Reactor's sparking solution

[AMLOVER]

Hi to all,

I was wondering what would be the pos or neg in adding a resistor (about 10x mod resistance, 25k-50k) in parallel with the reactor in order to minimize or even cancel the reactor's sparking effect by lowering its Q?
Could this affect the modulation and the reactor's role in a modified Heising circuit?
At the moment I turn off the transmitter by parallelizing such a resistor by a vac relay few millisecs before I turn off the driver.
My question is if it is safe to leave the resistor permanently in parallel with the reactor without any negative effect during transmitting.

Regards
Stefano

[AB2EZ]

Stefano

The effect of keeping this resistor in place all of the time will be negligible... particularly since your setup has lot's of spare audio power (high power audio amplifier driving a step-up transformer).

If the value of the resistor is about 10x the modulation resistance of the rf output stage, then the audio power flowing into this resistor will be about 10% of the audio power flowing into the rf output stage.

For voice operation (i.e., not using an audio sine wave test signal) the average audio power being applied is much less than 50% of the DC input power to the rf output stage. Assuming you are using a wirewound resistor (which heats up and cools down slowly compared to audio frequencies) for this purpose, you can use a wattage rating that is much less than 10% x 50%  of the DC input power to the rf output stage.

Stu

[AMLOVER]

Stu,

with what you suggest in my mind i'll leave the resistor (35K/350w) permanently in the circuit. I'll check its temperature after a long transmitting and see what's happening.
5% of audio is a reasonable sacrifice for keeping reactor peaceful and I'll use the vac relay in a new hv project.

Thank you.
Stefano

[AMLOVER]

Googling last night about plate modulation, I found a very "interesting" high level modulation circuit, the attached "parallel-fed plate modulation".
This circuit completely isolates the mod. transformer from the dc without grounding one leg and so make it easy to use a toroidal transformer with all the positives as mod. transformer even the af amp is a hv pp b class.
I am wondering if it is safe to give it a try and if anybody has an experience from that circuit before.

Stefano

[W2PFY]

wow, I never saw that before. Hopefully Stu and perhaps others who have seen this will comment.

[AB2EZ]

The main advantage I see for this configuration, compared to connecting the bottom capacitor to ground (instead of connecting it to the bottom of the Heising reactor), is that, if both the modulator B+ and the rf stage B+ are on, then the DC voltage across each of the two capacitors will be (after a sufficient amount of RC charging time... controlled by the DC leakage resistance from primary to secondary) the difference between the modulator B+ and the rf stage B+ voltages. I.e. during transmit conditions, there is less steady-state DC voltage across each capacitor

With respect to AC (audio) it will make no difference.

Since the product of the primary-to-secondary leakage resistance and the capacitance of the two capacitors in series (this product has the units of seconds) will likely be more than 1 second... the difference between the DC potential of the primary of the modulation transformer and the DC potential of the secondary could still be comparable to the larger of the two B+ supply voltages (modulator and rf) during transitions from standby to transmit, and vice-versa.... depending upon how the sequencing is done. Likewise, depending upon how the sequencing is done, the steady state DC voltage across each capacitor could be as large as the larger of the two B+ supplies... during the transitions between transmit and standby.

For example, if one starts in transmit...and if the rf B+ is turned off, and the modulator B+ is left on... then the primary-to-secondary leakage resistance will cause the voltages across each of the two capacitors to slowly charge up to the modulator B+ voltage value. At the beginning of that charging period, the secondary of the modulation transformer will jump to a DC voltage level that is lower than the primary ... by an amount equal to the value of the rf B+

If you use the configuration where the second capacitor is connected to ground, then the steady state potential of the secondary of the modulation transformer will be the modulator B+ (as it is in this alternate configuration). If you turn off the rf B+, and you leave the modulator B+ on, then the the secondary of the modulation transformer will jump to a DC voltage level that is lower than the primary ... by an amount equal to 1/2 the value of the rf B+ (assuming the two capacitors are of equal value).

Bottom line:

It appears to me that this is a good configuration to use in applications where one is always transmitting, because it minimizes the DC voltage across each capacitor... and it lets the secondary of the modulation transformer float at the same DC potential as the primary (which would also be true if the lower capacitor was connected to ground). When switching from transmit to standby, the DC (slowly decaying) voltage across each capacitor, and the voltage between the primary and the secondary windings of the modulation transformer can still reach levels comparable to the larger of the rf B+ and the modulator B+... depending upon how one does the sequencing.

Stu

[N8UH]

Huh... that is interesting. I wonder what the advantage of that layout would be besides what's stated in the pic? I don't think I have ever seen this in a commercial BC TX, but then again, I haven't seen a whole lot of them compared to most on this board...  

edit: oops, posted the same time Stu did.  

