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Old 5th Dec 2017, 8:28 pm   #1
Skywave
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Question 'Kalmus' wideband RFPA: info.?

A really long shot this - but always worth a try . . .

I have a wide-band RF power amplifier of which I would like to know a bit more about it. What I do know is as follows:

Full manufacturer's name: Kalmus Engineering International (U.S.A.).
Freq. range: 1 - 200 MHz.
Max. power out: 50-watts.
Gain: 45 dB
Model: 250C.

Detail.
This amp. does not seem to have ALC - or if it does, it isn't working. And thereby hangs the rub: "does not seem". If I can discover if this item does - or does not - have ALC, then I can - or otherwise - contemplate investigating it with a view to doing a (possible) repair.
As it stands, it is indeed useful, but the fact that the O/P power varies substantially with frequency does make multi-freq. usage with a constant drive level somewhat tiresome.

Anyone here know anything (other than that stated above) about this item?

Al.
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Old 5th Dec 2017, 8:51 pm   #2
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Default Re: 'Kalmus' wideband RFPA: info.?

Seems that Kalmus were taken over a while back - they are now these guys:

http://www.arww-modularrf.com/home_modular_rf.cfm

maybe dropping them an email and asking if there's anyone still around from the old company who might still have access to schematics etc?

In the past I've found US avionics/mil-spec-gear companies to be surprisingly good at providing help to hobbyists and hams.
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Old 6th Dec 2017, 1:19 am   #3
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Default Re: 'Kalmus' wideband RFPA: info.?

At work we have quite a few of the classic AR (Amplifier Research) range of amplifiers that are very similar to this type of amplifier. These tend to cover a couple of decades of bandwidth and are typically a big and rugged (and very heavy!) class A amplifier.

If this amplifier is of a similar type then I would not expect it to have ALC. ALC generally isn't desirable in a big general purpose class A amplifier like this. I think some of the amplifiers in the AR amplifier range are run in class AB and these tend to have inferior linearity but are presumably cheaper and probably weigh a lot less. I'm not sure if the class AB versions will have ALC but I doubt it. There will probably be some form of overload cutout or limiter but not any form of classic ALC. But there will probably be exceptions to this.

These amplifiers generally require about 0dBm to +5dBm drive level so they often have a gain in the region of 45dB. Because these amplifiers are meant to be used as a general purpose (class A) lab amp for research purposes, I'd expect the small signal gain to be quite flat across the whole range. Maybe +/-2dB maximum variation in gain? A key selling point of these amplifiers is the instantaneous bandwidth and the flat gain. So they would be good when used for wideband pulsed/swept applications.
It's probably worth finding out if your amplifier is meant to run in class A or class AB. Also, try and spot any feedback circuits that would be there to help control the gain response. Maybe the gain response isn't as flat for the class AB versions but I'd expect all of these amplifiers to use feedback to provide controlled input and output impedance and flat gain across the full range.
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Old 6th Dec 2017, 2:15 am   #4
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Default Re: 'Kalmus' wideband RFPA: info.?

If it does have ALC it will probably be obvious from the front panel. I'd expect the ALC to have an adjustable threshold to allow the user to set the output level over a fairly wide range. This would allow the amplifier to be used for something like formal immunity testing where the amp is driven with a slow swept cw signal and it will level the power to the user requirement. This would correct for the gain flatness of the amplifier and also for any variation in drive level from whatever is being used to provide the swept signal. So the ALC could flatten the output level to within a dB or maybe better.

But usually this type of amplifier will have been designed to have flat gain anyway (for the reasons I gave earlier). So it could be that yours is faulty if the gain variation is worse than about +/- 2dB. But I'm more used to the classic (old!) AR range of amplifiers and the typical specs they offer for gain flatness.
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Old 6th Dec 2017, 1:34 pm   #5
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Default Re: 'Kalmus' wideband RFPA: info.?

+/- 2dB gain flatness in a 50W amp used as an RFPA for transmitting could be a pain- power out could vary between 50W and 20W whilst still being "in spec" for a particular input level.

Though in practice the difference in received signal wouldn't be particularly noticeable unless you were right on the limit for signal/noise at the receiver.
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Old 6th Dec 2017, 10:07 pm   #6
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Question Re: 'Kalmus' wideband RFPA: info.?

