ag6k amplifier mods (rebuttal)

Wed Apr 12 17:02:22 EDT 1995

Wednesday, 7 September, 1994

From: Richard L. Measures, AG6K
6455 La Cumbre Road
Somis, CA  93066
1-805-386-3734

To: Paul Pagel, N1FB
Technical Correspondence Editor, QST

Re: Technical Correspondence, page 71, September 1994 QST, Revisiting "The
Nearly Perfect Amplifier": 

   No one person knows everything about high power RF amplifiers-so there are
bound to be some issues to resolve. Thus, I am grateful for Paul's column in
QST. It's a good venue for separating wheat from chaff. 
   This discussion involves an unusual phenomenon-intermittent push-push 
VHF - UHF parasitic oscillation - that presently fuels passionate, sometimes
odious, debate. If you are not familiar with this subject, "Parasitics
Revisited" (9/90 & 10/90 QST) might be worth reading. 
   Readers are invited to compare statements made by the six Contributors with
what was actually written in "The Nearly Perfect Amplifier."

   To begin with, Fred Telewski is correct about my amplifier design goals. I
am not a performance person. Long-term freedom from grief and linearity are
more important to me than the last watt of performance. Murphy's Law is
displayed prominently in my radio room. I try to design with Murphy in mind. 

FILAMENT VOLTAGE

   I recommended using regulated DC voltage for indirectly-heated cathode
[IHC] tubes. How did Fred come up with his 5V, 30A figure? The only amplifier
using IHC tubes which comes to mind that could use a 5V, 30A regulated supply
is (3) 8877s in parallel. 

FILAMENT INRUSH CURRENT

   On page 4 in the 3-500Z Technical Data, Eimac states that "For best tube
life the inrush current to the filament should be limited to two times the
normal current during turn on. This will minimize thermal stress (Tom Rauch
is correct) on the thoriated-tungsten filament wire, which can cause internal
geometry changes during repeated cycling." The 3-400Z/8163 carries a similar
proscription. Eimac says nothing about limiting filament inrush current for
IHC tubes up to and including the 290 (5kW anode dissipation), Do some people
obtain their information from a higher source than Eimac?
   Although it's comforting to hear that it's "extremely unlikely" that an
amplifier could have excessive inrush current, neither Tom nor Fred provide
specific measurements. From measurements, I know that some do and some do not. 

