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Author Topic: 3-400Z No Parasitic Supressors  (Read 12631 times)
KO4NR
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« on: March 27, 2015, 11:10:48 AM »

My Swan Mark 1 has no parasitic suppressors and the amp was unmodified when I got it.   I have read the 3-400Z is a very stable tube and doesn't require suppressors.  Is this case or is it the Mark 1 design?

Always willing to learn from you guys!!

73,
Bill
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WA8UEG
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« Reply #1 on: March 27, 2015, 11:47:07 AM »

I'm not sure about the MK I but my Mark II with 3-400's does have them. The MK I and Mark II RF deck are very similar as I recall. The major difference between the 2 was the MK I had a light power supply and of course the MK II had a very beefed up and remote supply. The Mark II originally came with 3-400's then with 3-400's and a bias board that you could remove a jumper from and use 3-500's. When Swan ran out of 3-400's they shipped with the jumper removed and 3-500's.
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W8JI
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« Reply #2 on: March 27, 2015, 11:48:49 AM »

The 3-400Z is not that stable.

There are certainly cases where tubes do not require parasitic suppressors, it is very common with flanged or ringed tubes with conical grid supports. As a matter of fact, the higher the full power operating frequency the more likely stable operation at lower frequencies becomes.

The 3-400 does not fall into that class.

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

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W1QJ
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« Reply #3 on: March 27, 2015, 01:18:01 PM »

The 3-400Z is not that stable.

There are certainly cases where tubes do not require parasitic suppressors, it is very common with flanged or ringed tubes with conical grid supports. As a matter of fact, the higher the full power operating frequency the more likely stable operation at lower frequencies becomes.

The 3-400 does not fall into that class.

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



He's right Tom, the Swan Mark 1 came through with no parasitic suppressors but they did have a ferrite bead over the plate lead at the tube.  It always amazed me but I suppose at 2500 volts it was just under the gain level.  Go figure.
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KM1H
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« Reply #4 on: March 27, 2015, 02:45:16 PM »

More snake oil trying to save a few pennies for real suppressors.

Wind some real ones.

Carl
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KO4NR
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« Reply #5 on: March 27, 2015, 03:44:48 PM »

Would suppressors for the SB220 work ok?
Bill
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AH6RR
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« Reply #6 on: March 27, 2015, 03:53:27 PM »

I repaired the power supply in a MK1 a few years ago and saw that the tubes had no suppressors they had a ferrite bead and scratched my head on that one. Told the owner that they should have real suppressors and made some for it using some silver plated #16 wire and if I can remember correctly 3 220 3W carbon resistors as in the MKII. It is still chugging along today.
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KM1H
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« Reply #7 on: March 27, 2015, 05:35:26 PM »

Quote
Would suppressors for the SB220 work ok?
Bill

Same tube basically so same suppressors. Use an Ohmite OY resistor

Carl
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K8AXW
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« Reply #8 on: March 27, 2015, 09:08:30 PM »

JI:  Tom, didn't I read someplace on your website that you couldn't take a few resistors of a particular value and wind X number of turns on one of them and use them for a parasitic suppressor for any amplifier??

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W8JI
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« Reply #9 on: March 27, 2015, 09:44:23 PM »

JI:  Tom, didn't I read someplace on your website that you couldn't take a few resistors of a particular value and wind X number of turns on one of them and use them for a parasitic suppressor for any amplifier??



Every different style of amplifier has different requirements.

I've seen some with notoriously bad tubes be stable when you wouldn't think they would be.

Tubes usually oscillate because the grid is resonant somewhere near where the anode circuit has a high impedance. I can't recall offhand which way it has to be, but I think the anode has to be above the grid resonance slightly. If it is below the grid, it will likely be stable.

I wouldn't chance a tube like the 3-400, even if it was working at one moment of time. I suppose a bead could be enough to dampen it, but I wouldn't go down that road.

A tube like an 8877, if the grid ring was well grounded, I wouldn't worry about. That tube has a grid up way high near UHF. By that frequency, the anode circuit is a mile below it in frequency.

