Subject: GLOWBUGS digest 488

From: [email-masked]
Date: Fri, 28 Mar 1997 00:36:50 GMT

GLOWBUGS Digest 488

Topics covered in this issue include:

  1) Using a tetrode or pentode as a triode
	by Jeffrey Herman <[email-masked]>
  2) Sweep tube linearity
	by Jeffrey Herman <[email-masked]>
  3) Re: Sweep tube linearity
	by Jan Axing <[email-masked]>

----------------------------------------------------------------------

Date: 	Wed, 26 Mar 1997 19:06:02 -1000
From: Jeffrey Herman <[email-masked]>
To: Glowbugs List <[email-masked]>
Subject: Using a tetrode or pentode as a triode
Message-ID: <Pine.GSO.3.95q.970326190403.20446D-100000@uhunix3>

Thought this might be of interest to many of you; picked it off
the ARRL email server.
7.3W, Jeff KH2PZ

From: [email-masked]

Subject: Re: Synthesized Triode from Tetrode or Pentode

             By Dave Newkirk/ July 28, 1993

  Connecting  together the screen and control grid of  a  tetrode 
(or  the suppressor, screen and control grid of a  true  pentode) 
turns the tube into a high-mu triode. The resulting tube may  be, 
but  is  not necessarily, a *zero-bias*  triode.  True  zero-bias 
tubes  are designed to be so, and must be operated  according  to 
their  designer's  specs to work well with zero  bias.  Too  much 
plate  voltage  on  a zero-bias triode can  overcome  its  grid's 
control and make it draw too much plate current when idling. So a 
high-mu  triode synthesized by connecting together  the  multiple 
grids  of  a screen-grid tube *may or may not* be operated  as  a 
zero-bias    tube,   depending   on   its    resulting    control 
characteristics and how you power the tube.

  Relatedly, a screen-grid tube connected as a high-mu triode may 
not  necessarily exhibit the low-IMD characteristics of  a  good, 
designed-for-low-IMD  zero-bias  triode.  In  other  words,   you 
wouldn't  necessarily  synthesize a 3-400Z  by  high-mu-  triode-
connnecting a 4-400. (Even a 3-400Z is old hat. There are  newer, 
better   zero-bias  tubes,  where  *better*  equates  to   "lower 
intermodulation distortion.")

  I haven't mentioned so far what is probably obvious: Connecting 
a  well-screened screen-grid tube as a triode means that it's  no 
longer  a  screen-grid  tube.  An amplifier  built  with  a  tube 
connected  in  this way must be (A) neutralized if  the  tube  is 
operated grounded-cathode or (B) operated in grounded-grid so its 
connected-together grids can act as a screen or (C) configured as 
a  cathode  follower (which is largely impractical  if  you  want 
significant power output).

This sets us up for the next question:

  What would happen to a beam-power tube, such as a 6DQ5, if  its 
screen is connected to its control grid? It would act as a  high-
mu  triode. *But*--and this is very important--it  *couldn't*  be 
operated grounded-grid (without neutralization, that is)! This is 
so  because, like many beam power tubes, the 6DQ5's  beam-forming 
elements  are internally hardwired to its cathode. This  bypasses 
the shielding afforded by grounding its control and screen grids. 
Beam power tubes that cannot be operated in grounded-grid without 
neutralization  include  the 807, 1625, 6146 and  many  TV  sweep 
tubes. *Some* TV sweep tubes and RF beam power tubes (6KD6, 5763) 
bring their beam-forming elements out to a separate pin.  *These* 
can  be operated in grounded-grid without  neutralization;  you'd 
ground  their control grids, screens and beam-forming plates  for 
RF. You have the option of tying all the grids together for dc or 
feeding them separately. This gives on one last issue:

