Print Page - Reactor's sparking when carrier goes off

Last night reactor flashed when I unkeyed even it is isolated from the sassis, this morning I checked and was black between the wirings, I hope not dead...
I am using modified Heising with a modulator 30:1, 3.6K to 4 ohm using a solid state amp and having grounded one side of the modulator. The other side is connected with reactor and final due to a 1.5uf/10kv cap.
I unkey in the same moment with one order to 3 different solid state relays, the drive, the screen and add in the cathode a 50K resistor.
When I key no problem at all, everything goes right.
My questions are:
a) is my sequence right or I should have some certain delay between drive and screen?
b) do I use the right Heising configuration by grounding one side of the iron as soon as I use a 4ohm af amp with no high voltage in the primary of the iron or I should ground one side of the modulator through the cap and connect with the reactor the other one?
c) I put off the solid state amp first and then I unkey the transmitter but I hear a loud noise. What is the right way to put off the af amp without this "boop"?
d) Is it a good idea for avoiding sparking to put mod and reactor irons in oil suitable for transformers or to keep them out as they are?

Thanks in advance for any help
Stefanos

Tom,

Thanks for the thread, I read it 3 times.
The results are :
a) I take the solid state amp far from the rf amp, in my case different room 30' away and I'll carry the 4 ohm energy by 2x30' RG213 to the rf amp mod department using the inners for af and the braids for rf isolation.
b) I must drop the fixed bias to as less as possible, now is -210v and goes to -390v with 2.5k resistor. I'll leave this for last because I feel very safe with high fixed bias as far I keep HV always on the tube.
c) I'll add a 100uf in paralel with the 50K cathode resistor, that's the easiest part, I'll do first.
And finally I'll keep on the drive for 1/2 sec after screen is off and cathode is grounded through the 50k.
I have not a T/R relay because I listen from an external receiver and the transmitter is monobander.
I must add that I unkey screen from the ac side so I notice the screen volts coming down in 1/2 second from unkeying whatever this means...

If there is smthing more that I lost please let me know. After the changes I'll let you know the results and post some photos of the "Lady in red".

If somebody can help me with the right Heising configuration for my case will be highly appreciated.

Stefanos

Stefanos,

With the items you listed, it looks like you have a good start and the knowledge to pull it off. Sometimes the sequencing takes some time to get right. You might even buy a 4-step sequencing board from Jay/W1VD. That's what I needed to do for my class E AM rig, but was able to get the 4X1 rig working with just a few R/C delays in the ant and keying relays.

Since that thread, I switched over to a homebrew solid state FET audio driver designed by WA1GFZ. The 100 watt 8 ohm amp with the backwards driver transformer did finally work well when placed far from the 4X1 rig. However, this FET driver sits right next to the rig and is far superior with less phase shift. You might build one up later.

http://amfone.net/Amforum/index.php?topic=23632.0

BTW, about your Heising question... will this simple Heising schematic approach help?

http://www.amwindow.org/tech/htm/813/813.htm

Another recommended approach is to tie the Heising cap to ground. There is a good thread on this subject about three months ago. I can find it if you can't.

T

Bill, you are very right. Energy stored in a big inductor is unpredictable sometimes.
The story is that this happened suddenly one night when was working without problems for more than 6 months.
It is possible that the reactor could handle the enrgy till a point after that it sparked.

Tom, thanks for the recomendations but in my case the af amp is not a driver for a tube af amp, it is the final af amp. In few words it is a Phonic 5100 the same like Crown XLS 5000 a huge amp with 5000w bridged 4 ohm output...That's why I thought to ground one end of the iron. There isn't any high voltage in the primary but just the pp af voltage 360v-400v. The schematic of the 813's is considering that the pri and sec of the iron have the same voltage, in my case this configuration would give 5000v to the sec of the iron and big difference with the pri...I thought that this could be dangerous or not?
I attach my configuration to become more clear.
The bad news are that the reactor is off...I already opened the lamination on the bench and found lot of smoke between the upper windings and the inside of the E iron...New winding is ordered with 25kv isolation between windings and iron.
Unfortunately I can't see the difference of the 3 changes I already did in the transmitter because all even before the changes worked perfect without the reactor in line. Change a. I added a 100uf/450v in parallel with the 50K catode resistor. Change b. I added aan RC delay to the drive on off relay and Change c. I moved the heavy af amp closed to the audio chain about 10m away from transmitter and in diff room. Tomorrow I must connect with RG213 the bridged 4ohm output to the transmitter and wait for the new reactor winding. The reactor has all E and all I laminates separated with a 2mm fat isolation material like a choke, is that right or it could work better if i mix the laminations like a transformer?
I still need your opinions about the rightest Heising configuration for my special case.
Thank you,
Stefanos

Stefanos

Hi!

I have been running a similar configuration for a couple of years now... with no problems. I'm using a 700W QSC ampliier to plate modulate a pair of GS35b's. The transformer is a 1:25 step up transformer*, the Heising reactor is a Peter Dahl 50H 300mA, and the coupling capacitor is 1.4uF. The plate voltage at carrier is 1800 Volts, and the plate current at carrier is 300mA.

*The transformer is a ferrite core toroidal power transformer which has nearly ideal characteristics. It's magnetizing inductance is much larger than 50H, so the reactor dominates. The frequency response of the modulated RF amplifier (as measured with an off-the-air monitor) is 3dB down at 20Hz, and flat out to beyond 10kHz.

I believe that because the impedance looking back through the transformer into the amplifier is very low (the amplifier looks almost like an ideal voltage source), the current that is flowing in the reactor will not have to change as rapidly when the rf tube(s) is (are) turned off or cut off on negative modulation peaks (compared to a traditional design). The current will just be rerouted into the amplifier (via the coupling capacitor and the transformer). Therefore, di/dt is not anywhere near as large as it would be if the modulator were a traditional tube design (with a relatively high output impedance).

Like Tom, I believe that the effect you are observing is being caused by a transient generated by the amplifier. This would be of particular concern with a 5000 watt amplifier.

