The AM Forum

A few questions - but first a summary:

I'm in the process of wiring up a 4-1000A modulated by a triode-connected pair of 4X1’s. I'm using a 1KW RCA BC modulation transformer with a 1.42:1 impedance ratio.  Have about 70H of Heising choke reactance. The plate power supply will be about 3500V, so the whole thing will be run conservatively.  It will have negative feedback from the modulator tubes to a low level audio circuit. Uses vacuum variable caps and turns counters for band agility

The modulator tubes and RF final will be on the same sub-chassis to permit cooling with the same blower.  I’ll post some pics when it’s working and finished in a week or so.


Questions:

Anyway, I'm wiring the RF and mod decks and had some questions on what values to use for bypass caps.


The triode–connected modulator tubes:  What value should I use for bypass caps to ground at the socket for the filament? (cathode)  There is audio there and I was thinking of about 0.1, but not sure. I remember someone once saying they didn't bypass the fils at all and just let the audio go through the fil transformer to ground. Any opinions?

The RF deck:  What value should the screen bypass cap be? I now have two .001 door knobs right at the socket pins to ground, then an RF choke going to the 20H screen self-modulating choke and to the 500v supply. Does the screen need further bypassing on the other side of the RF choke too?   Also, I remember someone once suggesting to bypass the screen circuit with an electrolytic cap that will pass audio. Where would that go in the circuit and what value, if desired? (This doesn't seem logical to me - maybe it's only for a linear amp screen)

What value caps are suggested for the RF tubes fil leads to ground?  .01 or .001?


Probably a lot of this won’t really matter, but it’s been a coupla years since I’ve built a plate modulated rig and have spent my time on linear amps. So the cap values are foggy as a result.

Thanks!

Tom, K1JJ

Tom...Very interesting...I had no bypass on the fils of triode connected 813's and then added .01 and could tell no difference .I had gobs of audio either way..That's my only experience...Not wanting to hijack your original question  but I am interested in the modulator negative feedback system you are going to use wid the 4x1s.....coupling technique and feedback voltage etc.I know you will keep us posted...Gud Luk   Steve

try this, OM.  :D

also sent U a e-type-mail with more.

As far as the screen bypass in the rf section goes, more bypass capacitance will lower the high frequency response of the rig, not bad up to a point.
.002 should be ok, unless you want one of those real wide signals....

The screen has to self modulate, and a lot of bypass cap will stop it at high frequencies, which would cut the high end some, not all, because the plate would still be modulating.

Strapping rig, a pair of 4-1000's in triode connection will take a lot of drive, what will you be using to do that?
How many watts will it take?

A real winter shack warmer!

Brett

 

With the mod tubes on the RF chassis you may have to bypass the filaments depending on how hot things are. Just keep it in mind in case of cracklies.

I pretty much stripped a local 5KW BCB 4x1 rig in the 70's with a pair modulating a pair and built for 80-20M. Around 85-6 I sold it to a guy in CT but cant remember who. Does it ring a bell?

Carl
KM1H

Vu, does that mod iron have a screen winding?

I agree with Carl on the mod bypass since they will share the same chassis and I think the transformer is external. On the RF side I would put a .001  or 470pf in parallel with the .01s to shunt any VHF resonant effects of the .01.

Excellent info!  Especially timely cuz I'm gonna finish the wiring on the sub-chassis today.

Derb:  Got the schematic. So looks like .002 at the screen pins AND a 4ufd on the other side of the choke, the power supply side.  I can see how that wud be fine, tnx.

Frank - Thanks for the bypass suggestions above - will do.

Steve/OF:  For audio negative feedback I'm using a solid state stage taken from the WA3KLR design for a cathode audio driver. See the schematic below. I input my 1 volt of audio along with the feedback from a resistive voltage divider taken off the 4X1 modulator plate cap.  They mix into the IC and then I feed the audio driver with it. I've used this same scheme on my last 4X1 X 833A rig and it worked perfectly.

Brett: The triode connected 4X1's will be fed by a backwards connected 8 ohm to 2K,  20-20K audio transformer driven by a 100W SS amp. I'm not sure how much drive it takes, but probably under 50w.  

This new project has been constructed from modular homebrew things from past rigs.. ;D  I used Quadzilla as the RF deck/mod deck. Also used the same vacuum variables and tank assy. Used already built screen and grid supplies as well as using the same negative feedback board and resistive divider.  Had to rewire a lot of stuff and make a new cooling system as well as wire the full modulator interconnects. The power supply is my old 5KVA pole pig that I’ve used for many years with a switchable 3500 or  6KV system.  The 4X1 would run on 6KV, but it wud likely crap out the mod xfmr – so I’m gonna stay wid 3500V.

Here’s a previous short thread on the RCA mod transformer:
http://amfone.net/Amforum/index.php?topic=6180.0
I don’t know what the p-p impedance is of the triode-connected 4X1’s but the according to the handbook, the 4X1 final should be about 5K ohms.  It usually takes a 2:1 step down for a pair of modulating one final of the same type.  This mod xfmr is about 1:1.42  step down, so reasonably close I’m hoping. I cud always adjust the final loading for a different impedance.

Bud – YES, the mod transformer does have a tap. I measured about 9 volts coming off it with 97 volts off the main sec. I was inputting 120V into the pri at the time. So maybe it cud be used instead of the screen choke? Any advantages to doing it this way? I’ve always used a screen choke in the past.

W5HRO:  What is the reason for the twisted leads on both the RF and mod fils?  I’m using Teflon wire and could twist the short leads under the chassis and also twist the incoming leads… but have never done this in the past. Right now they are not twisted. Please let me know what you think.


Carl:  I think I will bypass the fils of both the mods and RF tubes. Can’t hurt.

Steve, I  also saw no difference in the past when bypassing the modulators. When un-bypassed, you wud think it wud have an effect on the lower freqs, or cause some kind of phase shift with the higher audio freqs, but maybe not.


One more QUESTION:  The grid leak calculation calls for a 5K resistor. (200V grid leak bias / 40 ma = 5K)    It takes only about 10 watts to drive since I am neutralizing it and have found I usually don’t need grid swamping as a result.  What if I made the grid leak resistor a higher value like, 8K to send the final deeper into class C, thus more efficient? I have the extra drive available. Does this make sense and is there some disadvantage to doing this?  I wonder if there is a limit. It wouldn’t be right be knocking on the class E door using tubes… HA!

T

BTW, for whomever is interested:

In the past, I have found triode-connected 4X1's to be very stable and clean as modulators. Tron ran them for years and wants to duplicate his rig again. They require no biasing at usual plate voltages - just tie the grids and screens together and feed them directly with the hi-fi audio transformer. The CT of the audio transformer goes directly to ground.  3-4KV usually idles the tubes FB. If not, a small bias, either positive or negative could be put on the CT to place them right in the proper idling current.

