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807 at 1kV




 
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W4AMV
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« on: January 15, 2020, 10:53:59 AM »

Looking at this 4//807 amplifier and the PS doing a bit better than expected.
No load at 1kV and loaded at 400 mA,  around 800 V. The tubes are cutoff with the screens at 300 V and grid at -100 V. My concern is flash over. With the tube cutoff no appreciable plate current, is that an issue?

Anyone run the 807 at 1 kV? Many of the older QST papers routinely took these tubes well above their 750 V max rating, as long as you do not exceed plate dissipation.
Thoughts?

Thanks, Alan

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« Reply #1 on: January 15, 2020, 02:30:49 PM »

For a while i ran a single 807 modulated by a pair of 811s with 720 volts on the plates and would routinely hit 150-200% positive peaks, so B+ and audio together was probably well in excess of 1500 volts, and never had any issues with the tubes flashing over. Did arc the plate tuning cap a few times though.
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« Reply #2 on: January 15, 2020, 03:34:54 PM »

Alan,

   The 807 has not gotten scarce, or expensive yet. That said, the supply is limited, and all it takes is a run from the DIY audio folks, and that will quickly change things.

The other issues I have is when the plate voltage is high like that, a little off resonance tuning can lead to a red glow on the ceiling of your shack. About 5 seconds of that, and the tube is gassy, or otherwise damaged forever. The need for an overload circuit is high, NOT optional.

Also, the 807 is known to make TVI with just the filament on (/sarcasm). Point be, the risk of a parasitic rises with a higher plate voltage. Then if you run the screen from a dropping resistor off the plate supply, then without excitation, or CW Key UP, the screen voltage will rise to the plate voltage potential...with likely disastrous results, i.e. the need for a Screen Grid Clamp tube. The Clamp tube should also be able to withstand 1KV as well, something like a 6BQ6 would do....NOT a 6AQ5.

Perhaps use a 12-24V at 3A filament transformer to "Buck" the Plate transformer primary voltage to 12-24Vac less than it is now. A 3A winding should be fine.

Jim
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W4AMV
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« Reply #3 on: January 15, 2020, 08:03:02 PM »

Thanks for the replies!

Yes, looking at class C operation at a plate V of 600 V with 100 % modulation, the 1200 V peak is apparently tolerated. So I suspect then flash over is not an issue. Papers show the 807W operating at 1100 V DC input. Its Ebb max no different than the 807 or 1625. Jim, on the screen bias, the approach I am taking is screen regulation from the center tap of the HV xmfr. A FWB is used for HV and there and I am using of half of that bridge for screen V plus adding a pair of VR150's for 300 V screen.

The control grids are heavily loaded with 2 k and the use of 4 tubes in parallel sets the Zin at ~ 500 ohms. A PI net which is shielded will match 500 ohm to 50 ohm input. The grid input network is below chassis as well the 807 tube bases are brought below chassis level. I do not plan on shielding the tubes other than isolation done through chassis layout. By the way, all this is CW.

Alan
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« Reply #4 on: January 16, 2020, 01:17:56 PM »

Quote
...The control grids are heavily loaded with 2 k and the use of 4 tubes in parallel sets the Zin at ~ 500 ohms. A PI net which is shielded will match 500 ohm to 50 ohm input.

How did you arrive at those values?


I think I would use grid block Keying with 4 807s if you are going to use grid leak bias during KeyDown.

For CW you don't need a highly regulated screen voltage Vsg.

If you feel you need regulated Vsg, reduce the Rsg to 22k and place your series gas regulators at the screen pin(s),


Phil - AC0OB

* 807 4X807 Transmitter.pdf (109.39 KB - downloaded 53 times.)
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« Reply #5 on: January 16, 2020, 03:29:22 PM »

Hi Phil,

Thank you for your notes.

On the screen regulation, I may use the amplifier in another service. So I decided to add the overhead for gas regulators. Currently the shift in screen V is not bad w/o gas regulators, between no load and full load for this "economy" PS. Approximately 60 volts, 350-to-410V. While the HV output targets ~ 900 V (850-1kV) from no load to full load.