GRID PROTECTION

I have seen and heard of dozens of cases where a 0.5W carbon-film grid fuse
resistor popped during a glitch. Subsequently, the tube usually proved to be
undamaged. I have seen cases where a 1A 1mH grid choke on a 3-500Z failed to
open during a glitch - and the tube shorted. Such chokes sometimes open during
a glitch - apparently saving the tube. Since the fusing current of #27 copper
wire is >15A, obviously, an unusual event took place. Such amounts of grid
current can not be explained by high SWR, tube gas (the vacuum subsequently
tested good), or operator error. The transistors that are used in grid
protection circuitry are typically rated at a few amperes maximum. Does it
seem possible that >15A could cause a C-E short in such transistors?  Would
>15A be likely to open a 0.25A 3AG fuse or a 0.5W resistor? Sometimes simpler
is better. 
   In most of the grid-shorted IHC tubes I have opened, I saw evidence of
gold-sputtering on the cathode, missing gold areas on the grid, and yet no
sign of heat stress on the grid structure - pretty unusual in my opinion.
About half of these grid-shorted tubes came from commercial amplifiers that
had electronic grid over-current protection circuitry that reportedly shorted
during the event that shorted the grid. The remainder of IHC grid-shorted
tubes were damaged by over-driving the cathodes. Doing so causes the cathode
coating to flake off - creating a grid to cathode short.
   I have not found a bent grid in any grid to cathode shorted tube. 100% of
the grid to cathode shorted directly-heated cathode [DHC] tubes I opened had
bent filament helices.[1]  Some of these tubes had low operating hours. I can
not believe that Eimac installs bent filament helices in 3-500Zs. If they did,
such tubes could not pass their rigid final inspection standards. Tungsten
wire is incredibly difficult to bend. To re-straighten bent filament helices
in a 3-500Z I found that it takes about 2000 deg-K and a lateral force of 10
to 11G for about 30 seconds. Obviously, it took an unusual event to bend the
filament.
   Tom, Fred, and Reid tell us that grids can not be protected by a frangible
element such as a fuse or fuse-resistor. If a fuse melts before the grid it
is in series with overheats, why won't the flow of grid current stop?
Considering that the mass of a typical 0.25A 3AG fuse element is <1% of the
mass of a typical grid - and the melting point of a fuse is at least 1600 
deg-F less than the boiling point of gold, I don't follow their logic. 
   Are fuses obsolete? John Fluke Co. uses a fuse to protect their DMMs.
Thanks to its 2A fuse, my Fluke has so far survived every time I made a
mistake - and I am well into my second box of fuses. 
   Re: Page 71, column 3, #1: Reid states that "We do not understand the
reference to sudden bursts of VHF or UHF grid current." Then Reid issues a
blanket clean bill of health to every commercial amplifier made. However,
one commercial amplifier manufacturer now admits to customers that their
amplifier[2] occasionally has parasitic arcing at the bandswitch-but not
anymore so than other manufacturers' amplifiers. This is an improvement over
what they used to tell us - that burned bandswitch contacts were caused by
rapidly switching the bandswitch while transmitting at full power.[3]
   At the urging of Paul Pagel, Tom telephoned me after "Parasitics Revisited"
was published in the Fall of 1990. Tom said that he was angry because
customers who read my article were telephoning him complaining that their
burned bandswitches looked like the ones in the article's photographs. Tom
put the blame for the burned bandswitches on cheap coax, operator error, and
bad antennas. During our conversation, Tom told me that he had repaired 400
Heath SB-220s. I asked him if he ever saw parasitic damage. He said that
SB-220s have lots of parasitic problems. I asked him why the amplifiers he
designed did not have parasitics. He replied: because Tom Rauch designed them.
After I pointed out that his parasitic suppressor design was virtually
identical to the SB-220's, it sounded like someone switched on a rather
stentorian speech processor. 
   It was suggested that I discovered/invented intermittent push-push VHF/UHF
parasitic oscillations in order to make money selling Low VHF-Q Parasitic
Suppressor Retrofit Kits. I started selling these kits around December of
1988. It was not my idea. It was the idea of amplifier owners who had
experienced the problems I discussed in the October 1988 QST article. I have
not made money selling suppressor retrofit kits because that's the way I
decided to operate the business from day one. 

   A brief history of AG6K and parasitics: I did not discover them. They
found me. Parasitic oscillations appeared in my SB-220 amplifier - quite
uninvited. Intermittent arcing, an occasional big bang, tune-up vagaries and
smoking suppressor resistors were the manifestations. I tried replacing the
resistors, but the new ones subsequently became crispy-crittered. Initially,
I had some hunches, but I was not confident what was at the root of these
problems. I am a curious guy, so I began working on other amplifiers that had
similar problems. The denouement began when I agreed to work on an unruly
Dentron MLA-2500 in the Winter of 1985-1986. After some experimentation, it
appeared that there might be something wrong with the (2) 8875 tubes. I
telephoned Eimac. They connected me with Willis B. Foote, Eimac's Chief
Specifications Engineer of the Power Grid Division. I told him what I had
observed. He said that he needed to evaluate the tubes. I sent the tubes in.
Subsequently, I received a phone call from Mr. Foote. He said that both tubes
were found to be defective due to erratic leakage and emission problems. An
Eimac tube engineer cut open the bad 8875s for inspection. He found that the
gold plating on the grids had evaporated/sputtered and contaminated the 
insides of the tubes. Mr. Foote said that the damage was probably caused by
a high frequency oscillation. I asked how high was high. He said perhaps very,
perhaps ultra. He said that Eimac tube engineers encountered the gold-
sputtering phenomenon during testing of the 8877. I asked him if an 
engineering bulletin had ever been published on the subject. He said no -
adding that finding a way to improve the stability of this amplifier was my
job. He said it was good we were having this conversation because he was 
retiring shortly - and all of the members of the 8877 team who had knowledge
of this subject were gone. Later, I received a letter from Mr. Foote that
outlined the oscillation/gold-sputtering problem. Mr. Foote also sent me a
gratis pair of new tubes. I was surprised because the bad tubes were long out
of warranty and the oscillation problem was not caused by a fault in Eimac's
design of the 8875. Power grid tubes don't oscillate by themselves. A tuned
circuit in the amplifier allows them to oscillate.