Best regards
Stu

Stu,
First of all many many thanks for all I have learnt years now from you and all the amers in this forum.
I used to use the traditional mod style till I met all of you and I found the right way to avoid saturation etc etc...
You are absolutely right, the trancient came for some reason from the amplifier and wounded heavily the reactor.
After your reply I thought to check the amplifier and found that the protections lamps are always on now, the amp takes too many amperes with no load and after 1 minute about, one of the two fuses on its back comes out...
For sure the amp got ac or input rfied and created the transient on the reactor, after I didn't start the amp because I focused on the reactor's sparking and so I thought that the problem is sequencing or anything else.
Today I'll bring the amp to the service, middle of next week I'll get the new reactor with only 30H inductance because of heavier isolation and I'll keep all the new changes that Tom suggested.
The only doubt is for the 10m long audio cables from the amp's bridged output to the mod iron, they seem to me long but definately may be not...
I am thinking to key the af amp by interrupting a relay on its 4ohm output, now I key it with relay in the ac line and hear an annoying "boop" in my monitor, is it dangerous for the amp to stay on but unload even with all pre audio chain off or not?
Sorry for all boring questions but it is completely new to me how to use fragile solid state amp with heising.

Thanks,
Stefanos

Stefanos

1. With respect to the cables from the bridged output of the amplifier to the transformer

The impedance at that point is very low (essentially zero), and therefore the capacitance of the cables (the capacitance of each cable's center conductor to the grounded shield) will have no effect on the audio.

2. With respect to the output relay

Do not open the amplifier's output relay. What keeps the reactor and the mod iron from arcing over when the plate current to the rf tube suddenly turns off (either because you have removed the rf drive, or because the tube has been driven into cutoff on a negative modulation peak, or because you have inserted a large resistor between the cathode and the negative side of the B+ circuit) is the relatively low impedance path through the audio amplifier that the reactor's current can take.

3. I don't think that the audio amplifier cares if it is looking into an open circuit (i.e. when the rf tube is turned off). However, it might be a good precaution to put a resistive load (for example: 20 Ohms) across the output of the amplifier, in parallel with the modulation transformer. Since the modulation resistance of the rf amplifier is around 5000 Ohms (B+/plate current at carrier), the audio amplifier sees a load (from the rf amplifier) of around 5000 Ohms/[30 x 30] = 5.5 Ohms. Putting a 20 Ohm resistor in parallel will make the amplifier deliver an extra 25% of audio power; but you have plenty of extra power in the audio amplifier.

A key question to ask... which is specific to this particular audio amplifier, is whether the overload protection circuitry of the audio amplifier is sensitive (as designed) to the sudden increase in output current that results when the reactor current is diverted from the r.f. tube(s) to the audio amplifier. In fact, you may find that the overload protection circuit is trying to respond to that by opening up the output relay (just a hypothesis). You may want to consider bypassing the output relay.

Stu

Stefano

I few more thoughts:

1. With respect to the 100uF capacitor being placed across the 50k Ohm resistor

It makes sense to me to open the relay (RL1) across the 50k Ohm resistor + 100uF capacitor first, because doing so will cause the plate current and the screen current to drop slowly, toward 0mA, when unkeying. With the plate current and the screen current at 0mA, it should make little difference as to what you turn off next. Note that the 100uF capacitor will slow down the turn off of the plate and screen current. If the total plate + screen current is (for example) 1A, then the 100uF capacitor will initially charge up at a rate of 1A/100uF Volts per second = 10 Volts per millisecond. Thus, it will take several 10's of milliseconds for the tube(s) to turn off. This will, of course, reduce the concerns regarding the current in the reactor having to be diverted into audio amplifier during the turnoff process.

Note: with the 100uF capacitor charged to whatever the cathode cutoff bias is... closing the relay, RL1, is going to make a "bang" unless you place some resistance in series with the relay contacts.

2. With respect to turning the audio amplifier on and off

I think it is not a good idea to turn the audio amplifier on and off by turning its AC power on and off. What I do is to just open the audio input line to my audio amplifier. Even that may be unnecessary, but it seemed prudent to do. [I.e., in case I decided to set my audio chain to produce a high-level sine wave test signal, I didn't want that to go into the input of the audio amplifier until I was ready to transmit]. If you turn off the AC power to the audio amplifier, then there will be an open circuit across the input to the modulation transformer... and I think it is better to keep a very low impedance (the impedance looking back into the audio amplifier) across the input to the modulation transformer.

3. With respect to the Heising configuration

Since the primary of the modulation transformer is at 0 volts DC relative to ground, you should keep your present configuration. I.e. ground one side of the secondary of the modulation transformer, and couple the other side to the Heising reactor/plate using the Heising capacitor. This will keep the entire modulation transformer (primary and secondary windings) a 0 volts DC.

Stu

Stu,
I thought about the addition of the 100uf and found it needless for my case. The reason is that I turn off the screen supply from its ac side, this supply is a C (150uf/1200v) input one and with 170ma on the screen it comes down to 0ma in about 100ms when I turn it off. I think that if I turn off first the audio chain, then the screen's supply from ac pri + 50k to cathode and finally the driver, it will be right and safe.
The suggestion for not turning off the amp's pri was my wish for that Christmas and thank you for that precious present...It was my nightmare that "boop" but I was very afraid to leave the amp on and a huge backward trancient to destroy it. Definately that happened but because of different reason.
So I thought to add a switch between the output of my comp/limiter and the amp's input and keep the amp undriven due to receiving period.
The reactor is with separated E from I laminations by a 2mm thick isolation material, is that better or a mixed laminations core would give more inductance and work properly,too? Now that I have openned it is a good opportunity to corect it in case it is wrong.
Ending I decided to manually open and close the 3 switches for on and off untill I feel sure and safe for the sequencing. I'll also construct 2 pair of spark gaps as W2VW suggested to protect the irons.
Give me please an idea about the wattage of the 20ohm resistor and keep an eye to the new schematic that I am oriented to follow.
Of course all your comments and suggestions will be highly appreciated.

Thanks,
Stefanos

Stefanos

I agree that turning off the screen voltage first, by removing the AC input to the screen supply, is an excellent strategy for avoiding a sudden change in the current through the Heising reactor.

The 20 Ohm auxilliary audio load resistor would have to dissipate about 25% of the average audio power that is going into the transmitter. As long as you don't apply a sine wave at 100% modulation (at least not for a long period of time), the average audio power will be much less than the peak audio power.
In the end, you may decide that including this resistor is not necessary. Recall that in the normal audio amplifier power-on sequence, the audio amplifier's output relay is open until the amplifier gets through its power on transient. Therefore, operating without any load is nothing unusual for a modern audio amplifier.
But, if you do include the resistor, I would suggest a resistor capable of dissipating 100 Watts of average power. You might consider using a spare 1kW peak / 100 Watt average dissipation 50 Ohm RF dummy load... since all you are trying to do is to keep some load on the audio amp when the transmitter is off.
I don't know as much as the other folks on this board about the subject of how to lay out the laminations of the reactor.