On the grid loading, prior work with a pair of 807W's was done at 1k ohm/ 2 tubes. From the data sheet a typical grid Zin for 2 tubes with out loading is 30 k. And I targeted ~ 1/10th that value. There is no neutralizing in this amplifier. The total Pin drive is about 10 W. There is an input PI match from 50 to 500 ohm.

The grid 1 is held negative at ~ -100 V in standby and the standing plate current is set via a bias pot. In Tx, the pot R is reduced slightly and the exciter drive and pot R set for ~ class C operation.

Alan
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« Reply #6 on: January 16, 2020, 08:00:18 PM »

Hi Phil,

Thank you for your notes.

On the grid loading, prior work with a pair of 807W's was done at 1k ohm/ 2 tubes. From the data sheet a typical grid Zin for 2 tubes with out loading is 30 k. And I targeted ~ 1/10th that value. There is no neutralizing in this amplifier. The total Pin drive is about 10 W. There is an input PI match from 50 to 500 ohm.

Alan

Here is a similar bias switching (Lower Left Corner) circuit I implemented for my 175 Watt SG modulated transmitter to cut off the 813 in Standby.

For the grid leak bias resistor you want to make sure the grid current stays under 3.5 mA per tube or 14.0 mA for four tubes.

The recommended bias from the 807W/5933 tube specs is -45 volts. R = 45V/14.0 mA = 3.2k. I chose 3.9k to keep the control grid current within limits.

This way, you won't be wasting drive power driving into a low impedance so 1.25 Watt drive power should be sufficient.


Phil

 

* 807 4X807 Transmitter.pdf (109.56 KB - downloaded 41 times.)
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« Reply #7 on: January 16, 2020, 10:22:08 PM »

Excellent Phil.

This is similar to what I have. The Rps is used to assist in joining (or power combining) the 4 signal grids. I have it in as well the place holder for a VHF parasitic choke on the grid side for each tube. I will try to post my revised arrangement. I did post an earlier scheme under the prior topic "3-to-4 Parallel 807 HF Amp". 

Thanks, Alan
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« Reply #8 on: January 17, 2020, 10:08:05 AM »

If you are planning on screen modualting this PA you need to get rid of the .01 screen bypas or you are going to sound like you have a sock in your mouth. I'm using .001 caps for this- one on each of two tubes on a set of screen modulated ARC-5 transmitters here. Not up to 1000 volts on the plates yet but at 900 I'm able to get a 40 watt carrier100% modulated with no color on the 1625 (12 volt 807) plates. Your plate bypass can also be .001 if using plate modulation. Too much bypass capacity will roll off the highs very quickly.
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« Reply #9 on: January 17, 2020, 10:56:56 AM »

If you are planning on screen modualting this PA you need to get rid of the .01 screen bypas or you are going to sound like you have a sock in your mouth.

I am pretty sure Phil is showing this schematic based on CW operation.

And the calculations are based on 4 tubes. I agree on the Z values assigned to the plate.

So you are looking at 16 k grid termination per tube. My concern here is the high input Z and the possibility of feedback-instability. The low Zin is an attempt to use a grounded cathode with the stability associated with a GG. And not the higher drive requirements of GG. I have the input PI match in a shielded enclosure, mounted under chassis. The tube(s) are to be mounted about 1 inch below chassis as well and with the plate output above chassis gain some isolation. I was not planning on using the Millen shielded wrapper about the tubes!

One other note, on the schematic using external fixed bias, you show the choke of 470 uH isolating the 3.9 k termination... Are these elements reversed? While on the first schematic, the Zin would be set by the 3.9 k. Thanks.


Alan
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« Reply #10 on: January 17, 2020, 03:28:38 PM »

If you are planning on screen modualting this PA you need to get rid of the .01 screen bypas or you are going to sound like you have a sock in your mouth.

I am pretty sure Phil is showing this schematic based on CW operation.

And the calculations are based on 4 tubes. I agree on the Z values assigned to the plate.

So you are looking at 16 k grid termination per tube. My concern here is the high input Z and the possibility of feedback-instability. The low Zin is an attempt to use a grounded cathode with the stability associated with a GG. And not the higher drive requirements of GG. I have the input PI match in a shielded enclosure, mounted under chassis. The tube(s) are to be mounted about 1 inch below chassis as well and with the plate output above chassis gain some isolation. I was not planning on using the Millen shielded wrapper about the tubes!