  There's one more important issue in using screen-grid tubes  as 
high-mu triodes, grounded-grid or not. By construction, a  tube's 
control  grid exerts more control over its electron  stream  than 
its  screen; it's closer to the cathode than the screen and  made 
of  finer  wire.  Further, screen-grid  tubes  are  intended  for 
applications in which the screen is *considerably more  positive* 
than  the control grid. *Because of this, severe  grid  overdrive 
will  likely  result* if the control grid and screen  are  merely 
tied  together (for dc *and RF) during  high-mu-triode  operation 
because the screen operates at the same potential as the grid and 
therefore  doesn't  draw them past and away from the grid  as  it 
does when it's significantly more positive than the grid. *GE Ham 
Notes*  (see  enclosed) goes into detail on this, as does  V.  S. 
Campbell  and  W. S. Skeen, "Grounded Screen-Grid  Operation  for 
Tetrodes,"  *QST*, Nov 1959, pp 37-39. (The introductory box  for 
this  article  goes like this: "A tetrode with control  grid  and 
screen tied together to form a high-mu triode for a grounded-grid 
circuit  makes a very simple arrangement. However, this  type  of 
operation   invariably   results   in   excessive    control-grid 
dissipation. This article shows a simple method of avoiding  this 
difficulty.")   It's  therefore  important  to  monitor   element 
currents  individually, at least until the circuit is  finalized, 
when operating a screen-grid tube as a high-mu triode.

  Calculating  the operating conditions for a tube  operating  in 
grounded-grid is relatively straightforward for true triodes,  as 
explained  in G. Grammer, "Input Impedance and Fed-Through  Power 
in Grounded-Grid Amplifiers," Technical Topics, *QST*, Dec  1958, 
pp    32-35,   184.   Calculating   these   parameters    *isn't* 
straightforward if you want to run a screen-grid tube in grounded 
grid  and  apply  normal  screen  voltage  to  the  screen  while 
grounding  it  for  RF; experiment will likely  be  necessary  to 
determine particulars in this case.

------------------------------

Date: 	Wed, 26 Mar 1997 20:11:32 -1000
From: Jeffrey Herman <[email-masked]>
To: Glowbugs List <[email-masked]>
Subject: Sweep tube linearity
Message-ID: <Pine.GSO.3.95q.970326201045.3740A-100000@uhunix3>

From: [email-masked]
Subject: INFO response: TUBEDATA.SUM

From: [email-masked] (Barry Ornitz)
Newsgroups: rec.radio.amateur.misc
Subject: Sweep tube linearity, etc. (Was: tube finals melt a lot)
Date: 5 Jun 91 22:55:45 GMT
 
 In an earlier article, I had written about the linearity of sweep tubes and 
 small transmitting tubes.
 
 >A real problem with all of the sweep tubes - including the 6146 too, was the
 >poor linearity of the tubes.  I always wondered why no manufacturer ever used
 >6550's.  The power level and voltage ratings of these high linearity tubes
 >would have been ideal.  The Hi-Fi manufacturers were the only companies that
 >seemed to use them.
 
 Perry Scott, AA0ET, replied:
 
 >The Radio Handbook contains a table of distortion products for various
 >sweep tubes.  The 6146 is rated at -24 dB of 3rd order IMD, while the
 >6LQ6 is rated at -18 dB.  Some of the linearity problems occur because
 >of lack of tuned input, which presents a varying load to the exciter in
 >a grounded grid configuration.  Another remedy for nonlinearity is to
 >use the venerable pi-L to keep the neighbors happy.
 
 >I have never heard bad reports on the sweeps I use in the 520 or the
 >6x6LQ6 linear.  If the nonlinearity is just a specsmanship thing, then
 >I'll be nonlinear.  In general, I think sweeps have been much maligned
 >by people that didn't understand their limitations.  They are less
 >forgiving of design blunders than real transmitting tubes.
 
 I am not sure whether Perry is talking about data from the ARRL Handbook or 
 Bill Orr's book.  The table in Bill Orr's "Handbook" is from an article he 
 initially published in Ham Radio Magazine (back in 1968, I think).  The 
 data given were for Class AB1 service, grid driven.  It should be noted 
 that some of the tubes were pushed very hard, often well beyond their 
 normal ratings.  In general, the intermodulation distortion of these tubes 
 would be 3 to 5 dB better if they were operated in a cathode-driven mode 
 due to the negative feedback inherent in this operation.  However, we are 
 talking about sweep tubes as final amplifier tubes in older SSB exciters, 
 where usually two or more sweep tubes are operated in parallel, and are 
 grid driven with a tuned circuit on the grid and plate.  External sweep 
 tube kilowatt amplifiers are a different issue altogether.
 
