The AM Forum
March 29, 2024, 08:16:43 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Output power 6146  (Read 13294 times)
0 Members and 1 Guest are viewing this topic.
AB3FL
Guest
« on: June 27, 2013, 11:40:04 AM »

I am working on a Valiant.  All of the bands have close to 150W except for 10M.  Input power is 650V X 350mA = 227 Watts.  Output power is 68 Watts. Grid is 6mA.  So about 160 Watts or  53 Watts per tube are heating up the inside.

Tom - AB3FL
Logged
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #1 on: June 27, 2013, 12:45:46 PM »

Possibilities:

1. The grid waveform contains a large amount of 20m sub-harmonic of the 10m frequency because the 2nd multiplier stage (V4) is not working properly on this band. Therefore the plate current waveform contains mostly this sub-harmonic (e.g. half of the 10m fundamental frequency) component, and a correspondingly much lower (than it should be) component of current at the 10m fundamental frequency.

or

2. The loading is not adjusted properly. There should be a significant dip in the plate current, and a corresponding peak in the output power, when the output circuit is tuned to resonance.

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
w4bfs
W4 Beans For Supper
Contributing
Member
*
Offline Offline

Posts: 1433


more inpoot often yields more outpoot


« Reply #2 on: June 27, 2013, 01:06:52 PM »

I don't know where yer Valiumhunt came from but it's possible that some 11 mtr cowboy was trying to bump the outpoot pwr .... check 10 mtr coil for correctness ...73 ...John
Logged

Beefus

O would some power the gift give us
to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
AB3FL
Guest
« Reply #3 on: June 27, 2013, 03:06:49 PM »

I don't know where yer Valiumhunt came from but it's possible that some 11 mtr cowboy was trying to bump the outpoot pwr .... check 10 mtr coil for correctness ...73 ...John

The valiant had 11M standard on it.  No golden screwdriver needed.

I do have a sharp dip in plate current at which point I get about 68 watts.  The waveform looked good on my scope.

Tom - AB3FL
Logged
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #4 on: June 27, 2013, 03:09:32 PM »

How does the grid waveform look? The tuned tank circuit will make the output waveform look okay (i.e. a sine wave at the fundamental frequency), even if the grid waveform is not okay (i.e. even if the grid waveform contains a large component at half of the fundamental frequency).

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
AB3FL
Guest
« Reply #5 on: June 27, 2013, 03:19:44 PM »

How does the grid waveform look? The tuned tank circuit will make the output waveform look okay, even if the grid waveform is not okay (i.e. even if the grid waveform contains a large component at half of the fundamental frequency).

Stu
Waveform looked good on scope.  Could one of the 6146 finals be weak?


Tom
Logged
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #6 on: June 27, 2013, 03:26:54 PM »

Alternatively:

A sharp dip in plate current, accompanied by low RF output might indicate that the loading capacitance is set at much too high a value [too large a value of loading capacitance => too high a value of load impedance on the output tubes at the fundamental RF frequency] or that there is too much inductance in the tank circuit [too large a value of inductance => too high a Q => too high a value of load impedance on the output tubes at the fundamental RF frequency]

Check the band switch to make sure that it is properly shorting out the unneeded turns of the tank coil on this band. For example terminal 3 on SW3B might not be making proper electrical contact to the shorting bar in the 10m position

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
Detroit47
Member

Offline Offline

Posts: 644



« Reply #7 on: June 29, 2013, 10:37:29 AM »

How much grid current? Not enough grid will result in low output while still pulling plate current.
Logged
kb3ouk
Member

Offline Offline

Posts: 1640

The Voice of Fulton County


« Reply #8 on: June 29, 2013, 11:14:51 AM »

Says 6 ma in the first post, which isn't enough for 3 tubes, should be more like 9 or 10 ma (6146s run about 3 ma per tube in class C phone service at 600 volts).
Logged

Clarke's Second Law: The only way of discovering the limits of the possible is by venturing a little past them into the impossible
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #9 on: June 29, 2013, 12:59:41 PM »

Looking at the Valiant schematic:

The grids of the 6146 output tubes are biased by a combination of fixed (negative) bias and self biasing associated with the average grid current that is flowing through the resistance between the grids and ground (i.e. via the voltage divider of the fixed bias supply).

If the (adjustable) fixed bias is not adjusted properly (i.e. not negative enough), then the tubes may be conducting plate current during too much of the RF cycle (e.g. not operating in class C...) even though the average grid current looks okay.

It is difficult for me to see why this effect would depend upon the selected band of operation... but there may be some subtle effect that I'm not seeing.

