The AM Forum
March 28, 2024, 05:58:36 AM *
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: Modulation Transfomer Spark Gaps  (Read 4893 times)
0 Members and 1 Guest are viewing this topic.
W8UJX
Member

Offline Offline

Posts: 90


WWW
« on: April 22, 2014, 07:09:41 PM »

I notice the modulation transformer in my Globe King has no spark gaps across the primary or secondary.  Anybody have any idea of the gap size and where they should be placed.

The rig runs 500 watts DC input to a 4-400A and the modulators are 811A's with 1000 VDC on the plates.

Thanks,
Jerry, W8UJX 
Logged
WD8KDG
Member

Offline Offline

Posts: 262



« Reply #1 on: April 22, 2014, 08:18:24 PM »

For what it is worth: The Johnson Viking 500 runs 2000VDC on the plates of the 4-400A and the pair of 811A's. There is a spark gap on the secondary of the modulation transformer, gap is set at 0.018".

No doubt, owners of GK500's have info.

Craig,
Logged

Ham radio is now like the surprise in a box of "Cracker-Jacks". There is a new source of RFI every day.
Jim, W5JO
Member

Offline Offline

Posts: 2506


« Reply #2 on: April 23, 2014, 12:04:28 PM »

Jerry, I don't know which model you have but the 500 B and C versions had spark gaps.  What they did was use a 350 K 2 watt resistor and solder a piece of ~#12 wire to each lead.  The loose ends were sharpened to a point then they bent the sharpened ends  across the resistor until they points were about the same distance apart as Craig says.

You don't need the resistor just put the gap on the secondary as in the Johnson 500.  WRL put the resistor across the B+ input and B+ output terminals near the back of the chassis.
Logged
W8UJX
Member

Offline Offline

Posts: 90


WWW
« Reply #3 on: April 23, 2014, 01:26:45 PM »

Tks Craig and Jim. 

The GK I have has an 'A' version RF and modulator section and a 'C' version power supply.  I had the 'B' version when I was a kid in the 50's.
 
I imagine the resistor was used to hold the gap so the gap could easily be adjusted on the bench.  When the gap was correct, the resistor was soldered across the mod tranny secondary.

I think I remember someone in this forum saying the primary was a better place for the gap.  Anyone remember that?

Jerry, W8UJX
Logged
w1vtp
Member

Offline Offline

Posts: 2638



« Reply #4 on: April 23, 2014, 01:42:56 PM »

If you run a little math, you can calculate the peak voltage on both the input and the output of the modulation signal.  Then perhaps this breakdown voltage graph could help you with the first settings of the gap spacing.

The graph was generated on an Excel spreadsheet based on a chart found in 1978 ARRL handbook, page 165, table 6-111.

I'm sure there would be a safety setting where the gap spacing would be moved a bit further apart.  You would need to know what your % positive peak modulation is at that maximum modulation level.

Al

PS:  Oh, the Y axis if the peak voltage.  The X axis is the breakdown distance in inches.


* BREAKDOWN VOLTAGE GRAPH.jpg (100.06 KB, 1200x817 - viewed 375 times.)
Logged
W3GMS
Contributing
Member
*
Offline Offline

Posts: 3063



« Reply #5 on: April 23, 2014, 01:47:50 PM »

I dug up an old post that Don, K4KYV commented on the subject:  

You only need them on the primary side of the mod xfmr, from the plates of the modulator tubes to the primary midtap.  Putting them on the secondary side will not protect the transformer.  If they arc over on the secondary side, the inductive kick  from the abrupt short circuit reflects back through the transformer and may cause insulation break down. The RCA broadcast transformers have them only on the primary side.  The 5500Z:5500Z 1:1 ratio transformers designed for MCW use on Liberty ships have a set on both the primary and secondary.

I don't think it matters whether the gap material is made of tungsten, copper, nickel plated or bare brass.  Even steel nails would probably do just as well.

Joe, GMS
Logged

Simplicity is the Elegance of Design---W3GMS
AB2EZ
Member

Offline Offline

Posts: 1722


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


« Reply #6 on: April 23, 2014, 03:34:06 PM »

Jerry
et al.

