On Thu, 23 Apr 98 14:40:09 -0500 Jon Ogden <jono@webspun.com> writes:
>> The main
>>difference is that Rich tries to get by with #16 wire whereas
>National
>>used 1/2" wide strap around a big 50 Ohm non-inductive resistor.
>>I do object to using that material in the complete plate to blocking
>cap
>>path and prefer the traditional low inductance wide silver plated
>>strap/tubing strap. If the suppressor is doing its proper job right
>at
>>the tube then there is no reason to continue nichrome to the blocking
>>cap.
>
>Carl, I wondered about this as well. The first thought in my mind
>was:
>Just 16 Ga. wire...will that work? Then I realized the the coil in my
>
>original supressor was made with 20 Ga.wire! If 20 Ga works, then
>certainly 16 will.
The last time I saw #20 in a suppressor was for a 6146. Any suppressor
for a 3-500Z that I am aware of uses #12 or 14 and real quality ( read
stable) amps use strap.
I would not even attempt anything under #10 for a 4-1000A.
BTW, also note that those little bitty #16 wire kits include silver
solder since they run so hot...not a good engineering practice IMO.
Another problem I have with Rich's suppressors is that they will not work
on 6M. I get lots of broken amps in here with those suppressors. I
normally rip them out as a matter of course and try and cure the
problem...not the symptom. Anyway, I have tried and fried those
suppressors almost instantaneously in a 6M conversion even with 572B's.
In a SB-220 they flash and explode in a few seconds. That tells me there
is something inherently wrong with their design. To be honest I have not
bothered to really dig into the reason for the failures since my own
design works at HF and 6M and as I said earlier I am loath to waste time
(money) trying to fix some add-on gadget.
Additionally, the standard supressor might work
>fine
>today. But what if it is marginal and as the tube ages, something
>changes to decrease its effectiveness. Or what if you put in a tube
>that
>has better gain which again does the same thing. I have always
>over-designed all my circuits and I am just trying to bullet-proof
>things. Lowering the Q is never a bad thing.
Once you can divorce yourself from the old carbon composition resistance
and changing values and switch to a better R; then tube aging, or
whatever will not be an issue. In fact, as a tube ages the gain and
therefore the susceptibility to oscillation decreases. A metal oxide
resistor does not have the thermal problem of carbon comps....neither
does a Globar or that 20W pure carbon that Peter likes.
Do yoursel a favor....go to Dayton and pick up a 50 Ohm 20-50W
Carborundum style resistor. Wind the L from 1/2" wide copper strap and I
will bet you a dinner you will never have a parasitic or overheated R.
One of my better finds at Dayton a few years ago was a case of 100
Carborundum 50 Ohm resistors. They were all sealed in individual foil
envelopes and had the designation " 5905-107-8237, Resistor, Fixed Comp,
1ea Mfr P/N SP5X5/8 and dated 04/66. The seller was an obvious Duh What
type and it took under 2 minutes to buy the whole lot for $10. Heck, he
probably got them for nothing at some military dump. I believe these are
25W rated and are a compact 5" L and .75" wide.
I save these for serious size tubes and a few turns of 1/2" copper strap
has tamed anything that I have had the nerve to build including tubes
with 5 digits in the dissapation designator.
>
>>Layout is critical and in a sloppy version it may be necessary to use
>>more nichrome or whatever. But in a good layout I would think that
>the
>>lowest possible inductance in order to move resonance above the tubes
>>ability to oscillate would be the "good engineering" approach.
>
>Yes, layout even at HF is EXTREMELY critical and can solve many a
>problem. Agreed. However, a 4-1000 still has gain up at 150 MHz and
>many of the tubes today go much higher. If the anode circuit is
>resonant
>at 75 or 80 MHz, please tell me how you are going to move it 70 MHz
>higher, let alone 20 MHz higher with a coil and resistor.
You are missing the point entirely Jon....I am not talking about the
tubes parasitic frequency....that should be taken care of by the
suppressor. What I am talking about is other circuit resonances that are
typically well over 100Mhz and can then possibly be manipulated to where
the gain drops in the case of most glass tubes.
From what I
>
>can figure there is no way to move the resonant frequency that much
>(at
>least easily). Sure, maybe you can move it from 80 to 90 MHz, but I
>doubt the tube gain is that much different there. So what you are
>left
>with is having to reduce the Q.
>>
>>Secondly, I have harped for a year about VHF resonances in the tank
>>circuit with nary a bit of comment from anyone. Now I see that Rich
>has
>>finally added that possibility to his ever varying repertoire.