In contrast, I've found the 833A's to be more sensitive to human errors. I've lost a few when an audio parasitic took off.  At 3500-4KV, they can be a handful. The 4X1's seem to handle abuse quite well, being bigger tubes with operating ratings as high as 5500V in mod service.  At 3500V, they will be running very conservatively, giving good IMD numbers and require low air blower settings.

So, that's my thinking for using the bigger modulator tubes. We'll have to see how the rig sweeps out.

T

Makes sense. This also applies to directly heated cathodes, like the 4-1000A?   It will be easy to do on this rig. Thanks for the suggestion.

T

I always go past the recomended bias settings on my rigs.
I usualy use a big wire wound pot, and have the grid metered for voltage and current, and know you can go WAY beyond what the books say, with not much more drive needed.
Past a point, there is no improvement in the modulation, but it does not hurt anything, and might make things a bit more efficient?

Use the 8k....

Brett

Tom, consider doing some trapezoid tests to find the best bias point. The info could be informative for others. I've heard some claims of more linear moduation of finals when using mucho grid leak and heavy drive but I've never seen any measurements.

Tom a triode connected 4-1000A with zero bias will be the the same resting current as a GG. At 3500 volts I figure 100 to 125 ma per tube. That is well into AB and should be easy to drive. You could use a Zener at the CT of the heater transformer to knock in down a bit unless you need a nice room heater.

Tom,
Working with Jamie on my 4x1 x 833's we didnt find any problems with the 833'3 at 3400 volts. I think 833's in the rf section is a different story. Jamie designed feedback around the 833"s and the whole transmitter is a joy to operate......very forgiving no surprises.
Bill

We do have separate bias adjustment for each 833 and separate cathode meters for each toob. Cant have to many meters!

Steve,

Yes, I think I'll experiment with different grid leak bias and even modulator idle current. Using a trapezoid is a good idea to see what's happening. I could even vary the audio neg feedback to find the overall sweet spot.

Frank, 125ma at 3500V is around 1/3 plate diss, so that wud be a good idle, esp during the cold winter months... :-)

Bill, yes, the 833A's are time tested in BC transmitters for years, so they work well when set up correctly. I'm always changing things around and experimenting, so things tend to get stressed. I was using new Chinese 833A's and made a few operating "human errors" with them.  I've never had a pair of 4X1's blow out though.

Well, got the subchassis all wired up now with the bypassing we all talked about above - and now concentrating on the plate tank and interconnections. Got on AM briefly tonight with the Tron and company. He said the RCA mod transformer will be OK up to the limit of 3500V, so that's the limit or bottleneck to higher power...  It will be nice to be back on with a plate modulated rig after a coupla years of using linears - they're so frickin boring. As Tron said, linears just lie there like sick whores... ;D

Maybe in a week I'll have it working.

T

Tom,
The research and careful construction u do with anything u build is first rate. U are not afraid to ask questions about something u are not sure of.......that is a quality not often found!
JMHO....

Bill

Tnx, OM... ;D

I do axe some stupid questions at times...  ;)   It tends to get the thread going and some people involved who would normally not respond.   In the end I always learn a ton of new stuff and proceed with confidence on the project - as wid this 4X1 rig tonight.

BTW, I heard your 4X1 X 833A's on the air about a week ago. It sounded flawless.  Nice BC audio and full quieting over here. You guys did a good job getting it worked out. There's so few of them on that I'd decided it's a good choice. It's always been my favorite lashup, the 4X1 final modulated by 833A's or 4X1's. Over the decades, I keep coming back to it after going with simpler linears for awhile.

At this point for amplifiers, I'm down to the HB solid state eight FET linear, my big 8877 linear for ssb and now this 4X1 X 4X1's for AM.  Everything else has been either torn down for parts or put off line in moth balls. My life was getting too complicated maintaining all this other stuff and I started making operating mistakes after being away from a particular rig for a while. Just a big rig for ssb and one for AM that are band agile is my new goal.  I'll bet you know the feeling.

T

So Quadzilla is an organ donor for this new rig? ....... Mod Quad?....I follow with great interest...Steve

I believe Ive mentioned that on here in the past and Tom asked me about it at the May Nearfest. I can drive the 250TH's on CW at around 50W and for AM I cranked the bias to deep Class C (thats the term Ive been using) so that it takes 75W from the Viking I and the scope looks much better than at 50W. Im also burning a little power with resistive swamping and series grid resistors because the Viking doesnt look that good when backed down. The other amps are run similar.

I should be back on the lower bands in a few weeks after a few more tower climbs before the snow arrives.

Carl
KM1H

Tom,
That is exactly why I'm planning the dual 8 erbs for the future. KISS
After the big rig is back up.

Tom,
Heard you on with Tim asking about the mod iron.
I use 2 of those same transformers (at lower voltages) and they sure seem great, much better than the cvm5 I used to use.

I suspect its a bit light for your rig, but should work well if you limit the power and voltage a bit.
Who knows what it can actualy take, but it has no problem at 2500 volts, 400 ma, 600 or 700 watts of audio.

Brett

Steve/OF:  Yes, Quadzilla was a major organ donor.  I actually removed one 4X1 and left three - one for the final and the two modulators.  The air flow is better without the end tube close to the blower.   I never did like having two 4X1's in the final in the past. Too much fil, screen and grid current - a waste. Heck, one 4X1 will do 2600w carrier out with 5500v on it, so why go wid more than one?   ;D   I just finished up making a new band switch and tank coil to cover the major bands. Also installed a pair of those good metal turns counters. Those black plasdick ones seem to fail after a while. Their turns counter gears jam up.

Carl: That deep class C is intriguing. I located the grid leak resistor  (a big 100W windwound) in an easy to get to clip-on board so I can change it. I might axe Tron what he thinks of the idea too.  I've found that running more than recommended grid current always made the tube come to life in plate modulated service. Better audio peaks. Maybe deep class C will work out too.

Brett: Yes, I like the look of that transformer too. You have two of them? How are you using them in service?  I’ll bet it weighs at least 130 pounds.   What amount of Heising reactance are you using and how low do the lows look good until it falls apart? I found with 70H, sweeping the old 4X1 rig was good to about 40hz or so. My old big RCA transformer was better. What have you found?


I used to have an RCA 10KW 1:1 piece of iron here. What a beauty. It had a matching Heising reactor. I got it from the Apeman W2VW.  Sold it off maybe 10 years ago. Too big and heavy, like 1300 pounds for the pair.