[kb3ouk]

Googling last night about plate modulation, I found a very "interesting" high level modulation circuit, the attached "parallel-fed plate modulation".
This circuit completely isolates the mod. transformer from the dc without grounding one leg and so make it easy to use a toroidal transformer with all the positives as mod. transformer even the af amp is a hv pp b class.
I am wondering if it is safe to give it a try and if anybody has an experience from that circuit before.

Stefano

I'd say it's ok to try it, take a close look at this schematic, notice anything familiar? only difference is this rig used a single cap on the bottom of the mod transformer secondary compared to one on each end.
http://amwindow.org/tech/htm/813/813.htm

[AMLOVER]

Stu,

In my case, during receiving, the Rf Hv stays permanently on as it also does the Af Hv.
So there are not changes in DC on the mod.transformer during keying on - off.
My point is that with both caps the mod. transformer is completely isolated from DC and in the same time is not grounded which means that there is not voltage difference stretching on the "iron" as it happens if one leg is grounded. The iron being isolated could be now exchanged with a low leakage, low resistance, high gauss, light weight and minimized interleaving capacitance toroidal transformer.
The 2 caps are in series so they must be twice the uf to get the proper capacitance right?
As far the AC can pass and the DC will stay away from the mod.transformer then we don't need a lift to move our mod. transformers from the rack to the bench 
Till before this circuit toroidals could be used only with solid state af amps ( no dc in the primary) or with tube af amps but with lot of care on the isolation between primary and secondary.

kb3ouk,

This is a very common modified Heising circuit that even very succesfull, doesn't isolate the mod. transformer's secondary from DC and so no benefits in this direction.

N8UH,

I was also thinking exactly as you did...I have never seen it before in any schematic. If it works right from AC (audio) side as Stu says, then or it was a very well hidden secret or it didn't make any difference in sound, they didn't have the option of the toroidals and just adding one more capacitor was the reason to be rejected.
There are few good broadcasting engineers here, let's have their opinion, too.
I hope Stu you will be the first to try it.
A happy weekend to all of you. 

Regards
Stefano

[AB2EZ]

Stefano

If the B+ on the modulator is not the same value as the B+ on the rf output stage... then using 2 capacitors, as shown in the schematic, will remove the voltage between the windings of the modulation transformer (assuming that there is enough leakage current between the two windings to keep the capacitors charged).

However, what is more important, in my opinion, is that this approach will eliminate the small amount of average current (DC) that would flow through the secondary winding due to the leakage current of each electrolyic capacitor... because from a DC perspective, both capacitors have one side connected to the B+ on the rf output stage.

This (removing the small amount of DC in the secondary winding) would also work with a single capacitor ... with one side of the transformer's secondary directly connected to either the top or the bottom of the rf B+, and the other side of the transformer's secondary connected to the rf B+ with a capacitor.

Stu

[Opcom]

Is there a concern about the audio current through the capacitors C1 and C2? The peak current might be high, like 2x the carrier value of the RF stage DC current. Would the average at 100% modulation be 1.4X the DC value? Just thinking about it because of discussions of ripple current.

[AB2EZ]

Opcom

Your comment made me think about something that I wasn't thinking about before.

If the capacitors are not electrolytic types (i.e. if the capacitors are non-polar) then there is no issue... but if they are electrolytic capacitors... then the circuit under discussion will result in problems because of the lack of DC bias across the capacitors.

Since the optimal values are usually around 1uF, I guess it would be possible to use non-polar types (but maybe expensive)

If using an electrolytic capacitor, and a configuration that places B+ on the primary side of the modulation transformer, the best approach is to use a single capacitor between the bottom of the secondary of the modulation transformer and ground (i.e. what many would consider the standard configuration). This will keep the average voltage on the secondary of the transformer equal to the rf B+, and it will keep the voltage across the capacitor (but not the current flowing through it) roughly constant (and equal to the RF B+ voltage).

If using an electrolytic capacitor, and a configuration that does not place B+ on the primary side of the modulation transformer (e.g. when using a low impedance audio amplifier), the best approach is to use a single capacitor between the top of the secondary of the modulation transformer and the top of the Heising choke (i.e. what many would consider a non- standard configuration). This will keep the average voltage on the secondary of the transformer equal 0, and it will keep the voltage across the capacitor (but not the current flowing through it) roughly constant (and equal to the RF B+ voltage).

The downside of the single capacitor approach is that the capacitor's average leakage current will flow through the secondary of the toroidal transformer. I haven't experienced any problems with this in my legal limit, plate modulated amplifier, which uses a low impedance audio modulator. I.e. my toroidal modulation transformer has not saturated.

I also agree with Stefano's initial observation: if you use a traditional class B vacuum tube audio modulator, then keeping the primary of the toroidal transformer from saturating the core is a difficult thing to do. You have to either use some circuitry to actively adjust the average current in each half of the primary winding... or you have to use a Heising configuration to feed the plate current to each of the modulator tubes.