 I have listed some of W6SAI's data on sweep tubes below, along with data on 
 some other older tubes, and some data on a few higher power transmitting 
 tubes and even one power FET.  The data is for grid-driven, AB1 operation 
 except where marked with an asterisk which signifies cathode driven 
 operation or with a caret which signifies class AB2 operation.

                    RF LINEAR AMPLIFIER SERVICE FOR SSB AND CW
             GRID DRIVEN, CLASS AB1 (Except * Cathode Driven,^ AB2)
 
 TUBE PLATE SCREEN GRID  ZERO SIG MAX SIG MAX SIG PLATE INPUT USEFUL AVG 3-ORD
 TYPE VOLTS VOLTS  VOLTS    Ib0     Ib      Ic2    LOAD  PWR  PWR Po DISS IMD
        V     V      V      ma      ma      ma     ohms    W     W     W   dB
 ------------------------------------------------------------------------------
 6146  600   200    -46     25     103       9     3570   61    41    16   -25
       750   200    -51     25     118       7     2825   88    55    28   -22
       800   290    -69     30     125      10     3620   100   59    35   -24
       800   290    -77     25     180      13     2300   145   91    45   -19
 ------------------------------------------------------------------------------
 807   600   300    -34     18     70        8     4300   42    28    12   -23
       750   300    -35     15     70        8     5200   53    36    14   -23
 ------------------------------------------------------------------------------
 6DQ5  500   150    -46     48     170      17     1800   85    54    27   -28
       600   150    -46     48     182      13     1625   91    56    29   -26
       700   150    -49     35     182      11     2210   127   78    41   -23
       800   180    -67     30     250      13     1710   200   121   70   -19
 ------------------------------------------------------------------------------
 6550  680   340    -39     48     140      20     3010   95    67    26   -32
       800   290    -33     45     127      15     3920   102   70    29   -30
 ------------------------------------------------------------------------------
 6HF5  500   140    -46     40     133       5     1900   67    35    29   -27
       800   125    -45     30     197       7     2170   158   100   48   -21
 ------------------------------------------------------------------------------
 6JE6  500   125    -44     40     110       4     2300   55    30    24   -26
       750   175    -63     27     218      15     1850   163   102   51   -20
 ------------------------------------------------------------------------------
 6MJ6/ 750   175    -60     25     215       9     1850   161   102   49   -18
 6LQ6  800   200    -69     25     242      13     1850   197   124   60   -18
 ------------------------------------------------------------------------------
 4CX300 2K   350    -55     100    250       5            500   300        -27
 ------------------------------------------------------------------------------
 8930   2K   350    -63     90     290      30     4000   580   350        -27
 ------------------------------------------------------------------------------
 *^8877 2.7K triode -8.2    92     740      --     1820   2000  1085       -40
 *^    3.5K  ---    -8.2    182    1000     --     2000   3500  2075       -38
 ------------------------------------------------------------------------------
 *3CX500A7 4K triode -9     400    1250     --     1800   5000  3000       -37
 *     5K    --     -11     400    1450     --     1800   7250  4000       -35
 *     6K    --     -12     400    1550     --     1800   9300  5000       -38
 ------------------------------------------------------------------------------
 MRF150 50   TMOS           250    3750     --            333   150PEP     -32
  
 A few points should be noted from the data: 1) as the tubes are pushed 
 harder to get higher outputs, the distortion products increase; 2) the 6550 
 tube shows considerably lower IMD than do comparable sweep tubes at a 
 similar power level (this is expected since the 6550 was designed for low 
 distortion); and 3) _real_ transmitting tubes, i.e. those designed for high 
 power service, show far superior IMD ratings.  Unfortunately, Bill did not 
 publish data on the 6JB6A's and 6JS6C's popular in many early SSB rigs 
 (Drake and Yaesu).
 
 I included the data on the MRF150 TMOS power FET to show how far solid-
 state devices have come.  It has excellent IMD characteristics.
 
