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Amplifier Advice




 
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ag4rz
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« on: April 07, 2022, 09:20:26 PM »

I have recently ventured into the world of AM.  Long story short, I decided to finally get some of my hamfest finds working, and put all my Heathkit stuff on the air, to include my DX60/HG10 and HR10.

The DX60 is stock, with the exception of cap replacement.  I do need to do the audio mods, but I wanted to run it stock first to be sure everything worked before I started messing around with it.  I also have downloaded all the other documentation I could find about other mods as well, which brings me to my question:

My Father-In-Law, who is also a ham, has offered to let me "exercise" one of his spare amplifiers.  I have my choice between a Dentron DTR-2000L, stock, or a single 4-400 built from the January 1961 QST article.  The 4-400 is a grounded cathode, tuned grid input, AB1/Class C switchable, and has a Variac in series with the screen supply primary.

Obviously, the DTR-2000L with the 3CX-1500 is a powerhouse, but if something happened to the tube..ouch!

The 4-400 based amp, while being lower output, is well constructed and there are several spare tubes already on-hand for it.

He is the original owner of the DTR, and while it has been used, it wasn't abused.  The 4-400 was built by a fellow club member, and has an rf deck for 80-10, as well as a deck for 160. 

I have used both of the amps at various times at his place, and both have done well.  They are currently backups, having been replaced by an Alpha 99, and now see intermittent use just to keep them "active."

I am leaning toward the 4-400, I know it's lower power, but the design comes from the time period that AM was still popular, but SSB was coming into play.  The 4-400 has all the original builder's note, to include his notes on running it on AM, CW, and SSB.  Plus, it's in a cool Bud rack that just looks good. 

Also, I can't find much info about running the 2000L on AM.  If I were to go with the the 2000L, what should I look out for specifically in regards to AM operation?

Thanks in advance,

Tim
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« Reply #1 on: April 08, 2022, 09:58:54 PM »

The 4-400 AB1 amp would be my choice unless you need max power. AB1 means low drive power. Does it need an attenuator?
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flintstone mop
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« Reply #2 on: April 08, 2022, 10:08:32 PM »

Try to get on AM with at least 200 Watts carrier. And a very tall inverted VEE 60 foot apex lottsa wire fed by ladder line or an inverted "L". Dipole is ok but needs lottsa wire and at least 60 feet high. Ladder line is the best for easy matching with a good tuner.
Verticals will skip over the "local guys"

I personally don't care for 4-400's...I like triodes.
Was it the "SLAB BACON"....SK  who had a screen modulated TX? Sounded fabulous

I have a FLEX 6400 and my trusty AL1500. That ceramic tube will run forever making 250Watt carrier. I bought that amp 2nd hand and in use often during the winter since 2009.
Fred
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Fred KC4MOP
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« Reply #3 on: April 08, 2022, 11:07:50 PM »

Hi Tim,

I would go with the 8877 DTR-2000L amplifier by Dentron.

If you blow a tube, replace it with an equivalent Russian tube.   (new socket and chimney, but reasonable cost)

The 8877 AM carrier power will depend on how well it is cooled. I understand the DTR cooling is just fair.   The general AM linear rule is the MAXIMUM carrier power output to run is equivalent to about 1/2 the tube's dissipation.  The 8877 is a 1500 watt tube and I have built two 8877 amps here and ran them on AM. In this case I would reduce the rule and not run it more than about 400 watts carrier to give you ample audio peak headroom. Figure 2KW pep out with a good 350-400 watt linear AM carrier.  I wud also run it in class  B/C using more cathode diodes to bias it below cut-off. This technique can be found on this BB and will make the amplifier more efficient on AM. This trick will not work well on ssb and will cause severe distortion.

In contrast, a single 4-400 amplifier is more of a 125-200 watt maximum carrier tube on linear AM. It also needs a chimney and ample air to stand up to the continuous heat. The grid driven input means it will be easy to drive, maybe 10 watts or so. You will need an attenuator for most drivers.  In contrast, the 8877 is a nice direct tuned cathode match for a 100 watt pep rice box rig.