Another question, this time real stupid….  I realize that to modulate say, a 1KW carrier 100%, it takes only about 500w of audio power, though, the RF power increases four times or to 4KW pep.  Now, where is all that extra peak power (2500W) being drawn from? … is  the power supply supplying it thru the Heising reactor if it’s not coming from the mod xfmr?   I can’t get it outa my head that if the modulation iron is causing the voltage to double in series with the plate circuit, why it is not supplying more than 500w of that 4KW?  I’ve read the reason a few times in the past but can’t remember it now that I need to apply it to this new rig… sigh.  Don’t beat me up too bad now….

T

Yes, it takes 500 Watts average power for 100% modulation. But it takes much more than 500 watts peak power. Apples must be compared to apples. If the plate voltage is doubled (the definition of 100 percent modulation), the power must increase by 4x.

Maybe I have a different mod trans, but it sounded the same.
Mine is RCA, its big, maybe 75 pounds, and had the impedances you have I think, but its designed to have DC through it, I think they made two versions of it.
I tried the reactor, and got no improvement, so leave the dc on it.

One is in the 2x813 rig, modulated by a pair of 4cx250b tubes in ab1, at 2000 volts, or 4x100th tubes.
The other is in the 3x4D32 rig, modulated by 811a's all running 1200 volts, 300 watts carrier out.

I had a third one, in a push pull 812a modulated by 811a's rig, but I sold that rig on ebay.

I friend bought a warehouse of RCA transformers, lots of those mod transformers, and various matching power transformers, some quite large.
I guess the guy that had them worked at RCA, since it was close (in Camden NJ).

I never swept it, last time I tried that I blew out the mod iron in the 30K1 when it got down to 30 Hz!
I should sweep it (at low power) to see how good it is, but it SOUNDS good...

My rule of thumb is to have as much audio power as RF output power, things are real clean that way, with no limit of the positive peaks. More is better.

The power comes from the fact that you double the plate voltage (or more on peaks) to the RF final.
So say my 2x813 rig runs 2000 volts and 400 ma, under modulation, the plate voltage swings up past 4000 volts on peaks. If I put out a carrier at 2000 volts and 400 ma, then turned the variac up to 4500 or 5000 volts, that would be the same as modulating it....

A far cry from these crappy linyar amplifiers, I have four 813's in grounded grid and can get about 150 watts carrier out at rated plate disapation!
Four 813's in plate modulated service would do 1200 watts carrier and 4800 watts pep all day long without getting hot!

Now, for sideband service, the duty cycle is low, so I could crank the power up to 3000 volts and get 1500 or more pep watts out with the tubes being cool....

I would never consider running an amp on AM, but the flex demands it....
Lots of power in for little result, but it will make a good space heater for the shck in the winter.


Brett
 

looking at 100% modulation on a spectrum analyzer you have 1 KW carrier with 2 sidebands 6 dB down. Each sideband is 250 watts since it is 6 dB down from 1 KW. It takes 2x voltage peak modulation to create 250 Watts RMS sidebands. The final is a high level mixer in this case. F1+F2, F1 and F1-F2
F2 (audio) won't come through the plate coupling cap

Tom

The output power (at 100% sine wave modulation) increases to 4000 watts on positive modulation peaks... but it also drops to 0 watts on negative modulation peaks.

When you average over one cycle of the modulation [and you need to do the averaging carefully, using the fact the the power is proportional to the square of the voltage waveform... and the voltage is 1+ cos (2pi ft), where f is the modulation audio frequency]...

You find that the average power is 1500 watts. 1000 watts of power coming from the B+ supply and 500 watts coming from the modulator.

Also note: since the rf stage is probably operating at around 75% efficiency, and the modulator has to deliver 50% of the input carrier power, the required audio power is 500 watts / 0.75 = 667 watts of audio

Looking forward to hearing more.

Stu

Thanks for the replies.


OK, I understand the AVERAGE power required from the modulator. That's what hams have been using for years and is probably the reason why many plate modulated rigs flat topped and ran out of headroom.  I think PEP power is a better indication of the modulator requirements.

Let's talk in terms of pep power that the modulator must suppy at any one instant. This is where the damaging voltages which crap out transformers occur. The average power parameter is OK for measuring current and heating effects. But peak voltages and core saturation are greatly affected by peak power.

So if we have a 1000w constant AM carrier input, how much PEAK power is required from the MODULATOR to modulate 100%?    

I'm trying to deteremine just how much peak power I can pull out of this mod transforer before worrying about flashing it over. It's big enuff to handle 1000w of average power, in my opinion, but at 3500V using 4X1's the peak voltages could get out of hand depending on the peak power required, or mistakes made.  It has a "1 KVA" tag on it.


Brett: Good on the RCA transformers. I'll post some pics of the one I have and see if you are using the same ones. Yes, tone tests can be really stressful. I've blown only one mod transformer here over the years. It was a 500W BC unit when I was sweeping it at about 1KC. Suddenly the secondary opened up and that was it. Really sucked.  So I try to limit the % of modulation  when testing, but that is hard cuz we need to know how the extreme peaks act at 120%, etc.

I just moved the brother to that REAL mod xfmr you had Tom. It lives in the crawl space below one end of the house now. I used an engine crane to lift it high enough to fit into the entrance.

Why not suspend your mod iron in oil and hook the thing up with the series cap on the cold end of the secondary?

Another thing I just found out....

Derb was nice eniff to send me an old 1951 manual regarding big tubes.  One chapter was on cooling. In the past I advocated drilling holes in a circle around the inside chimney. That's OK for 813, etc - tubes w/o chimneys, etc. But for chimney tubes it says not to do this OR do not sub-mount the tube. This applies to tubes with chimneys like the 4-400, 4X1, 3-500Z, etc.  The air is supposed to pass only thru the seals base holes on its way to the plate glass.

I remounted my 4X1's flush and resealed the holes. The air flow is somewhat reduced, as expected, but it is now flowing through more critical areas, like the seals.  The added back pressure also equalized out the flow between tubes, so a plus there.

Thanks for the dope, Derb!

BTW, Stu, one of the 4X1 modulator tubes I'm using is one I got from you. It's one of the best of the bunch and is a good match for its mate.

T

PEP audio power would be half the carrier power, plus whatever asymetrical positive peaks you have (say 10%), plus losses in the mod transformer (10%?).
My guess would be 600 or 650 watts of audio.
Average power will be much lower, unless you transmit sine waves....

Brett


 

Tom,
Now that you have the tubes mounted correctly measure the air pressure under the tubes to see if you have enough flow through the tube bases.
Glass tubes like the 4-400 and 4-1000A need a fair amount of back pressure to cool them properly in AM service. My old 4-1000A rig I pressurized the chassis and pulled a vacuum on the top side and was still a bit light but was ok at linear duty cycles.

I'm still not clear on this...

