Johnson DKW screen bias & screen supplies

[VE3AJM]

Hi Don and all,

My Johnson Desk KW amplifier is S/N 100-626. I do have the original manual, so I can post the schematic etc. when I get home today. A lot of great information here as usual.

Al VE3AJM

S/N 100-626...........and the plot thickens!  

Al, any idea when that unit was first purchased??

Craig,

Hi Craig,

I'm curious too about the serial numbers.

Off the top of my head, the unit was first put into service in 1961. I'll let you know for sure later today. I have the part of the original warranty card with that info on it etc. in the manual.

Al VE3AJM

[W9BHI]

Looking at the latest schematic, I see that R213 would adjust the screen regulator current.
I would guess that that would be about 40 Ma. @525 volts (VR150, VR150, VR150, VR75).
R202 would adjust the SSB bias regulator current, 40Ma. @90 volts (VR90).
I believe this is the correct current for these tubes.


Don W9BHI

[VE3AJM]

Here is the information from my manual etc. First in operation Sept. 4 1961.

Al VE3AJM

[AB2EZ]

Don

Yes...

Assuming these resistors can handle the extra power dissipation, they serve the purpose of compensating for the increased output voltages of the solid stated supplies (at least with respect to protecting the voltage regulators).

Note:

Across the output of the regulated screen supply, there is a fixed 8k ohm load resistor... in addition to the load presented by the tubes. This resistor, by itself will draw 65 mA through R213. If you add the screen current drawn by the tubes (around 60mA) and the current flowing through the voltage regulator chain, you will have more than 150mA flowing through R213.

An extra 25 volts of voltage drop across R213 will add an extra 25V x 0.150A = 3.75 watts of dissipation in R213

Stu

Looking at the latest schematic, I see that R213 would adjust the screen regulator current.
I would guess that that would be about 40 Ma. @525 volts (VR150, VR150, VR150, VR75).
R202 would adjust the SSB bias regulator current, 40Ma. @90 volts (VR90).
I believe this is the correct current for these tubes.


Don W9BHI

[W9BHI]

Thanks Al,
The table "C" is what I needed.

Stu,
So the total current through R213 should be set to 105Ma when in CW HI and no excitation?

Don W9BHI

[AB2EZ]

Don

Let me preface my answer by saying that the Desk KW screen voltage regulation design appears (to me) to be marginal to begin with. Each VR-150 is rated for an operating current between 5mA and 40mA. However, the "typical value" of the screen current, on key down in CW mode, drawn by a pair of 4-400 RF tubes is (per the data sheet) 92mA. So, if the regulator is doing its job when the RF tubes are drawing their recommended 92mA of screen current, at key down in CW mode, it will be overloaded (excessive current through the regulator tube chain) on key up... when the RF tubes are not drawing any screen current.

A similar problem will occur in AM mode; where the 4-400 tube specification sheet indicates that the "typical" 2-tube screen current, at carrier, is 56mA. [The actual Desk KW screen current in "hi-power AM" mode is probably somewhat less than 56mA, because: the target "hi power AM" screen voltage in the Desk KW, at carrier, is 450V versus the 500V value specified in the 4-400 tube data sheet; and the grid bias applied to the 4-400 tubes in the Desk KW is somewhat less negative than the grid bias specified in the 4-400 tube data sheet].

Fortunately, regulation of the screen voltage is not necessary in CW mode; and regulation of the screen voltage is also not necessary in AM mode (because the screen voltage is "self modulated" by the 12H choke... which draws an approximately constant current from the screen regulator output during modulation). Screen voltage regulation is only necessary in linear (SSB) mode.

All of this is apparent from the table that Al posted. Note that: in that table, regulation of the screen voltage only takes place in SSB mode and (maybe) in hi-power AM mode.

Having said that, I would adjust R213, in SSB mode, with no excitation (and, therefore, with the 4-400 RF tubes drawing minimum screen current)... to set the current flowing through the regulator chain (not the current flowing through R213) to be approximately 35 mA.

This should ensure reasonable regulation of the screen voltage on the RF tubes, over the range of screen currents that will be drawn by the RF tubes in linear/SSB operation.

The voltage at the output of the screen voltage regulator will drop below the target regulated value in low power AM mode, in CW mode, and possibly in hi-power AM mode, whenever the screen current drawn by the pair of 4-400 RF tubes exceeds 35mA. For example, this will occur on key down in CW mode.

Note: the drop in the output voltage of the screen voltage regulator, at high screen current draws, will be exacerbated by the solid stating of the 5R4 supply... because the value of R213 will be higher.

All of the above is another reason to "think twice" about solid stating the 5R4 power supply.

Stu

[WD5JKO]

Let me preface my answer by saying that the Desk KW screen voltage regulation design appears (to me) to be marginal to begin with.

   It is interesting that other companies building RF amplifiers back during the same time period took dramatically differing approaches. Look at the plate and screen supplies for the Central Electronics 600L amplifier. I attach schematics here for comparison. The power supplies must provide what the RF stages need, and if the supply is incapable of following the demand, then reduced power output, and higher IMD products are the result. One big gotcha are those that boost the low frequency syllabic current draw from a power supply where the supply in stock form is a choke input filter with a self resonance somewhere in the lower audio range. This causes a surge in ripple as resonance is passed through.  CE used a choke input filter with a swinging choke and a 55 uf oil capacitor. I'm not sure about the JDK's choke, but the capacitor is only 8 uf.

  The 600L can peak 400 watts from a single 813 in linear mode. It will also do 100% modulation of a 100 watt am carrier. Here the 813 is dissipating beyond ratings, but it will do it. A more conservative AM carrier power for a 600L is about 70 watts carrier. This gives lots of headroom for extended positive peaks.

Jim
Wd5JKO

[AB2EZ]

Jim

An interesting approach! Much better, in my opinion, than the design E.F. Johnson used for the screen voltage regulator in the Desk KW. Thanks for pointing it out to me.

It appears to be the vacuum tube equivalent of building a high current voltage regulator using a Zener diode in combination with a "pass" transistor.

The approach used to make the reference voltage on the grid of the 812 higher than the 300V reference voltage (from the series pair of VR-150 voltage regulators) is neat. I guess that is the vacuum tube equivalent of how one makes a solid state, adjustable voltage regulator... whose adjustable, regulated output voltage can be higher than the voltage of the Zener diode reference.

Best regards
Stu

P.S.

As an aside, Collins did not employ a screen voltage regulator in the KW-1 (which, as I understand it, was designed by the same engineer who designed the Desk KW). Of course, the KW-1 was not designed to operate as a linear amplifier, and (as in the Desk KW) the screen current is essentially constant in AM mode (because of the reactor used for self modulation of the screen voltage). As a matter of curiosity, I just checked the KWS-1 schematic that is available on-line... and it appears that Collins used a screen regulator circuit, in the KWS-1, that is similar to the one you posted (i.e. as used in the Central Electronics 600L).