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Author Topic: Convert Two 813 Tube SSB Amp To "AM"  (Read 26675 times)
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WB4AM
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« on: May 28, 2015, 12:52:29 PM »

Hello,

I had bought a home brew amp that uses two 813's.  The power supply is huge and is separate from the Deck itself.  Actually the supply is so heavy that it is on wheels.  This amp is in storage at the moment.  Maybe later I can go there and snap a "Pic" of it.

Anyway, it was designed for SSB for 1KW output.  The supply output is somewhere around 2500 volts.  I think it has 6 or 8 large caps that equal roughly 45 pf altogether, in Series of course.  I had tried to use it on "AM" and ended up blowing one of the caps!  A very loud whistle letting out all the steam!

Is it possible to covert this amp in a way that it can be used on "AM" without any crazy redesigning the whole thing?

Thank you,
Ken



 
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W7SOE
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« Reply #1 on: May 28, 2015, 03:35:49 PM »

Ken,
   I can't help you with your question but, as I am building a 2x813 amp now I sure would like to see some pictures of yours.

Rich
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K1JJ
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« Reply #2 on: May 28, 2015, 05:05:21 PM »

Hello,

Anyway, it was designed for SSB for 1KW output.  The supply output is somewhere around 2500 volts.  I think it has 6 or 8 large caps that equal roughly 45 pf altogether, in Parallel of course.  I had tried to use it on "AM" and ended up blowing one of the caps!  A very loud whistle letting out all the steam!

Is it possible to covert this amp in a way that it can be used on "AM" without any crazy redesigning the whole thing?

Thank you,
Ken

Hi Ken,

2500 VDC  should work just fine in linear service.

45 pF on what band, 40 M?  

It would be of great advantage to replace those fixed plate tuning caps with one big single variable type. Many of the doorknob types will explode, if that's what is in there now.

Any RF linear designed for ssb is already set to run AM linear (or any mode for that matter) as far as biasing and tuning are concerned. The only problem may be getting adequate cooling for AM, if run to full power. The tube plates should not show any color if they are graphite.  I have used lantern chimneys over 813s to provide good air flow using a modest fan.  Just a small breeze does wonders for power capability and seal longevity.

Post a few pictures and we'll go from there.

Tom, K1JJ



 
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W1ITT
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« Reply #3 on: May 28, 2015, 05:22:01 PM »

Instead of 45 pf I think he meant to say 45 microfarads, referring to a collection of electrolytics in the HV supply.  That would best explain the pop and whistling sound.  The characteristic bouquet was not mentioned.
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K1JJ
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« Reply #4 on: May 28, 2015, 05:39:53 PM »

Instead of 45 pf I think he meant to say 45 microfarads, referring to a collection of electrolytics in the HV supply.  That would best explain the pop and whistling sound.  The characteristic bouquet was not mentioned.



 Grin  Yep, that makes more sense.

Ken,

Just to be clear, the filter caps would not blow just because the linear and power supply were running AM.  Though there is a chance that some RF was leaking into the power supply through the linear plate choke, however. But most likely the filter caps were old, dried out  and needed replacement anyway - and should work FB when refreshed new.

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

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

There's nothing like an old dog.
WB4AM
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« Reply #5 on: May 28, 2015, 06:23:54 PM »

Ken,
   I can't help you with your question but, as I am building a 2x813 amp now I sure would like to see some pictures of yours.

Rich

Hello Rich,
I would be glad to take some pics.  If I remember its pretty basic.  Remember the deck is separate from the supply.

I will try to get to the storage this weekend.
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WB4AM
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« Reply #6 on: May 28, 2015, 06:36:13 PM »


Hi Ken,

2500 VDC  should work just fine in linear service.

45 pF on what band, 40 M?   

It would be of great advantage to replace those fixed plate tuning caps with one big single variable type. Many of the doorknob types will explode, if that's what is in there now.

Any RF linear designed for ssb is already set to run AM linear (or any mode for that matter) as far as biasing and other parameters are concerned. The only problem may be getting adequate cooling for AM, if run to full power. The tube plates should not show any color if they are carbon.  I have used lantern chimneys over 813s to provide good air flow using a modest fan.  Just a small breeze does wonders for power capability and seal longevity.

Post a few pictures and we'll go from there.

Tom, K1JJ


Hello Tom,

Thanks for the reply. 

The Caps are those big can types not the door knobs.  When I say 45 PF altogether in parallel, I mean I added all the values of the caps using a capacitor calculator program here on the net.  I think this amp was built for 10-80 without the WARC bands in mind?

I will see if I can post some pics this weekend.  Oh, the owner had said when I purchased it way back when, it doesn't have enough capacitance to work on "AM".  I really didn't know if he met in the circuit with the can capacitors or not?  As I mentioned I did use it on "AM" and that is when one of the "Caps" had blown it's top!  Also I am talking about the "Caps" in the Power Supply.

Again Thanks.