Stu

[KM1H]

The 1uF oil caps are readilly available surplus in about any voltage needed. Im using a pair of 10KV in series for my Hi-pot tester which runs almost 14KV when fully cranked up.

Carl

[AMLOVER]

Back to the bench...

Considering my sequencing on-off I definately decided to use the attached circuit.

Receiving :

a) Resistor R=Rload involves in the circuit by disactivating the vacuum relay and
b) Rf exiter-driver off.
c) audio chain off + receiver on

Transmitting :

a) Rf exiter-driver on
b) Resistor R=Rload comes out of circuit by activating the vacuum relay.
c) audio chain on + monitor receiver on

I let all voltages to the Rf (plate + screen) and to the Af amp (plate) ON during receiving having a very reliable screen overload protection circuit.
My question is that if the pushpull side is perfectly balanced, is it right that the toroidal in the attached circuit will not saturate from DC appearance in its primary (Rf side)?
Any better suggestion for avoiding possible saturation is very welcome.


Stefano

[IN3IEX]

I am very skeptical about all these Heising methods. In fact the "inductor" must have (high) inductance, must have high DC current capabilities (a gap and a lot of iron), must be free of self resonances like good modulation transformers (if you want audio quality). It follows that it will ba as big and expensive as a modulation transformer of similar quality. And it also needs capacitors, a gapless modulation transformer, a detailed study of transients....and two times the chassis space compared to a real modulation trasnformer.
Power supply inductors are designed for 50-60-100-120 Hz, not for audio.... the inductance changes with frequency and there are resonances in the audio band.
My conclusion: no Heising for class AB/B push-pull modulators, maybe good for class A single tube modulators using a single (high quality audio) inductor for AF and RF stages (like many tube AM transceivers for CB and 144). 
Sorry for the pessimistic view.

Giorgio

[WA1GFZ]

Look at the voltage rating of any resistor you put across the modulation reactor.
On the negative peak it will see the plate voltage and on modulation positive peak the peak final voltage - power supply. A single WW resistor may not be up to the task to stand off thousands of volts. You might be better off with a number of resistors in series.

[kb3ouk]

I have saw circuits which use a regular modulation transformer but still have a heising choke. a lot of broadcast rigs use a heising choke in parallel to the mod transformer, with a cap hooked to one side of the mod transformer secondary. a DC power supply filter can be used as a heising choke.

[WD5JKO]

My conclusion: no Heising for class AB/B push-pull modulators, maybe good for class A single tube modulators using a single (high quality audio) inductor for AF and RF stages (like many tube AM transceivers for CB and 144). 
Sorry for the pessimistic view.
Giorgio

    I have looked up your web space, and you are a very smart man. Having that depth of knowledge gives you more insight into all the trade-offs and compromises when topics such as this come up. That said, even with all those trade-off's, using a modulation reactor to compliment a modulation transformer does work, and often does so very well.

   I have used power supply chokes for this purpose as the reactor choke, and you are right these are designed for filtering low frequency ripple, not AC up to 5 Khz or higher. So in my case I had some "dumb luck" where I used three 8H 400ma chokes in series. This breaks up the winding capacity which ordinarily shunts the higher audio frequencies. So the end result was the best plate modulator I ever had. I used four 808 triodes in push-pull-parallel to make 600 watts of audio with 2250V B+. The modulation transformer was only rated for 120 watts audio from 200-20,000hz. By removing the DC magnetizing current from the mod transformer secondary this thing took 5X rated power, and did so down below 200hz. I used a BC-610 4UF AC coupling capacitor. Never had any sparking.

Jim
WD5JKO

[AMLOVER]

Frank,

the resistor is 9 resistors in series (390R/100w) but this combound resistor is out of circuit during transmitting and when it comes in during receiving, it sees the same voltage in both sides. Till now I have used 4x15k/50w in parallel with no problem at all. Its role is to prevent an image load to the AF amp during receiving just in case anything goes wrong. In the same time I found it very helping to keep reactor choke silent when the driver turns off. Without this resistor the reactor choke's energy jumped left right, remember I keep plate and screen voltages permanently on and the rig is deep biased for class C. This resistor was Steeve's recommendation and from the moment I used it I have solved the reactor's sparking when turning off the driver.

Giorgio,

As Jim WD5JKO and Kb3ouk have already said, I will add that my attraction to the traditional way of amplitude modulating my transmitter is worthing all the efforts of modified Heising needs. My point is how we can improve this circuit for each ones different needs. At this time I am very much thinking on how to overisolate the mod transformer from DC in order to use a toroidal transformer with all the benefits in its place. Caps is not a special cost, choke for 50Hz is proven to work and the roll off can start as high as 7khz, I use 2 in series 28H/2A + 13H/2A and I am more than satisfied from my sound. My only concern is safety.

So my question is still here, with 2 caps instead of 1 is it right that I could use a toroidal modulation transformer without saturation?

Stefano