The 8877 is also a cleaner tube for linear service since it was designed for it. -35 to -40 dB 3rd order IMD is a good figure.

There is no question that I would choose the DTR-2000L. With a few mods, changing out the old electrolytic caps and proper maintenance, it will be an amp that can take you forward for decades. You might eventually buy it from him rather than have it as a loaner.

Tom, K1JJ


* dentron_dtr2000l.jpg (65.82 KB, 750x508 - viewed 73 times.)
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ag4rz
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« Reply #4 on: April 09, 2022, 04:04:03 PM »

Thanks for the tips.  The 4-400 amp is indeed low drive, 12-15 watts will drive it to full output on SSB. 

The DTR gives it's full output with 50-60 watts SSB.

Seems like there's good points for both.. maybe I need to negotiate to get both of them, and use one for my SSB stuff and the other for AM.

With the exception of my SB-101 and the DX-60, my entire station is QRP, much of it being homebrew.  I have enjoyed the challenge of SSB QRP for several years now, but I'd also like to see what's it's like to not be the last guy to get through a pileup. 

I'll let everyone know how it all works out.

73,

Tim
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Detroit47
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« Reply #5 on: April 09, 2022, 06:49:55 PM »

Hi Tim,

I would go with the 8877 DTR-2000L amplifier by Dentron.

If you blow a tube, replace it with an equivalent Russian tube.   (new socket and chimney, but reasonable cost)




Not so cheap Russian tubes anymore. Putin put an end to that with the war over there.

Johnathan N8QPC
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flintstone mop
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« Reply #6 on: April 09, 2022, 08:05:53 PM »

I guess I shudda mentioned that driving the ceramic tube in my AL1500 for AM mode for 250 watts out is about 12 watts of drive, from the Flex radio. And watch the positive peaks go crazy.
Trapezoid pattern on 'scope shows 1200 W PEP when modulated. I'm using a Chinese condenture microphone going through a DBX 286 processor and moving on to an Aphex Dominator and finally to a Broadcast Electronics AM Limiter

And I have heard fabulous audio from a DX-Hundred and the D104 microphonium. 100 W makes it really tough to copy on 160M..........80M has a lot of challenges..............40 is a little less of a battle

Fred
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« Reply #7 on: April 09, 2022, 11:19:09 PM »

Thanks for the tips.  The 4-400 amp is indeed low drive, 12-15 watts will drive it to full output on SSB. 

The DTR gives it's full output with 50-60 watts SSB.

Seems like there's good points for both.. maybe I need to negotiate to get both of them, and use one for my SSB stuff and the other for AM.

With the exception of my SB-101 and the DX-60, my entire station is QRP, much of it being homebrew.  I have enjoyed the challenge of SSB QRP for several years now, but I'd also like to see what's it's like to not be the last guy to get through a pileup. 

I'll let everyone know how it all works out.

73,

Tim
If you are going to feed an amp you will need some method of power control.

Phil


* AC0OB DX-60 Variable Power 6DE7 Modulator.pdf (40.87 KB - downloaded 71 times.)
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ag4rz
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« Reply #8 on: April 10, 2022, 08:42:03 AM »

Phil,

That's the mod I have printed and on the bench already for the DX-60, in preparation for using it with an amp.
From what I've read on various sites, it seems to be the best way to go.

73,

Tim
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kc2we
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« Reply #9 on: April 11, 2022, 06:09:11 PM »

Remember 250 watts am is like 1000 Watts PEP. So in linear service an amplifier that has an SSB output capability of 1000 watts should only be driven to 250 watts out in AM linear service. Likewise, an amplifier rated at 1500 watts output, CW or SSB again, should be derated to about 450 watts output in am linear service. AM has the carrier plus two sidebands. Also, older amplifiers pre 1980's were rated for watts input not output. So an older 2000 watts pep  rated amplifier is good for about 1000 watts CW, less on am. It's all about the plate dissipation and duty cycle on the tube.  The only way to get around the am linear limit on any tube is run it class C and plate modulate. Seth KC2WE
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Seth Taylor
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« Reply #10 on: April 12, 2022, 12:51:28 AM »