One other note, on the schematic using external fixed bias, you show the choke of 470 uH isolating the 3.9 k termination... Are these elements reversed? While on the first schematic, the Zin would be set by the 3.9 k. Thanks.

Alan

I only mentioned screen modulation as I took this bias example (lower Left) from my screen modulated transmitter written up in ER magazine. This circuit for Alan is for a grid driven CW operation system.

No, elements are not reversed. The choke was added to RF isolate the grid-leak/protective bias circuit. I think you may be confusing RF AC with DC bias.

As I mentioned above the Rcg was determined by the tube specs:

The recommended bias from the 807W/5933 tube specs is -45 volts. R = 45V/14.0 mA = 3.2k. I chose 3.9k to keep the control grid current within limits.

Your operational/protective bias is set by RB for required power output as you requested. In case the operational/protective bias (- 120V or whatever) fails (goes to 0V) Rcg limits grid current.

The operational/protective bias does add to the total control grid bias which is why I made it adjustable. Grid leak bias due to Class C grid conduction + operational/protective bias = total negative bias voltage on the control grid.

I prefer adjusting the RF drive from the exciter to set power output and adjusting the operational/protective bias for sweet spot operation, but that's just me.  


Phil


 

* 807 4X807 Transmitter.pdf (113.45 KB - downloaded 31 times.)
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« Reply #11 on: January 17, 2020, 03:48:16 PM »

...So you are looking at 16 k grid termination per tube. My concern here is the high input Z and the possibility of feedback-instability. The low Zin is an attempt to use a grounded cathode with the stability associated with a GG. And not the higher drive requirements of GG. I have the input PI match in a shielded enclosure, mounted under chassis. The tube(s) are to be mounted about 1 inch below chassis as well and with the plate output above chassis gain some isolation...

Alan

The design you said you needed was for a 4 tube RF system using 807's and nothing about a Grounded Grid amplifier.

The implication here was that you wanted a grid driven Final stage since you said you would be using a Pi input filter.

So which is it? Grid driven or GG?? Please clarify your requirements.

If your exciter output stage has an Rsource of say 2500 ohms (have you taken that into account for your input Pi filter design?) or so then using a load of 3.9k should be no problem.

Rps is a parasitic grid resistor to dampen any parasitic's.


Phil
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« Reply #12 on: January 17, 2020, 03:59:45 PM »

Hi Phil. Thanks again.

Let me clarify. My design selection is grounded cathode. My previous statement was suggestion that concerns stability. The heavy grid loading was prompted to obtain that stability. The exciter is 50 ohm output and the PI match currently set up to handle a 50 ohm to 500 ohm match // with the total Cgk of the 4 tubes, about 50 pF.

I mention that in one case it looks like your input Z was 3.9 k and the other significantly larger with the series choke for isolation.

Alan
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« Reply #13 on: January 17, 2020, 04:44:02 PM »

Hi Phil. Thanks again.

Let me clarify. My design selection is grounded cathode. My previous statement was suggestion that concerns stability. The heavy grid loading was prompted to obtain that stability. The exciter is 50 ohm output and the PI match currently set up to handle a 50 ohm to 500 ohm match // with the total Cgk of the 4 tubes, about 50 pF.

I mention that in one case it looks like your input Z was 3.9 k and the other significantly larger with the series choke for isolation.

Alan

Okay, look at both Pages from this PDF:

If page 2 is what you are proposing then if the adjustable bias supply fails you will have - 7 volts for grid bias which is 40 volts less than the tube spec requires for safe operation.


Phil

* 807 4X807 Transmitter.pdf (154.72 KB - downloaded 36 times.)
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« Reply #14 on: January 17, 2020, 07:52:24 PM »

Thank you again Phil.

I have attached what I had in mind. I have included a R5 which needs to cover the bias protection if loss in active bias supply occurs. I show only ONE of the 4 tubes. I left off the details of the output match and screen supply, some cathode "stuff"... we are on the same page there. It is the input item that is desired for focus. The grid input per tube is loaded to a couple of k, 2.2 k. It is in the RF path, not decoupled. While the bias R, in this case provides the required added ~ 12 k needed for protection. It is RF decoupled. The net in this case is ~ 500 ohm total Zin // with the reactance C of 3 other tubes.

