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Author Topic: Copper Strip For Grid Grounding in TL922A  (Read 21710 times)
G3RZP
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« Reply #15 on: July 25, 2015, 10:30:42 AM »

How much trouble is given by too small a plate choke and plate bypass cap? I did some calculation for the 30L1 with its 40 microhenry plate choke. On 3.5 MHz, I reckon you have about 1 amp of RF current in the choke. The HV end of that has a 5000pF to ground, then a 2.2 microhenry and another 5000 pF to ground, and my calculations say that there is still something like 2.5 volts of RF on the HV line and the electrolytics.

Doing similar sums for 3-500Z and the like and a 250 microhenry choke bypassed with 5000pF, there's still nearly 3 volts across that 5000pf at 3.5 MHz: a 22ohm glitch resistor and another 5000pf drops that to about 1 volt. On 160 the situation is worse, of course. But is does suggest that two chokes are needed as well as somewhat bigger bypass caps if the HV line is to be RF dead.....which could be an explanation of things not always being as stable as one would expect.
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W1BR
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« Reply #16 on: July 25, 2015, 11:54:05 AM »

What was the purpose of the VHF choke in the SB-220 amplifier to begin with?  The plate choke is bypassed..

I also added a glitch resistor in place of the VHF choke when I did the extensive mods to my SB-220.

Pete
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KM1H
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« Reply #17 on: July 25, 2015, 01:59:02 PM »

Quote
The concept may have been your idea but, I used the selection info provided by Tom W8JI (link).

Some like to claim ideas as their own and not give credit. I never claimed originality as I first saw it used when on the NCL-2000 design team in 1963 and it was included in the RCA spec sheet for the 8072/8121/8122 and who knows how long they knew about current limiting. I mentioned it many times on the old AMPS reflector along with other things that were copied/borrowed/take your pick, and wound up on a web page or in an amp.
I believe Rich Measures, AG6K, also suggested them with his TL-922/SB-220 mods in the late 80's or early 90's.

Quote
What was the purpose of the VHF choke in the SB-220 amplifier to begin with?  The plate choke is bypassed..

I also added a glitch resistor in place of the VHF choke when I did the extensive mods to my SB-220.

Pete

A carry over from QST/HB of the 50's when a high value choke was used and then followed by a small one to stop VHF energy. The RFC2 in the SB-220 is self resonant right near the tube parasitic and can turn the amp into an oscillator. I use a WW resistor with an inductance close to the choke and becomes another suppressor as well as a current limiter, the B+ end is already bypassed with a 1000pf disc by Heath but the main 1000pf choke bypass is woefully undersized and is replaced by a pair of 4700pf/10 KV USA discs.

RF sniffing with a SA shows no more strange stuff floating around but I still wouldnt recommend directly grounding grids in that amp; some that do are just lucky and others you dont hear from again to mention that the amp blew up on 10M.

Somebody, maybe Measures, once said that the SB-220 RF cage was a VHF cavity; I might have confirmed it with a GDO but dont remember. Rich did a lot of good work before he got into seeing parasitics in his sleep.

Quote
Doing similar sums for 3-500Z and the like and a 250 microhenry choke bypassed with 5000pF, there's still nearly 3 volts across that 5000pf at 3.5 MHz: a 22ohm glitch resistor and another 5000pf drops that to about 1 volt. On 160 the situation is worse, of course. But is does suggest that two chokes are needed as well as somewhat bigger bypass caps if the HV line is to be RF dead.....which could be an explanation of things not always being as stable as one would expect.

Dentron used a 90uH choke and a 500pf bypass in several of their 160-10M amps that were also well known to prematurely blow the filter caps.

Carl
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N1UK
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« Reply #18 on: July 27, 2015, 03:01:09 PM »

I ordered this to replace the TL 922A 470 uH grid to cathode choke. Will this work OK or am I missing something? It is quite small compared to the original Kenwood part.

http://www.mouser.com/ProductDetail/EPCOS-TDK/B82144F2474J000/?qs=sGAEpiMZZMsg%252by3WlYCkU0ufu97XRvnv8k4YSyfWRVE%3d


I am not really sure what this part does in the circuit since it is bypassed by a  three 220pF caps and a 4K7 resistor.

73 Mark N1UK


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K9AXN
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« Reply #19 on: July 27, 2015, 08:12:03 PM »

Anyone had experience with the band switch in the TL-922A failing?  Did it ever fail when running other than 20 or 40 Metres and which wafer failed?

Isn't the choke on the grid circuit just to DC ground the grid?  If you should decide to replace the choke, test the resonant spots along with the caps installed.  Then again why not just ground the grid probably the best answer.  Used direct ground with a 4-1000 all bands for years --- no squirrely stuff ever detected.

Used and tested 8122s for years.  First use flashovers were frequent with new or unused 8122's.  The system was designed using tube designer's protection specs, a 15 ohm current limiter and filter capacity not exceeding 10uf.  Never had a flashover do any harm.  Think about this.  The 8122 has a screen capacity of 8 watts and that is a close spaced tube.  The radio is AB1 high impedance grid drive.  Uses 2700v and capable of 1250 watts delivered to the antenna if using high bias 8122s.  Over the years the radio used to test 8122s has had probably 200 flash overs and not one tube that tested good at low power before high power testing or component in the radio has failed.