>National
>>Radio solved that problem very neatly in the NCL-2000 and NCX-1000 by
>>adding a SERIES cap...not a shunt as has been discussed about here
>the
>>past few days. The National engineers added a 10pf capacitor across
>the
>>20 and 40M shorting bandswitch contacts which eliminated a
>particularly
>>nasty proclivity to arcing.; perhaps you would wish to analyze that
>>circuit.
>
>It's an interesting idea. Although, I do believe that you could put a
>
>cap in series or in shunt. And series circuit can be turned into a
>parallel equivalent circuit and vice versa. Although a series cap
>across
>the bandswitch might be easier.
Adding 10-15pf in shunt in many amps would then not permit use on 10M.
The typical 250pf air variable already has a min C of 20-25pf. Trying to
find the proper shunt C with both the HV and current ratings along with
low L will be a chore above the 6146 class of tube. The series C can be
a lot less robust, the NCL-2000 used a small 1KV silver mica and that is
the same as I use today.
Yes, filters can have resonances
>outside
>their passband and that could be a problem. Adding the caps could
>help
>that.
I agree on filter resonances but in the pure sense I do not believe that
is the problem. In a real world tube amp with discreet components, point
to point wiring, etc you have many, many sources of spurious resonances.
Not only do you have all the various coupling from component to component
and to ground but in some cases the actual dimensions of the plate
circuit enclosure come into play. The possibilities can be overwhelming
and therefore the low Z series cap is a rather simple way out in
many...of course not all...cases.
>>I use that idea in stock SB-220's and have had users report no more
>>switch or Tune cap arcing. I've used it in various Alpha 8874 amps
>that
>>had arcing problems...and suppressor R that was as new.
>>
>Very possible. So since the cap removed the VHF resonance in the tank
>
>circuit, I would conclude that you do admit the arcing was caused by a
>
>VHF oscillation. So oscillations can cause arcing.
Of course...I have never denied that nor has anyone else that I am aware
of. BUT there is a big difference between parasitic caused problems due
to a tubes internal geometry and a purely external resonance source. You
and others have to think about keeping the two isolated...at least for
discussion and the individual cures required.
>>
>>Thirdly, my dis-belief centers around the big bang and that is where
>I
>>suggested ( or that was my intent anyway) an article. You are the
>>engineer, show me how a VHF parasitic has enough energy to bend a
>3-500Z
>>filament.
>>I say it is a gas/plasma discharge and partially caused by barnacling
>AND
>>the lack of proper PS surge protection. It is this area only that I
>>refered to junk science.
>
>Well, as I said earlier, this I cannot comment on since I do not have
>knowledge of tube structure. My experience with parasitic
>oscillations
>has not been of the "big bang" type, but I do not dispute that it
>could
>happen.
Well, there we differ...I do dispute that.
So I would not want to write a paper on anything that I don't
>
>have knowledge of or understand. The part of Rich's theories that I
>support are about lowering the Q of the resonances in the tank.
But yet according to Wes Hayward's web page and Tom Rauch's input, Rich's
stuff does nothing for lowering VHF Q. We all know Rich claims otherwise
but how come Hayward disavows his conclusions and has gone so far as to
publish his own page in refute????? Something has a decided dead fish
smell IMO.
Hey Jon, at least this is more fun then telling some senile citizen to
bugger off!
73 Carl KM1H
>>
>>The BIGGEST problem in many ham amps IMO is the save a buck attitude
>of
>>the manufacturers. They use marginal components, leave out key safety
>>features and would be laughed out of anything but the ham market.
>
>I'd agree. However, you'd be surprised how many people have the
>"we've
>always done it this way before attitude."
>>
>>My own suggestions for an amp would include:
>>
>>1. HV surge suppressor resistor of a value and wattage that can limit
>the
>>instantaneous PS discharge current to a safe non-destructive value.
>>2. Use of a suitably placed series capacitor in the tank circuit to
>>provide a low impedence VHF path to the load.
>>3. Use of a suitably sized parasitic suppressor resistor and phase
>out
>>carbon composition. A 5W metal oxide should be the minimum for
>3-500Z
>>size tubes; a pair of them for the 4-1000A.
>>4. Use of a resistive suppressor L material only if necessary to tame
>a
>>known problem tube such as the 3CX1200A7 or to compensate for layout
>>problems.
>
>Certainly not bad ideas at all.
>
>To those who are tired or reading about parasitics: I am sorry. But
>it
>is an area some of us are interested in and I have been learning a lot
>
>about them.
>
>73,
>
>Jon
>KE9NA
>
>
>--------------------------------------------------------------------------
>Jon Ogden
>
>jono@webspun.com
>www.qsl.net/ke9na
>
>"A life lived in fear is a life half lived."
>
>
>
>
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