Modifying W Plugin to use 6DJ8 Tubes


Dave Wise
 

Addendum. Pictures are in album "Type W Plugin 8416-to-6DJ8 Conversion".


Dave Wise
 

6DJ8 CONVERSION SUCCESS

CAUTION: I prefer a constant-current supply, even though you can't
interchange 6DJ8/7DJ8/PCC88/E88CC/6922 due to heater current.
They all have to be the same and the regulator set for that type.
You can also do a constant-voltage supply, but a Nuvistor Type W
will see more drift with mains voltage. (FET Type W doesn't care
either way.)

1) Change R290 from 150 ohm to 250 ohm.
2) Add isolated current source across V124-V214-V114.
It's just an LM317 with a resistor between OUT and ADJUST,
and take output from ADJUST. Two 15 ohm 1/4W resistors in
parallel gives 170mA, close enough. This gives the 6DJ8's
350mA so they drop about 18V.
3) Annotate new tube types.

You have to handle 100V backfeed if a tube burns out. Just put
a diode in series.

I've tried a few sets of 6DJ8, and everything makes 26-27MHz
vs 27-28MHz on the 8416's. I thought they were identical.
FWIW, Sylvania made a single datasheet for 6DJ8/7DJ8/12DJ8.

OTHER 6V TUBES

6BQ7 and 6BS8 fail with Q154 starved.
6ES8 gives Q154 2-3V but only makes 17MHz, same as 12AU7. What's up with that?
Looks like 6DJ8 is *the* way to meet spec.

TRANSFORMER POWER SUPPLY FOR 6DJ8's

6V-24V WORKS

I fitted S/N 7465 with 120-0252-00, which is used in Type 3T77 and 3T77A.
I have several. The best position is the rear bulkhead upper right
corner with the long axis of the core vertical. I can't see any hum
at 1mV and line triggering. I put the rectifier/filter/regulator
high on the rear bulkhead to the left of the transformer, securing
the rectifier and cap with hot glue to facilitate short leads. I
strung the diode between the cap and the LM317 to avoid a tie point.
With 200uF/50V (two 100's in parallel), I see 4V peak to peak ripple
at IN, with the trough 8V above OUT. It's a symmetrical triangle wave.

OTHER TRANSFORMERS

TEK

Maybe can use 120-0481-00, used in Type 1A4. It has a 20V winding
that puts out 135mA, should be okay at 180mA since the other
winding won't be used.

OTHER

Nobody sells a 6V-24V transformer AFAIK.

Maybe a 110V-36V transformer on pins 13-14 with
primary and secondary reversed, rectifier, filter, and LM317.
We need at least a 20VA rating to stay within spec for copper loss.
That means a big transformer and hum shielding.
Specify 110V|110V primary and 15+15 secondary.
Convenient available space in the plugin is 65x50x50mm.
Here are YHDC 20VA transformers.

PE5424E-M is PCB mount, 57x48x44mm. Not a toroid.
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 25VA PCB with hole toroid 60x60x38mm.


Dave Wise
 

Correction to previous post number 179890. With 12AU7 I see 17MHz not 22MHz. I was counting graticule minor divisions wrong.


Dave Wise
 

Correction to previous post number 179890. With 8416 I see 28MHz not 33MHz. I was counting graticule minor divisions wrong.


Dave Wise
 

Background:


Production below S/N 7000 uses a pair of 8056 Nuvistor at the input.

At 7000 they are replaced by a dual JFET.


V114 is on the A (plus) side, V214 is on the B (minus) side.??

One section is a cathode follower that bootstraps the first-stage amp anode or drain,

the other section is the top half of the second-stage hybrid transistor-tube cascode amp.


V124 is current sinks on the first-stage cathodes or sources.


Update:


I'm still investigating powering 6DJ8's, but I also experimented with a couple of common 12V tubes.


First I tried 12AT7. It's a no-go. The cascode top's grid bias is Q154's Vce. 12AT7 is only 0.5V, so Q154 doesn't work.


Then I tried 12AU7.

Like 8416, it's about 2V and Q154 is happy.


12AU7 works! Albeit with reduced 3dB bandwidth.