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WB4AM
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« Reply #7 on: May 28, 2015, 06:39:51 PM »

Instead of 45 pf I think he meant to say 45 microfarads, referring to a collection of electrolytics in the HV supply.  That would best explain the pop and whistling sound.  The characteristic bouquet was not mentioned.


Yes that is correct!

I stand corrected!

Thank you.
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WB4AM
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« Reply #8 on: May 28, 2015, 06:43:19 PM »

Instead of 45 pf I think he meant to say 45 microfarads, referring to a collection of electrolytics in the HV supply.  That would best explain the pop and whistling sound.  The characteristic bouquet was not mentioned.



 Grin  Yep, that makes more sense.

Ken,

Just to be clear, the filter caps would not blow just because the linear and power supply were running AM.  Though there is a chance that some RF was leaking into the power supply through the linear plate choke, however. But most likely the filter caps were old, dried out  and needed replacement anyway - and should work FB when refreshed new.

T

Oh!  That sounds great!  I will have to look for some new "Caps".

Although the owner/builder said there would not be enough capacitance for "AM" Huh

What could he be talking about there?

As always, Thank You.
 
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AB2EZ
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« Reply #9 on: May 28, 2015, 07:26:39 PM »

Ken

Perhaps what the previous owner meant to say is that there would not be enough power supply "capacity" for AM.

That is, the average current drawn from the power supply (averaged over several minutes) will be much higher in AM operation than it will be in in SSB operation. As a result, the heating of the HV power supply transformer will be significantly greater in AM operation. The transformer will run at a significantly higher temperature during "old buzzard" transmissions... shortening its lifetime, at a minimum, and, perhaps causing it to fail very quickly.

AM operation is very tough on linear amplifiers... even if the carrier level power output is set to be lower than 25% of the peak power output specification of the amplifier.

In SSB operation (with normal voice characteristics), the average current (averaged over several minutes) drawn from the power supply will be just a little more than the bias current for the output tubes + the current drawn by the power supply bleeder resistor.

In AM operation, the average current drawn from the power supply will be around 1/2 the peak current. I.e. the output tubes will draw 1/2 the plate current, at carrier, that they will draw on 100% positive peaks.

Stu
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WB4AM
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« Reply #10 on: May 28, 2015, 07:46:16 PM »

Hello Stu,

I have always known "AM" is rough on amps due to the constant carrier but I never thought of it this way for this situation.

The supply transformer is huge and heavy.  Is there anyway of knowing if the supply can take the current if there were some changes for the purpose of "AM"?

Maybe some info on the transformer?  I don't know if there is any information on the transformer or not. (Stamped Plate)

I definitely need to go to storage and check this out.

Thank you Stu.


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N2DTS
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« Reply #11 on: May 28, 2015, 08:15:15 PM »

About 100 watts carrier output in amplifier service?

I made one with four 813's and it was good for about 200 watts carrier I think.
Lots of heat.
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K1JJ
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« Reply #12 on: May 28, 2015, 09:27:41 PM »

Over the years based on practical on-air experience and testing, I've found the following rule of thumb generally applies to plate dissipation vs: power output for AM linear operation:

Simply take the plate dissipation and divide it by 2 to arrive at the maximum AM carrier output to run the tube on AM.  This assumes adequate cooling for the tube and its seals.


Based on this rule:

A pair of 6146Bs in linear = 35W plate dissipation  X 2   / 2  = 35 watts carrier output.

Quad 811As are good for about  130 watts carrier output. (65 watts X 4 / 2)  Like in a Collins 30L-1

A pair of 813s:  125 W X 2 / 2 = 125 watts output carrier. (An 813 is actually good for more than 125W plate dissipation  with some air, like maybe 150-160 W diss)

A single 4-400A = 400W / 2 = 200w out.

A single 4-1000A =  1000 W / 2 = 500 carrier out.

Again, these are maximum, full strap parameters and it assumes the power supply can supply the peak power (X4 or X5 carrier).   I would normally not run the tubes this hard, and de-rate the above figures by 20% or so for better longevity. Lots of air is recommended for whatever is done in AM linear. In addition, a linear amplifier needs to be loaded very heavily for clean and high peak power, thus, the efficiency will be reduced even more  (more heat).

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

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There's nothing like an old dog.
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« Reply #13 on: May 28, 2015, 10:26:09 PM »

And that assumes good efficiency of the RF deck.
Higher voltage and less current often improves things for me.

Screen modulation is about the same.
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« Reply #14 on: May 28, 2015, 11:39:10 PM »

A pair of 813s:  125 W X 2 / 2 = 125 watts output carrier.

    If the 813's are in grounded grid, then some of the driving power will feed through which will add to the power output. With forced air cooling and chimneys, perhaps 150-200 watts out are obtainable at the expense of reduced tube life.

    My Central Electronics 600L with a grid driven 813 will do 100 watts out on AM with one tube and still be able to modulate 100%. That said, the graphite anode 813 is blushing red. In that amplifier on AM, 70 watts AM out is without any blush, and a better place to be.

