Discussion of screen dropping resistors on my 813 rig

[w9jsw]

I use screen dropping resistors on my 813 K1JJ inspired rig and plan to use them on my 4-400 RF deck as well. Originally, this 813 rig was built to follow Tom's design using a screen LV supply and choke, as well as shutting off the cathode and HV. HV is shut off at the trans primary, so it fades to zero as the bleeder resistor does its job. Takes a few 10's of seconds. I also use Tom's idea of a snubber on the mod transformer secondary. The rig is well behaved (but is hiding a problem).

As I was doing 813 bring-up, I had concern with how to control the screen current in the event of loss of HV without loss of LV. Try as I might, I never found a solution that I was happy with. I decided to go with the screen dropping R since I never move off of 2KV of HV. When the HV goes away, the screen current does also. I determined this to be a good failsafe solution if the HV craps out. The rig has been running that way for over 2 years. The 813 rig does not have a screen voltmeter, the 4-400 one does. Therein lies the problem.

Recently as I have been bringing up the 4-400 rig, I observed that the screen voltage would soar as I went from TX to RX, pegging a 1000V meter. Not good for the meter and not good for the screens - that are rated at 800V max. Thinking about that led me to conclude that as the current drops to zero on the screens very quickly due to the cathode cutoff, the voltage drop decreases and the screen potential then follows the plate voltage as it drains off.

This bothers me. Even at zero current, I don't want the screens to be over spec. I hooked up a voltmeter to the 813 and it is doing the same, as I suspected. Although 2 years of operation has not yet blown the screens due to this problem. May be true, what Tom has often said - 813s are stout. 4-400s may be less so.

I posed this issue to Chuck, K1KW, as I knew that he used a screen dropping resistor on his 572/813 rig. Chuck was kind enough to explain that yes, my concern is valid. His solution is, along with the snubber, to keep the cathodes on all the time and switch the HV on the B+ side with a vac relay. This extinguishes the HV immediately along with the screen voltage, preventing it from soaring to the plate voltage value. He keeps the HV primary on all the time. Using proper sequencing of the snubber WRT the HV, all activity is quenched quickly and smoothly.

Long story short, if you are using a screen dropping resistor, pay attention to what the screens are doing after TX to insure they are not being abused by excessive voltage.

I have some re-work to do...

John

[n8fvj]

Use an array of Transorbs (zeners) and regulate the screens.

[K8DI]

While I’ve not seen it done on a big rig, there’s plenty of examples on smaller tubes (6146, etc) where the screen is fed with a voltage divider. Such a divider also reduces the amount of self-modulation, which may or may not hurt or help linearity. I read someplace in a scan of an old article that this was a way to optimise the percentage that the screen modulates.  It will waste more power.  But….it would prevent this issue of the screen voltage shooting upwards when the cathode circuit is opened.

Ed

[KD1SH]

  Definitely food for thought for anyone designing a big plate modulated tetrode rig. I'm wondering, though, if the soaring screen voltage is as big a problem as it might appear at first glance. Sounds like a Bill Orr/Eimac question but, unfortunately, we can't ask him anymore. My thinking is that both the screen voltage and the plate voltage effectively double under 100% modulation, and with Eimac recommended voltages of 600V on the screen and 3.2KV on the plate, we get peak values of 1.2KV on the screen and 6.4KV on the plates under full modulation. These conditions are transitory, not steady state, but your tube isn't actually operating when you've cathode-blocked it, either.
  I share your concerns, and I'll be keeping them in mind when I start work on my 813 rigs, but I'm going to climb out onto a limb here and figure that roughly doubling the screen voltage under quiescent conditions is likely not a problem, at least not with 813's; but with possibly more delicate (and more expensive) 4-400's, maybe not so much.

I use screen dropping resistors on my 813 K1JJ inspired rig and plan to use them on my 4-400 RF deck as well. Originally, this 813 rig was built to follow Tom's design using a screen LV supply and choke, as well as shutting off the cathode and HV. HV is shut off at the trans primary, so it fades to zero as the bleeder resistor does its job. Takes a few 10's of seconds. I also use Tom's idea of a snubber on the mod transformer secondary. The rig is well behaved (but is hiding a problem).

As I was doing 813 bring-up, I had concern with how to control the screen current in the event of loss of HV without loss of LV. Try as I might, I never found a solution that I was happy with. I decided to go with the screen dropping R since I never move off of 2KV of HV. When the HV goes away, the screen current does also. I determined this to be a good failsafe solution if the HV craps out. The rig has been running that way for over 2 years. The 813 rig does not have a screen voltmeter, the 4-400 one does. Therein lies the problem.