Not a 3-400 or 3-500, and the worse tubes of all probably are an 811 or 572.
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G3RZP
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« Reply #10 on: March 28, 2015, 02:41:54 AM »

I have seen commercial amplifiers using 6146s with ferrite bead parasitic suppressors, although they had a pi network with a fixed capacitor of the Erie type with screw terminals from plate to ground with short leads, inductive tuning and switched extra capacitors for lower frequencies - they only went up to 16 MHz. I could never make ferrite bead suppressors work, though.
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W8JI
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« Reply #11 on: March 28, 2015, 05:21:56 AM »

I have seen commercial amplifiers using 6146s with ferrite bead parasitic suppressors, although they had a pi network with a fixed capacitor of the Erie type with screw terminals from plate to ground with short leads, inductive tuning and switched extra capacitors for lower frequencies - they only went up to 16 MHz. I could never make ferrite bead suppressors work, though.

My homebrew 6146 transmitter is 100% stable with no suppressor at all, at least it is with a five inch long anode lead hanging on it.

I'm not sure I would do an 80-10M high power 3-400 without a normal suppressor, because the tendency is for that class of tube to run away at 100-160 MHz, depending on socketing. I would think it tough to get enough slope on the transition from low inductive reactance to high series R to keep ten meters good while loading the plate on 100 MHz and up. 
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KM1H
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« Reply #12 on: March 28, 2015, 08:54:44 AM »

Triodes were often very stable back when the mu was way down in the 15-40 range and once neutralized they were fine, parasitic suppressors werent used. My first amp back when in HS was PP HF-200's out of a diathermy machine and were so well used I barely got to 800W input on 20M and didnt even try any higher.

The next amp was thanks to being in the AF MARS program so a pair of BC-610's were used for parts and I wound up with PP 250TH's modulated by NIB 810's from the same source. That worked fine right to 10M but operating was seriously curtailed due to 10 and 15M TVI !! The NYC-LI area was Channel 2 and 4 territory plus B&W TV IF's were around 21mc.  Going to HB 3 el yagis and coax feed with a pair of Johnson LPF plus some amp shielding cleaned up most of that and being on during Cycle 19 was an absolute blast with even 10 being open all night to exotic DX places. That was also my last PP amp and plug in coils.

When the 807 came out in the late 30's was when parasitics became a ham buzz word as high gain, wide open construction and little understanding of the issues was the norm. When special sockets with partial shields came out and bypassing and shielding was followed it was very stable, parasitic suppressors were still in the future but low value resistors in the grid and screen leads were known 10+ years earlier in receiver circuits.

The 6146 and real parasitic suppressors came along at the same time (plus many other high gain power tubes into the 4-1000A size and beyond) and allowed even some sloppy construction to be stable, especially in some commercial gear and kits. I built and sold several 6M transverters in the 60's with a single 6146 that were very stable without a suppressor but I included them anyway. Having permission to use the National Radio machine and sheet metal shops after hours and weekends were prime reasons for the stability as RF only went where it was supposed to go  Shocked

Ive used ferrite beads on grid and screen leads but never trusted them on the plate and later experiencing them on the SB-230 just reinforced that as I built several to order from a pair of Boston area Heathkit shops; as well as numerous SB-201 and SB-221/HL-2200's.

Many confuse parasitics and neutralization and often consider them the same thing, its a shame the subjects are pretty much ignored in the various ham publications. Toss in those who fall into the voodoo nichrome trap when simply replacing the ancient suppressor resistor would be a lot better. Many kit amps need serious attention to sloppy construction, cold solder joints on coax shields and more.

Carl
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G3RZP
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« Reply #13 on: March 28, 2015, 09:00:28 AM »

Tom, Carl,

Surely you both know that nichrome is the universal panacea for all ills? Infallible in preventing hurricanes, good for babies in teething, prevents all instabilities and turns lead into gold.....and has its very own physical laws applicable to it, unlike other materials..
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K8AXW
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« Reply #14 on: March 28, 2015, 09:34:14 AM »

Peter:  There was a story about a guy in NYC who wore an amulet around his neck, every day.

When asked about it he said it was to ward off lions. The guy that asked the question said, "Hell man, there's no lions in NYC!"

The man with the necklace said, "You see, it works!"

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