Alan


* Phil_bias_protection.jpg (40.1 KB, 960x580 - viewed 79 times.)
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« Reply #15 on: January 17, 2020, 08:39:59 PM »

ok, i can see where you're getting the ~ 550 ohm grid impedance, it is the 2.2k/4.

However, you're going to pay a power penalty because the impedance at the grid node is going to be really low since you are attenuating most of the RF voltage at this point. You'll need at least 4.5 Watts but if you have 5.5 watts to spare then, ok.

I still don't know how you are coming up with 12k for R5. In my schematic at the top it says 4X807 which means all four 807's are in parallel and my R5 value is accounting for the 14 mA of total control grid current for a grid leak bias of at least -45V. Now you can use a 2.7k if you are going to make up some bias with the adjustable 10k. 0.014X 2.7k = 37.8 volts so your 10k pot can supply the rest of the bias or an additional -7.2 volts.   

Also in the last schematic the 27k screen grid resistor is accounting for the total screen grid current of all 4 tubes, which is why there is a "4" near Csg which means there is a 0.01 uF at each individual screen (Pin 2) node.

Phil
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« Reply #16 on: January 17, 2020, 09:33:17 PM »

Understood Phil.

I was showing only 1 of the 4 tubes. I see the confusion for R5. I can BUS the grids at the decoupled -DC node, C2 location. Then use a single R5, your 3.9 k (~ 12k/4). That would make sense to setup in that manner. The required additional bias if needed is provided by the 10 k bias control pot. Its setting would be made in the TX mode. Standby, the tubes would be cutoff.
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« Reply #17 on: January 17, 2020, 10:54:39 PM »

In keeping with your topology take a look at this:

Suggested values, etc., all for free.  Grin

RGL is 3 Ohmite 152KE "OY" Resistors in Parallel.

Lower Rcg to 3.3k 2W.

I would suggest a relay for STDBY bias and OP bias and go to a 25k 5W pot for RB.


Phil

* 807 4X807 Transmitter.pdf (118.99 KB - downloaded 35 times.)
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« Reply #18 on: January 18, 2020, 07:28:22 AM »

Very good Phil. This looks fine.
The only comment is the Zin reactance. The tubes input C plus stray still dominate. So this would amount to 50 or 60 pF that would need to be absorbed into the PI match.

The parasitic choke(s) is something I looked into. After review of a dozen or so others work at 100 W, the common arrangement was 7 T # 22 AWG on 1 W space wound on 68 ohm carbon composition resistor.  Closest I had was a 33 ohm. I don't have any of the ceramic R's. Nice. Anyway, I built this arrangement up and looked at its Z value as shunt S11 on the chart. As well, the RX value with frequency. Noted the rapid rise in R value above 30 MHz. Sweep stops at 200 MHz. So it appears this arrangement is not to bad. A circuit equivalent shows why the bend in Z and the increased R value with frequency.



* Smith_807_PC.png (12.73 KB, 495x495 - viewed 63 times.)

* 807_parasitic_choke_RX_2.png (24.12 KB, 1366x705 - viewed 66 times.)

* Choke_Ckt.jpg (40.37 KB, 650x443 - viewed 68 times.)
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« Reply #19 on: January 18, 2020, 11:34:22 AM »

Very good Phil. This looks fine.
The only comment is the Zin reactance. The tubes input C plus stray still dominate. So this would amount to 50 or 60 pF that would need to be absorbed into the PI match.

The parasitic choke(s) is something I looked into. After review of a dozen or so others work at 100 W, the common arrangement was 7 T # 22 AWG on 1 W space wound on 68 ohm carbon composition resistor.  Closest I had was a 33 ohm. I don't have any of the ceramic R's. Nice. Anyway, I built this arrangement up and looked at its Z value as shunt S11 on the chart. As well, the RX value with frequency. Noted the rapid rise in R value above 30 MHz. Sweep stops at 200 MHz. So it appears this arrangement is not to bad. A circuit equivalent shows why the bend in Z and the increased R value with frequency.



I guess you missed it but there have been long discussions about parasitic chokes both here and on other blogs, but I have never heard of a "common arrangement" of 7T #22 1 Watt CC.

The one I suggested on the schematic was modeled by me and has worked on various rigs.

 

Good luck with the 4X.


Phil
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