If a peak reading wattmeter shows 1000w carrier and 4KW pep at 100% modulation, the peak power has to be coming from somewhere. If the mod xfmr is in series with the plate circuit and is adding say 3500V to the series plate circuit, and the series current is the same, then it is responsible for its 2,000 watts peak power share of the circuit, no?

IE, It seems to me that if the mod xfmr secondary is in series with the plate circuit (no Heising in this case) it is delivering the full 3500v and a full series current of the plate circuit when the power hits 4KW pep. This equates to ½ of the peak power, which is 2KW peak from the modulator.

Someone set me straight before I hang myself, please.  ;D

T

Well, if you increase the voltage, current does not stay the same, its not regulated....
So figure when the plate voltage doubles, the current doubles as well, 4x the power.

This all happens so fast, the average does not change much, and is offset by the power going down to almost zero on neg peaks.

On my rigs, the meter movements dont change under voice modulation, rms rf output power might increase a little, pep goes way up...

Brett

Yes, but regardless of the duty cycle or power coming down to zero throughout it's cycle, what is the peak power of the modulator's output? If the modulator cannot supply this peak power it will flat top and saturate, causing distortion.  It's just like a linear amplifier.

Actually, to put out 2KW peak modulator power, the modulator tubes have to take in about 3.3kw at 60% efficiency to modulate an input power of  1333w carrier (75% final eff).

This peak power requirement means the power supply has to handle the heavy dynamic regulation as well as the mod xfmr core being able to handle and pass the instantaneous peaks.

I just need someone to admit that the peak power coming from the modulator is 1/2 of the peak power of the RF envelope. IE, 4KW peak RF output = 2KW modulator's share.  Or show me why it's not without math formulas.


BTW, Brett - I saw your post about not finding the plate meter of much value in a linear. I see the same thing. The tank tuning looks lazy, but not if you're watching the power output meter.. I use the power output peak reading meter as a guide.  You can see everything on a scope and power meter to load it correctly. I use the plate current meter mainly to set and monitor the idling current of the finals.
T

I have always visualized cooling air as just like flowing water. If you need water to go in a certain spot, you direct it there and make sure you have enough force to get enough of it across the target, but getting more flow by widening your 'drain' size does not help much because all you are doing is giving the cooling agent more of a chance to miss your target.

Tom

Let's put out a few numbers. To keep it simple, let's assume that with only carrier (no modulation), the B+ is 3500 volts, and the plate current is 400 mA. Therefore the power is 1400 watts (input).

Now let's add 100% sine wave modulation.

And (important) let's assume (temporarily) that we not using a Heising configuration. We'll come back to the Heising configuration after we consider the non-Heising configuration

On positive modulation peaks, we have the following:

The plate voltage is 3500 volts x 2 = 7000 volts
The plate current is 400mA x 2 = 800 mA.

The total input power is 5600 watts (i.e. 4 x the input power when there is no modulation).

The B+ supply is providing 3500 volts at 800 mA = 2800 watts (2 x the input power it provides with no modulation)

The modulator is (also) providing 3500 volts at 800 mA = 2800 watts

Thus on positive modulation peaks, half of the total input power (2800 watts) is coming from the DC supply. The other half of the input power (2800 watts) is coming from the modulator.

Since the DC supply is providing 800 mA, its output capacitor is discharging twice as fast as it does when there is no modulation. But, that's not a problem provided it isn't going to discharge very much is in the time it takes for the modulation to complete 50% of one audio cycle. If it does discharge (and recharge) more than a few percent in each audio cycle... at the lowest audio frequency you care about... then you need more capacitance at the output of the B+ supply.

Now let's consider the negative peak of the modulation cycle.

The B+ is 3500 volts. The modulation is -3500 volts. So the total voltage on the plate of the rf output tube is 0. The modulator is putting out 0 mA. The power supply is putting out 0 mA. The r.f. output tube has zero plate current.

So, for the non-Heising case, the peak voltage across the secondary winding of the mod transformer is 3500 volts. The peak current (that occurs at the same time as the peak voltage) through the secondary winding of the mod transformer is 800 mA.  The peak power that is being delivered by the mod transformer secondary is 3500 volts x 800mA = 2800 watts (as stated above).  But, this number can be very deceptive and/or confusing, because half of that peak power (1400 watts) is being delivered during the positive half of the modulation cycle by the stored energy in the transformer, and only the other half of that peak power (1400 watts) is being delivered by the modulator tubes to the primary of the transformer. This will become clearer when we "discuss" the Heising case (next).  

Next, let's consider the case of Heising modulation.

On modulation peaks, the peak voltage across the secondary of the mod transformer is 3500 volts (same as non-Heising case)

The peak current flowing through the secondary of the mod transformer is 400 mA (half as much as in the non Heising case).

The peak power being delivered by the secondary of the mod transformer is therefore 1400 watts (half as much as in the non-Heising case).

The B+ supply is delivering 3500 volts and 400 mA... independent of the modulation peaks. So the B+ supply is delivering a constant 1400 watts.

So.... if the modulation transformer secondary is delivering 1400 watts on modulation peaks (both positive and negative), and the B+ supply is delivering 1400 watts (all the time), and if the total input power to the rf stage is 5600 watts on modulation peaks... then: where does the remaining 2800 watts come from (in the Heising configuration)?

Answer: it comes from the energy that is stored in the magnetic field of the Heising choke (E=1/2 LI**2), and the energy stored in the electric field of the Heising capacitor (E= 1/2 CV**2).
 
The Heising choke is always carrying 400 mA (except a low modulation frequencies). But, on positive modulation peaks, there is +7000 volts  (the plates of the rf tubes) on one side of the Heising choke, and 3500 volts (the B+ supply) on the other side of the Heising choke. So the Heising choke is delivering 3500 volts x 400 ma = 1400 watts of power to the rf output tubes on positive modulation peaks. On negative modulation peaks, the Heising choke has 0 volts on one side (the plates of the rf tubes), and 3500 volts on the other side... so there is 1400 watts flowing into the Heising choke (and being stored in its magnetic field). So during each audio cycle, the Heising choke delivers power to the rf tubes on the first half of the cycle, and gets "recharged" with energy stored in its magnetic field on the 2nd half of the modulation cycle.

That accounts for half of the remaining 2800 watts of peak rf power.

Now for the final piece, the remaining 1400 watts:

The Heising capacitor always has 3500 volts across it (except at low modulation frequencies). However, the current flowing through the Heising capacitor varies from +400mA on positive modulation peaks to -400mA on negative modulation peaks.

So... on positive modulation peaks, the Heising capacitor delivers 1400 watts to the rf tubes (the missing piece of the total) by drawing down the stored energy in its electric field (E= 1/2 CV**2). On negative modulation peaks, the energy stored in the Heising capacitor is replenished.