I see 22MHz with 12AU7, and 20ns rise time. Compare with the spec: 26MHz and 14ns.

I see a flat top on the square wave. I didn't bother adjusting amplifier HF comp. Maybe I can speed it up.

I see 33MHz with 8416.


Common-mode rejection spec is 500:1 at 500kHz; both tubes make 1000:1.

(Just for fun I tried 20MHz. Atrocious with either tube. Don't throw out your Type G.)


It's a simple mod. Unless you're a purist or a stickler for specs, I say go for it.


To modify your Type W to use 12AU7:


1) Cut off XV214 pin 9 solder tail. For assembly convenience Tek ran a component lead through it.

This 8416 pin is internal shield but for 12AU7 it's the heater center tap and must be disconnected.

To reach the solder tail to cut it, I had to lift the grounded terminal next to the socket, with a skinny right-angle #1 Phillips.

This step is backward compatible.


2) Add 1200 ohms 2W across the V114-V214-V124 heater string. I hooked one end to the chassis end of R290 and, with a wire, extended the other to XV124 pin 4 . To pass shake test, add a tie point at the flying end of the resistor.


3) Adjust DC Level. I hit spec with R280 at minimum R, i.e. full CW. (Same as 8416.)


4) Adjust amplifier HF comp. I have not done this.


Dave Wise


My signal sources were HP 8640B and HP 8601A. Using an HP 436A/8482A power meter I confirmed less than 0.5dB variation from 1MHz to 35MHz.

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Dave Wise via groups.io <david_wise=phoenix.com@groups.io>
Sent: Tuesday, February 23, 2021 9:28 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Modifying W Plugin to use 6DJ8 Tubes

In post #108873 (https://groups.io/g/TekScopes/message/108873<https://groups.io/g/TekScopes/message/108873>), David Hess suggested wiring 6DJ8's in parallel and running them off 6.3VAC from pin 13-14.

Unfortunately, that's out. Pin 13-14 is elevated to +100V and that will exceed the heater to cathode voltage rating on V124.

To recap, today I see exactly two approaches.

One, reverse a transformer. Unless you get lucky and find one with 6V and 18V windings like the 1A4's 120-0481-00, you have to wind your own, or put up with wire sized for 110V, i.e., at least 20VA even though the load is less than 4W.

Two, run a switching power supply off +225V. I will investigate this if the 12AT7 sub fails.


Dave Wise
 

In post #108873 (https://groups.io/g/TekScopes/message/108873), David Hess suggested wiring 6DJ8's in parallel and running them off 6.3VAC from pin 13-14.

Unfortunately, that's out. Pin 13-14 is elevated to +100V and that will exceed the heater to cathode voltage rating on V124.

To recap, today I see exactly two approaches.

One, reverse a transformer. Unless you get lucky and find one with 6V and 18V windings like the 1A4's 120-0481-00, you have to wind your own, or put up with wire sized for 110V, i.e., at least 20VA even though the load is less than 4W.

Two, run a switching power supply off +225V. I will investigate this if the 12AT7 sub fails.


Dave Wise
 

?I don't consider $30 a pop "rational".

Anyway, improvising and adapting are sports I enjoy.


Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Bill Perkins via groups.io <sales=pearl-hifi.com@groups.io>
Sent: Sunday, February 21, 2021 9:28 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Modifying W Plugin to use 6DJ8 Tubes



On 21/02/2021 10:19 PM, Bill Perkins wrote:
I know, it's seven years later. But I just acquired a Type W in the usual state, to wit, 8416's stolen.

It looks to me like there's enough empty space to fit a small mains-powered 18V regulated supply.

6DJ8 heater is 360mA vs 180mA 8416.
Normally R290 drops 77-50 = 27V at 180mA. We need to drop 20V more.

1) Add R9290 110 ohm 5W (20V at 180mA) in series with R290
2) Add isolated regulated 18V 6.3VAC-powered supply
across V124-V214-V114. It will put out 180mA, raising
V124-V214-V114 to its nominal 360mA so they will drop their
nominal 18V.

This can be done with a 24V transformer on pins 13-14 with
primary and secondary reversed, rectifier, filter, and LM317.
The transformer needs to have low magnetic field because it's
right next to the tubes.