Recently as I have been bringing up the 4-400 rig, I observed that the screen voltage would soar as I went from TX to RX, pegging a 1000V meter. Not good for the meter and not good for the screens - that are rated at 800V max. Thinking about that led me to conclude that as the current drops to zero on the screens very quickly due to the cathode cutoff, the voltage drop decreases and the screen potential then follows the plate voltage as it drains off.

This bothers me. Even at zero current, I don't want the screens to be over spec. I hooked up a voltmeter to the 813 and it is doing the same, as I suspected. Although 2 years of operation has not yet blown the screens due to this problem. May be true, what Tom has often said - 813s are stout. 4-400s may be less so.

I posed this issue to Chuck, K1KW, as I knew that he used a screen dropping resistor on his 572/813 rig. Chuck was kind enough to explain that yes, my concern is valid. His solution is, along with the snubber, to keep the cathodes on all the time and switch the HV on the B+ side with a vac relay. This extinguishes the HV immediately along with the screen voltage, preventing it from soaring to the plate voltage value. He keeps the HV primary on all the time. Using proper sequencing of the snubber WRT the HV, all activity is quenched quickly and smoothly.

Long story short, if you are using a screen dropping resistor, pay attention to what the screens are doing after TX to insure they are not being abused by excessive voltage.

I have some re-work to do...

John

[w9jsw]

Ed,

In conjunction with talking with a colleague, this may be a solid solution. The variation is to place a NC vac relay in the center of the divider that disconnects the resistor during TX. That way it will not affect any self-modulation. When in RX, the dropping resistor divider effectively becomes a part of the PS bleeder. The bleeder is 200K. If I set the divider R to 7.5K, it will hold the screens at a max of 460V while bleeding down the HV. The new bleeder becomes 28K overall, with the 7.5K R needing to handle 61ma max at 2KV peak. So 30W. A 7.5K 50W unit should be perfect. Mount it right next to the current dropping R that are on the top of the chassis on ceramic standoffs.

I think this is a KISS solution...

John

[w9jsw]

  I share your concerns, and I'll be keeping them in mind when I start work on my 813 rigs, but I'm going to climb out onto a limb here and figure that roughly doubling the screen voltage under quiescent conditions is likely not a problem, at least not with 813's; but with possibly more delicate (and more expensive) 4-400's, maybe not so much.

I kind of agree, but

On the 813, which runs at approximately 350-400 volts DC, the modulation would be a max of 800V at 100% on my rig. Jumping to 2kv with no cathode current still feels sketchy to me. RCA specs it at 1.1KV. The relay solution seems a simple way to resolve this.

John

[KD1SH]

  I agree; I like the relay idea.

I kind of agree, but

On the 813, which runs at approximately 350-400 volts DC, the modulation would be a max of 800V at 100% on my rig. Jumping to 2kv with no cathode current still feels sketchy to me. RCA specs it at 1.1KV. The relay solution seems a simple way to resolve this.

John

[w9jsw]

The relay is a solid fix. Using a 20W 7500 ohm WW with a NC vac relay. Screen is nominally 350V during TX. When I go to RX, it jumps just a bit to 450V, then tapers off quickly to 0 in around 5-7 seconds. I expected a slight jump as the divider will put out 460V if the plate is at 2KV.

Happy with the solution and plan to implement the same on the 4-400 deck.

John

[KD1SH]

Excellent. I will very likely employ the same approach when I get to that point. Thanks—this is the sort of thing that makes AMfone such a terrific resource.

The relay is a solid fix. Using a 20W 7500 ohm WW with a NC vac relay. Screen is nominally 350V during TX. When I go to RX, it jumps just a bit to 450V, then tapers off quickly to 0 in around 5-7 seconds. I expected a slight jump as the divider will put out 460V if the plate is at 2KV.

Happy with the solution and plan to implement the same on the 4-400 deck.

John

[K8DI]

One last note on John’s implementation for those who read this later on…

John is a big fan of proper sequencing, as we all should be. The relay that connects the resistor when it drops needs to drop before the cathode opens by enough time such that there’s not an instantaneous spike.  I’d bet John has this handled, but it should be mentioned here for completeness.

Ed

[K1JJ]

John, W9JSW said:
"I use screen dropping resistors on my 813 K1JJ inspired rig and plan to use them on my 4-400 RF deck as well. Originally, this 813 rig was built to follow Tom's design using a screen LV supply and choke, as well as shutting off the cathode and HV. HV is shut off at the trans primary, so it fades to zero as the bleeder resistor does its job. Takes a few 10's of seconds. I also use Tom's idea of a snubber on the mod transformer secondary. The rig is well behaved (but is hiding a problem)."


Hi John,

Yes, there's many ways to do it. Each has advantages and disadvantages. Having a few methods helps tailor the technique to the particular job.  Relay keying the HV B+ while leaving the primary on is a great solution too.