So, in summary... for the Heising configuration... where does the total of 5600 watts of rf input power on modulation peaks come from?

1400 watts from the B+ supply
1400 watts from the modulator
1400 watts by drawing down the energy stored in the Heising inductor
1400 watts by drawing down the energy stored in the Heising capacitor.

SDP

Stu,
Thats the best description so far.....
I hope it helps Tom.......
It sure helped me and my simple mind :D


Bill

Tom if you think you are asking stupid questions, I'm the guy who didn't know what this meant at the start of the thread:

"triode-connected pair of 4X1’s."

I eventually managed to decode it and figure out we're converting tetrodes to triodes and 4X1 is shorthand (the X instead of a hyphen threw me).  So I'm learning a lot and trying to not get discouraged hi hi.

To confuse things, I attempted to answer your question for myself before looking at the answers.

I took 1000 w. times 50 ohms (feedline Z) and took the square root of that for the voltage and divided by 2 for the positive half of the cycle.  Figuring this was RMS I multiplied it by 1.414 and wound up with 158 v.  I figured the audio straight for dc as sqrt(500 * 50) and also got 158 v.  the sum of those two v. squared / 50 gave 2 kw.  I think for pep that is doubled? 

I have a heck of a time getting this also, and as many times as I have read all the math about modulation and pep I never can get it.   I like Brett's headroom suggestion about having modulation power equal to carrier.  Seems like a lot of the broadcast rigs have the same tube lineup in the modulator that they have in the RF.  I would like that especially for going 120 or more % positive.  I once calculated (somehow) that 130% positive is 5.2 times carrier.  I don't know for sure if that is correct but I use that in setting my carrier so as to not push my amp too hard.

73

Rob
K5UJ

The total average power of each sideband in AM is Pc*(m^2/4). m is the modulation percentage.

The total average power of the carrier plus sidebands is Pt = Pc + Pusb + Plsb, Pc is carrier power, m^2 is modulation percentage squared, and Pusb or Plsb = Pc*(m^2/4)

so Pt = Pc + Pc*(m^2/4) + Pc*(m^2/4)

or combining terms:

Pt = Pc + Pc*[2*(m^2/4)] = Pc*[1+ (m^2/2)]

For example for 10% modulation with a Pc = 1000 Watts,

Pt10% = Pc*[1 + (0.1^2/2)] = Pc*[1 + (0.01/2)] = 1000 Watts*[1 + 0.005] = 1000*(1.005) = 1005 Watts

for 100% modulation with a Pc = 1,000 Watts, m = 1, Pt100% = Pc*[1+ 1/2) = 1000 Watts*[1.5] = 1,500 Watts

Using the formula for other modulation percentages:

Pt25% = 1031.25 Watts

Pt50% = 1125 Watts

Pt75% = 1281.25 Watts

Pt90% = 1405 Watts

Pt120% = 1720 Watts

Pt125% = 1781.25 Watts

Pt130% = 1845 Watts

Pt150% = 2125 Watts

DMOD,

   Your calculations clearly illustrate the need to achieve the maximum percentage of modulation possible with the least amount of distortion. Equalizing that modulation is also important depending on a persons voice, and microphone.

   So those figures you show for lower modulation percentages clearly illustrate why some of us might transmit a big fat carrier across the globe, but with that carrier we are running QRP sideband power. That explains to me why I often hear strong carriers on 75M across the AM window here in Texas, but I cannot copy a single word due to the low sideband power being transmitted, and the receiving S/N ratio. Then other stations come in, from the same area, sometimes with the same or lower signal strength (carrier), and I understand every word spoken.

   So instead of modulating an 833 with a pair of 6AQ5's, can we modulate a 6AQ5 with a pair of 833's?  :P

Jim
WD5JKO

Its not just modulation percentage.
Its more the average modulation level on the carrier.

Many stations have a very low average modulation level.
I was listening on 80 yesterday and heard someone using what sounded like a very clean setup of some sort of modulated  solid state amplifier I think it was, something like an 8 ohm to whatever ohm transformer in the power feed to a solid state amp, or some crazy setup like that.
The audio sounded VERY good, which has to do with the voice as much as the transmitter, and the way the operator speaks. But as the operator talked about the setup, I noticed that the average amount of modulation was quite low, but with very high positive peaks. The other station commented on the unusual modulation showing on his scope, and it did look like 200% positive modulation, but only in very short duration spikes.

It sounded real clean, and very nice, but not loud, despite the 200% modulation.
I did not hear any sort of compressor or limiter described in the equipment lineup, a good mic, a preamp, a 3 band EQ, and some sort of audio power amp, no compressor/limiter.
I also noticed it got quite wide on peaks, no limiting of the extreme high end audio I assume.


I have talked with other guys who ran 75% modulation, and when I told them, they said it sounded cleaner that way and there was no need to run more....

With a normal rig, without any compression, the average modulation level will be quite low at 95% neg and 115% positive modulation.
With a strapping signal, this sounds good, but as the signal gets weak, it does get hard to copy.
Back away from the mic a little and the modulation level drops off severly.

If you are using vintage gear with a D104, you cant do much about it without changing things a lot, but at 100 watts, many guys just crank it up a bit and overmodulate a little in the neg direction.
With 100 watts or less, that is no big deal.

With all the modern gear avalable cheap, it is very easy to add some good compression, I use a beringer vx2000 that I got new for $100.00, it does limiting, compression, noise compander, voice freq EQ, and other things, plus has phantom power for good microphones, and a phaze reversal switch, a lot of bang for the buck.
In the past, I used an old DBX unit that compressed above an adjustable set point, and compressed at any ratio you want, from nothing to hard limit. That was a $10.00 hamfest buy.

One interesting thing I want to play with is the carrier adjustment on the flex, since I am running it into my amplifier with limited plate dissipation, I might try to lower the carrier and increase the modulation.
At 50% carrier, I should be able to get 200% positive modulation, 99% negitive, and reduce the amp load.
If I run 150 watts carrier and 1000 watts pep, that might sound quite loud with compression!
With it set at 90% I get 150 watts carrier andabout 800 watts pep.
It seems to sound loud in the mod monitor at 90.

The other thing I have been noticing is the WAY some guys talk on the air.
Some seem to assume some sort of 'personality' which I wont comment on, some sound VERY distracted, like talking to someone who is surfing the web or watching TV while talking. They often talk about themselves only, or their equipment, at very long length, dont seem interested in what anyone has to say, and spend lots of time adjusting things. Do they know they sound like this? Do they care? Are you just an excuse for them to listen to themselves talking on the air?

Sometimes it 'might be' a flex thing, since one operator I heard was using one, and sounded very odd, like I do if I try and monitor myself with its delay, like a bad echo on the telephone, you have   to   talk   sloooowwwwly,    and    sound     like   someone         with      brain            damage      .....