Remember 250 watts am is like 1000 Watts PEP. So in linear service an amplifier that has an SSB output capability of 1000 watts should only be driven to 250 watts out in AM linear service. Likewise, an amplifier rated at 1500 watts output, CW or SSB again, should be derated to about 450 watts output in am linear service. AM has the carrier plus two sidebands. Also, older amplifiers pre 1980's were rated for watts input not output. So an older 2000 watts pep  rated amplifier is good for about 1000 watts CW, less on am. It's all about the plate dissipation and duty cycle on the tube.  The only way to get around the am linear limit on any tube is run it class C and plate modulate. Seth KC2WE

PEP really means nothing in AM service as it is an SSB measurement.

A plate modulated amplifier with 1kW input and 75% efficiency will have a 750 watt output or 3000 Watts PEP according to the silly 4 X Carrier power rule of thumb.

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kc2we
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« Reply #11 on: April 12, 2022, 11:13:38 AM »

 All radio signals, whether AM or SSB or anything in between all have a PEP value. To state that an AM signal doesn't have a PEP value is a myth. In fact when you mathematically integrate the power value of an AM envelope you can  prove that an AM envelope is nominally 4 times in peak power value over an SSB signal with the same modulation waveform. That's why an AM'er when running under FCC rules cannot "legally" use a transmit power exceeding 375 watts OUTPUT using the FCC 1500 watts PEP limit.  375 watts carrier power into 50 ohms at 387 volts RMS will achieve a PEP of 1500 watts when modulated 100%. That's about 460 watts INPUT running class C, plate modulated (about 82% efficiency), like a single 4-250A or 4-400AA (as in a T-368 transmitter). If you're using a linear amp to transmit an AM signal, the same FCC rule applies. No more than 375 watts output. In reality, anything higher than 375 watts out (carrier before modulation occurs) will shorten tube life and exceed plate dissipation of most tubes rated for 1500 watt SSB service. Most AM'ers don't use the PEP terminology in a plate modulated world, but the AM signal does have a PEP value that can be represented by a mathematical expression. The attached chart from the W8JI web site lists various SSB service tubes running AB1, AB2 or class B and the maximum recommended values without exceeding plate dissipation.  Check this website for the math expressions. https://www.w8ji.com/am_linear_amplifiers.htm


* Screenshot w8ji.png (23.78 KB, 1294x185 - viewed 72 times.)
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Seth Taylor
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« Reply #12 on: April 12, 2022, 11:28:48 AM »

Even Bird states that the correct way to check operations of a AM transmitter is by checking carrier and pep output.

--Shane
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« Reply #13 on: April 12, 2022, 12:28:10 PM »

With all due respect to W8JI - who's probably forgotten more about radio than I'll ever know - I think he's a bit overly cautious. Don't get me wrong, he's a great resource, but I sometimes think his data is more theoretical than practical. Tom, K1JJ, shared some of his own guidelines here:

http://amfone.net/Amforum/index.php?topic=43615.msg314897#msg314897

I think those are pretty realistic. W8JI says 125 watts max for single 3-500, but I figure it a little more optimistically: figure a lousy efficiency of 28% when tuned for AM, so 185 watts output would require 660 watts DC input. That leaves 475 watts dissipated by the anode. Close - you'll require optimal cooling - but not over the fence, and a single 3-500 can easily come up with the PEP.