Unless a miracle drops a few 8416's in my pocket, I might try this sometime.
But I'll try 12AT7s first.
I wouldn't, those parts aren't even remotely alike. The 'DJ has half
the mu and three times the gm.

Bill
8416s on eBay for rational money compared to the rest of the offerings:

https://tinyurl.com/sg6acmw1<https://tinyurl.com/sg6acmw1>

Bill


Dave Wise
 

?They're being used as: 1) current source, 2) cathode follower, 3) top half of a cascode. Within limits, mu and gm aren't critical. It is so easy to try I'd be ashamed not to.?


Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Bill Perkins via groups.io <sales=pearl-hifi.com@groups.io>
Sent: Sunday, February 21, 2021 9:19 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Modifying W Plugin to use 6DJ8 Tubes

I know, it's seven years later. But I just acquired a Type W in the usual state, to wit, 8416's stolen.

It looks to me like there's enough empty space to fit a small mains-powered 18V regulated supply.

6DJ8 heater is 360mA vs 180mA 8416.
Normally R290 drops 77-50 = 27V at 180mA. We need to drop 20V more.

1) Add R9290 110 ohm 5W (20V at 180mA) in series with R290
2) Add isolated regulated 18V 6.3VAC-powered supply
across V124-V214-V114. It will put out 180mA, raising
V124-V214-V114 to its nominal 360mA so they will drop their
nominal 18V.

This can be done with a 24V transformer on pins 13-14 with
primary and secondary reversed, rectifier, filter, and LM317.
The transformer needs to have low magnetic field because it's
right next to the tubes.

Unless a miracle drops a few 8416's in my pocket, I might try this sometime.
But I'll try 12AT7s first.
I wouldn't, those parts aren't even remotely alike. The 'DJ has half
the mu and three times the gm.

Bull


Bill Perkins <sales@...>
 

On 21/02/2021 10:19 PM, Bill Perkins wrote:
I know, it's seven years later. But I just acquired a Type W in the usual state, to wit, 8416's stolen.

It looks to me like there's enough empty space to fit a small mains-powered 18V regulated supply.

6DJ8 heater is 360mA vs 180mA 8416.
Normally R290 drops 77-50 = 27V at 180mA. We need to drop 20V more.

1) Add R9290 110 ohm 5W (20V at 180mA) in series with R290
2) Add isolated regulated 18V 6.3VAC-powered supply
across V124-V214-V114. It will put out 180mA, raising
V124-V214-V114 to its nominal 360mA so they will drop their
nominal 18V.

This can be done with a 24V transformer on pins 13-14 with
primary and secondary reversed, rectifier, filter, and LM317.
The transformer needs to have low magnetic field because it's
right next to the tubes.

Unless a miracle drops a few 8416's in my pocket, I might try this sometime.
But I'll try 12AT7s first.
I wouldn't, those parts aren't even remotely alike. The 'DJ has half
the mu and three times the gm.
Bill
8416s on eBay for rational money compared to the rest of the offerings:

https://tinyurl.com/sg6acmw1

Bill


Bill Perkins <sales@...>
 

I know, it's seven years later. But I just acquired a Type W in the usual state, to wit, 8416's stolen.
It looks to me like there's enough empty space to fit a small mains-powered 18V regulated supply.
6DJ8 heater is 360mA vs 180mA 8416.
Normally R290 drops 77-50 = 27V at 180mA. We need to drop 20V more.
1) Add R9290 110 ohm 5W (20V at 180mA) in series with R290
2) Add isolated regulated 18V 6.3VAC-powered supply
across V124-V214-V114. It will put out 180mA, raising
V124-V214-V114 to its nominal 360mA so they will drop their
nominal 18V.
This can be done with a 24V transformer on pins 13-14 with
primary and secondary reversed, rectifier, filter, and LM317.
The transformer needs to have low magnetic field because it's
right next to the tubes.
Unless a miracle drops a few 8416's in my pocket, I might try this sometime.
But I'll try 12AT7s first.
I wouldn't, those parts aren't even remotely alike. The 'DJ has half the mu and three times the gm.