Quote:
Originally Posted by Herald1360 View Post
+/- 2dB gain flatness in a 50W amp used as an RFPA for transmitting could be a pain - power out could vary between 50W and 20W whilst still being "in spec" for a particular input level.
By my arithmetic, +2 dB on 50-watts is 80-watts; -2 dB on 50-watts is 32-watts (all to nearest watt). So the output could vary between 32 to 80 watts. OTOH, 20-watts to 50-watts is a change of +4dB. But that corresponds to a gain variation of +4 dB / -0 dB. That is not the same as 2 dB.

Al.
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Old 6th Dec 2017, 10:17 pm   #7
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Thumbs up Re: 'Kalmus' wideband RFPA: info.?

Extract from post #2:

Quote:
Originally Posted by G6Tanuki View Post
. . . maybe dropping them an e-mail and asking if there's anyone still around from the old company who might still have access to schematics etc?
Good idea: thanks. I've just done that.

Al. / Dec. 6th.
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Old 7th Dec 2017, 12:35 am   #8
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Default Re: 'Kalmus' wideband RFPA: info.?

One thing to bear in mind is that you need to be careful what test gear you use to measure the RF power level from some of these amplifiers if you run them close to full power. This is because the test gear could give false readings in the presence of relatively high harmonic distortion produced in the amplifier.

You can typically expect to see three or four stages within these amplifiers. If this is a budget 50W amplifier it may use a fairly simple design for the final amplifier stage. It could be as simple as a push pull output stage. It could be a BJT design but it could also be a VDMOS design.

Push pull amplifiers like this tend to produce a lot of odd order harmonic distortion when driven up towards the onset of compression. This can cause a lot of uncertainty in the test gear if you are using something like a classic (ham) power meter that uses a simple/linear diode detector. You can get the same issue if you use a simple analogue scope to measure Vpk voltage. The harmonics can cause confusing results because of the influence of the harmonics on the peak (or peak to peak) voltage of the overall RF waveform.

A typical push pull amplifier just running into compression can easily have a third harmonic at -15 to -20dBc. Depending on the phase of the harmonic relative to the fundamental this can cause a lot of uncertainty in a typical ham power meter. Easily adding +/- 1dB of measurement uncertainty because of the limitations of this type of power meter. So this can make the amplifier frequency response look worse than it really is.
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Old 8th Dec 2017, 12:29 am   #9
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Arrow Re: 'Kalmus' wideband RFPA: info.?

An update to post #7.
I contacted the company who have acquired Kalmus Eng. I received a very polite reply - and very promptly, too! Unfortunately, my respondent was unable to help me directly - but I am still in communication with him.

Al.
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Old 8th Dec 2017, 12:55 am   #10
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Default Re: 'Kalmus' wideband RFPA: info.?

Quote:
Originally Posted by Skywave View Post
Quote:
Originally Posted by Herald1360 View Post
+/- 2dB gain flatness in a 50W amp used as an RFPA for transmitting could be a pain - power out could vary between 50W and 20W whilst still being "in spec" for a particular input level.
By my arithmetic, +2 dB on 50-watts is 80-watts; -2 dB on 50-watts is 32-watts (all to nearest watt). So the output could vary between 32 to 80 watts. OTOH, 20-watts to 50-watts is a change of +4dB. But that corresponds to a gain variation of +4 dB / -0 dB. That is not the same as 2 dB.

Al.
I was assuming that the amp was unlikely to produce MORE than 50W at any frequency, so given a dose of typical specmanship, gain "flatness" is just that- you can draw a straight line at 2dB down from 50W and the output would vary no more than 2dB above and below when a constant drive level is applied. It could be driven harder to get 50W at a low gain frequency, but doing the same at a high gain one would overdrive it with consequent seriously distorted output.

Another way to look at it would be- At some (carefully selected) frequency in the operating range it will need a particular drive power, P. To get 50W output anywhere else in the operating range, P will vary by no more than +/- 2dB from its original value.

Maybe I'm a bit cynical, but..........


Re Jeremy and power measurement- doing it calorimetrically is one way to know just how much altogether is coming out.
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Old 8th Dec 2017, 12:58 am   #11
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Arrow Re: 'Kalmus' wideband RFPA: info.?