In none of the posts to this thread do I see any mention of the simple solution I have used for years, and which has been in use by AM broadcast transmitters since the advent of plate modulation in high power transmitters, dating back to the 1920s.

Use a spark gap across the modulation reactor, set just slightly wider than what will spark over on modulation peaks. The idea is to let the voltage breakdown occur harmlessly at the spark gap, rather than in the wiring of the transmitter, or worse, through the internal insulation in the modulation transformer and/or reactor.

To protect the modulation transformer, a similar gap should be installed across the primary, not secondary of the modulation transformer. A gap across the secondary and/or across the mod reactor, without one across the mod transformer primary, may actually increase the risk of insulation break down in the mod transformer, due to the voltage generated by the dI/dt that occurs in the pulse that results when the gap sparks over. You want the dI/dt to occur before the modulation transformer, not allow it to be reflected back through the transformer to the primary from the secondary load.

I also install a gap across the power supply filter chokes.

Quote from: k4kyv on February 20, 2011, 01:21:34 PM

To protect the modulation transformer, a similar gap should be installed across the primary, not secondary of the modulation transformer. A gap across the secondary and/or across the mod reactor, without one across the mod transformer primary, may actually increase the risk of insulation break down in the mod transformer, due to the voltage generated by the dI/dt that occurs in the pulse that results when the gap sparks over. You want the dI/dt to occur before the modulation transformer, not allow it to be reflected back through the transformer to the primary from the secondary load.

Don,

RCA installed spark gaps across both the primary and the secondary windings on some of their broadcast modulation xfmrs (at least the ones I happen to be personally familiar with).

I have an RCA type 900777-502 BC mod xfmr (I had two at one point; gave one away many years ago to another AM'er), and both of them had factory-installed gaps across both windings. This xfmr was used in the RCA model 250K 250 watt-class AM rig, and the xfmr was designed to handle the full 250-300 ma. unbalanced secondary current; there was no mod reactor used in this rig. This xfmr easily weighs in at over 150 lbs.

Perhaps RCA knew something here that you may have overlooked???

By the same token, I am not aware of any Gates/Harris AM BC rigs that used spark gaps on either winding of the mod xfmr.

Just my 2 cents.

73,

Bruce

Don et. al.

One has to adjust the various rules of thumb that apply to a traditional plate modulated AM transmitter, when one is employing a plate modulated transmitter that uses an audio amplifier whose output impedance is essentially zero.

Consider the case of a classical audio amplifier that employs a pair of tetrodes in push pull.

The current that will flow through each tetrode is essentially (not exactly) independent of the plate voltage on each tetrode. Therefore, the (output) impedance looking back toward the audio amplifier... from the modulation transformer... is essentially infinite (in any event, very large compared to the modulation resistance of the rf amplifier).

As a result, if the rf tubes are cut off, the current flowing in the mod reactor (for the case of modified Heising modulation) or in the secondary winding of the modulation transformer (for a non-Heising configuration) has no place to go. Therefore, di/dt is essentially infinite... and the reactor or transformer needs a spark gap to limit the voltage and to dissipate the energy stored in the magnetic field of the reactor or transformer.

If the audio amplifier employs triodes in puish pull, then the situation is not quite as bad. The (output) impedance looking back from the modulation transformer toward the audio amplifer (adjusted for the turns ratio) will be comparable to the the modulation resistance of the rf amplifer. Thus, if the rf stage plate current is disrupted, the current in the reactor or the secondary of the modulation transformer has a path to flow into (to allow the stored energy in the magnetic field to be dissipated, and to limit di/dt to a safe value)

For the case of a modern audio amplifier, whose output impedance is essentially zero (because it is a feedback-controlled voltage source), a disruption of the plate current in the rf stage will not cause a disruption of the current flowing in the Heising reactor or in the secondary of the modulation transformer. The current will be diverted into the audio amplifier. As long as the audio amplifier does not shut down (e.g. by opening up its output relay), the problems associated with a sudden cutoff of the rf output stage plate current will not cause a large voltage to develop across the modulation transformer or (in the case of a Heising configuration) the modulation reactor.

Stu

Quote from: W2XR on February 20, 2011, 01:57:13 PM

RCA installed spark gaps across both the primary and the secondary windings on some of their broadcast modulation xfmrs (at least the ones I happen to be personally aware of).

I have an RCA type 900777-502 BC mod xfmr (I had two at one point; gave one away many years ago to another AM'er), and both of them had factory-installed gaps across both windings. This xfmr was used in the RCA model 250K 250 watt-class AM rig, and the xfmr was designed to handle the full 250-300 ma. unbalanced secondary current; there was no mod reactor used in this rig. This xfmr easily weighs in at over 150 lbs.

Perhaps RCA knew something here that you may have overlooked???

By the same token, I am not aware of any Gates/Harris AM BC rigs that used spark gaps on either winding of the mod xfmr.

Nothing wrong with placing gaps across both windings. The problem is when there is a gap across the secondary but none across the primary.

With a reactor (assuming a gap there), a gap across the mod xfmr secondary would serve no useful purpose.

On my BC1-T, I re-mounted the mod transformer on stand-off insulators, and added spark gaps across the primary.

My HF-300 rig uses the mod xfmr from an old RCA 250-watt transmitter, although I have seen it modulate as high as 150% positive at 1 kw DC input. That transformer has gaps across the primary, but none on the secondary. I have a reactor to match that transformer, but the one I use in the rig is a UTC LS-103, to which I added gaps across the terminals. I'll have to check to see if the RCA reactor has a set of gaps.

I have one of those RCA 250 watt mod xfmrs designed for use without a reactor; it's the same size as their 1 kw transfomers designed for use with a reactor. I don't use it; if someone wants it, I would be willing to swap it for something, or even sell it, but it has to be pick-up only; I refuse to try to pack the thing and then haul it to a ~~destruction~~ shipping company, since it weighs 75 lbs or more. One of the problems with it is that its turns ratio (1.7:1) is too much step-down @ 3:1 Z ratio.

One danger may exist in ham transmitters without gaps across the transformer, in the case of a relay across the secondary to short it out for CW or linear amplifier operation. The relay contacts may inadvertently serve as an arc gap, with none across the primary.

Regardless of whether I used triodes or tetrodes, I would still feel safer with a mod transformer protected by gaps.

Quote from: AB2EZ on February 20, 2011, 02:39:31 PM

Hi Stu,

Thank you for your usual detailed and insightful response.

This would probably explain in large part why Gates, who almost exclusively used triodes as modulator tubes, did not see the need to include spark gaps on the windings of the modulation xfmrs or reactors used in their plate modulated BC rigs.