 Perry stated that the nonlinearity of the tubes could be handled with some 
 extra tuned circuits.  This is only partly true.  Higher levels of 
 harmonics, caused by the tube nonlinearity, were usually no problem with 
 the tube rigs because most had excellent Pi-network outputs which required 
 tuning (not broadbanded) and were fairly high-Q.  However, the nonlinearity 
 also presents itself as INTERMODULATION distortion, an in-band phenomena 
 which the extra tuned circuits can do nothing about.  Consider a simple 
 case of a two-tone signal on 80 meters where two signals are to be 
 amplified by the tube, one at 3.900 MHz and one at 3.901 MHz.  Third order 
 IMD will create new signals at 2*f1 - f2 and 2*f2 - f1, or in this case 
 3.899 MHz and 3.902 MHz.  Fifth order IMD will produce signals at 3.898 MHz 
 and 3.903 MHz.  These are in-band signals and the extra tuned circuits will 
 do nothing to filter these; however, they might eliminate the problems with 
 the third and fifth harmonics.  Excessive IMD can be easily heard - the SSB 
 people call it flat-topping.  Your signal gets "mushy" and your bandwidth 
 becomes excessive; other people up and down the band suddenly hear 
 "buckshot" from your signals.  IMD increases rapidly as the tubes are 
 driven to saturation.
 
 So one important thing to consider if you have one of the older tube rigs 
 is NEVER try to push the power output higher than the manufacturer 
 recommends if you want to remain popular with others on the band (the same 
 thing applies to solid-state rigs too).
 
 Nonlinearity is not a specmanship thing, it causes real problems.  But do 
 not let this scare you off from using an older rig however.  Most of these 
 rigs were capable of good IMD performance by reducing their drive slightly.  
 If you have access to an oscilloscope, it is easy to adjust the rig for the 
 best drive levels and power output consistent with low distortion.  The 
 reduction in power will usually be slight, barely enough to show on your 
 contact's S-meter, but the IMD will be much lower.  You will also get 
 considerably better tube life.  Also keep the tubes cool if you want them 
 to last.  Usually all it takes is just turning the microphone gain down a 
 little.  I might add that you must be especially careful when using an 
 external speech processor with many of the older rigs.  By decreasing the 
 peak to average power ratio, speech processors effectively run up the 
 average power.  Many of the older rigs using sweep tubes were not rated for 
 this increase in average power.  If you have the manufacturers original 
 specifications, look at the difference between the SSB PEP power levels and 
 the CW/RTTY power levels to see the effect of increased average power.
 
 External, multiple sweep tube kilowatt amplifiers, on the other hand, are 
 an atrocity in my opinion.  The increase in IMD performance obtained by 
 cathode driven operation is almost always lost because of the use of 
 parallel tubes.  Unless perfectly matched tubes are used in these 
 amplifiers, distortion performance will suffer.  The only thing going for 
 these amplifiers was the low tube cost (in their day).  Look at the 8877 
 specs if you want to see low distortion.  External amplifiers are a 
 different issue than we have been discussing, however, and I prefer to
 leave the discussions on this to another day.
 
 Paul Nix, WB5AGF, asked about RF feedback to reduce distortion.  The answer 
 is yes, it works.  The best references to this subject are as he suggested, 
 Bill Orr's "Radio Handbook".  {While I find W6SAI's discussions on antennas 
 often wrong, Bill KNOWS transmitting tubes, amplifiers, and feedback}.  Paul 
 is correct in his statements that feedback lowers stage gains while 
 enhancing linearity.
 
 RF negative feedback is an entirely different design problem from negative 
 feedback at audio frequencies.  To cover all of the HF amateur bands, the 
 feedback network need cover only slightly more than one decade in 
 frequency.  With good audio equipment, the feedback network must work over 
 approximately four decades of frequency range.  This is a much tougher 
 problem to do properly.  For this reason, many of the audio phreaks have 
 abandoned negative feedback around multiple stages in favor of inherent 
 feedback in individual stages.  Collins was about the only amateur radio 
 equipment manufacturer that did much with negative RF feedback in their 
 rigs that I know about.  Drake in the L4 amplifier also used the trick of
 smaller than normal values of the grid bypassing capacitors to obtain some
 additional negative feedback.
 