Quote:
Originally Posted by G0HZU_JMR View Post
One thing to bear in mind is that you need to be careful what test gear you use to measure the RF power level from some of these amplifiers if you run them close to full power.
Ah yes! Of that - and things related - I am well aware. In the past, I spent several years working as a technician for a firm who designed, sold and serviced VHF FM radio transmission equipment. Amongst all manner of other items, typically transmitters up to 10 kW O/P. Permanently, on my bench, was an AR wide-band RFPA (lots of orange on the front panel!): that one had an O/P power control and, IIRC, also had ALC.

Anyway, back on topic. I tested this Kalmus amp. at an O/P power level of approx. 25-W. Two methods:
(1) O/P to a 50 Ω, 50-W. dummy load, with a BNC 'T' to feed a 10 'scope probe (flat to 100 MHz), then to a Tek. 2465 'scope.
(2) O/P direct to a Marconi R.F. power meter, model TF2512. That has a max. FSD of 30-W and claims a 'flat' response to 500 MHz. I have no reasons to disbelieve its indications.
The coax cable for these measurements was 'thick, 50 Ω' coax with N-type connectors. IIRC, that cable was RG-215. The test freqs. were between 5 and 50 MHz. Drive source was an HP 8640B.

Al.
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Old 8th Dec 2017, 11:56 am   #12
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Default Re: 'Kalmus' wideband RFPA: info.?

I work for AR, if you need any info on AR or Kalmus products just ask...well for the next 5 months then I'm retiring to Devon.


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Old 8th Dec 2017, 12:10 pm   #13
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Default Re: 'Kalmus' wideband RFPA: info.?

Quote:
Originally Posted by Skywave View Post
This amp. does not seem to have ALC - or if it does, it isn't working. And thereby hangs the rub: "does not seem". If I can discover if this item does - or does not - have ALC, then I can - or otherwise - contemplate investigating it with a view to doing a (possible) repair.
As it stands, it is indeed useful, but the fact that the O/P power varies substantially with frequency does make multi-freq. usage with a constant drive level somewhat tiresome.
I've worked for AR for 34 years!, The Kalmus 250C does not have ALC, some of the valve units had internal levelling which was used when a customer wanted to carry out a swept flat power test. The flatness spec is basically 50W -2dB, +/- 2dB. This particular amp is Class AB and requires a maximum of 0dbm of drive for full output. The harmonic spec is -20dBc in band but I would treat that with a pinch of salt being Class AB. These old Kalmus amps were not that good hence they were discontinued years ago.
I don't need to tell you that if you want to use it on the air then you need a decent filter on the output.


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Old 9th Dec 2017, 12:17 pm   #14
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Arrow Re: 'Kalmus' wideband RFPA: info.?

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Originally Posted by M0AFJ, Tim View Post
The harmonic spec is -20dBc in band but I would treat that with a pinch of salt being Class AB. These old Kalmus amps were not that good hence they were discontinued years ago.
Thanks, Tim: a definite answer to my Q. So it doesn't have ALC: I thought as much. Moreover, what you've written goes a long way to explain how I acquired this item. It came from an ex-employer:
"This thing is quite unsuitable for our (and your) needs: would you like it? Otherwise, it's going in the skip!"
Needless to say, my reply was "Yes please: thank you!"

Aside:
Quote:
Originally Posted by M0AFJ, Tim View Post
I work for AR, if you need any info on AR or Kalmus products just ask...well for the next 5 months then I'm retiring to Devon.
So you're contemplating a relocation to Devon, eh? Excellent choice! I relocated here to south Somerset about 3 years ago and have no regrets about doing that. You may recall that we had an eye-ball many years ago when I lived in Evesham, Worcs. So, when you do so re-locate, if you'd like a repeat meeting, please send me a P.M. or e-mail accordingly: it'll be pleasant to meet you again.

Cheers,
Al.
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Old 9th Dec 2017, 9:59 pm   #15
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Default Re: 'Kalmus' wideband RFPA: info.?

Hi Al, yes I remember visiting you in Pershore, did you ever get that Vanguard running?.
I’m looking at a cottage just outside of Sidmouth, going down on Wednesday to give it the once over and make an offer, should be moved end March. My current QTH is sold, taking all the antennas down over Christmas and packing the gear away.
I’ll give you a yell when I’m ensconced!