To carry this one step further, and the answer is probably obvious, but would this also suggest that plate moduated rigs employing triode modulator tubes would be less likely to exhibit modulation transformer or modulation reactor failure in the event of periods of over-modulation, where the carrier is cut-off and the modulation transformer is effectively unloaded?

73,

Bruce

I have never worried about crapping out a modulation transformer with negative overmodulation. At the moment of overmodulation, the plate current is minuscule, approaching zero, and at exactly 100% it is zero. IMO you would have to drive the plate of the final hundreds of volts into the negative region before this would happen. However, it could happen with separate power supplies if you somehow lost plate voltage to the final but maintained full plate voltage to the modulator tubes. I have accidentally done that a few times and nothing happened, but I still would not intentionally allow it to happen.

I never cared for the ultramodulation circuit or negative cycle loading. But a variation would protect the modulation transformer during negative overmodulation. Obtain a wirewound power resistor the same value as the modulating impedance (DC plate voltage/DC plate current), and a diode with PIV at least twice the DC plate voltage, plus substantial margin for safety. Connect the resistor from the positive plate lead coming off the modulation transformer, and ground the resistor through the diode, with the anode grounded and the cathode going to the resistor. The diode keeps the resistor disconnected as long as the voltage to the final is positive or zero. As soon as the final is overmodulated, the plate lead goes negative and the diode conducts, placing the resistive load on the modulator just as the final cuts off.

Yup. Doing an ultramodulation circuit here since ER #3, 7/99.

No matter how hard I hammer the audio, I can't push it beyond 95% negative. And doing about 110%-120% positive. I absolutely cannot cut off the RF final.

I recently smoked one of the 15 yo ultramod diode stacks, which I just replaced today. Six 6 Amp, 1000 volt fast diodes in series now in place instead of the 1 amp stack. Looks great on the scope.

if you've got excess audio power, and good iron, Ultramodulation is a good way to go and helps protect your iron. Negative cycle loading is a cheap way to protect against human screw ups.

Don, you should write up an article on the technique. Actually, you already sort of did.

But I don't like to load down the modulator over the negative cycle before 100% modulation. That's what the classic 3-diode ultra modulation circuit does, actually attenuating the entire negative half of the audio cycle via resistive losses. The abrupt discontinuity as the audio waveform crosses the baseline point produces splatter if a low pass filter is not inserted between the modulator and final. In practical use, splatter with this circuit is reduced to a near-satisfactory level by the rudimentary filter formed by the leakage inductance of the mod transformer combined with the rf bypass and plate tuning capacitors in the final, aka "building out" the mod transformer.

Any time you tamper with an audio waveform, by definition, you generate distortion and greater bandwidth. The ultramod circuit produces a positive carrier shift along with even harmonic distortion. Better to take advantage of the natural asymmetry of the human voice to generate enhanced positive peaks. The increased output power you see with ultramod mostly comes from the rectified audio voltage added to the DC plate voltage, plus even harmonic distortion audio products. This would be identical to the effects of running controlled carrier modulation while deliberately introducing even harmonic distortion elsewhere in the audio chain.

Years ago I tried the ultramod circuit. It looked good on the rf ammeter, but signal reports told me the audio was not any louder than without it, but the additional distortion was noticeable.

The single diode and resistor circuit I described produces no distortion, and can serve as a handy overmodulation indicator when an 866 or 866-A, or other MV rectifier tube, is used as the diode. At the point of overmodulation, you see the 866 flash blue. I once used one for this purpose, with the 866 mounted inside a darkened box with a viewing window cut in the side. If desired, the plate of the rectifier can be biased positive in order to get it to flash at less than 100%, for example at 95% negative.

I agree with Don,

I use a single diode limiter with a bias voltage. I don't use the load resistor, the impedance of the bias supply RC filters creates a load to ground. Works perfect and it doesn't create any distortion.

As far as the OP's problem, how are you putting the xmtr into standby?? Switching off the HV plate supply? Or, leaving the plate supply on and biasing the finals into cutoff by opening the cathode circuit??

Opening the cathode circuit is almost the same as opening the HV B+ line to the mod reactor with a relay. Doing so will create very high voltages in the reactor with the sudden loss of current.

A single-diode-negative-peak-limiter circuit connected to the Class C terminal of the mod reactor will help to absorb the negative spikes. Positive spikes should be absorbed the the Class C stage. Another problem are the spikes (+ or -)that appear at the B+ terminal of the mod reactor. You might think that the filter caps in the supply would absorb them. Probably not, those caps have too much inductance to absorb fast-rise-time spikes. You would need to have a number of HV disc caps connected to the B+ terminal and ground to help absorb the spikes. Using a spark gap across the mod iron is another way (as mentioned by others) to absorb the spikes.

I think JMO that it's better to switch off the HV B+ supply on standby and use a step-start to restart the B+ supply on transmit. The filter caps in the supply allow for a time constant for the current to drop to zero. This should greatly reduce the HV spikes across the mod reactor.

Just some of my thoughts which may or may not be useful for your transmitter.

Fred

Bruce

Hi!

In response to your question, pasted below.

If the modulated B+ tries to go low enough for the rf tubes to stop conducting, then the average current that is still flowing in the secondary of the modulation transformer (or in the reactor, in the case of Heising modulation) would force the modulated B+ to go back up (positive)... and the rf tubes will continue to conduct.

So, the "hitting the baseline" effect is an issue with distortion on the modulated envelope (splatter, etc.).

The transformer/reactor damage problem occurs if something other than a drop in the modulated B+ causes the rf tubes to stop conducting. As examples: suddenly forcing the screen voltage to zero, or suddenly disconnecting the grid drive, or something that causes the B+ line to suddenly open up.

Best regards
Stu

Hi Stu,

Thank you for your usual detailed and insightful response.

This would probably explain in large part why Gates, who almost exclusively used triodes as modulator tubes, did not see the need to include spark gaps on the windings of the modulation xfmrs or reactors used in their plate modulated BC rigs.

To carry this one step further, and the answer is probably obvious, but would this also suggest that plate moduated rigs employing triode modulator tubes would be less likely to exhibit modulation transformer or modulation reactor failure in the event of periods of over-modulation, where the carrier is cut-off and the modulation transformer is effectively unloaded?

73,

Bruce

I never put the transmitter in stand-by by interrupting the plate current to the final. That's just asking for trouble from transients and soaring power supply voltage, plus the safety hazard of keeping a transmitter running for hours in stand-by with the full +HV present. Not only might it be easy to forget that it's there, this also causes unnecessary stress wear on the HV insulation in certain components, particularly the mod xfmr and reactor if their case is grounded.

