W/Repairs: Difference between revisions

==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.

The 8416 is the industrial version of the [[12DJ8]]. 8416 and 12DJ8 are crazy-expensive when you can find them at all.

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

 

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

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

 

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

The two mods are about equal in difficulty.

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

You can use any type but you can't mix them.

 

Adjust the series resistor and current supply for the type you use.

 

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

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

 

You might think you could just wire them in parallel and run off 6.3 V_AC,

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.

: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

: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.

: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.

: C) V114B and V214B grids must be biased 3 V positive for successful operation of the Q154-12AT7 cascode. Details below.

: 12AZ7 requires supplement and grid bias.  Curves == 12AT7.  Meets V_hk.

: 12BZ7 requires supplement and grid bias.  Meets V_hk.

===Heater-Cathode breakdown===

Most types exceed their V_hk spec, including the 8416 itself, assuming

===Tube types thar do not work ===

 

 

===Effect OF a heater-cathode short on V124===

===Transformer power supply for 6DJ8===

I used a [[120-0252-00]] transformer which is used in Type [[3T77]] and [[3T77A]]. I have several.

I strung the diode between the cap and the LM317 to avoid a tie point.

 

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

I can't see any hum at 1 mV/div and line triggering.

 

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

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

Maybe a 110 V - 36 V transformer on pins 13-14 with primary and secondary reversed, rectifier, filter, and an LM317 would work. The transformer needs to have low magnetic field because it's right next to the tubes.

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 110V|110V primary and 15+15 secondary.

Convenient available space in the plugin is 65 mm × 50 mm × 50 mm.<br />

Here are YHDC 20 VA transformers:

* PE5424E-M is PCB mount, 57 48 44.  Not a toroid.<br />

The following don't fit:

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

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

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

*PTC25 25 VA PCB with hole toroid 60x60x38mm.

Batteries work.  One CR2032 in series with R152/R252.

 

Minus side to Q154 emitter, plus side to the resistor.