   When I spoke with Paul Pagel on 24 August 1994, Paul said that Reid Brandon
was aware of the Foote Letter. Paul told me that Reid said Mr. Foote was not
authorized to release the information to me. 
   Did I state that parasitics bend grids, destroy tank capacitors or cause
anode to cathode arcing?  
   Re: the list of..."recognized experts in the amplifier community"... A
Hughes employee told me that one of their 100kW RF amplifier designs incorpor-
ates several low Q devices to thwart anode circuit parasitic oscillation. 
I noticed that Hughes was not on the list of "recognized experts". 
   Tom states that it is impossible to perform failure analysis of a tube and
determine if the failure resulted from excessive VHF or UHF grid current.
Eimac was doing it before 1986. You should try looking at the insides of a
bad tube yourself, Tom. It's fun and educational. A low power microscope is
helpful. Details and photographs are provided in "Parasitics Revisited."
   Tom tells us it's possible to damage an 8877 in "only a moment" by applying
100W drive. Normal drive is around 80W. In a vacuum, gold boils at about
2000 deg-F. The 8877's grid is made from bar-stock, not wire. Thus, its mass
is substantial - probably on the order of 40g. How can an extra 20W of drive
boil gold off of the grid?  

GLITCH PROTECTION

   I agree with Fred that it's a matter of designer choice. For long-term
freedom from grief, planning for unusual events hardly seems like a matter of
choice. Using a glitch protection resistor to limit fault current is not
Rich's idea. Beginning sometime around 1978, Eimac began stressing the 
importance of limiting fault current. In the Eimac 8989 Technical Data (1985),
page 3: "A protective resistance should always be connected in series with
each tube anode to help absorb power supply energy if an internal arc should
occur."  More information is provided in Eimac Application Bulletin #17,
Fault Protection. For their 5CX1500B, Svetlana Electron Devices in St.
Petersburg, Russia says "The tube and associated circuitry should be protected
against surge current in the event of an arc with a current limiting resist-
ance of 10 - 25 ohms..."
   While it is true that glass-coated wirewound resistors are not as high-tech
as pulse-energy absorbing resistors, in practice 10W 10 ohm glass-coated
resistors seem to hold up pretty well in legal-limit amplifiers - and they
are much less costly. For above-average anode voltages, using two such
resistors in series is indicated. 
   Tom is correct about the ESR of electrolytic filters. For a typical HV
supply and HV-RFC, a figure of about 9 ohms total is representative. However,
several kV divided by 9 ohms is still a substantial amount of peak current.
>From my experiences - as well as Svetlana's and Eimac's, a series resistance
is needed to reduce the peak discharge current. 