Another advantage not yet mentioned - of keying the HV 240VAC primary and keying a tetrode tube's cathodes:   As you said above, when unkeyed, the HV filter capacitors take a long time to discharge, like 15-60 seconds depending on the cap values and bleeders. So when in a lively PTT QSO, [ssb or AM]  this power is conserved and stored across the filter caps, only sucked down by the slow bleeders when in RX. Instead of running continuously or having to charge the caps up fully every time, the HV stays up near full value - kinda a free ride. In my case I use 10KV at 140 uF filter caps, which esentially acts like a HV supply on all the time when doing fast PTT operation. Otherwise, when just monitoring, the HV power bill is zero.

I like to simulate VOX using PTT on ssb.  The keyed HV works FB there. I mask the loud shack relay clack using a 300 mS audio delay.

FB on the ideas concerning protecting the screen.   I haven't lost a screen yet on any of my bigger rigs, but it is certainly a concern to watch the unkey transients.

BTW, a very light HV bleeder load is OK, mainly used for safety. Regulation benefits drawdown is satisfied with a keyed linear amplifier idle or AM carrier load.

T

[w9jsw]

Ed has a point. I am keying both relays from the same sequencer output. The new vac relay has a 12ms turn off time. The cathode relay shuts off in 10ms. Thus, I could possibly have a 2ms spike occurring. If we look at this from a practical standpoint, the spike was huge before and now we have shortened it tremendously. I am going to put a scope on it to see if it is really there. Not sure how long it takes from the cathode ground to being removed to seeing the tube shut down.

Also, specs on relay actuation times are averages, I bet. Given that I could add a reverse diode on the cathode relay and lengthen it's close time a bit. That is what I will do if I find a pulse.

We may be splitting hairs or may not. The scope will tell.

By the way, this is making me consider a new scope. I have an old Tek 454. Hard to see this sort of stuff on it. I think I need a new whizbang storage scope like a Siglent 1104X-E. Not that i have been looking at them... 

[KD6VXI]

I bought a 150 mhz owon used off ebay.

I'm honestly amazed at what 100 bucks bought me.  I'd check them out.

--Shane
WP2ASS / ex KD6VXI

[w9jsw]

Let me know how it handles ham work in an RF environment. Teardowns show not a lot of shielding. DS1102?

I keep thinking that 4 probes would be nice for watching sequencing in my rig. Two may be enough as I could save multiple waveforms and compare them, I guess.

Interesting...

John

[KL7OF]

At least this is complicated......

[KD1SH]

At the risk of hijacking the thread (not that it ever happens on AMfone), I've had a DS1102 for years now, and it's a great little scope. I used a DS1104 at work—also great. Other than looking at sequencing, I'm seldom hampered by having only two channels. Actually, when I built my last sequencer, I kluged up a little LED panel so that I could visually observe the sequence.

Let me know how it handles ham work in an RF environment. Teardowns show not a lot of shielding. DS1102?

I keep thinking that 4 probes would be nice for watching sequencing in my rig. Two may be enough as I could save multiple waveforms and compare them, I guess.

Interesting...

John

[w9jsw]

KD1SH,

That's what I did with my sequencer also. But got to thinking that it would be nice to see exactly what is going on with rise and fall times.

Is your DS1102 an OWON? Work ok in a QRO RF environment?

John

[KD1SH]

Mine's a Rigol. I don't know how much QRO you're thinking, but I've used it with 1KW nearby with no problems yet.

KD1SH,

That's what I did with my sequencer also. But got to thinking that it would be nice to see exactly what is going on with rise and fall times.

Is your DS1102 an OWON? Work ok in a QRO RF environment?

John

[w9jsw]

Lusting over the new Rigol DHO804 scope...

https://www.tequipment.net/Rigol/DHO804/Digital-Oscilloscopes/?Source=googleshopping&/?utm_content=rigol%208000%20series%20scopes&utm_term=&utm_campaign=Shopping%20Campaign(BSC-Main)&utm_source=Bing_Yahoo&utm_medium=cpc&msclkid=e8cfd03d65341d1e4d54b56eda428a42

[KD1SH]

Oooh, hadn't seen that one! Nice price, and for $100 more you get back up to 100Mhz.

Lusting over the new Rigol DHO804 scope...

https://www.tequipment.net/Rigol/DHO804/Digital-Oscilloscopes/?Source=googleshopping&/?utm_content=rigol%208000%20series%20scopes&utm_term=&utm_campaign=Shopping%20Campaign(BSC-Main)&utm_source=Bing_Yahoo&utm_medium=cpc&msclkid=e8cfd03d65341d1e4d54b56eda428a42

[w9jsw]

Pulled the trigger on a Siglent SDS1104X-E. The new Rigol is having birthing pains. I want to eventually be able to run a bode plot on some of my audio chain and the Rigol's Bode capability is currently broken. Siglent has two ADCs so you don't lose capability running more than one probe.

John