I am likely guilty of some of this, but I am aware of it and TRY to interact on the air like a real person.
I wont even bother to talk with some of these people, since a simple question about anything results in a 1/2 hour transmission about every piece of gear in the shack, with its history, cost, choices, etc.


I try to remember that it is nice to interact and talk with someone like you would in person, instead of addressing a convention or making a documentry for TV.

I dont mind long transmissions, its not that, its long transmissions to the audience about nothing that bug me.

If you catch me doing any of the above, please let me know, as the xyl says I DO tend to go off...

Brett

Stu,

Thanks so much for taking the time to write that up. It has answered my question - it does appear that for a standard plate modulated set-up, (non-Heising setup)  1/2 the peak power is needed from the modulator and 1/2 the power from the RF power supply. But what is surprising is your analysis about the "flywheel effect" from the inductor and capacitor when using Heising!  The integration/electromagnetic and differential/electrostatic effects certainly take a load off the modulator!   The break down of power contribution is an eye opener.   I suppose this is expected cuz the DC current is taken off the mod xfmr, but I've never seen it expressed in this particular way.

I've read it through a few times and it has added to my understanding of what's going on stress-wise in the system. BTW, I don't know if many of the newer guys (or farther away) know that Stu is/was a college professor of electrical engineering at a leading university.  That well-written and thought-out treatise shows it and should be put into the AM BB handbook.

While we're at it, Dave/APE, interesting to hear about the twin 10KW iron needing a small crane to move it... :-)  If I cud snap my fingers, I wud have its brother back here again.  

QUESTION: Dave, you mentioned using the Heising coupling cap in the ground lead of the mod xfmr. I realize that is used to reduce the HV insulation to ground requirements.   I already have all the mod related iron sitting on a Plexiglass sheet isolated from ground. Is there a STILL another reason for me to place the cap in the ground lead?


Good comments about keeping our  % of modulation high. There’s been a lot of threads about this in the past. The bottom line is to keep a hawk-eye on the scope and be sure you are constantly approaching negative 100% modulation most of the time. There’s many ways to maintain a high level. But the most important thing is to be sure your transmitter is capable of a clean 120-130% positive modulation. This can be determined by sweeping it with an audio gen. There is a short article in the East Coast section about doing this.


Brett: Your comments about some ops who talk to themselves are a scream…   The way to determine if we are guilty of it is to tape ourselves when on the air. I have some tapes of myself from 20 years ago and I cringe when I hear some of them. I made some old buzzard 10 minute transmissions that just rambled on about useless crap. I was bored to death listening to them a few months ago. Over the last 10 years I’ve made it a rule that if I’m gonna get on the air, it’s gonna be an “event” and I will have something to add or material to entertain with. There’s got to be an agenda of some kind or we all end up just hanging out and boring ourselves. It takes some thought to keep things interesting and we can’t let others pull all the load…  

Well, back to wiring up the new rig. The shack is a disaster zone. I’ve decided to rebuild most of the rigs here, some with major changes, so still have a lot of work left.

T

It's always a special event when you get on, Tommy.  I can honestly say I've never had a boring QSO with you  ;D

Boy you nailed me on that one Brett! Sounds like how I operate. I didn't think anyone would notice. :'( :'(

Great dissertation Stu, I always learn from you ;D ;D

Tom, it seems you never can stop building and experimenting. Me old hat is off to you ;D ;D

Well, I think some of what people talk about on the air is booring to someone, that is not the big point, and you cant help that, but when someone only talks about their stuff and does not interact, or ask questions about the other guys stuff, or talk vintage gear, car stuff, some crazy homebrew project plans, etc, it gets old very fast.
Talking one way, and taking no interest, or attaching no importance to the people you are talking at is just plain rude.
So is the attitude of "my stuff is great/and or expensive and nothing you have or do interests me at all".

Another thing that seems rude to me is chatting over top of someone, is anything more rude?
Where does that come from?

I have enjoyed some of the qso's lately about hot rod cars, guns, Tim-trons yard cleanup, etc.
I always love hearing about the guy running a screen modulated 4cx250b, or water cooled 6aq5's, and so on, sdr is always interesting, and vintage gear is interesting to hear about, the really good sounding dx60B is very interesting, antenna's and tuners, even current news items.

Brett

 

Back to the topic. Where is the best place to have the hysing cap placed? Some people put it to ground or common at the so called cold end of the mod transformer. Others like me in the case of the Westinghouse MW-2 have it from the so called hot end of the mod transformer to the hot end of the mod reactor. I have the B+ to the bottom of the reactor and mod transformer tied together. My thinking was that I wanted to avoid current surges going through the secondary of the mod transformer every time I keyed the transmitter. This doesn't happen all that often on a broadcast transmitter because they are always on. I was also lead to believe that putting the cap to ground gave additional dc filtering. So to get all that energy stored in the reactor and audio coupling capacitor, which way is best? I've seen it done both ways by manufactures, so is it the same? Seems to me the cap to ground would make more sense even if I don't do it that way.         

Terry

If you put the cap between the transformer and ground, then you will have the transformer's secondary winding "floating" at the B+ voltage. This increases the voltage between the secondary winding and the case (assuming the case of the transformer is grounded). On the other hand, it reduces the DC voltage between the secondary winding of the transformer and the primary winding of the transformer (assuming the center tap of the primary winding of the transformer is connected to B+).

As a compromise, I've seen some recommendations to place twice the desired Heising capacitance on either side of the secondary (one to ground, and the other to the Heising choke). You need to include balancing resistors to split the DC voltage equally.

Another compromise is to place the capacitor between the secondary and ground... and to float the transformer's case on appropriate insulators... with rubber shock absorbing grommets to dampen "talk back" while you are at it. In this approach, one has to be concerned about the danger of the B+ appearing on the case of the transformer if a short occurs (or even a very small amount of leakage from the transformer's secondary or primary winding to the case)

In my KW-1, I have the mod transformer sitting on a "mouse pad" to dampen the talk back. Gravity, of course, keeps the transformer on the bottom of the cabinet. Opening the cabinet door when the B+ power supply is on, and poking my hand around where the mod transformer and the plate transformer are located, is not a good idea in any event... so I decided that floating the KW-1's modulation transformer's case is okay.  :)

Best regards (and thanks to all for the kind words!)
Stu

When I was playing with this stuff, I think it was Don C who told me which way was better and why, but I dont remember!
I tried it both ways and could tell no difference.
What I DID find was no matter what value of cap i used, it always sounded different.
Small, med, large, really large, each one had a different 'sound'.

That was the main reason I scraped it, I could not figure out what I liked best...