POWER SUPPLIES

   Transformers - Re: Potting: Polyester laminating resin is designed to 
allow air bubbles to escape from fiberglass laminates. I use the stuff - 
sans-vacuum - for potting transformers. I see air bubbles cease rising to the
surface before the resin gels. For specifics, send me a SASE.  
   Rectifiers - I agree that rectifiers should be tested. I covered this
subject in a QEX article on building and using a HV breakdown tester.
Manufacturers of HV rectifiers stopped using so-called "equalizers" at least
a decade ago. Old habits die a slow death. 
   Fred said he did not understand my comments about choke filters.
A question: Why don't Collins Radio, Gates, Henry Radio, Continental, Harris,
Hughes, ... use swinging choke filters or non-resonant fixed choke filters in
the HV power supplies for their linear amplifiers? Such filters try to
maintain a constant current into the load - not a constant voltage across it.
To amplify SSB linearly, an amplifier needs a fairly constant supply voltage.
The only type of choke filter that tries to maintain a constant voltage across
the load is the resonant-choke. Once I asked an old timer, Mac, W6SDM, what he
thought about using choke filters. He told me that choke filters have poor 
load-transient V regulation. He recommended a test. Mac said to connect a DC
oscilloscope - not a meter - across the output of a choke filter power supply,
change the load resistance, and watch the scope. I tried it. When the load R
decreased, there was a temporary dip in output voltage. When R increased there
was a temporary surge in output voltage. But isn't that exactly what an
inductance is supposed to do - i.e., maintain constant current? The larger the
change in load current, the larger the voltage transient. When the load went
from zero to 100%, the output voltage temporarily dipped more than 50%. The
"old goat" was right on - as usual. Although transient regulation can be
somewhat improved by increasing the C of the filter capacitor, the best 
solution is to resonate the choke - like the "big boys". 
   Electrolytic Capacitor Equalizing Resistors-Tom says that the resistance
of such resistors is very important due to leakage current considerations.
I can't find anything like that in Sprague's Engineering Bulletin 3431D for
electrolytic capacitors. It seems to me that it would be nice if the equalizer
resistors did not bake the life out of the capacitors. Currently manufactured
450V 300uF electrolytic capacitors typically exhibit a leakage of <300uA @
25 deg-C & 450V. With 100K ohm equalizers, the ratio of the equalizer and
leakage currents is >15 to 1. In wide practice, Matsushita 100K ohm, 3W, MOF
resistors have, to the best of my knowledge, worked flawlessly in this 
application. 

BIASING

  Did my article say that there was anything wrong with electronic bias
switches? I said that there were problems with RF-actuated electronic bias
switches when using SSB. I use an electronic bias switch - but it is NOT
controlled by the amplitude of the RF driving the amplifier. Thus, it is not
possible for the amplifier to switch into non-linear bias during transmit. 

HIGH SPEED RELAYS

  Fred says he isn't sure the relays are sequenced properly. I don't see how
anyone could be sure unless he measures (no pun intended) the make/break
timing with a dual-trace oscilloscope. I did. 
  Steven Katz wrote to me extolling the virtues of PIN diodes. It sounded
pretty good. I suggested that he write an article for QST. How about it Steven?
Incidentally, Steven lives in the same area of So. CA I live in. We don't have
much lightning. A number of people who live in areas that have severe light-
ning, told me of problems with PIN diodes. A legal-limit amplifier develops
about 800Vp-p into a 50+/-j0 ohm load. With a >= 1 to 1 SWR, the p-p voltage
can go down or it can go up depending on the Z of the load presented to the
output terminals of the amplifier. If the load was 10+/-j0 ohm,[4] the SWR
would be 5 to 1 and the potential would be about 350Vp-p. If the load was
250+/-j0 ohm, the SWR would be 5 to 1 and the potential would be about 
1600Vp-p. If the inverse rating of the PIN diodes is 1000V, there might be a
problem.

VHF STABILITY

   Did I say anything about striking gongs? Spark gap transmitters operate on
a principle that can be compared to ringing a bell. Both generate damped wave
oscillations. If there is a better analogy, I would like to hear it. If you
have doubts that the anode resonant circuit in HF amplifiers generates energy
at its VHF resonance, loosely couple a spectrum analyzer to one, key the
amplifier on and off, and watch the analyzer.[5] If there is no damped wave
VHF signal present during anode current transients, then AG6K is a contempor-
ary version of the infamous Larson E. Rapp! 
   The main obstacles to a better understanding of push-push VHF - UHF
parasitic oscillations is their propensity for transience and lack of
cooperation. When a parasitic decides to appear, the observer doesn't have a
chance to observe and make scientific measurements. He hears a (typically
stentorian) noise, perhaps sees a flash in the corner of his eye - and it's
all over. What follows is like investigating the scene of a fire. 
   Tom assures us that there is little difference between silver and 
nickel-chromium alloys at VHF - but that nickel-chromium alloys produce dire
consequences at 28MHz. In an HF amplifier, my dipmeter says otherwise. 
   In my opinion, parasitic oscillations in HF amplifiers can not be completely
eliminated by lowering the Q of the VHF resonant anode circuit - but they can
be made less likely to take place. And if one does occur, glitch protection 
can prevent damage. 

IS MORE GAIN ALWAYS BETTER?

