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Author Topic: Neutralising  (Read 13029 times)
G3RZP
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« on: July 24, 2015, 07:51:49 AM »

It appears that successful 811 and 572 gg amplifiers have some neutralising to avoid being of somewhat dubious  stability on the higher bands. Way back when, amplifiers with modified 1625 tubes were around: not all 1625s could be used and neither could 807s, because they had the beam plates internally connected to cathodes.

Leaving aside the obvious 'Why would you want to?' question, is it possible that a gg 807 or multiples thereof could be neutralised in the same way as 572Bs and 811s - a small capacitance from the plate to a winding on the filament choke?

It seems to me that it should be possible, or is there an inherent snag that I have missed?
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W1BR
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« Reply #1 on: July 24, 2015, 08:00:52 AM »

Peter

I thought it was just the 1625 that had the suppressor internally wired to the cathode inside of the bakellite tube base shell? 

I'm debating whether neutralization is worth pursuing on an 813 amp design.

Pete
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W8JI
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« Reply #2 on: July 24, 2015, 08:01:23 AM »

I have successfully neutralized 6146 and 807's for GG operation. Some 1625's can have the base removed and the beam formers are on an isolated wire than can be moved to a grounded pin.
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G3RZP
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« Reply #3 on: July 24, 2015, 08:52:01 AM »

Some brands of 1625 had the beam plates connected internally and some had the beam plates on a wire which you could get at. On the surplus market, they all were CHEAP!! I wonder how many million were churned out between 1942 and 1946?

Interestingly, over here, despite the number of 807s produced in the war, 1625s were cheaper, because I believe, the sockets were harder to get and people didn't like 12 volt filaments - most transformers were 6.3 volts.

It's an interesting thought that anyone with a stash of 807s could make a very cheap amplifier....I think I have 6 or so.
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W8JI
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« Reply #4 on: July 25, 2015, 05:31:29 AM »

I built a GG 1625 amp for my DSB transmitter in the late sixties. 1625's that could be modified were affordable for me at a local surplus store. They had huge bins of tubes of all types.

I looked at 6146's in the 80's as an alternative for sweep tubes in a GG amp. It was just too expensive to commercially build. That project went nowhere because of size and cost.
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G3RZP
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« Reply #5 on: July 25, 2015, 05:46:36 AM »

Tom,

Was it because modifiable 1625s were so available that nobody built a neutralised 1625 or 807 amp using the 1625s you couldn't modify?

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KM1H
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« Reply #6 on: July 25, 2015, 07:43:21 AM »

It is the same as today Peter, an AB1 amp runs less efficient and is more prone to being overdriven and become a splatter machine.
With GG in AB2 or almost B the efficiency is up plus the feedthru power helps to contribute to the warm and fuzzies feeling of much higher efficiency.

Ive built several 2 to 4 813's in GG and never needed neutralization 160-10M as long as the layout was good plus I submounted the sockets and grounded the metal shells.

I still have the five 1625's I modified by cutting a notch in the side of the base and separating the beam forming (not a suppressor grid) leads; .25 each NIB at NYC Radio Row. I lived a short bus/subway and later car ride away back then. A Central Electronics 10A drove a single 1625 which drove 4 more. With virtually no test equipment it seemed to work fine 80-15M and once I got the hang of setting the mike gain the reports were good but that was also back when IMD wasnt a concern. Ive no idea of the output and ran them around 650V.
The P&H LA-400 was a commercial version of that 1956 magazine amp which also offered it with 837's which were real pentodes. Im not aware of any 807's or 6146's that could be modified for true GG, super cathode drive yes.

Other somewhat uncommon tubes Ive run in true GG are 715C, 803, 814. Even tried the 100TH, 250TH, and 304TH which werent good choices.

Carl

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G3RZP
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« Reply #7 on: July 25, 2015, 10:00:24 AM »

Carl,

My thoughts were on the lines of 'Unmodified 1625s had feedback because of beam plates connected to cathode. If you neutralise 572Bs and 811s, why not 807s?' In gg they'd still be AB2. It also leads one to wonder if there ever was a manufacturer who built 807s the same way as 1625s with the beam plates connected in the tube base.

AB2 can actually be a bit cleaner than AB1, because the approach to grid current gives curvature the other way: I definitely found with my pair of 4-250s - or 4-400s - that AB2 with regulated screen and shunt regulated bias was cleaner on the spectrum analyser at all levels than AB1 - only a few dB, though.

My present amplifier is definitely AB1 - it's an Acom 1500 - and driven to grid current, it switches off!  Just as well, considering that the 4CX1000 has a delicate grid...
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W8JI
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« Reply #8 on: July 26, 2015, 05:45:35 AM »

Tom,

Was it because modifiable 1625s were so available that nobody built a neutralised 1625 or 807 amp using the 1625s you couldn't modify?



I think the general public didn't understand neutralizing a GG amplifier. Better engineers certainly did, but not the general homebrew people. Heathkit neutralized, as did Gonset and others, but not Collins!! That's why Collins had stability and IMD issues with the 30L1, and why it was critical for exciter cable length.

Another factor was lack of ferrites to make wideband inversion transformers when 1625's were around by the bushel basket, and 807's were being produced in bulk. You just didn't see that many powdered iron or iron composition cores, so neutralizing would have been bulky.