Stu


 
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
AB3FL
Guest
« Reply #10 on: June 29, 2013, 01:13:21 PM »

the biass was way too negative.  I set that correctly and can get about 110 watts
Logged
kb3ouk
Member

Offline Offline

Posts: 1640

The Voice of Fulton County


« Reply #11 on: June 29, 2013, 05:13:16 PM »

The deeper you have the tube biased into class C, the more drive it takes to drive it, so what was probably causing your problem the driver couldn't put out enough power on 10 meters to satisfy the finals' drive requirements with that amount of bias.
Logged

Clarke's Second Law: The only way of discovering the limits of the possible is by venturing a little past them into the impossible
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #12 on: June 29, 2013, 06:28:27 PM »

 But:

If the fixed bias was too negative and the drive was inadequate, then the average plate current would have been low.

In the case at hand, the average plate current was normal, but the rf output was low.

This suggests that the fixed bias was not negative enough.

Even if the RF output is low (e.g. due to insufficient drive) the efficiency ( rf out / electrical power being delivered to the final subsystem) should increase as you bias the finals deeper into class C.

Something is not consistent.

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
AB3FL
Guest
« Reply #13 on: June 29, 2013, 06:49:14 PM »

The biass with grid drive at 7.5ma was about -130.  I raised the fixed bias so it was -70 with 7.5ma of drive.  I also adjusted the neutralization a little following the manual.


Tom - AB3FL
Logged
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #14 on: June 29, 2013, 07:27:06 PM »

Something doesn't seem right:

If the 6146's were drawing 360ma of average current (your first post) and if the dc level (with drive applied) of the grid bias was -170v, and if the grid current was greater than zero, that implies that the peak RF drive voltage was in excess of 170V. If that were the case, the RF output with proper loading would be a high percentage of the DC input power.

Perhaps the adjustment of the neutralization played the key role in making the observed improvement, by eliminating a parasitic oscillation

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
kb3ouk
Member

Offline Offline

Posts: 1640

The Voice of Fulton County


« Reply #15 on: June 29, 2013, 09:43:43 PM »

That might explain why it was only acting up on 10 meters, if neutralizing it helped as part of the fix.
Logged

Clarke's Second Law: The only way of discovering the limits of the possible is by venturing a little past them into the impossible
AB3FL
Guest
« Reply #16 on: June 30, 2013, 10:45:33 AM »

here is the latest.   10 & 15 both have low output.  I tuned it up at 29mc. 660v and 400ma on plate.  The output was about 80w.  After a full minute of key down, no red plates.  But there was significant rf at 14.5mc.  I am thinking something wrong with the bandswitch.


Tom - AB3FL
Logged
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #17 on: June 30, 2013, 11:23:31 AM »



Tom,

   A few posts back you had 110 watts out on 10m after adjusting the bias. Now you say 80 watts on 10, 15m with 400 ma plate current. What changed?

   I don't have specific Valiant experience, but other rigs of that era when pushed for rated output on 10m result in lower output as something gets HOT. That something is usually the band switch, or loading capacitor(s). Going for that extra 1 db in RF power out often results in 10DB more heating. I am basing this on a Globe King 500 that I once had on 10m.

   I wonder if the efficiency is better at 300ma dipped plate current? For sure the modulation would be better there.

  The 10m band is pretty much a dummy load these days except for some sporadic E. Are you getting ready for fall when we might get some better openings?

Jim
WD5JKO
Logged
AB3FL
Guest
« Reply #18 on: June 30, 2013, 11:36:26 AM »

I have to put it on the bench today to figure out the problem
Logged
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #19 on: June 30, 2013, 11:37:02 AM »

Repeating my earlier posts in this thread

Take a close look at the output (waveform) of the 2nd multiplier (V4), which is also the input to the 6146 grid bus.

On 29MHz, the 2nd multiplier should have a 14.5 MHz input signal (on the grid of V4). If the 2nd multiplier is working properly, it should be producing a sequence of plate current pulses (i.e. current flowing from plate to cathode through V4) with a repetition frequency of 14.5MHz. This sequence of plate current pulses is not a sine wave... and therefore contains a strong harmonic component at 29MHz. It is not easy to measure the V4 plate current waveform directly without disturbing the circuit.

The turned circuit that is the output load for V4  (i.e. C7 and L6) is supposed to select out the 2nd harmonic (29MHz) of the V4 plate current waveform (i.e. the circuit should be tuned to resonance at 29MHz, and therefore have a high impedance at 29MHz; while it should have a low impedance at 14.5MHz, 43.5MHz, and higher harmonics of 14.5 MHz). Therefore, the voltage across the tuned circuit (on the grid bus of the 6146's) should be approximately a 29MHz sine wave.