Bull


snapdiode
 

There are interesting devices on ebay

https://www.ebay.ca/itm/ECC83-12AX7-12AU7-To-6N2-6N6-6DJ8-Vacuum-Tube-Amp-Convert-Socket-Adapter/233701614445?hash=item3669b3a36d:g:UZEAAOSwiSBfT1SR

Maybe you can find something there that can help? If only to alert you to the presence of such adapters. There seems to be some with extra wires to bring in filament power.


Dave Wise
 

I know, it's seven years later. But I just acquired a Type W in the usual state, to wit, 8416's stolen.

It looks to me like there's enough empty space to fit a small mains-powered 18V regulated supply.

6DJ8 heater is 360mA vs 180mA 8416.
Normally R290 drops 77-50 = 27V at 180mA. We need to drop 20V more.

1) Add R9290 110 ohm 5W (20V at 180mA) in series with R290
2) Add isolated regulated 18V 6.3VAC-powered supply
across V124-V214-V114. It will put out 180mA, raising
V124-V214-V114 to its nominal 360mA so they will drop their
nominal 18V.

This can be done with a 24V transformer on pins 13-14 with
primary and secondary reversed, rectifier, filter, and LM317.
The transformer needs to have low magnetic field because it's
right next to the tubes.

Unless a miracle drops a few 8416's in my pocket, I might try this sometime.
But I'll try 12AT7's first.


 

I was intrigued enough by this idea to clean up some Type W schematics and
forward them to Kurt who made them available on the TekWiki page for the Type W:

http://w140.com/tek_type_w_schematics.pdf

I do not see any way to use the +75 volt 150 milliamp heater power from pin 15
however 6.3 volts AC (elevated to +100 volts for the vertical amplifier) is
available between pins 13 and 14 and some plug-ins directly use this as a heater
source; for example the 1A4 uses it to power a single 6DJ8 and an auxiliary
transformer isolated power supply. The +100 volt elevation will reduce coupling
but if it is a problem, I would try rectifying it to produce about 8.1 volts DC
peak with schottky diodes for low forward voltage drop and using a low dropout
regulator yielding 6.3 volts DC.

On Mon, 18 Aug 2014 12:49:28 -0700, you wrote:

Because it represents +100V after (in some mainframe models) passing through two 12.6V 150mA tubes, pin 15 is 75V and must be loaded with exactly 150mA. Plugins that need more current use AC. I think one even contains a filament transformer. Since V124 operates at low level (current sink for 8056 input cathode follower), it probably needs that quiet DC heat. You could rewire for AC powered 6DJ8's but I think you'd have trouble with hum. <shrug> Try it and see. Be sure to keep the 8056 heater balancer.

Dave Wise


Dave Wise
 

Because it represents +100V after (in some mainframe models) passing through two 12.6V 150mA tubes, pin 15 is 75V and must be loaded with exactly 150mA. Plugins that need more current use AC. I think one even contains a filament transformer. Since V124 operates at low level (current sink for 8056 input cathode follower), it probably needs that quiet DC heat. You could rewire for AC powered 6DJ8's but I think you'd have trouble with hum. <shrug> Try it and see. Be sure to keep the 8056 heater balancer.

Dave Wise

-----Original Message-----
From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: Monday, August 18, 2014 12:07 PM
To: TekScopes@yahoogroups.com
Subject: Re: [TekScopes] Modifying W Plugin to use 6DJ8 Tubes

http://w140.com/tekwiki/images/e/ea/Tek_w_differential_comparator.png


On Mon, Aug 18, 2014 at 3:04 PM, Cliff White cn.white@sunbelt-plb.com
[TekScopes] <TekScopes@yahoogroups.com> wrote:



Did you mean to say 8416? I can't seem to find a datasheet for an 8416.


Respectfully,
Cliff White, W5CNW
w5cnw@arrl.net <mailto:w5cnw@arrl.net>
On 08/18/2014 01:42 PM, Kurt Rosenfeld kurt.harlem@gmail.com [TekScopes]
wrote:


The filaments are in series, and a 6DJ8 has higher filament current
than an
8056. So the heater wiring topology will need to be altered. Furthermore,
how much current can be safely drawn from pin 15?