 Mark Bitterlich, WA3JPY, and Gary Coffman, KE4ZV, also brought up the 
 important point that the matching network on your rig must be tuned at the 
 actual desired power level.  You cannot tune up at low power and expect the 
 same matching conditions at high power.  One good way to handle this is to 
 tune your rig into a dummy load at the desired power level.  Then switch 
 your rig to an antenna tuner to match the antenna.  Do not adjust the 
 transmitter at all after switching to the tuner - only adjust the tuner.  I 
 have often used an antenna noise bridge preset to 50 ohms to adjust the 
 antenna tuner.  In this way, I can tune up without ever putting a signal on 
 the air.  Gary mentions the special difficulty in tuning legal-limit 
 amplifiers.  One good way to do this involves a pulsed two-tone generator 
 and a scope.  You have to use a dummy load for the tests; it is not simple 
 but it works.
 
 Finally Perry Scott, AA0ET, suggested that I consider designing a 6550 
 based project and he also asked the price for the 6550.
 
 To start with the second issue, Newark lists the 6550 for around $32.  I 
 have seen some audio importers price some 6550's at $15 for Chinese 
 imports.  I would tend to stick with the known manufacturers.  Now for 
 around $70, I can order the MRF150 transistors.  So for new design, I would 
 rather use the single TMOS device to get essentially the same power as a 
 pair of 6550's.  With proper care, it will never wear out and I like the 
 lower voltages because of safety.  Heat dissipation is easier with the 
 transistor too since radiation cooling and forced convection are seldom 
 needed.
 
 As far as retrofitting 6550's into older rigs that used sweep tubes, you 
 will have several problems.  First the 6550 has a higher input capacitance 
 than most of the tubes it will replace.  The increase in feedback 
 capacitance of the 6550 over most sweep tubes would also usually 
 necessitate changes in the neutralization circuits.  Thus you would have to 
 make extensive modifications to the rigs.  Finally the physical size of 
 6550's would mean having to greatly enlarge the final amplifier assemblies 
 in most existing rigs.
 
 In conclusion, I would agree with many others - if you can get one of the 
 older tube ham rigs at a reasonable price that works well, do not worry 
 excessively about the tubes and nonlinearity.  Crank the power down a 
 little and most of these tube rigs will work fine.  Tune quickly though!  
 Ask for HONEST signal reports from others you talk to on the ham bands.  
 Make sure the rig is in proper neutralization and always replace the finals 
 with matched sets (and re-neutralize after any change in finals or driver 
 tubes).  As good advice to newcomers to ham radio, find an older ham who 
 may have used these rigs many years ago.  They can often remember how to 
 tune these rigs without the long-lost instruction manual, and they can help 
 when you need parts or troubleshooting help too.
 
                                 73, Barry   WA4VZQ
                                        [email-masked]

------------------------------

Date: Thu, 27 Mar 1997 11:26:12 +0100
From: Jan Axing <[email-masked]>
To: [email-masked]
Subject: Re: Sweep tube linearity
Message-ID: <[email-masked]>

Jeffrey Herman wrote:
<see his excellent post, a little too long to include in a reply>

As a curiosity, McIntosh used 8 6LQ6 in parallel PP in the MC3500 audio
amplifier capable of 350 W out running the tubes in class AB2. However,
I must admit that this amplifier is extremely complex with one of the
most complex output transformers I've ever seen.

Next curiosity, 6550 is almost equivalent to the British KT88, another
famous audio tube. The Brits also made a transmitting tube called TT21
and if I'm not totally lost, TT21 is exactly the same as KT88 except for
the top plate connector. Its power performance is similar to 6146B.
Can't find any IM performance data, unfortunately. Maybe someone has?
TT21 is still available and should be easier to retrofit than 6550
thanks
to the plate cap.

One semi-modern rig using RF feedback is Kenwood TS-830. It's rated at
220 W PEP input using a pair of 6146B.

73's
--
Jan, SM5GNN
Linkoeping, Sweden
[email-masked]
[email-masked]

------------------------------

End of GLOWBUGS Digest 488
**************************