   On the air - in SSB mode - ALC does not perform as well as it does on paper.
The intrinsic problem is finite attack time and subsequent overshoot. One look
at the typical ALC path in a transceiver shows why - a series of decoupling
resistors and bypass capacitors. Does a "systems view" obviate T=RC ? 

ADJUSTABLE TUNED INPUTS

   If what Tom and Fred are saying is correct - i.e., that the radio's output
filter reactance and the amplifier's tuned input reactance do not interact,
then it should make no difference what length of coax is used between the
radio and the amplifier. 

FINALLY

   I believe that technical disputes are best resolved by hands - on
measurements and the laws of physics - not philosophy or opinions - even if
they be "expert."  
   At the risk of plagiarizing McGeorge Bundy, I would like to close by saying
that there is no safety in technological hubris.[6]

                        R. L. Measures, AG6K

Copies to:
Al Brogdon, QST
David Newkirk, QST
Paul Pagel, QST
Mark Wilson, QST
Reid Brandon
Bill Clemow
John Fakan
Steven Katz
Tom Rauch
Fred Telewski

Notes
1 Photographs of grid-shorted IHC and DHC tubes appeared in "Parasitics
  Revisited", September-October 1990 QST.
2 Uses silver-plated parasitic suppressors. 
3 If the RF relays in an amplifier switch slower than the transceiver,
  non-parasitic related bandswitch arcing can occur during R-T transition. 
4 To be able to match such an impedance, an amplifier's tank would need a
  higher than average Q. A pi-L tank would be helpful in this regard. 
5 Use a short probe wire and start with high input attenuation. The probe
  should not be within 5cm of the anode resonant circuit. 
6 "There is no safety in unlimited technological hubris" (McGeorge Bundy). 

>From rmarosko at bcm.tmc.edu (Ron Marosko)  Wed Apr 12 23:04:49 1995
From: rmarosko at bcm.tmc.edu (Ron Marosko) (Ron Marosko)
Date: Wed, 12 Apr 1995 17:04:49 -0500
Subject: TDXS April Bullsheet now available
Message-ID: <199504122204.RAA14775 at bcm.tmc.edu>

Greetings!

The April issue of the Texas DX Society Bullsheet is now available via the
electronic media. It may be reached through the Texas DX Society home page,
at http://pegasus.ssctr.bcm.tmc.edu/tdxs.html . This month issue features
the results of the 1994 Texas QSO Party!

Catch it here, even before I take it to the printer for printing and mailing.

Vy 73,
    Ron KK5DK
    Secretary, TDXS

---
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>From Larry Tyree <tree at cmicro.com>  Wed Apr 12 23:05:12 1995
From: Larry Tyree <tree at cmicro.com> (Larry Tyree)
Date: Wed, 12 Apr 1995 15:05:12 -0700
Subject: SprINT logs wanted
Message-ID: <199504122205.PAA02327 at cascade.cmicro.com>

The 72 hour "deadline" is approching.

We still are looking for logs from KR0Y, K7GM, AB1T, K6TZ, W5XD and NA4K.

The 72 hour deadline is somewhat flexible...  but if you can contact me
with your plans, that will help.

Thanks

Tree N6TR
tree at cmicro.com

>From Larry Tyree <tree at cmicro.com>  Wed Apr 12 23:23:29 1995
From: Larry Tyree <tree at cmicro.com> (Larry Tyree)
Date: Wed, 12 Apr 1995 15:23:29 -0700
Subject: Message format
Message-ID: <199504122223.PAA02577 at cascade.cmicro.com>

This message might not have the word CONTEST in it.  I have noticed a 
growing number of messages having strange formatting for the paragraphs.

I think these are being caused by some WINDOWS based tools that are 
formatting the messages for other than 80 characters per line.  They
may look fine on your screen, but when they are displayed on a 80
character per line terminal, they are very fragmented and hard to 
read.

Also, it is suggested in the INTERNET COMPANION book that Treu suggests
we all read, that the maximum number of characters per line be about 
70.  This allows people to respond to the message without the lines
overflowing.

If you want people to read your message, you should be concerned with
the format.  It is a LOT more work to read something that is poorly
formatted.

Please spread the word and if you receive a message that looks garbled,
you might let the person sending it know so they can take corrective
action.

Tree N6TR
tree at cmicro.com