTE: These types are often advertised as identical, but their heater current is different.<br />

You can use any type but you can't mix them.<br />

Adjust the series resistor and current supply for the type you use.<br />

The numbers below are for 6DJ8.

The 6DJ8's heater is half the voltage (6.3V) and twice the current (365mA).<br />

It requires series resistance to drop the extra 19V, and a power supply for the extra current.<br />

The supply must be isolated and electrically quiet.

You might think you could just wire them in parallel and run off 6.3VAC,<br />

but some mainframes elevate that rail to +100V, which will put an uncomfortable 200V between heater and cathode on V124.<br />

Besides, Tek used DC to avoid hum.

:A) Add 100 ohm 5W 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.

:Start with either a 6V-24V step-up transformer (details below), rectifier and reservoir, or a shielded switching supply powered by +350V.

:The current regulator is just an LM317 with a resistor between OUT and ADJUST, and take output from ADJUST.

:Two 1/4W resistors in parallel, one 18 ohm, the other 12 ohm, gives 180mA, raising the total to 360mA.  Or two 15's and a 100.

;The supply must handle 100V backfeed if a tube burns out.

;Just put a diode in series.  That's why I regulate current instead of voltage.

===2. 12AT7===

:A) Add 1200 ohm 2W in parallel with V124-V214-V114.  This bypasses 30mA around the tubes, reducing the heater current from 180mA to 150mA.

   

:B) Tek used XV214 pin 9 as a tie point.  It must float when 12AT7 is used, because it's heater center-tap.

: Details below.

===3. OTHER TYPES - NOT TESTED===

 

===HEATER-CATHODE BREAKDOWN===

===TUBE TYPES THAT DO NOT WORK===

===EFFECT OF V124 HEATER-CATHODE SHORT===

 

The best position is the rear bulkhead upper right corner with the long axis vertical.<br />

I put the rectifier/cap/regulator

the rectifier and cap with hot glue to facilitate short leads.<br />

I strung the diode between the cap and the LM317 to avoid a tie point.<br />

at IN, with the trough 8V above OUT.  It's a symmetrical triangle wave.<br />

 

Maybe can use 120-0481-00, used in Type 1A4.<br />

It has a 20V winding that puts out 135mA, should be okay at 180mA since the other winding won't be used.

120-0177-00 (used in Type Z) is 6V-6V, so it's no good.

Maybe a 110V-36V transformer on pins 13-14 with primary and secondary reversed, rectifier, filter, and LM317.<br />

The transformer needs to have low magnetic field because it's right next to the tubes.

One aliexpress candidate is YHDC.<br />

If we reverse a 110V primary, we need at least a 20VA rating to stay within spec for copper loss.<br />

That means a big transformer, and hum unless you shield.<br />

Specify 110V|110V primary and 15+15 secondary.<br />

Convenient available space in the plugin is 65x50x50mm.<br />

Here are YHDC 20VA transformers.<br />

PE5424E-M is PCB mount, 57x48x44mm.  Not a toroid.<br />

:PE5424K-M PCB mount plus holes, 75x49x44mm.

: LKB5424-L bolt-down with wire leads, 75x43x50mm.

:PU3917B low-profile bolt-down, 68x55x33mm.

: PTC25 25VA PCB with hole toroid 60x60x38mm.

===SWITCHING POWER SUPPLY FOR 6DJ8===

Here's a $3 6W model.<br />

Sanmim SM-PLG06A-24.<br />

 

Several stores sell this.  Just search for "sanmim" or "sanmin" and grovel through the hundreds of hits for 24V and 5W or 6W.<br />

I bought mine at "Your Cee".

Mounted in a steel box, with simple filtering on input (1K) and output lines (0.33uF), it causes barely perceptible trace thickening at 1mV.

===GRID BIAS INCREASE FOR 12AT7===

12AT7 is fine as-is in V124 and V114A/V214A, but V114B/V214B need an additional mod.<br />

 

Q154's Vce is the tube's grid bias.<br />

The 6DJ8 operates at -2.5V, which gives Q154 2V.<br />

The 12AT7 is only -0.5V, so Q154 starves.<br />

 

I tried a resistive divider. (100K/100nF to 10Meg to +350V.)<br />

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.<br />

Minus side to Q154 emitter, plus side to the resistor.<br />

Out of RF paranoia I put a cap across each battery.<br />

100nF axial-lead MLCC fits in the small space.<br />

Ig varies from 10uA to 70uA depending on the tube.<br />

the batteries should last for their shelf life.<br />

I put them in insulated wire-lead holders anyway, to make replacement easy and to keep them from shorting.<br />

The batteries dangle below the ceramic strips to minimize stray capacitance.<br />

I unsoldered 5 terminals (silver-bearing solder!) and levered parts and wires out of the way to reach R152/R252.<br />

 

It's easier than it sounds.<br />

The grid bias mod is necessary for the 12AT7, but it's compatible with all tube types.

===End of ADAPTING THE TYPE W PLUGIN TO USE 6DJ8 or 12AT7 INSTEAD OF 8416===