I think the best approach is to use theoretical limits to get you in the ballpark, and then go empirical: accurately meter your plate voltage and current, do the math, and - assuming you're running glass bottles - actually observe your plates for color. A dull tomato red is fine; a brighter cherry red is okay; and even venturing into some subdued orange isn't terrible. Bright yellow is for people with deep pockets who can afford new tubes on a whim.
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« Reply #14 on: April 12, 2022, 04:52:49 PM »

...  Check this website for the math expressions. https://www.w8ji.com/am_linear_amplifiers.htm

And an SSB PEP power rating of 1500 Watts from a Linear Amplifier sure sounds more impressive than 750 Watts plate modulated AM, which, by the way, under the pre-1990 ruling, was 3000 Watts according to the 4XPc rule of thumb. W8JI and the company were selling what, "Linear Amplifiers?"  Roll Eyes

Do you know any non-commercial Linear Amplifiers that will function with 750 to 1,125 Watts input?

Thanks, I am well aware of the math expressions and the rule of thumb of 4 X Pc = PEP, so see this, especially the second to the last page.

The PEP rule of thumb is an obfuscation and a miscarriage of electronic theory.

Obfuscate comes from the Latin prefix ob- (meaning "over" or "completely") and fuscus ("dark-colored"). That fact gives an idea as to how the word can refer to making something difficult to see or understand—much like how dark, dirty water makes it hard to see the bottom.
Grin
 

Phil - AC0OB


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« Reply #15 on: April 12, 2022, 04:56:48 PM »

Even Bird states that the correct way to check operations of a AM transmitter is by checking carrier and pep output.

--Shane
WP2ASS / ex KD6VXI

Most if not all directional watt meters (whether 'passive' or active) are based on Warren Bruene's Directional Coupler design of April 1959, QST, pages 24-28.

If sufficient RF is captured on the secondary then there is plenty of DC to drive the integration circuits and meter. There are two integration circuits and are calibrated to indicate Average or Peak voltages. Each integration circuit is designed with a specific time constant to capture average or peak voltages with enough 'dwell' time for display. Furthermore, each RWD or FWD signal has its own calibration circuit as well.

The point is, with enough rectified RF and DC I can make the meter(s) indicate whatever I set them to, so there is nothing special about the PEP reading.

Phil

* Bruene Dirctional Coupler and Watt Meter.pdf (36.47 KB - downloaded 59 times.)
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« Reply #16 on: April 12, 2022, 06:19:38 PM »

The thing is, the FCC doesn't care what your am carrier power is.  It cares that you aren't hitting over 1.5kw pep.

Which to me is what to watch out for.

Both thr regulatory agency and Bird who is one of the most respected manufacturers of RF measuring equipment agree.

You can argue semantics all day long about the old carrier power vs the pep rules, but the FeeCees is going to show up with a 43P and a slew of slugs.  Been there, done that.

At the time, I asked the FCC agent why I was allowed to run 16 watts pep on am, but only 12 on ssb.

Deer in the headlights.  That was in the 80s when I was killing Ota with a bone stock tram D201 and a big stick 15 feet off the ground.

--Shane
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« Reply #17 on: April 12, 2022, 06:48:48 PM »


You can argue semantics all day long about the old carrier power vs the pep rules, but the FeeCees is going to show up with a 43P and a slew of slugs.  Been there, done that.

--Shane
WP2ASS / ex KD6VXI

It is not a matter of semantics but a matter of not bastardizing the clearly defined terms used in electronic theory. It is not proper to try and redefine the terms used in communication electronics theory for the purpose of targeting a group who wants to use the AM mode.

At the time, I asked the FCC agent why I was allowed to run 16 watts pep on am, but only 12 on ssb.

--Shane
WP2ASS / ex KD6VXI

Did this FCC "Birdman" actually explain any of the theory or understand it?

What does CB radio have to do with this topic other than the FCC has arbitrarily set those limits for Title 47 Part 95 D:

"§ 95.967 CBRS transmitter power limits.

Each CBRS transmitter type must be designed such that the transmitter power can not exceed the following limits:

(a) When transmitting amplitude modulated (AM) voice signals or frequency modulated (FM) voice signals, the mean carrier power must not exceed 4 Watts.

(b) When transmitting single sideband (SSB) voice signals, the peak envelope power must not exceed 12 Watts.

[82 FR 41104, Aug. 29, 2017, as amended at 86 FR 53565, Sept. 28, 2021]"


Phil - AC0OB
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