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Author Topic: Copper Strip For Grid Grounding in TL922A  (Read 21704 times)
W1QJ
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« Reply #60 on: August 22, 2015, 03:54:36 AM »


You infer that Bill Orr was able to exert some kind of uncanny influence on so many of those designers that the concept became nearly universal.

Jim, if you look at the facts, you'll see that is probably exactly what happened.  Way back when the 3-400/3-500 was offered by Eimac.  They (Eimac) showed a typical grounded grid circuit with a pi input network that they suggested.  That circuit was to ground the grids with a short as possible path.  Early amplifiers followed that direction.  All you have to do is go back and look at the original amplifiers using these tubes.  They all followed Eimac's direction to ground the grids.  Then comes Orr and Collins with floating the grids.  According to Tom, who seems to have been knowledgeable  to what was going on with the 30L1 amplifier that was widely used by the military.  Accordingly Collins was trying to deal with the instability problem with the 4 tubes on the higher bands.  They came up with floating the grids as some sort of solution aside from neutralizing.  Once they came up with floating the grids, you suddenly see all the other 3-500 based amps using that design.  Why did almost all the companies use that new design?  It is easier to name the ones that did not as the ones that did.  To my knowledge, Eimac never changed their stance on how to ground the grids other than to stick to their original design.  So how is it that  so many companies followed suit, I would have to say there was some influence there by someone.  According to Tom, and I have no reason to doubt him, he and the engineers at Heathkit were pressured to follow suit.  They did, but apparently against the advice of Tom.  Dentron and Amp Supply did not.  This post is not to be taken as technical, but rather a simple observation of the highlighted comment you made.
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K9AXN
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« Reply #61 on: August 22, 2015, 08:25:57 AM »

Good morning Louis,
First, thanks for the courteous response.

Thanks for the reference back in history and verifying to some extent Tom’s commentary.  When I first started reading this thread and having read many of Tom’s comments I thought there was some logic to his narrative.
  
Early on I agreed with the ground’em and be done with it scenario.  I was however very uncomfortable with the progression through the time line of events and awestruck by the unbelievable  influence that Bill is said to have exerted.
  
I’m still interested in the time line and maybe you can help.  The information that I’ve found is the first implementation of negative feedback was with the Collins 30S1 --- late 50’s early 60’s which was not to stabilize the radio but to reduce IMD.  Also understand it was a successful design.
    
About the same time the 30L1 came to be using the same concept although with 4 811 triodes.  I have found no evidence that the feedback system was for anything but IMD reduction, not stability:  Also no evidence that Bill Orr had any influence with the Collins people.

Reviewing opinions and historical notes, I find an overwhelming endorsement of the 30L1 and have found only one change to the drive system; not the horror stories written.
  
Would I choose 4 811’s parallel for my amp?  Nope, but the Collins folks seem to have made it work.

From a tech point of view, I just couldn’t accept the notion that the plate to grid capacity either didn’t exist, or had no effect on negative feedback, in neither the tetrode or triode.  That one thing disqualified Tom’s narrative.

Thanks for the info and appreciate your responding with courtesy Louis.

Please have a super day --- Kindest regards Jim
« Last Edit: August 22, 2015, 09:50:24 AM by K9AXN » Logged
W1BR
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« Reply #62 on: August 22, 2015, 09:03:54 AM »

Hello Jim,

Regarding 811A tubes, they'd be my last choice for a linear amplifier, but only because the supply of RCA or other American tubes has long dried up... back in the 50's and 60's thousands of hams built home-brew amps using a quad array of 811A tubes, with good results. 

My next amp is going to use two or three RCA graphite plate 813 tubes.. only because I have a good stash of them, and that I'v found that 700 watts of RF output is all I need to work DX, even in pileups.

Pete
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W8JI
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« Reply #63 on: August 23, 2015, 05:51:59 AM »

The Collins 4x 811A amplifier is a good example of an amplifier with stability issues. The 30L1 has a long history of Collins field corrections for stability and IMD, where Collins played with the grid circuit values and the exciter to amplifier cable length.

When the input cable length becomes critical for IMD, it indicates a severe regenerative feedback problem in the amplifier.

This problem really all centers around the lack of neutralization and the small value grid bypass in the 30L1. They attempted to correct it by adding loss to the input circuit, by specifying an abnormally long cable from exciter to amplifier.

If the amplifier was stable with a good low-pass input circuit, the input cable length would be meaningless for amplifier generated IMD.
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N3QE
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« Reply #64 on: August 23, 2015, 05:59:33 AM »

You infer that Bill Orr was able to exert some kind of uncanny influence on so many of those designers that the concept became nearly universal.
 
Sounds like a visit to a séance where uncle Harry blew smoke through a now hear this tube torn from a 1904 wind jammer and quietly whispered Beware the ides of March --- Bill Orr – Bill Orr.