I think that one set of rules of thumb regarding the use of spark gaps will not fit all situations.

One of the critical issues regarding arc overs (or lack thereof) in a modulation transformer is the amount of leakage inductance associated with each winding (considered separately).

When the input and output voltages are low-to-moderate (i.e. no issues regarding arc-over between the turns, between the windings, or from a winding to the case of the transformer) we can model (i.e. the equivalent circuit) the leakage inductance by a single series inductor on either the input side or the output side of the transformer.

However, to deal with potential arc-over situations, we need to (at a minimum) take into account the leakage inductance associated with the input winding, and (separately) the leakage inductance associated with the output winding.

If we (partially) model the transformer as an ideal transformer plus a (leakage) inductor in series with the input side winding and a (leakage) inductor in series with the output side winding... we can begin to see the physical effects of a sudden increase in the load impedance across the output winding of the transformer. I'm using a sudden increase in the load impedance across the output winding of the transformer, rather than imagining that the load impedance across the output winding of the transformer suddenly goes to infinity... because it is easier to model, and also more realistic. I am also presenting this analysis in the context of a modulation transformer whose output side winding has DC as well a audio current flowing through it (i.e. no separate Heising reactor)

The input winding (leakage) inductor will store energy if there is current flowing through it... and will produce a large voltage across itself if the current flowing through it tries to suddenly change. Likewise for the output winding (leakage) inductor. Saying the same thing in a different way: the current flowing through either inductor will start to change (but not change instantly) if a voltage is applied across that inductor.

The current through an inductor cannot change instantly because the energy stored in the magnetic field of the inductor (Energy = 0.5 x L x I x I) cannot change instantly.

A sudden increase in the impedance of the load across the output winding of the transformer, will result in a proportional, sudden increase in the voltage across the output winding of the transformer. I.e. the current through the load cannot change instantly, and the voltage across the load = I x R (in the case of a resistive load).

This sudden increase in the voltage across the output winding of the real transformer will be split between: a sudden increase of the voltage across the output winding's leakage inductance, and a sudden increase in the voltage across the output winding of the ideal transformer in the real transformer's equivalent circuit. [I.e. the sum of the voltages around the output side loop has to equal zero] This implies that there will be a sudden increase in the rate of change of the current flowing through the output winding's leakage inductance.

The sudden increase in the voltage across the output winding of the ideal transformer will be caused by a sudden increase in the voltage across the input winding of the ideal transformer (multiplied by the turns ratio)

The sudden increase in the voltage across the input winding of the ideal transformer will be caused by the sudden increase in rate of change of the current flowing through the leakage inductance on the input side of the real transformer.

The rate of change of the current flowing through the leakage inductance on the input side of the real transformer will equal the rate of change of the current flowing through the leakage inductor on the output side of the real transformer (divided by the turns ratio).

Another key factor to consider is the output impedance of the modulator (not including the modulation transformer). If it is a typical tetrode or pentode based modulator, it will have a very high output impedance... which will act essentially as an open circuit across the input winding of the real transformer. If it is a triode-based modulator (or a modulator with negative feedback around it) it will have a low to moderate output impedance... and as long as one of the tubes is still conducting plate current, it will provide a low or a moderate impedance across the input winding of the real transformer. The output impedance of the modulator, i.e. looking into the modulator from the input side of the transformer, will affect the voltage surge that occurs across each side of the real transformer if there is a sudden increase in the impedance of the load across the output winding of the real transformer.

A spark gap across only the input winding will protect the entire transformer if the rising voltage across the input winding causes the spark gap to arc over before the rising voltage across the output winding (or the voltage from the output winding-to-case ground) increases to the point where the output winding arcs... i.e. due to the voltage being produced by the flow of (slowly decreasing, but initially unchanged) output side current through the suddenly larger output side load impedance.