Brett

Stu,

OK on placing the Heising cap in various positions and the compromises.  Maybe the two-cap approach is the best. Or, as I intend to do, float the case above ground and put the cap in the secondary to ground.

Another question:  In the distant past I used a large value Heising cap, like 10-20uf. But I was told it should be a small value, like about 2uf. I have since used 2uf.

What is the reason for the smaller value?  (If you agree with this idea)

T

Tom,

I found Stu's detailed description in this thread to be extremely helpful regarding the Heising cap.

http://amfone.net/Amforum/index.php?topic=18722.0



Jeff

Jeff,
You have one nice sounding BC 610.

Thanks, Jeff - that did the trick on the Heising cap value. . In addition, I wasn't aware that when using Heising, no matter how large the reactor,  the inductance of the mod transformer was the limiting factor to the low end.  Heck I have 70H of reactor going in there and thought that was hot sh*t... ;D  But it makes sense, of course.

BTW, I meant to mention that the other night when we chatted briefly your rig sounded quite pristine. It was almost 60 over and the audio was back to the normal transparent norm. Was that the newer BC rig or the BC-610 as Frank mentioned?


Johnny - Yep we've had some crazy QSOs when you're aboard too. When Big Country, The Derb, Brentina, you  and a few others (like the chairman of the board and the prime minister) get going, you never know what's gonna happen. I shud of known you were crazy the first time you intrduced yourself at Timonium in a serious manner as W3FSH -" W3 Fat Smelly Ham"... ;D

T

Tom
Jeff

Et al. Again... I very much appreciate the kind words.

Jeff provided a pointer to my prior post on this subject... which makes this post a little redundant. [I posted this before I saw Jeff's post]. However... I'll leave it here anyway. Note the factor "K" which I have added here, and which is defined below.

On the subject of the size of the Heising capacitor. You may not notice the difference in the sound... between using a 10-20 uF capacitor and a 2uF capacitor, but the reason for using the smaller capacitor is as follows:

If you use a large capacitor, then frequency response of the transmitter will be 3dB down at the point where the reactance of the Heising choke in parallel with the magnetizing reactance of the modulation transformer (as measured on the secondary side of the modulation transformer)  equals the modulation resistance of the rf output stage.

For example:

Suppose: R= 3500 volts/0.4A = 8750 ohms; the Heising choke has an inductance of 70H; and the magnetizing inductance of the modulation transformer (as measured from the secondary side) is 35H.

The combined inductance (in parallel) of the Heising choke and the magnetizing inductance of the modulation transformer is: 23.33H [Note that making the Heising choke larger is not going to increase the value of the parallel combination very much]  

Then the low frequency 3dB cut off is 8750 ohms/[2 x pi x 23.33H) = 59.7 Hz.

[That's how the numbers come out when the modulation resistance of the RF output stage is that high]

If the Heising capacitor has a value of 20uF, then the combination of the Heising capacitor, in series with the Heising choke, and in series with the magnetizing inductance of the modulation transformer will have a series resonance at a frequency of  3.5 Hz. The "Q" of this series resonant circuit will be around 3.85. I.e., at the resonant frequency of  3.5 Hz the impedance of the 20 uF Heising capacitor will be around 2274 ohms; which is (1/3.85) x the modulation resistance of the rf stage.

What this means is that a sub-sonic oscillation of the modulator (at 3.5 Hz), or any other sub-sonic audio injected at a frequency near 3.5 Hz, can cause a very large voltage to appear across the secondary of the modulation transformer, across the Heising choke, and across the Heising capacitor.

If that never happens (sunny day scenario), then its not a problem. If it happens ... e.g. a truck goes rumbling by your house causing ~3.5 Hz mechanical vibrations of your transmitter... that get converted into low frequency audio by microphonic effects (rainy day scenario)... then your expensive components may destroy themselves. This is the Tacoma Narrows Bridge scenario... playing out in your transmitter.

The small capacitor approach has two benefits

1. It moves the series resonant frequency up to a higher frequency... where the Q of the series resonant circuit will be close to 1 (no huge voltages produced by an audio input at that series resonant frequency).

2. The low frequency rolloff characteristic is actually slightly better... i.e. the modulation frequency response will be flat down the the frequency of 59.7Hz (instead of 3dB down). Below that point, it will roll off fairly quickly.

The size of the small capacitor is chosen as follows (this is an approximation):

The impedance of capacitor at the low frequency rolloff point (59.7Hz in this example) should equal (1/K) x the modulation resistance of the rf stage (8750 ohms in this example), where K is [the inductance of the Heising choke and the magnetizing inductance of the mod transformer in series / [the inductance of the Heising reactor and the magnetizing inductance of the mod transformer in parallel]

In this example: K = [70H + 35H]/[23.33H] = 4.5

So the impedance of the Heising capacitor should be 8750 ohms/ 4.5, at the frequency 59.7 Hz.

Therefore, in this example, the Heising capacitor should have a value of around: 1.37 uF.

As a second example:

In my Viking Ranger, the stock modulation transformer has a magnetizing inductance (referenced to the secondary) of around 7H. Using the 9-pin socket on the back, I can run the Ranger in Heising configuration. I'm using a Heising choke with a value of 20H. The Ranger has a modulation resistance of around 600 volts / 0.120A = 5000 ohms. The parallel combination of the Heising choke (20H) and the Ranger's mod transformer's magnetizing inductance (7H) is 5.2H. The low frequency response, therefore, is flat down to around 5000 ohms /(2 x pi x 5.2H) = 153 Hz. The value of K is 27/5.2 = 5.2. The best value for the Heising capacitor is around 1.1 uF. I am using four (4) 4.7uF capacitors in series, so the Heising capacitance I am actually using with my Ranger is 1.2 uF.  At the lower rolloff frequency (153Hz), the impedance of the Heising capacitor I am actually using with my Ranger is around 833 ohms. It resonates with the series combination of the Heising inductor and the transformer's magnetizing inductance at a frequency of around 28Hz. The Q of the series combination at resonance is around 1 (i.e. the impedance of the Heising capacitor I am using, at 28Hz, is 4840 ohms)

As a third example:

My home brew plate modulated transmitter uses a pair of Russian GS-35b triodes.

The plate voltage (at carrier) is 1700 volts. The plate current (at carrier) is 300 ma. The modulation resistance is 1700 volts/300ma = 5667 ohms.

The Heising choke has a value of 50H. The magnetizing inductance of the ferrite core toroidal modulation transformer is much larger than 50H.