Bill Orr published a handbook. ARRL published a handbook. Both handbooks have many examples of "bad or incorrect ideas", applied by followers as if they were pursuing the holy grail, over the decades. It doesn't mean the authors were bad people or technically incompetent. I think the lesson to learn is that after something is published it is hard to get corrected.
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W8JI
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« Reply #65 on: August 23, 2015, 08:50:45 AM »

You infer that Bill Orr was able to exert some kind of uncanny influence on so many of those designers that the concept became nearly universal.
 
Sounds like a visit to a séance where uncle Harry blew smoke through a now hear this tube torn from a 1904 wind jammer and quietly whispered Beware the ides of March --- Bill Orr – Bill Orr.

Bill Orr published a handbook. ARRL published a handbook. Both handbooks have many examples of "bad or incorrect ideas", applied by followers as if they were pursuing the holy grail, over the decades. It doesn't mean the authors were bad people or technically incompetent. I think the lesson to learn is that after something is published it is hard to get corrected.

Tubes are going away but it is still worth learning how this circuit can work in a few limited cases, and how it does not work in almost all common cases.

The 8877 looks like this for low frequency passive capacitances:



The Super Cathode System looks like this:



When the tube is driven into grid current, resistance from control grid to cathode shunts the cathode to grid path. This screws up the divider, making it non-linear with drive level and even over parts of an individual cycle.

It is actually a terrible idea unless the tube is a tetrode or pentode, and the control grid never gets close to conduction.

It is pretty simple to see how bad it is, and why it causes problems.

73 Tom
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K9AXN
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« Reply #66 on: August 23, 2015, 06:20:35 PM »

Hello Jim,

Regarding 811A tubes, they'd be my last choice for a linear amplifier, but only because the supply of RCA or other American tubes has long dried up... back in the 50's and 60's thousands of hams built home-brew amps using a quad array of 811A tubes, with good results. 

My next amp is going to use two or three RCA graphite plate 813 tubes.. only because I have a good stash of them, and that I'v found that 700 watts of RF output is all I need to work DX, even in pileups.

Pete

Good evening Pete,

Your bringing back old memories.  I built a plate modulated grid driven single 813 rig back in 1955.  I was 15 years on the planet and so excited that I couldn't stand myself.  Don't think I slept for weeks.  Makes me think I should drag that radio out and bring some of those memories back.  Send some photos as you go.

Kindest regards Jim
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W1BR
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« Reply #67 on: August 24, 2015, 08:15:33 AM »

Jim... I have so many projects, and so little time, and at my age I suspect many will remain uncompleted dreams!!!  LOL!  I did assemble all of the parts for the 813 Amp project, so it might be a reality. I have several repairs in the cue that I need to get done for friends... there never seems to be enough time. I should have  had build two or three work benches, so several works-in-progress could be ongoing at any one time.

Take care

Pete
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KM1H
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« Reply #68 on: August 24, 2015, 01:22:44 PM »

There was little to no trouble with Collins, Dentron, Heath, Gonset and likely other 4 X 811A amps using US built tubes. Excessive tuneup times brought runaway conditions and destroyed tubes no matter what brand of amp or grid configuration. Ive worked on dozens of 30L1's of many production runs as well as USAF versions released to surplus where it was near impossible to train servicemen at the MARS and phone patch stations how to tune. I witnessed the lack of proper training at several USAF facilities in Southern EU, Near East, and North Africa while in the USN. As a guest op I did my own tuning and thought I saw a glimmer of understanding at times.

The Russian and Chinese 811A's were never up to US quality and specifications, and building an amp around them was a formula for disaster. Svetlana quit reselling them (built in the Ukraine) and the Chinese quality kept going downhill; I wonder if the latter was because Ameritron, RFP, and others were continuously pushing for lower prices and the Chinese provided them..... Id hate to place all the blame on the Chinese.

Carl
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KD0REQ
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« Reply #69 on: August 24, 2015, 02:14:25 PM »

with all the audiophools drooling over tubes, and the bigger the better ("ooh, much bigger than a 2A3, this must be a hell of a tube!"), the Chinese were very happy to give them what they wanted.  at 450 volts instead of 600+, you don't need to pump 'em down.  no flashovers feeding audio transformers, so the odd fingerprint inside the glass doesn't matter.

that's how you get creepy tubes... the primary market doesn't need quality.
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KM1H
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« Reply #70 on: August 24, 2015, 05:07:41 PM »

Quote
that's how you get creepy tubes... the primary market doesn't need quality.

At one point there were 3 Chinese 572B manufacturers and 2 were damn good. I used dozens of them in SB-200, FL-2100B, Clipperton 1200 and other 2x 572B amps converted to 6M. When the audiophool biz really cranked up they discontinued the 572B in favor of lots more profits.
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