I prefer to use large "contactor" relays to interrupt the a.c. voltage to the primary of the plate transformer(s). I use choke input filtering, and as much filter capacitance as I can without having to resort to step-start, while using a low enough resistance in the bleeders to maintain critical inductance in the filter choke in the event of loss of load on the power supply, either inadvertently or intentionally, as in the case of operating CW. Depending on the voltage and current drawn from the power supply, I have found between 25 and 50 mfd to be optimum for the filter capacitance.

If you do interrupt the +HV to the final but leave it on all the time, it would better to break the cathode circuit as long as the insulation in the filament transformer and filament by-pass capacitors can withstand the voltage. You will not see the full HV appear across the switch or relay contacts. When the cathode circuit is opened, assuming the grid resistor and bias supply are taken directly to ground, the open contacts on the relay act like an infinitely high cathode resistance. Current ceases to flow through the tube as soon as the cathode voltage rises above the grid bias cut-off voltage of the tube, so you might experience a couple of hundred volts, depending on the tube, not the full plate voltage that may exceed several kilovolts. If, OTOH, the switch or relay is placed in the plate side of the circuit, the full HV will appear across the contacts when they open. The contacts in the HV switch or relay are much more likely arc over upon stand-by if it is placed in the plate side than if it is placed in the cathode side.

The same is true for cathode keying a CW transmitter. Contrary to common belief, the full HV does not appear across the contacts of a hand key. Of course, the full voltage will appear across the contacts in the event of a plate-grid short in the tube, so I would be careful when cathode keying a final with kilovolts of DC on the plate.

Hi everybody,

I have read very carefully all your comments about negative peak diode limiter and turning on-off the C class amplifier.
As it has to do with my situation there are some things I can do and some I can't because of the specific construction of mine.
I bet to myself to keep the solid state af amp as modulator even it is very difficult at the moment to fully protect it because :
a)I designed the modulator iron for 4-8ohms primary and
b) this class H mosfet amp has amazingly low dissipation/distortion in comparison with any B class push pull amp I've heard in the neighbourhood.

Steeve (Stu) gave me the best solution to my nr1 problem, how to turn on-off the transmitter without any spark or even any little flash by keeping also the Hv always on because it is complicate to step start it (2 phses with two HV transformers in series etc etc... anyway everything around it is super insulated-no chance for shortings, tested under very stressed situation.
Now I first turn off the audio chain output, then with the same switch I turn off screen and short the reactor's ends by a resistor=Rmod and finally I turn off the driver.As Steve K4HX said, I don't open the cathode as far I use -200v fixed bias and tub cuts off as soon screen and drive turn off.
Don,
spark gaps that I also have got, are fine for irons protection but for every spark they create a trancient travels through windings back to the solid state, where they shouldn't appear.
Fred,
I just noticed that I don't have any 0.001uf/10kv on the reactor's Hv supply side, I'll put some tomorrow that I'll also add the diodes with zeners in series to the relays' coils.
Are the solid state af amps very sensitive or there is still the possibility to block any trancient from jumping through the modulator's windings in the output of the solid state af amp and kill the mosfets??????????
I don't want to quit and go for a B class pp triode amp and let the trancient without punishment laughing at me... ??? I hope next time to catch it... >:(
Check please the attachment.

Stefano

Stefano,

Problems like this can be cured at the source, or you can beef up things where susceptibility occurs. Of course you can also do both.

The audio power amplifier at 3200W into 4 ohms puts out 113 vac rms. I would be tempted to order some varistors, like V130LA10A, and put across the audio lines at the amplifier, and from each audio line to ground, and also make a common mode choke going to the modulation transformer primary. Could use the choke in one of those 20Amp 250V AC line filters.

Does that modulation transformer have an electrostatic shield between primary and secondary windings? If so, ground it.

Jim
WD5JKO

I disagree with the 3.6k 100 watt resistor connected across the modulation reactor. That would waste 50% of your audio power. The whole purpose of the modulation reactor is to present an infinite impedance across the modulation transformer, while simultaneously providing a zero resistance DC path from power supply to final. A practical reactor won't quite make those ideal characteristics, but will come satisfactorily close. The modulation impedance of the final (plate voltage/plate current) already simulates a perfect resistor connected across the modulation transformer. A second equal resistance connected in parallel will result in the modulator working into half the nominal modulating impedance.

Don,
as Jim mentioned, the resistor is not there when transmit, it is entering the sircuit few milisecs before the drive switches off and greatly improves the sparking effect of the reactor. When I turn on the relay disactivate it and the reactor acts freely. It needs of course an extra vacuum relay but believe me worths every penny.
With 5-6kv and 5-7kw power on and off can not be easier than it is now, I believe.

Two questions only.
What is the diode-resistor configuration and the right position on the mod sircuit for better negative peaks cliping?
The irons are ringing-singing my voice back and this produce feedback to my condenser mic which pushes me to keep my audio 10-20% lower than could be. I'll soon move away from transmitter's place but I anyway prefer to improve this effect. Is it right to take the irons out and swim them in varnish or transformer oil or there is another trick?
Jim,
you are right about the varistors, as long they don't annoy audio quality I will put them in place as you suggested. I'll order 150v ones because the final amp to be used is a 5000w one.
Unfortunately there is not an electrostatic shield in the modulator because it is not a single pri to a single sec winding, it is a multiple sandwitch type with 5 primary windings between 5 secondary windings.
Didn't understand about the choke, could you explain me better what you mean "common mode" and where should I put it exactly?
Anything I can add is between the solid state output and modulator pri input, of course without annoying the pure ac audio signal, varistors to ground sound good.
I always thought that all those solid amps have clamping diodes on their outputs for arresting any dc presenting there but....or they don't have or those diodes don't work properly...or this trancient is too fast and huge... ???
I know that if I used a push pull tube amp I wouldn't realize any of these problems as long the tubes are elephants for such little ents-trancients...now I have the quality, the lowest distortion, the lowest dissipation but the highest sensitivity.
Anyway, I have 3 recomendetations to try and possibly improve the sircuit, some disc caps on the HV supply, diodes on the relays coils and the varistors on the solid af amp's outputs.

Thanks
Stefano

Quote from: w3jn on February 22, 2011, 10:55:57 AM

Don't neglect a purely acoustical reason for the transient.

My FT-1000MP has a terrible transient due to the "click" from the push to talk switch.

One more reason to avoid a microphone with attached PTT switch.