In this case, the approximate formula for K (given above) doesn't hold. At low frequencies, the impedance looking into the secondary of the mod transformer will be the modulation source impedance (referenced to the output of the modulation transformer); and not the impedance of the magnetizing inductance of the modulation transformer. This impedance is resistive.  Series resonance will occur at the frequency where the Heising capacitor resonates with the Heising choke. The Heising capacitor should be chosen so that frequency will be the same frequency at which the Heising choke has an impedance equal to the modulation impedance of the rf stage (5667 ohms in this example). I.e. for this case, we should set K=1

Going through the numbers...

The low frequency cutoff is: 5667 ohms/ (2 x pi x 50H) =  18 Hz
The optimum Heising capacitor is 1.6 uF

Summary

If the modulation transformer has an extremely high magnetizing inductance... i.e., much larger than the inductance of the Heising reactor... then use K=1

Otherwise use the formula for K given above

Stu

Another benefit of a smaller blocking cap is better time domain/impulse response. Larger caps can create large overshoot which can cause damage, at worst or limit your peak levels of modulation at best. Neither are good.

Still busy wiring stuff up tonight and came across another question:

Let's say I have two HV supply voltages available, 3500V and 6KV from the same supply.  I would like to keep the voltage on the 4X1 rig all the time whether it is being used or not.  It's operating limit is 3500V. Can I switch the HV to 6KV while the 4X1 rig has its filaments switched off? I know it won't hurt the tubes, but how about the mod iron and associated Heising parts that normally would crap out if operated at 6KV?  I plan to float the mod iron on Plexiglass so there is no HV path to ground. The other path is through the tube plates, but the fils are off.

Is this OK or am I missing something?

BTW, heard Steve/ HUZ and Frank/GFZ on tonight looking for west coast. Steve, for the vac variable flange you were talking about, just build a small frame outa alum angle and sheet and mount BOTH of them side by side or one over the other. (C1 and C2)  Look at the pics you took. I've done it for years this way - no big deal.  Strong and wide paths for conduction.

It was a good conversation - I wanted to join in but the shack is torn apart. Next week shud have the new rig running on AM.

T

Frank and Tom,

Thanks for the signal reports. You both heard different transmitters; Frank the BC-610, and Tom the Yellicrafters HT-37 done up with mods posted on the AM Window.. It definitely was great chatting with both of you!

Stu,

Your posts have really been educational, as are Don's, Tom's, Steve's, Dave's, Derb's, Slab's, Frank's, and too MANY others to mention on the Forum. The "search" function is your friend! 

Which brings me to another subject: a compilation of topics that could be gleaned from the VAST resource that this group has contributed to a HUGE archive that keeps growing (with all the server space, etc.)  For instance, the "Heising cap value" and its associated topics (all Heising related) could be collected and written out as an "open-source" book and chapter form, etc. The point is that MANY of these posts are worthy of serious documentation as they really are a legacy to the ART of AM radio..


Jeff

Tom

On the 6kv question:

1. The Heising capacitor is going to charge up to 6000 volts. It needs to be rated at about 3x the normal B+ anyway (3500volts x 3) ... so that probably won't be a problem.

2. You need to ask the question of whether there is any "rainy day scenario" where the high voltage applied to the rig can ever transition, abruptly, from 0 to 6000 volts. [For example: a loose HV connector that disconnects the supply from the rig for a short time when someone jiggles the HV cable].


2a If that happens, the Heising capacitor will be starting out with 0 volts across it, and the entire 6000 volts will appear as a 0=> 6000 volt step in voltage, across the secondary of the mod transformer. Although this is one of the reasons for using a small value of Heising capacitance (i.e., to shorten the duration of this transient), I would still worry about the implications of this.

2b.  I would worry about the effect, on the mod transformer, of having 6000 volts abruptly connected to the center tap of the mod transformer's primary.
Although one might think of the modulator tubes as open circuits (when the filaments are off), there are still many small, parasitic capacitors that need to charge up... and, for a brief period of time, each half of the primary winding may have 6000 volts across it.

3. Above 3000 volts, the electric fields in typical components become so high that there are additional types of breakdown effects that become problematic in those components (i.e. effects that are negligible at lower electric field levels). Things that we normally assume to be capable of handling these high voltages (like connectors) may deteriorate rapidly.

For the above reasons, I would be reluctant to double (almost) the voltage that is applied to the rig... even with the filaments off.

Stu

Tom,
A 6000V static charge can kill you. I don't like high voltage on things that are turned off for safety reasons. I would put a bleeder in the final just to be safe if there isn't a volt meter on the panel.
I think I have the flange HUZ needs, now if I could just find the next size bigger.

Stu,

Thanks for the info in favor of NOT keeping excessive HV on a rig that is turned off. Makes sense.  I will make provisions to manually select the HV for each rig with some kind of failsafe to prevent 6KV on the 4X1 AM rig by accident.

Normally I have the HV supply running all the time once turned on, but I do have a HV TR relay that sends this HV to the various rigs on key-up.  Yes, I don't like HV sitting on a rig, even when it is on but unkeyed.   There's other rainy day issues like, what if a person, animal or whatever situation found its way into the rig or interconnecting HV cable? Or if I had a heart attack and the rig was sitting there hot, even though it is sealed off?  At least with key-up HV the danger time window is greatly reduced and the HV supply is well sealed off.

Frank:  That cabinet will give HUZ a big start on his new project amp. It will make all the difference to doing a first class job in metalwork.

I've found I can build virtually anything with aluminum angle, sheet and 6-32 screws. All my air sub-chassis, vac variable flanges and other framework use the technique.  Very strapping.

Well, did up the 4X1 neutralization wiring and finished the grid circuit and plate tank/ vac variables, etc. Today it's the grid and screen supply wiring and might start on the modulator component wiring. Getting there little by little.  All parts are basically mounted and working mechanically.

Jeff: So that was an HT-37, huh? That's one of my favorite exciters from the past. It's amazing how good the AM can be generated with a rig like that, once you get into the guts. I see you're able to transistion from a big BC rig to the smaller ssb-oriented rigs. Not many guys want to do that. My fix is using an FT-102 modified with an NE-602 bal mod chip. Works DC-light, of course. But doesn't have the soul of a plate modulated dragon... ;D

T

Cool Tom. Yup angle aluminum is great stuff. I'm close on my layout and even found the spare rear panel I cut years ago. Good thing I'm going to need it.
I plan to do some work after I get something to eat and recover from all the trips between the antenna and rig. I raised and lowered the 40M antenna about a dozen times. Also Got the LPDA coax cable and rotor cable in the shack.

Just a quick note about aluminum chassis..
Welding aluminum is a problem IF u need to. A few years ago there was an infomercial about these welding rods called Alumiloy (spelling). All u had to do was use this stuff just like solder but with a torch. Well the stuff got my attention so I ordered some. For chassis work it works GREAT.
If u have to subdivide a chassis say to isolate the input from the output. this is the stuff for u.
If anyone needs some........let me know.

Bill