Band switch failures were mitigated by proper wafer design and circuit that verified the antenna relay had not only closed but had continuity before drivers were enabled. 

I don't always agree with Tom, but this time he's got it right --- ground the grid straight out.

     

   
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KM1H
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« Reply #20 on: July 28, 2015, 01:32:50 PM »

Quote
I ordered this to replace the TL 922A 470 uH grid to cathode choke. Will this work OK or am I missing something? It is quite small compared to the original Kenwood part.

http://www.mouser.com/ProductDetail/EPCOS-TDK/B82144F2474J000/?qs=sGAEpiMZZMsg%252by3WlYCkU0ufu97XRvnv8k4YSyfWRVE%3d

That is due to the large ferrite rod in the center, I dont know how that will react.

Quote
I am not really sure what this part does in the circuit since it is bypassed by a  three 220pF caps and a 4K7 resistor.

The choke holds the grid slightly above DC ground and the caps bypass RF. The resistor de-Q's the choke to dampen any resonance responses. There are several ways to skin a cat and Kenwood's version has survived over 30 years and thousands of amps sold. IMO they did something right but should have come out with an updated version to be more compatible with fast RF switching SS rigs. Rich Measures, AG6K, circuit works well and Ive converted dozens for customers.

Carl
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WA7PRC
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« Reply #21 on: July 28, 2015, 11:46:53 PM »

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IMO they ... should have come out with an updated version to be more compatible with fast RF switching SS rigs. Rich Measures, AG6K, circuit works well and Ive converted dozens for customers.
Jim W7RY's board does all that, as well as electronic bias regulation and switching. For the TL922, you also have LED drivers for the meters and TX & STBY indicators as well. He sells on ebay as user 'radioamplifiers'.

When you buy the board, he sends complete documentation and a link to the Bill Of Materials on Mouser. He can supply the input reed relay, and suggests a source for the output vacuum relay. PCB assembly is all through-hole components, and is very easy for most anyone to perform.
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K9AXN
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« Reply #22 on: July 29, 2015, 05:01:46 PM »

Thanks for the info Bryan,

Haven't heard one tangible explanation regarding the use of the 200pf caps, choke, and/or resistor used in the grid circuit of the amps.  There has to be somebody out there that can or will explain in detail what the technology accomplished.  Is it possible the tuned input or cathode bias configuration required neutralization.
   
G-day Jim 
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W8JI
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« Reply #23 on: July 29, 2015, 06:35:18 PM »

Thanks for the info Bryan,

Haven't heard one tangible explanation regarding the use of the 200pf caps, choke, and/or resistor used in the grid circuit of the amps.  There has to be somebody out there that can or will explain in detail what the technology accomplished.  Is it possible the tuned input or cathode bias configuration required neutralization.
   
G-day Jim 


The 220 caps are copied from Heathkit. Heathkit used the caps because Bill Orr pressured them to use the caps. Orr did the same thing with me over the years. This was Orr's pet idea called "super cathode drive". You can find it in any of his old Radio Handbooks.

When I was redoing the SB220 style amp for Heath, Orr insisted on floating the grids and using the caps. He also did the same with the SB1000 and AL80A.

Orr's idea what floating the grid added negative feedback. It appears to me, although I cannot be absolutely certain, that this was from Collins use of that system in the 30S1 amplifier. That circuit worked well in the 30S1 because it had a grounded screen grid for shielding, and the tube was AB1. The tube had zero grid current, so the grid was a very high resistance over the entire cycle. This let the caps from grid to chassis form a divider with the cathode/grid C. This floated the grid partly up for RF, and reduced G/K RF voltage.

If the PA tube in the 30S1 went into grid conduction, the G/K would go low impedance. This would immediately limit drive and protect the tube.

The problem started when that good idea for the 30S1 was used in the 30L1. The 30L1 was an AB2 4x triode. It had no shielding from grid to anode, and the grid cathode impedance was low and all over the place with grid current. The capacitive divider was shunted by the varying G-K resistance, and that caused multiple problems. It increased IMD, it helped destabilize the amp (which already was not neutralized),  and it made gain vary more between 80M and 10M because the grid voltage division was not uniform due to the shunting resistance between grid and cathode.

A person who worked at Eimac told me Orr liberated that Collins circuit and coined it "super cathode driven".

Orr, like Collins in the 30L1, did not test or think through the circuit behavior. He pressured (I know this as a direct fact, because I was party to conversations) Heath and others to use that messed up circuit.

I went to Benton Harbor and we ran an HL-2200 (later SB220) on a spectrum analyzer, and we made other tests. These were duplicates of tests I had made on other amps. Unequivocally, in every amp we tested, the amps had much worse gain flatness over the Ham band range and generally produced higher IMD. The behavior of the amp was that tuning for minimum distortion was much more critical, but never got better than with the grid grounded.