Another problem with the PTT mic switch is that the PTT line may magnetically induce an electrical pulse into the audio line, due to the long mic cord with parallel PTT and audio lines in close vicinity. I had that problem with my first D-104 with G-stand. It caused the tuning capacitor in the final to sometimes arc over just when I changed over to transmit. It's a wonder I didn't blow the modulation transformer. As soon as I discovered the cause of the problem I got rid of the chicken-choker once and for all.

I prefer to mount my microphone on a boom, with separate PTT switch attached to the end of a spiral cord, salvaged from some kind of old communications mic. Been using that system for decades. I don't like the idea of a foot switch, either. Nor a toggle or rotary T/R switch mounted on the panel of the transmitter or exciter.

My system allows me to walk over to the back of the transmitter and hit the transmit switch, so I can observe what is going on inside the rig just as it goes into transmit. It has helped me solve numerous flaky type problems and discover where arc-overs occurred. I can walk away from the operating position while holding the PTT switch, ready to instantly shut down the transmitter in case an intermittent short or arc-over occurs while I am trying to chase down the problem.

I'd leave the combination PTT microphone switch for the Good Buddies.

Jim,
I have a big T200 ferrite ring and few ferrite clamps to add but this would help mostly in hummmms and noises, a trancient will travel due to the cable continuity and reach the af amp output. A short talk with the modulator transformer brought up on the table the use of a capacitance coupling between af amp and modulation transformer. They suggested 400uf-800uf/440vac for 8 ohms and 30hz lowest frequency.
I have some 800uf/450vdc electrolytics and I could use in a combination to create an ac capacitance.
Should they be connected in series +- to -+ or oposite -+ to +- or doesn't matter?

Don,
I also use the independent ptt key with 3 teflon rg cables and the audio chain is totaly separated. Now that I am located next to the transmitter I have the problem of irons talking back, later the audio chain will be relocated in a separate room and of course it will be no problem at all. I can also move around with the ptt on hand and control the amp by being next to it, it is really very helpful.

Stu,
I didn't realize any problem by shuting down first with the same order the screen ac voltage & enter the resistor and shuting down second the drive but I can change it to as you suggest if this can make things even safer.

W2PFY,
I am also confused, it is a mixed style transmitter...power and modulation like broadcast, ptt like communication, use and problems like both of those types... :-\

Thanks
Stefano

I think you need to hook up the caps so they look at the circuit as non polarized.

In other words, the caps should be hooked up in parallel with one end both pos and neg tied together and likewise on the other end and the other pair in the other feeder would be wired the same. If you were to look at this as two flashlight D cells, it would look like a short but for capacitors of this type wired this way, they would be non polarized.

Quote

W2PFY,
I am also confused, it is a mixed style transmitter...power and modulation like broadcast, ptt like communication, use and problems like both of those types... Undecided

I guess my question is what frequencies do you operate this transmitter on? This has been an on going thread and perhaps you mentioned it before?

Stefano,

I once worked for a company that used a pair of Crowm M600's bridged to drive a HV step up transformer. They drove 7.5 Khz PWM (bipolar signal) to make 40,000 volts DC at 50 ma (2KW). They used a series cap setup like B or C as you draw, and the caps were 1600uf @ 450v computer grade with the big screw terminals. I got upset at the idea, "how could you guys be so dumb as to...".

I added some big diodes across each cap so each capacitor could not be reverse biased. It made NO difference, in fact without the diodes, and measuring with a hand held DVM, I could measure no Net DC voltage across either capacitor, even when the PWM drive had a DC component to it (< 50% duty cycle). I ate crow on that one.

Jim
WD5JKO

Hi

THe way I see it,when You used Heizing modulation,You used a cap ( 1.5uf 15 Kv ) to couple the secondary winding of the Mod.trans to uper connector of the reactor.

What happened at this C ,since the secondary winding is grounded via the winding to ground.and the other end of the C is connected to B+ above the reactor ,it is charged to to the B+ .

SO it is Ground ---- Secondary winding T.modulator----C (1.5 uF) ---Reactor ----B+---power supplu(HT).

If the B+ is 1000 VDC for instance, than The Coupling C is charged to +/- 1000 V

Now when You Switch off (Unkey) the transmitter ,What happen to capacitive load C1.5 uf (energy stored in it) ---- 1000v DC---,
It begin to discharge the load trough Secondary Mod transformer -----C --- Reactor --B+ supply (that is also dicharging) and caused a big EMF across the Reactor.

It is behaves like a CDI (capacitor discharge ignition),since the energi(EMF) has no way to go.Than Swap there's a spark from the reacktor to ground.

Gito

Hi

Another though comes to Me ,I have the conclusion after reading all the comments.in this article, is A storage energy in the reactor ,when the current is suddenly is gone, and there's noway to go ,it can cause a flash ,or spark occur in the reactor.

So Why don't we used a bias for the PA,that makes the PA tube conducting when idle ,a small standing current like 25 ma.

We still work at class C when this bias Voltage +(grid leak R X grid current) voltage = Class C operating voltage,

The difference when idle,no drive ,there's a small idle current, that makes the PA tube on conducting state

When You unkey the transmitter ,the current trough the Reactor is gone ,and make a high voltage across the reactor

, but since the tube is in conducting state, It will conduct this current,and limit it to 25 ma,so the positive peak voltage is shunt across the tube .

And we used also a negative peak limiter(3 diode negative peak limiter),if it is a negative peak voltage that can cause the spike to occur. It limits the negative spikes.

Just A though

Gito

Jim,
I also think the right way to make that ac capacitor by connecting some dc caps together is B or C but I saw your reply to this thread http://amfone.net/Amforum/index.php?topic=22341.msg161638#msg161638 that confused me because it seems like D which is a series dc capacitor connection.

W2PFY,
It seems like solution A hasn't that much popularity in the net, I couldn't find anything like that googling around but I'll give it a try connected the output to a speaker and listen to.