When I looked at 6146's and other tubes, the issue was amplifier case size (marketing department issue, they had a fetish about sales and size) and cost to build something. Remember at that time wholesale bulk on a 3-500Z was 45 dollars US, so a 6146 at $4 each with the added cost of neutralizing was a killer.

As a young Ham, I always looked at tubes carefully. I personally never found any 807's with beam forming plates on a separate lead. I commonly found 1625's with the beam plates out on their own wire.

So I think the issue, just guess, was a combination of lack of ferrites, lack of general population knowledge about neutralizing GG circuits, and how cheap it was to just do other things.

73 Tom
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N2EY
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« Reply #9 on: July 26, 2015, 08:09:10 AM »

I think the reason 1625s were so plentiful was because they were used in so many US aircraft radios. All of the "ARC-5" family used them, both in the transmitters and in the modulators. The ART-13 used the 1625 as well. These radios were produced in enormous quantities, and the tubes in even greater quantities.

It should be remembered that in mid-1945 the war against Japan was expected to last at least another year, if not longer. Huge quantities of materiel were being manufactured for the invasion of Japan, both for immediate use and for replacements and training.

Then the war ended suddenly, thanks to Fat Man and Little Boy, and all that materiel was suddenly surplus. It took decades to dispose of it all.

I used to wonder why the 1625 has the odd 7 pin Medium base, (0.866 inch pin circle) which was used on very few tubes. 1930s receiving tubes that needed 7 pins used the 7 pin Small base (0.75 inch pin circle).  AFAIK only the 53, 6A6, 837 and 1625 use the 7 pin Medium base, though there are probably a few others.

My best guess is that the choice was made in an attempt to keep the tubes in the sockets in aircraft radios, the fear being that a tube the size of an 807 would not be held securely by only 5 pins. Of course a tube clamp could be used, as was done in the ART-13, but the 7 pin base was simpler.

73 de Jim, N2EY
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WB6BYU
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« Reply #10 on: July 26, 2015, 08:50:15 AM »

QST ran a photo of an amp with a dozen 1625s years ago, but didn't
include details on neutralization.  Said the plate load impedance was 75 ohms
so it didn't need an output matching circuit other than a DC blocking capacitor.
No specs on harmonics or IMD, however...
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G3RZP
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« Reply #11 on: July 26, 2015, 09:12:50 AM »

I've seen it suggested that the 1625 had the 7 pin base so that in the rough and tumble of wartime spares supplies, 807s couldn't be substituted in error.

Another big user of the 1625 is the TCS transmitter, using 4 of them. There must have literally been millions of them made.
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W1BR
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« Reply #12 on: July 26, 2015, 11:46:03 AM »

Just like the 813 tube... zillions made, a few NOS military surplus are still readily available.  Any one have a good link for neutralizing an 813 GG amp?  I'm curious how it was done--assuming a winding on the filament choke was used...

Pete
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KM1H
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« Reply #13 on: July 26, 2015, 01:13:04 PM »

I never found it necessary to neutralize even 4 813's in GG when the sockets are submounted and the shells grounded and Ive run them close to 3000V on 10M.
Unlike 572B's which could do with a simple neutralization even in the SB-200 if run hard on 10M.

I dont know who pushed the ridiculous statement that GG never required neutralizing back in the 60's or so. Maybe Orr who wasnt working with ancient designs in the 4-125A to 4-1000A.

The graphite anode 813's appear to be good for about 225-250W PD on SSB and CW; a quiet muffin fan moves enough air to keep the anode seal happy along with a decent heat dissipating cap. Dont try this with the cost reduced late production with sheet metal plates, they act like Chinese 811A's! In any event they handle power much better than even Cetron 572B's and should make a good swap in some amps.

Carl
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G3RZP
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« Reply #14 on: July 26, 2015, 10:21:13 PM »

Quote
I dont know who pushed the ridiculous statement that GG never required neutralizing back in the 60's or so. Maybe Orr who wasnt working with ancient designs in the 4-125A to 4-1000A.

Carl,

At a guess it came from the concept of the Wallman cascode back in the early 1940s, with the classic 6AK5 - 6J4 low noise amplifier for radar IFs. GG stages in receivers weren't neutralised, but generally used triodes with multiple grid leads and in the 6J4 case, gold sputtered grids to minimise transfer of cathode material to the grid - it had a very small grid - cathode spacing to get the gm up. The other tube often used was 1/2 a 6J6 - why they didn't use both halves in parallel to get higher gm and lower input resistance, I don't know. So receivers could use non-neutralised grounded grid amplifiers and get low noise which requires no feedback - ergo, then so could transmitters! Without recognising the fact that the tubes were very different in construction.....

Even the RCA tube manual of 1962 shows an application circuit for a linear using four 811s that has neutralising.

I believe that 813s in grid driven linear service benefited from neutralising on 10: a lot of people neutralised them in Class C on 10 as well, but  at least one commercially UK built ham transmitter used one un-neutralised with no problems. No parasitic suppressor and a plate lead of heavy braid - no nichrome in sight! A Marconi tx with two in push-pull Class C didn't neutralise, either. Neither did the RCA ET4336, which used two in parallel.


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