If V4 is not working properly (for example, if it is biased incorrectly), then the plate current waveform flowing through V4 will not have enough harmonic content at 29MHz.

If the tuned circuit at the output of V4 is not working properly (e.g. tuned to 14.5 MHz instead of 29MHz), or, for some reason, has too low a Q to select out the 29MHz 2nd harmonic... then the voltage waveform applied to the grids of the 6146's will contain a large component at 14.5 MHz (instead of being a sine wave at 29MHz).

In that case, the 6146 plate current waveform (which is a sequence of pulses) will have a large 14.5 MHz component, and a relatively small 29MHz component. Therefore, when the output tank circuit is tuned to resonance at 29MHz, the RF voltage waveform will be much smaller than it should be... and the power output will be much lower than it should be.

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #20 on: June 30, 2013, 12:16:25 PM »

Addendum to comment above

Looking at the Valiant manual (on-line... BAMA), I cannot tell whether the multiplier tube (V4) is acting as a doubler (14.5 MHz => 29MHz) or a quadrupler (7.25MHz => 29MHz), when the Valiant is operating at 29MHz.

If V4 is acting as a doubler, then what I said in the previous post is valid.

If V4 is acting as a quadrupler (i.e., there is no doubling taking place in the buffer/oscillator stage), then the input to V4 will be approximately a sine wave at 7.25MHz, and the V4 plate current waveform (if V4 is working properly) will be a sequence of pulses at a repetition rate of 7.25MHz... containing harmonics of 7.25MHz. The tuned circuit at the output of V4 will have a high impedance at 29MHz, and a much lower impedance at 7.25MHz, 14.5MHz, 21.75, 36.25MHz, and higher harmonics of 7.25MHz.

Note, when adjusting the tuning and the drive to produce the recommended grid current, it would be important to adjust the tuning control to select out the proper harmonic of the V4 plate current waveform. Since the V4 plate current would contain components at 7.25MHz, 14.5MHz, 21.7MHz... and since the lower frequency harmonic components of the V4 plate current will be larger in amplitude... it would be easy to tune the V4 output circuit to resonance at the wrong harmonic of 7.25MHz.

If V4 is acting as a quadrupler when operating at 29MHz, then it is critical that V4 is properly biased (well into cutoff with no RF input applied to the V4 grid), in order to produce a sequence of plate current pulses that are very short in duration compared to half a cycle at the input frequency (7.25 MHz). This will ensure that the V4 plate current waveform contains enough 4th harmonic content.

Stu

Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
AB3FL
Guest
« Reply #21 on: June 30, 2013, 01:10:00 PM »

The oscillator does run at 7.25mc and V4 does quadruple it to 29mc


Tom - AB3FL
Logged
AB3FL
Guest
« Reply #22 on: July 07, 2013, 02:52:43 PM »

Latest info:

I can get 130W on 15M and about 105W on 10M.  I have seen several values for the 6146 Bias.  Anywhere from -52 up to -70 without excitation.  If I turn on the exciter, the bias goes down to anywhere from -100V to -130V.  What value is correct?  I still have to check the voltages on V4 and scope it to see if the waveform is good.


thanks

Tom - AB3FL
Logged
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #23 on: July 07, 2013, 10:49:03 PM »

As per the manual, and also by calculation from the schematic:

With no excitation, the grid bias voltage on the RF output tubes should be adjusted to -70V

With 8ma of grid current, the grid bias should be approximately 23V more negative. I.e.  about -93V.

Make sure to measure the DC grid voltage at a location that is on the meter side of the grid bias RF choke (L7). This will ensure that the RF component of the grid voltage does not disturb the measurement instrument, and that the capacitance of the measuring instrument does not disturb the grid drive circuit.

If the bias voltage drops (becomes more negative) by a lot more than 23V when you go from no excitation to 8mA of grid drive, then check the values of resistors R22 (5k ohms, with two adjustable taps), R23 (9k ohms) and R50 (1k ohm); and also make sure that the adjustable taps on R22 are making good contact with the resistor's winding.

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #24 on: July 08, 2013, 12:30:33 AM »

In addition to the above:

Refer to pages 10 and 11 of the Johnson Valiant manual. In particular, see section 5c on those pages. As per the instructions...check to see that L5 is adjusted for maximum grid current on 10 meters.

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.074 seconds with 18 queries.