    A pair of 6146B's plate modulated should do 125 watts AM....but here the efficiency is twice that of a linear amplifier. Imagine what a pair of 813's running class C with plate modulation will do.... Shocked

    We look forward to seeing that 813 RF deck and power supply.

Jim
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VE3AJM
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« Reply #15 on: May 29, 2015, 09:29:00 AM »

"Is it possible to covert this amp in a way that it can be used on "AM" without any crazy redesigning the whole thing?"

Why not change this ssb amp to a grid driven class C AM RF deck and build a real plate modulated transmitter with a high level modulator? I guess that's "crazy"? I've done it before with 4-250s and 813s Rf decks that had been in grounded grid configuration, if you can handle it? Grounded grid AM operation of a pair of 813s seems to be a huge waste of 813s for AM operation to me, same as screen modulation of such tubes, unless you have no other AM txs there, or don't have access to heavier rated parts or inclination to take that project on, etc. to do such a refit.

Its a free world of experimentation of course. Have lots of good spare tubes there. You may need them. Just my opinion.

Al VE3AJM
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« Reply #16 on: May 29, 2015, 09:52:53 AM »

I was never fond of running a linear in AM service, a huge waste of power it seems.
Two 813's will do 100 watts carrier in linear service if you don't beat on them, but will do 600 to 700 watts carrier plate modulated with less heat generated in the RF deck. 2400 watts pep or more.

But you need a modulator, and the mod transformer introduces frequency response issues and lots of phase shift unless you get a very good one.
Good ones for that power level are not easy to find.

A flex or some such nice low power exciter into a big amp can be very clean and flat.
Screen modulation can also be very clean and flat with no transformers in line at all.

Linear and screen modulation operation also do not need the extreme voltage components in the RF deck, the plate voltage is the maximum voltage the deck gets to.

Run 813's at 2000 volts in plate modulated service and you could see 5000 or more volts on peaks.
Run them at 2500 volts with screen modulation or in linear service and the most the deck ever gets to is 2500 volts.

Not sure how clean 813's are in linear service, but they do not seem to screen modulate well.
They plate modulate very well.

It is very easy to tune a plate modulated final, linear amps and screen modulation are more involved.

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WB4AM
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« Reply #17 on: May 29, 2015, 10:47:17 AM »


All Good stuff here to consider and most definitely a good learning too!

Maybe I should had mentioned that I don't have any real experience with iron and such.

Actually the reason I like "AM", is for the leaning!  You Guys and a like on "AM" actually talk "Meat & Potatoes" when it comes to radio!  So I am all ears to listen and take advice. 

My collection of Iron is next to none, so before going out and searching for materials I need to understand how things work when they are put together.  I recently purchased a BC1G that hasn't be converted to the Ham Bands.  And so it sits until I learn more about it?

Anyway I am listening, trying to take all of it in!   

Tomorrow I will take some pics of the deck and the supply.

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« Reply #18 on: May 29, 2015, 11:06:30 AM »

AM and some QRP guys are the only ones building anything for the most part, other then maybe amplifiers.

We get to fool around with vintage gear, home brew, class E stuff, old buzzard antenna tuna's and sdr's along with all the modern radios.

You find some all vintage, even all pre ww2 stations, all home brew and all tube stations on AM.
Gives people something to talk about besides complaining about their gout or the government.


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WD8BIL
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« Reply #19 on: May 29, 2015, 11:28:13 AM »

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

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WB4AM
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« Reply #20 on: May 29, 2015, 07:37:39 PM »

So I went to storage a day sooner!

I am having problems loading three pics at a time...going to try only one?


* Front Deck ReSgn.jpg (46.66 KB, 600x337 - viewed 467 times.)
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WB4AM
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« Reply #21 on: May 29, 2015, 07:39:48 PM »

Should be the back of the deck, okay and Top!


* Back Deck ReSgn.jpg (63.7 KB, 600x337 - viewed 480 times.)

* Top Deck.jpg (4542.52 KB, 5312x2988 - viewed 535 times.)
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WB4AM
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« Reply #22 on: May 29, 2015, 07:41:24 PM »

Bottom of Deck, and Filament transformer.  The two are the same in the bottom of the deck.


* Bottom Deck.jpg (4186.72 KB, 5312x2988 - viewed 530 times.)

* FilamentTrans.jpg (3898.58 KB, 5312x2988 - viewed 541 times.)
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WB4AM
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« Reply #23 on: May 29, 2015, 07:42:27 PM »

Front of Supply.


* Front Supply.jpg (4252.49 KB, 5312x2988 - viewed 443 times.)
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WB4AM
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« Reply #24 on: May 29, 2015, 07:43:55 PM »

Inside Supply....Iron, caps...and so forth.

I should had took a pic away from it more.  It doesn't look as big as it really is!


* Iron.jpg (4349.81 KB, 5312x2988 - viewed 478 times.)

* TransPlate.jpg (4401.99 KB, 5312x2988 - viewed 444 times.)
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