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Old 10th Dec 2017, 1:57 pm   #16
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Arrow Re: 'Kalmus' wideband RFPA: info.?

Tim: the 'Vanguard': no, I never got round to doing 'the necessary'. It came with me when I moved here, but this location is hopeless for erecting any sort of reasonably effective aerial, plus lots of overhead cables radiating PLT, and with demands on my time for non-hobby work all resulted in my eventually selling it - to a guy who collected it from Scotland!

I am familiar with Sidmouth: a nice place to live; I first 'discovered' it in 1976. It's about 30 minutes drive from here: A3052 and A358. I look forward to hearing from you in the New Year.

TTFN,
Al.
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Old 10th Dec 2017, 2:58 pm   #17
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Default Re: 'Kalmus' wideband RFPA: info.?

That Kalmus amp may be of more value to you boiled-down for power supply, heatsinks etc.

We used them to power the antennae in a screened room for EMC susceptibility testing. despite a tightly controlled environment, the frequency with which they blew sets of RF transistors was terrible.

If you do fix it, the question is how long will it be before it needs another set of transistors?

Sorry to sound negative, but I think you may be about to commit to a money sink. HP found it too expensive!

David
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Old 11th Dec 2017, 1:04 am   #18
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Default Re: 'Kalmus' wideband RFPA: info.?

This Kalmus amp. & it's reliability. In my hands, over a period of about 12 years, it hasn't been used for extended periods of time delivering substantial power (40 - 50 W.) into 50 Ohms. It gets used very occasionally: typically when I need a higher output voltage than my HP 8640B will produce - or other R.F. generating sources I have. OTOH, in a past employment, I regularly used an AR amp. of similar spec.: that was often blowing O/P transistors.

As for "fixing it", I'm not yet convinced that it actually needs fixing. The max. O/P is 50-W. (approx.) and the waveform looks O.K. too. Although the O/P power varies substantially with freq., since it doesn't have ALC, I suspect that that variation is simply a characteristic of the design.

Al.
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Old 11th Dec 2017, 6:53 am   #19
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Default Re: 'Kalmus' wideband RFPA: info.?

Ah, sorry, I thought you'd been given one because it had given up the ghost. Perhaps ours suffered so much because various people used it and not everyone knew enough about the care and feeding of RF power amps? It was used with ALC around it, from a detector feeding the levelling input of the sig gen driving the amp. I used to bring it up gently with the sig gen step attenuator locked, so there were no abrupt jumps or interruptions. It never failed in my hands, but whenever I wanted it, it always seemed to be in maintenance, waiting for new transistors. Eventually it got replaced with AR kit, which proved reliable.

Davd
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Old 11th Dec 2017, 11:29 pm   #20
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Default Re: 'Kalmus' wideband RFPA: info.?

It would be interesting to know if it uses BJTs or FETs in the final PA stage. At work, I've got a fair bit of experience designing wideband PAs in the 10W to 100W region for use across approx. 10-520MHz. In the old days I often used LDMOS or VDMOS parts for stuff like this. It's a long time ago but I worked closely with Semelab and PolyFET when designing these amplifiers. These amplifiers were meant for military use where linearity wasn't that important but efficiency and reliability were. These amplifiers were fine for swept/pulsed/cw signals but would not be suitable for SSB use unless the power was backed off a 'lot' from P1dB. I suspect that your amplifier could be similar. So it would be OK for FM or CW as long as a suitable LPF was fitted. But it might prove to be quite dirty/splattery across a ham band if used for high power SSB. A BJT design would typically be better here if you can get the bandwidth.

If your amplifier is of a similar vintage I wouldn't be surprised if it uses similar FET parts. I think I used Semelab D1008UK and D1009UK DMOS (28V) parts in my designs. I gave up with LDMOS for wideband high power stuff across HF to UHF as it was too fragile. The Semelab VDMOS parts were much more rugged. Note that VDMOS uses BeO inside the device so you have to be careful how you handle/dispose of any dead FETs if your PA uses similar parts. They are fine/safe to desolder etc but don't chuck them in a regular domestic bin and don't fracture the core of the device. If the top white ceramic cover pops off this isn't as scary as actually fracturing the area under the tabs/leads/flange. But best not to fracture these devices in any way. If it does use VDMOS and you do ever throw this PA away you should ideally dispose of it at a facility able to cope with BeO devices.
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