Question: HV Ratings for Heising cap, plate blocker, tuning and bypass?

[K1JJ]

Looking for opinions...

What  HV ratings would various capacitors need when used in a plate modulated Class C rig with 3000V DC on the final and modulated 100% ?  [6,000V peak at 100%]  ie, what is actually there and what safety margin would you use within reason?

1) The Heising capacitor - connected to the cold side of the mod transformer to ground OR connected to one side of of the Heising reactor.

2) The plate blocking cap

3) The plate bypass cap

4) The plate tuning cap [C1]

T

[KA8WTK]

Hi Tom,
  My "William Orr" Radio Handbook says that the blocking cap should be able to handle the plate and the audio voltage, or at least twice the DC plate supply.
  It also states that the instantanious RF voltage will reach nearly four times the plate voltage. In his charts he shows that for plate modulation with a plate voltage of 3000 volts you would use a 0.500" spaced capacitor for plate tuning. Such a gap, according to his chart, would have a breakdown of 15,000 volts!
  I would think that the plate side of the blocking capacitor is going to see the same voltages as the blocking cap. So, if it were me, I would go with at least a 6,000 volt rating for the bypass and Heising capacitor.
  Just my "take" from reading the book. Make any sense?
 

[K1JJ]

Thanks, Bill-

If the Heising cap is  located in the cold side of the MOD transformer secondary to ground, instead of in the reactor circuit, does this reduce the voltage to 3KV across the cap under modulation ? [not counting safety factor]  The audio path is now from the mod transformer cold end, thru the Heising capacitor, thru the power supply filter caps, etc.

BTW, I always thought 1/2" = 10KV for DC... possibly less for RF.....?

T

[WA1GFZ]

Tom,
Cap needs to be at the bottom of the mod tran secondary. Choke carries dc to the final. with the cap at ground then only the voltage across the cap due to reactance matters. Also have to consider the B+ now on the secondary. 2X plate voltage sounds like a fair value

[K1JJ]

I'm talking about the Heising circuit as shown here:


http://www.amwindow.org/tech/htm/50c5/50c5.htm

So what's across the cap, Frank... just 3KV and you're adding another 3KV for safety?

What confuses me is that I always thought that putting the cap at the bottom of the mod transformer compared to the Heising reactor circuit saved 1/2 the rating. To ground and back thru the power supply filter caps is the key...  check that circuit out.

I have the chance to get some 5KV at 2ufd caps for the job, but it looks like they might not be high enuff for a 3KV rig...??

T

[KA8WTK]

  I am really not sure about the rating for the Heising cap changing according to where it is located. I remember seeing a discussion about the advanatges of it located one place or the other, but I will have to find it and see if it addresses the rating.
  Orr's chart for "Breakdown Ratings of Common Plate Spacings" reads in part....
.250"         9000 volts
.350"        11000 volts
.500"        15000 volts
.700"        20000 volts  

  In this edition (18th), below that is a chart that reads, in part.....
    DC plate voltage                  CW                       Plate Mod.
            2000                        .100                         .250
            2500                        .175                         .375
            3000                        .200                         .500
            3500                        .250                         .600

  Now, it may be that the chart is in error. Let me look around at some other books and see if they read the same.

[WA1HZK]

Crude sketch but this is how I did it:
http://www.criticalradio.com/833%20Rig%20Project/833%20modulator%20Schematic.jpg
I'm using around 3 KV, The heising choke is two chokes in series for 37 H & the cap is 2 mf. at 10 KV.
Have not zorched anything here yet.
Keith

[K1JJ]

Interesting on the feedback circuit, Keith. I'll use that as a starting point for my 833A's.

Yes, you're using the same Heising cap circuit I'm talking about. I'm just not sure what voltage appears across it at 100% modulation with 3kv.  Is it 3kv or 6kv??


I can see the 3KV DC, but it appears that there is no audio voltage dropping across it too.... A virtual short for audio. Thus only 3KV DC there  and a 5KV cap I can get wud work there...

T

[KA8WTK]

 Hmm.... The 1987 ARRl Handbook says 1/2" for 13000 volts as does the 1972 ARRL book.