Gito,
I specially have 2 problems on the modulation-hv side of the transmitter. The first that was the EMF and the huge spiking on the reactor was solved by shorting reactor's ends by a resistor=Rdc, driving the stored energy in the reactor back to the HV capacitance bank. Keep in mind that I keep Hv on the tube all the time and I use fixed bias (-50v to -200v as much I wish) but with a parallel leaking resistor like in the sheme below.
In this configuration I can't really idle the tube with the screen voltage off in any fixed bias level from -50vdc to -200vdc. Screen voltage shuting down is a very important safety cure even I have added a very fast and reliable security sircuit in any case. I wish to keep screen off during receiving periods. You are very right though about the heising capacitor decharging energy but as long I keep HV always on, this capacitor is charged when I step start the HV begining of the evening and stay so till I turn off the amplifier on the end of the night and the HV supply is dicharged slowly through the bleeding resistors.
The second and very main problem is that sometimes, can't really say what from is depending, some trancient is coming through the modulation transformer back to the 4 ohms bridged mode output of the solid state af amp and mosfets are blown up.
That happened twice unreasonably after 40-50 on-off periods with lot of transmission in full power mode.
Now I'll try ato involve between the output of the AF amp and the primary of the modulation transformer some kind of ac capacitor to block any trancient or dc upseting direction Af solid state amp.
I know that you guys have not that sensitivity matters because of the push pull tubes af amps but in my case with solid state involving in a HV heising sircuit special care must be taken on.
Gito, what is your opinion about the ac capacitor?
Thanks

Stefano

Hi

You are right when You wrote since You have the B+ always on then the Coupling won't discharge.

But sometimes we forget The voltage in B+ loaded and without load,
reading Your reply ,the B+ is slowly discharging till the next morning,
than The B+ power supply has no or enough R Bleeder ,and if You used Capacitor input Power supply,Than what happen
When You Unkey the transmitter,The B+ soars up to maybe1.2% or 1.4%.(not loaded)

When it happened A current flow from B+(1200VDC) to Coupling C (1000VDC },. via the Reactor. So a back EMF is build around this reactor.

It just a theory.

When You used a resistor to short the Reactor ,That doesn't mean that the back EMF is gone,
You only reduced to a voltage to a low level that makes this voltage "low enough" that it cannot make a spark to jump in the reactor,but actually it's high enough , but not the enough to make the spark jump.

But the voltage is still there and coupled to secondary winding to the Amplifier.
Maybe when You short the reactor omitting The R (bypassing the resistor and reactor},than it won't happened.

The Idea of making the Amplifier in conduction state (by varying it's min bias/varying Cathode resistor) ,is making a path where this energy (EMF) can flow through the Tube
As the AC cap .it is believed that it's a "form" of low pass filter .limitiung the frequency response of the audio,so it deppents on the" Frequency "of the "spark"?

Gito

Johny,

I don't know him because there are many around this frequency. They don't belong to our team, they are "locals", they use vertical fishing rods 8-15m long 4 wires for top hat and lot of radials on their roof in order to almost broadcast for the city of Athens. I am 99% working 1670-1700 and only communication radios. The "rule" is 1600-1630 local level broadcasting by using the broaden receiving properties of the old analog receivers, 1630-1650 local chats with 50-200w, 1650-1670 medium power comms 250-1000w, 1670-1680 high power national comms 1000-3000w, 1690 the big guns 3000-10000w, 1700-1710 the Crete island power net and 1710-1730 empty for music tests, then a little 100khz jump and legal again. 8) Of course all are welcome to every frequency but it is difficult to survive next to those huge sidebands if less than 6db of them... :-\
Some are 24h broadcasting at 909, 1204 and almost in all channels every 9khz in the MW band.
The majority have bought the trs from few low level educated in electronics constructers and of course they don't use any Pi network, the letter Q is totally unknown to them. The most of them have connected the wire antenna straight on the anode blocking doorknob cap with max Q=0.001, 50 ohm and swr are allien terminologies...
See that plastic yellow can with the modulator swimming in extra virgin Greek olive oil... ;D

Stefano

Quote from: AMLOVER on February 25, 2011, 11:20:04 AM

Stefano, Maybe I did not say this well:

" If your voice has much asymmetry, running that Crown into a near DC short circuit will cause a DC current to flow into the transformer primary (4,8, or 16 ohm taps). This will use up some of the Crown power supply ampere budget. Crown in their manuals for the big amps recommend a series capacitor between the AMP and the load (if under 2-3 ohms DC). One possibility is dual 10,000mfd caps back to back with the + ends together to make a poor mans AC capacitor placed in series with the load. Remember that large loudspeakers usually have a DC resistance that is just under the AC impedance rating, so no series Cap is needed there. When you drive say the 4 ohm tap of a transformer, the DC resistance of that primary might be just a few tenths of an ohm.."

The capacitor value depends on the lowest audio frequency used. I just stated "dual 10,000mfd caps back to back with the + ends together" which was intended to be a series connection, not parallel.

Also your schematic shows two sets of capacitors. This is not necessary, just using one pair will accomplish the same thing with less reactive low frequency roll off.

Jim
WD5JKO

Quote from: AMLOVER on February 28, 2011, 04:06:20 PM

Stefano, that's one of the most interesting posts I've seen this year! The picture tells it all. It sounds like the wild west USA in the 1800's.. ;D I'll bet you guys are having a lot of fun. It wud be cool to tune in with a remote web global tuner and listen in to you guys some time.

T

Gito,
Its true that after shorting the reactor and having grounded one leg of the mod transf secondary all are very safe looking from modulator primary side. I am thinking that if trancient was the reason why the disaster happens only after 40-50 times on-offs?????Trancient should be there all the time but acts like a phantom after 40-50 on-offs?????
I start thinking that the problem is RFI that comes after some different amp tuning and loading. So I now see that Jim WD5JKO was very right to say that a big ferrite making a common mode choke with some inductance and 4 by passing caps (1-10nf), 2 in each side of the choke would act as an RF trap and keep the Af solid state amp undisturbed.
So I'll try when I have the time to make the Rf trap in conjuction with the dc trap, ac varistors+ac caps.
Jim,
I have understood you right but I have 16 caps of 800uf/450v each so I need to put every 8 of them in parallel to create 2 cap banks of 6400uf and then to connect those 2 banks back to back + with + together.
Or I'll look for some 10000-20000uf ones and make it with just 2 of them. Oh, it's good that I'll need to place the poor man's ac cap only to one side. It's more convinient.
Johny,
The guy is in middle of Greece, in the city of Larisa and I think that he uses 8xQB4-1100s, there are 2 rows of 4 tubes and modulates his plywood box with 2xTH250, I think.
The driver seems like 1x6146b.
Tom,
As far you like the photo I'll post few more with USA on 1700 and 1900... ;D
Have a look and much fun. Believe me some of them takes very seriously and they can almost die if their "beauties" get some sickness and can't sing any more. Sure we have lot of fun specially when we listen to all that .....proudness coming out of their crystalic microphones.... ;D

Stefano

Quote

Have a look and much fun. Believe me some of them takes very seriously and they can almost die if their "beauties" get some sickness and can't sing any more. Sure we have lot of fun specially when we listen to all that .....proudness coming out of their crystalic microphones....