Depending upon the design of the transformer (e.g. whether the input winding or the output winding has more leakage inductance, what the turns ratio is, and how the windings are physically arranged inside the transformer), the amount of DC flowing through the output winding, the output impedance of the modulator, and other factors, ... I believe that a spark gap across the input side may not protect the output side (and vice-versa).

Bottom line: if you are going to use spark gaps in conjunction with the modulation transformer, I (respectfully) suggest that you include a properly spaced spark gap on both the primary side and the secondary side.

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
K1JJ
Contributing
Member
*
Offline Offline

Posts: 8893


"Let's go kayaking, Tommy!" - Yaz


« Reply #7 on: April 23, 2014, 04:38:56 PM »

Bottom line: if you are going to use spark gaps in conjunction with the modulation transformer, I (respectfully) suggest that you include a properly spaced spark gap on both the primary side and the secondary side.
Stu


I agree.   I have always gapped both sides and find the gaps take turns arcing when set correctly.


Also, I once had a 10KW RCA broadcash  modulation transformer. It had gaps on both the pri and sec. These were factory installed gaps.  The matching Heising reactor also had its own gap, even though the mod sec was across the same point.   I also use gaps on my screen modulation reactor. Even the power supply choke uses a set I made myself. 

I'd rather have a known, preset voltage quenched with a burst of current - than an "unlimited" voltage  looking for a place to arc over the insulation.

T
Logged

Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
W8UJX
Member

Offline Offline

Posts: 90


WWW
« Reply #8 on: April 24, 2014, 02:03:17 PM »

I had wondered if it would be better to have gaps on both the primary and secondary.  I installed a gap on the secondary.  That was easy.  Just soldered the gap wires to the chassis feed thru connections.

The modulation transformer primary is a whole other story.  The wires from the plates of the 811A's go through feedthru insulators to get to the bottom of the chassis.  A gap across those connections will be easy.

The center tap of the primary winding is a whole other matter.  That wire is a long wire that goes directly to modulator tube plate current meter mounted on the front panel.

I could drill a hole in the chassis and mount a third insulator between the two existing primary winding insulators.  Connect the primary center tap wire to that insulator.  Then it would be easy to gap both sides of the primary.  Then reconnect the primary center tap wire to the plate current meter.

I wonder if anyone has gapped the primary winding of the mod tranny in a GK.  If so, how did you do it?

Jerry, W8UJX 
Logged
Jim, W5JO
Member

Offline Offline

Posts: 2506


« Reply #9 on: April 24, 2014, 02:13:25 PM »


I could drill a hole in the chassis and mount a third insulator between the two existing primary winding insulators.  Connect the primary center tap wire to that insulator.  Then it would be easy to gap both sides of the primary.  Then reconnect the primary center tap wire to the plate current meter.

I wonder if anyone has gapped the primary winding of the mod tranny in a GK.  If so, how did you do it?

Jerry, W8UJX 


If you have a near stock audio system of the GK there is no reason to put a gap on the primary.  I don't believe you can drive it enough to cause problems with stock configuration.  If you have a audio section that develops a lot of voltage then that would be another matter. 
Logged
W8UJX
Member

Offline Offline

Posts: 90


WWW
« Reply #10 on: April 25, 2014, 12:29:10 PM »

Tks Jim,

The modulator is stock except it has been recapped.  I use the REA modulation monitor and the rig will not quite do 100% positive so I will forget about gaps on the primary.  It still has the couplates, yetch.  I am playing around with an outboard modulator and want to leave the King stock as much as possible.

When I bought this rig the first thing I did was remove all the HV chokes and transformers and bake them in the oven at 250 degrees for three hours.

A week or so ago one lead in the filter choke in the modulator HV supply arced to the case.  There was a carbon track to the case but I was lucky.  I was able to clean it well enough to repair the choke.  The choke worked with the case grounded but to be sure it would not arc again I mounted the choke, as well as the modulation transformer on ceramic stand off insulators.

I realize this could cause a safety problem but as long as I am aware of the mod, there should be no problem.  If I ever sell the rig I will remove the ceramic stand off insulators.

Jerry, W8UJX
Logged
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 0.045 seconds with 19 queries.