 However, where it gets real confusing is when you look up the "puncture voltage" of air. The 1953 ARRL book lists it as 19.8 to 22.8 volts per 0.001". So, 500 times 19.8 = 9900 volts for 1/2" and 500 times 22.8 = 11,400 volts.

 The ARRL refernces on plate spacing don't agree with the above calculations and Orr is also high. The possible "fudge factor" here is that the 1953 book states that the plate design and state of polish is a contributing factor to how much voltage the plates can take before they arc over. Also, the fact that this is an RF voltage (and instantaneous) not a pure DC voltage must play into it.

[K1JJ]

Yeah, I think you're correct. Bill.

Certainly a rough surface will break down before a finely polished one.  And I notice that RF will jump a gap much easier than DC.  At least I've seen some RF arcs that were far greater than 1/2" and supposedly less than 10KV... :-0

Tnx for the input, OM.

T

[Ian VK3KRI]

What confuses me is that I always thought that putting the cap at the bottom of the mod transformer compared to the Heising reactor circuit saved 1/2 the rating. To ground and back thru the power supply filter caps is the key...  check that circuit out.

If the cap and transformer secondary are in series, it makes no difference to the capacitor breadown voltage - the voltage across tha cap will be the same regardless of where it is. The difference will be the voltage on the transformer winding to ground and  the cap to ground.
                                                           Ian VK3KRI

[W2PFY]

Most people and me included keep the cap above chasis ground.

[k4kyv]

I'm talking about the Heising circuit as shown here:


http://www.amwindow.org/tech/htm/50c5/50c5.htm

So what's across the cap, Frank... just 3KV and you're adding another 3KV for safety?

What confuses me is that I always thought that putting the cap at the bottom of the mod transformer compared to the Heising reactor circuit saved 1/2 the rating. To ground and back thru the power supply filter caps is the key...  check that circuit out.

In that circuit, if the cap is returned back to the power supply instead of ground, that removes the DC from across the capacitor.  In that case, it doesn't matter if the cap is in series with the top or bottom of the mod xfmr winding.

The disadvantage of returnig the cap to the power supply is that residual hum on the power supply DC will be induced through the mod xfmr secondary onto the signal.  No DC power supply is 100% hum free, so the hum  level WILL increase.  By taking the audio directly to ground through the capacitor, you bypass the power supply hum altogether, and the modulation reactor even acts as additional a.c. power supply filtering.

Also, with a common power supply for modulator and rf final, 2nd harmonic audio distortion components induced on the +HV by the class-B modulator may show up on the signal.  This occurs because of the load the modulator stage places on the power supply every half cycle as each modulator tube draws its peak current.  This varying load (two kicks every audio cycle) will show up as a corresponding audio voltage at the midtap of the modulation transformer primary.

This harmonic distortion may be severe in the case of a convetional mod xfmr circuit with no  reactor, and insufficient capacitance in the power supply output filter capacitor.  Many old-tyme plate modulated ham rigs built back when you were lucky to be able to acquire 2 mfd of power supply filtering, suffered from this little-known effect.

Returning the audio from the modulator directly to ground through the blocking cap, as shown in the above diagram, bypasses both these problems.  It is the standard circuit used in nearly all broadcast transmitters.  An alternative is to ground the bottom end of the xfmr winding and put the cap on the top side.  I have seen that shown in the circuit diagram of certain broadcast transmitters.  The problem with that is that the mod xfmr secondary is permanently at DC ground potential, so the full HV DC on the modulator appears between the mod xfmr windings.  This poses unnecessary stress on the insulation between windings of the mod xfmr.  This is especially true with broadcast iron, in which the primary and secondary windings may be split up into sections and interleaved.  You have multiple  layers of insulation, stressed by the full HV, with the potential to break down.

[K1JJ]

OK, tnx for the info Don.

I now see the advantages of bringing the cap to ground or to the power supply.

Based on that I will put two in series and to ground -  a 10KV rating at 1 uf.

73,
T

[WA1GFZ]

Tom Vu,
The secondary of the mod transformer is going to have DC at one end of the winding. so cap will see DC. The transformer will be above ground isolated by the cap which is a good thing for the transformer. The only thing that changes is the transients the cap sees.