That's pretty funny, Stefano. Yes, we get depressed when our precious creations crap out, for sure... ;D

From the pictures, I can see you prefer to build "broadcast style layout" in Greece... Easy to service. Back in the 1970's my homebrew stuff looked a lot like those rigs. We used the parts and cabinets that were available.

BTW, you might tell the owner of the 4CX-15,000 (pic #2) that it needs bigger air cooling as shown by the brown coloring on the top anode area. The seals may soon fail if not corrected.

All in all, those are creative looking rigs and you guys are certainly enthusiastic about radio.

T

Stefano et. al.

I am considering a new AM tranmmitter design concept that is inspired by various trends in supercomputing. I call it "massively parallel tetrode pooling".

The objective if to place a large number of readily available, low power tubes in parallel... so that the plate voltage is relatively low... and the plate current is relatively high ... therefore making the modulation resistance a perfect match to an 8 Ohm amplifier.

For example, consider 512 6CL6's in parallel, arranged in modules of 16 tubes each... running at 200 Volts B+ and drawing a total of 25 amperes (at carrier).

The input power, at carrier, would be 5kW. Assuming approximately k x75% efficiency, the output power at carrier would be k x 3.75 kW.... where k takes into account possible inefficiencies associated with combining the outputs of the 16 tube modules, tube mismatches, etc.

If k = 0.1, then one would have a US legal limit transmitter that can be directly modulated by a 2500 watt, 8 Ohm audio amplifier.

Comments?

Stu

Stu,

That's a good idea for very low Rdc, for low R input (grid driven closed to 50ohm) and Rload closed to 50ohm. Very safe arrangement with low high voltage, beautiful glassy shape, lot of heat and amazing ambience...
Many do it here with as many as 16 tubes but you can see the photo for 30x813...with 2500vdc and 6000ma (200ma each) this monster has Rdc=400ohm, Rload=200ohm and a modulator 1:7 for 6000w af solid state amp....15kva input and 11kw output power.....on that small low height antenna....poor neighbors... :-[

Johny, this is the famous "Playboy radio" that broadcasts on 945khz every Sunday.....

Stefano

HI Stefano

When You short the reactor with an R ,than there's still a current flow in the Reactor,that makes a back EMF that causes a voltage to appear ,this voltage/energy is absorb By The Resistance,but there's sill a voltage at this reactor right,before it was absorb/dissipated in the R resistor,This voltage is coupled back to the the Amplifier via the C coupling and the Modulation transformer ,So the "reactor" is the source of that voltage that comes into the output terminal of Your Amplifier,if it's high enough ,it can destroy your Amplifier.
But when You short/bypass the reactor ,there's no current flow trough it.Than there's no voltage appear across it,so there's no voltage getting to the amplifier.

To make sure ,used an extra switch/relay across the Mod transformer.to short it.

As long there's An Inductance (Sec Mod Trafo)--Coupling C -- B+ and ground connected in serie ,a voltage change in the B+ can charge or discharge the coupling C ,that makes a current flow trough the inductance ---- making a voltage appear across this inductance. and coupled to the primary winding of the Mod Trafo.

Maybe the voltage that's got into Your Amplifier is just small enough to destruct it,
But maybe sometimes when Your line voltage gets higher than normal,making also the voltage that appear in/at Your output terminal (voltage from the React---C ---Sec Mod Trafo....ground Coupled to.....primary Mod.Trafo...output terminal.Ampli} of Your Amplifier high enough, that makes /destroy Your Amplifier.

You wrote after 40 or 50 times ...... it happened ........

Stef I can asked You the same question ,if the RFI is there ,why it took so long till the Amplifier is destroyed.

And As I know when an RFI gets into an Amplifier ,that used to modulated ,it can oscilate ,because the audio that is rectified in the amplifier can becomes a positive feed back ,or distorted if it becomes a negatif feed back .(the phases of the rectified audio)

in my Expirience ,You can used the audio that is produced at the transmitter as a "simple judge " if there's RFI coming back in Your Amplifier,of course using an osciloscope is better.

if there's big enough RFI that comes into Your Amplifier/transmitter ,that can Destroy Your Amplifier.Than I think Your Amplifier/Transmitter Can't produced a good audio.

Of course Stef I can be wrong

Gito

I grew up in Greece and as a non-Greek citizen I couldn't get a ham license.
But I was into electronics and by age 14 I knew enough to become a "pirate"
(argh!). The pic is me at age 15 (1979) with my 6v6-6146 transmitter (no loading
cap, the antenna wire, about 200 feet long, went directly to the L-net coil and
loading was achieved by changing taps on it). I blew the horn to clear the
frequency ;D In 1980 I modified the xmtr with an 807 buffer to drive an 811
at about 1kV. But in those days the authorities were still hunting pirates with
the DF wagons so I got in touch with Charlie Jackson, SV0AA, who examined
me for the by-mail Novice and in 1980 I was KA2KEV/SV0BU and the rest is
history.

Those were the days my friend !

The last time I was in UK, about 10 years ago, my brother-in-law told me that hardly anyone listened to AM any more, whether long wave, medium wave or short wave, and he couldn't comprehend why I had even the slightest interest in exploring those bands on his car radio while we were riding. Now the UK is supposed to be phasing out analogue broadcasting, both AM and FM, and replacing it with some form of DAB. I wonder if in Greece and other European countries, so few people now listen to the AM broadcast bands that authorities don't really care who transmits there as long as they don't interfere with other services or become a general nuisance in the local area, kind of like what has happened here with CB and increasingly, ham radio.

I have heard it said that in the UK, more than one ham is running a converted broadcast transmitter on 160m at >1 KW, even though the power limit on that band is supposed to be 10 watts carrier output, and there has been no sign of a "crack down".

From my own past experience travelling around in Europe, I would suspect that authorities in UK, France, Germany, Netherlands, Switzerland and in a few other countries, radio laws would be strictly enforced as a matter of principle, but in certain others authorities might tend to look the other way, especially if the appropriate palms were properly greased.

But, OTOH, with all the paranoia these days about terrorism and political movements trying to overthrow governments (which appear to be having some success in N. Africa and the mid-east), I would think that illegal broadcasting might be an item high on most governments' priority list. Maybe they figure that the internet is a greater threat to public order than unlicensed broadcasters would be, so that's where they focus their attention. At least it is easier to control.

The AM Forum

THE AM BULLETIN BOARD => Technical Forum => Topic started by: AMLOVER on December 22, 2010, 03:40:03 PM