-Kurt

On Mon, Aug 18, 2014 at 1:51 PM, Cliff White cn.white@sunbelt-plb.com
[TekScopes] <TekScopes@yahoogroups.com> wrote:



The 6DJ8 is a 6-volt version of the very expensive 8416. I assume a mod
as simple as adding the appropriate resistor to the heaters. Has anyone
done this before?

--
Respectfully,
Cliff White, W5CNW
w5cnw@arrl.net <mailto:w5cnw@arrl.net>


Kurt Rosenfeld
 

http://w140.com/tekwiki/images/e/ea/Tek_w_differential_comparator.png


On Mon, Aug 18, 2014 at 3:04 PM, Cliff White cn.white@sunbelt-plb.com
[TekScopes] <TekScopes@yahoogroups.com> wrote:



Did you mean to say 8416? I can't seem to find a datasheet for an 8416.


Respectfully,
Cliff White, W5CNW
w5cnw@arrl.net <mailto:w5cnw@arrl.net>
On 08/18/2014 01:42 PM, Kurt Rosenfeld kurt.harlem@gmail.com [TekScopes]
wrote:


The filaments are in series, and a 6DJ8 has higher filament current
than an
8056. So the heater wiring topology will need to be altered. Furthermore,
how much current can be safely drawn from pin 15?

-Kurt

On Mon, Aug 18, 2014 at 1:51 PM, Cliff White cn.white@sunbelt-plb.com
[TekScopes] <TekScopes@yahoogroups.com> wrote:



The 6DJ8 is a 6-volt version of the very expensive 8416. I assume a mod
as simple as adding the appropriate resistor to the heaters. Has anyone
done this before?

--
Respectfully,
Cliff White, W5CNW
w5cnw@arrl.net <mailto:w5cnw@arrl.net>

[Non-text portions of this message have been removed]


[Non-text portions of this message have been removed]

[Non-text portions of this message have been removed]



[Non-text portions of this message have been removed]


Cliff White
 

Did you mean to say 8416? I can't seem to find a datasheet for an 8416.


Respectfully,
Cliff White, W5CNW
w5cnw@arrl.net <mailto:w5cnw@arrl.net>
On 08/18/2014 01:42 PM, Kurt Rosenfeld kurt.harlem@gmail.com [TekScopes]
wrote:

The filaments are in series, and a 6DJ8 has higher filament current
than an
8056. So the heater wiring topology will need to be altered. Furthermore,
how much current can be safely drawn from pin 15?

-Kurt

On Mon, Aug 18, 2014 at 1:51 PM, Cliff White cn.white@sunbelt-plb.com
[TekScopes] <TekScopes@yahoogroups.com> wrote:



The 6DJ8 is a 6-volt version of the very expensive 8416. I assume a mod
as simple as adding the appropriate resistor to the heaters. Has anyone
done this before?

--
Respectfully,
Cliff White, W5CNW
w5cnw@arrl.net <mailto:w5cnw@arrl.net>

[Non-text portions of this message have been removed]


[Non-text portions of this message have been removed]


Kurt Rosenfeld
 

The filaments are in series, and a 6DJ8 has higher filament current than an
8056. So the heater wiring topology will need to be altered. Furthermore,
how much current can be safely drawn from pin 15?

-Kurt


On Mon, Aug 18, 2014 at 1:51 PM, Cliff White cn.white@sunbelt-plb.com
[TekScopes] <TekScopes@yahoogroups.com> wrote:



The 6DJ8 is a 6-volt version of the very expensive 8416. I assume a mod
as simple as adding the appropriate resistor to the heaters. Has anyone
done this before?

--
Respectfully,
Cliff White, W5CNW
w5cnw@arrl.net <mailto:w5cnw@arrl.net>

[Non-text portions of this message have been removed]



[Non-text portions of this message have been removed]


Cliff White
 

The 6DJ8 is a 6-volt version of the very expensive 8416. I assume a mod
as simple as adding the appropriate resistor to the heaters. Has anyone
done this before?


--
Respectfully,
Cliff White, W5CNW
w5cnw@arrl.net <mailto:w5cnw@arrl.net>