We also had to use a larger inductor in the suppressor to stabilize the amp when the caps were added, a sign the grids were poorer grounded at VHF.

That is the actual history of the capacitors.

http://www.w8ji.com/vhf_stability.htm

http://www.w8ji.com/hf_stability.htm


73 Tom (don't care about who agrees or not)   
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K9AXN
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« Reply #24 on: July 29, 2015, 07:52:01 PM »

Thanks for the response.  The “don’t care who agrees was unnecessary”.
 
Interesting visit to the history of the origin.  The only difference between the Collins and Heathkit implementation is the Collins system used a 5pf cap to provide the feedback and the heathkit P to G inter-electrode capacity.
  
Seems that the negative feedback should have rendered neutralization unnecessary.
  
Thoughts?

Regards Jim
« Last Edit: August 12, 2015, 01:25:19 PM by K9AXN » Logged
KD0REQ
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« Reply #25 on: July 29, 2015, 08:07:57 PM »

read W8JI's website pages.  the tubes are marginally unstable to start with starting at the VHF boundary.  add extra positive feedback G-P through the antenna effect of those extra parts, and voila! -- it breaks into song.
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W8JI
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« Reply #26 on: July 29, 2015, 08:54:23 PM »


"I don't always agree with Tom, but this time he's got it right --- ground the grid straight out."




Interesting visit to the history of the origin.  The only difference between the Collins and Heathkit implementation is the Collins system used a 5pf cap to provide the feedback and the heathkit P to G inter-electrode capacity.
  
Both had to generate grid current though.  Seems that the negative feedback should have rendered neutralization unnecessary.
  
Thoughts?

Regards Jim


You may have missed my point. Let me try it differently.

The critical difference between the Collins that **worked**, the 30S1, and the triode amps that do not work as negative feedback is in the control grid to cathode **resistance**, and secondly in the lack of a shielding screen in the triodes.

The 30S1 is a tetrode with zero grid current, thus it has a virtually open circuit grid.

The triode amps are AB2, so they have significant shunt resistance across the G-K capacitance as the tube is in grid current. This creates all sorts of issues.  

The 30S1, that works, has a directly grounded screen. The directly grounded screen allows complete isolation of the input and output.

When the tube is a triode, lifting the grid removes some of the shielding afforded by the control grid.

Negative feedback, if it would occur, is not neutralization. They are different.  

Negative feedback in some way takes a sample of output and uses it to reduce gain. Floating the grid simply reduces G-K RF voltage. An example of NFB in that system would be adding a cathode resistor. Adding a cathode resistor will often improve IMD performance. Floating the grid in a GG triode just makes the system unstable.

There is a world of difference between a grounded-screen cathode-driven AB1 tetrode and an AB2 cathode driven triode.
« Last Edit: July 29, 2015, 08:57:23 PM by W8JI » Logged
G3RZP
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« Reply #27 on: July 30, 2015, 05:14:39 AM »

Once you have grid current, you must have a phase shift if the grids aren't grounded to RF.

There was a thread on here sometime back about the worst amplifiers and someone listed the 30L1. I can't say I was impressed with the 30L1, but they sold a hell of a lot of them! Rather better than the FL1000, however.

The RCA suggestion for a four 811A gg linear grounds each grid through a 0.01 microfarad 600 volt disc ceramic capacitor. One reason for the 600 volts may be that's what they use elsewhere. They use a 1mH plate choke, but only 1000pF to ground from the  base of it. It is neutralised. No glitch resistor - hmmm - and a choke input filter that isn't. It has  5-8H 300mA choke, and 75k of bleeder: that really is a capacitor input filter with a choke to limit the peak current in the 866s. Where I have real problems is that it claims 875 watts (approximately) output, while in their example of design calculations, one 811A in those conditions gives 105 watts, so it should be 420 watts output: the 875 must be PEP input.

But all in all, except for the lack of input tuned circuits, the RCA suggestion is rather more sanitary in many respects than the 30L1.


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W8JI
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« Reply #28 on: July 30, 2015, 10:48:34 AM »

Once you have grid current, you must have a phase shift if the grids aren't grounded to RF.

....and the grid current varies over the RF and envelope cycles!!!

 I've never seen such a unreliable performance mess as the mess created with the small value mica caps in control grid leads of AB2 triodes.

On 160 meters, 600 pF would be 147 ohms. On ten meters, 9 ohms. It not only is non-linear division across an RF cycle, it changes with envelope and with frequency.

The super cathode circuit pushed on manufacturers (and used by Collins in the 30L1) reduces gain more on the high end, where gain is already usually low, and less on low bands where gain is often higher.

It also floats the grids off chassis, and if a tube faults that increases chances of gas-arc damage in the grid and cathode systems.

It does almost everything the exact opposite of what anyone would want.

73 Tom
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W1BR
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« Reply #29 on: July 30, 2015, 10:53:20 AM »

Tom

What does a series resistor do in the cathode circuit to improve feedback?  I've seen comments where a series resistance will improve negative feedback, but I don't see the mechanism that provides this supposed advantage?

Pete
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