The AM Forum

Well, the Power Rock Collins Broadcast Transmitter on E bay is the second one I've seen advertised in the last couple of weeks.   I almost purchased one last month.  I almost, until I checked on the prices of the 5-500 tubes.  No wonder the price was so low.
At almost a grand apiece for these tubes,  It is no wonder these transmitters are coming up surplus.  They use 4 each.
Yow!!!
So, if one were to change the filament voltage supply to 5 volts I wonder if there would be an issue subbing 4-400A's instead.   I know the 5-500 is a tetrode but the pins connections are the same and the suppressor grid is grounded to the base ring of the tube.  The 4-400 just doesn't have a suppressor.

At 1.9 mhz, is one really needed? 
Just thinking that these transmitters might find their way into allot more shacks if the tube costs were
within reach or perhaps since I'm a nubie Broadcast transmitter owner I still have more info to absorb.

Thoughts?

Pete

Pete....I am converting a Macmartin Broadcash transmitter from 4-500's to 4-400's....other than changing the filament voltage and modifying the the necessary circuits for ham bands...it is a fairly easy conversion...I don't anticipate you having any trouble changing from tetrodes to pentodes in the collins TX.....You might consider other tube types as well....the iron in that TX will work with many types........ Good Luck... Steve

Someone would have to explain this............I had a Collins 20V with a quad of 4-400's. They were good tubes and that box could not make over 100% pos peaks.
I also owned a McMartin BA 1K that had the scary hard-to-find 4-500's and could shwang the munkey to 130% pos peaks easily. I never messed with those tubes as the transmitter made 1100W easily and the modulation was more than needed.
The audio quality of nice low end bass, was from the Collins transmitter. The BA 1K cut-off around 80hz. Very weak bass frequencies in that little oil filled modulation transformer. And the exciter was less than exciting to try to modify or order weird freq crystals. So I had my rice box and antenna tuner drive the RF PA's with external RF. That transmitter was a nice multiband TX 160-40M......Sorry I got rid of it. One easy to handle, not ball breaking heavy box.

Moving most broadcast transmitters to 160 is a piece of cake. There is one model of a Collins transmitter that is a nighmare to move. No Money Mike K6N??? Dorrough had a nice ER article how to move that monster to 160M. I can't remember the model... a 1980's or late 80' box??
Fred

Probably the Collins 828C1, which was a PWM rig and the one of the last rigs Collins made before Continental bought the transmitter line and the 828C1 became the Continental 314R1.

I wonder if your problem with the 20V might've been from the screens on the 4-400s not having enough modulation on them. How did they do that, self modulating through a choke, dropping resistor off of the modulated B+, or a tertiary winding on the mod iron? That would've been the first thing I checked, along with drive levels, but it wouldn't seem that low drive would cause as much trouble with a tetrode as it does with a triode.

I believe the 20V used a dropping resistor for screen voltage...It would make sense for them to drop the modulated B+ for the screen circuit....thus modulating the screens....I think that is how they did it.   The Mcmartian BA-1K has 2 separate screen supplies...one supply each for modulators and finals...Those supplies give about 750 volts.......Mcmartian uses taps on a resistor to set the screen voltage level and a small amount of self modulation of the screens is produced by the voltage drop across that resistor...I haven't looked at the schematic for the power rock..To see how that TX works...

Looks like the power rock uses a PDM modulator with a 3-500 as a switch tube...they claim 125% positive peaks.........

I think the 20V was before the loudness wars in AM radio and they were just happy to get a pretty looking Art Deco transmitter out there that made the station look good.
A lot of engineers cursed the 4-400's in broadcast use that they were not long-lived tubes 24/7.
And Pete keep checking here in AMFONE For Sale, for some BC transmitters already converted to the Ham bands.
 They seem to be reasonably priced. The bargain days are pretty much over. A T-368 can be a nice legal limit TX with nice audio if only a few modifications are performed. But they are usually over-priced and beyond $1,000.00

I was lucky to get that BA1K McMartin in 1999. A Canadian station in Vancouver was selling it. Very cheap! $900 complete and working. The Canadian FCC wanted all AM stations to maintain in perfect condition a 1 kw backup TX. The hitch was that it cost $900.00 for shipping. It was crated and shipped in an upright position and in perfect condition and a ton of dead black flies inside.
Cleaned up easily and was on the air on its crystal freq.
You might get lucky and find an older Gates TX using nice cheap Chinese 833's. Nice easy tube to operate.
Fred

I never worked with the 4-500 to 4-400 conversion but in conversation with WA1HLY, all one need do is wire the filaments in series therefore, one does not have to chance the filament transformer.

My 02C worth!

Wow,  am I confused.......

I pretty much thought that the 4-500 could just be dropped into a 4-400 socket.

What conversion needs to be done???? 

What I'm interested in finding out is a 5-500 conversion.   A 5-500 is a pentode tube and looks like a 4-500.   It has a 10 volt filament instead of the 5 volt that the 4-400 and 4-500 have.
It has a suppressor grid that comes out and is grounded to the base ring. 
The "Power Rock" uses two modulating two.

The cheapest I could find new was 980.00 and change per tube.   Yowzer!!!

Anyway,  I'm thinking of a second box but not going that direction unless I can decide if the swap is
workable.   Same socket,  same plate voltages and currents, just different filament voltages.
Wonder why they picked that tube?
Less iron for the filaments??  Dunno.

For those who are interested,   I did pick up a pretty nice 20 V and am in the process of setting it up
for operation at my place.   It has already been on 75 meters.   It also appears to have a new Peter Dahl modulation transformer.   I'm working on a keying circuit right now as I want to do it a bit differently than the way it's currently configured.

Thanks for the input, Guys...and Gals?

Pete
aka;  Power Pig

I got lucky with this transmitter as it came with suitable filament iron for 4-400's...
  This transmitter belongs to Brian W1LYD and we have it working, but not yet on the air in Lake Havasu City ,AZ  Brian hauled this thing from  Maine...It was stored  ..It had been hammy hamboned with a quick and dirty conversion for 75 meters and several important parts were missing...A couple vacuum finds at the Phoenix hamfest and from the very large junkbox of Brian's and it lives again on 75 and 160 meters....By the way the plate and Mod iron for this 1 KW TX is physically larger than Gates or Collins ..

Pete..Congrats on the 20V..That is a nice transmitter..Will I hear it on 160?..Gud Luk...Steve

Correction, I meant to say 5-500. well anyhow, if you have a rig that has 5-500, all you need do to run it as I understood it from Tim was to series the ten volt filament line so that the 4-400's would run in series for their filaments. I guess you could leave the 10 volt line in place and put a variac in series with the primary of the transformer and go that route? maybe it already has a variac in line? If the transformer is running on 240 perhaps running it on 115 would give you another way to get your 5 volts?

I'll sent Tim a note on this thread and maybe he'll chime in here and give you better ideas on your conversion......  I be backing on down and black out!

With the series 4-400A running from the 5-500 filament line you may end up with a little too much voltage so check that with the tubes in socket.  I assume a pair of 5-500 would have been run in parallel drawing 20.4 amps while the 4-400A pair will draw around 14.5 and that is enough of a current difference that the existing voltage is likely to be high unless adjusted downward.

There was a note in one of the old Hints and Kinks manuals concerning a Henry amp that ran the tubes in series and the voltage division was nowhere near equal-probably an issue with one of the tubes.  It would be worth measuring this also.  4-400A tubes are inexpensive compared to a lot of tubes but there aren't that many free ones around anymore.

I have several 4-500 pulls that I picked up at a hamfest for future use.  I believe those also have a 100 watt cathode structure.

Mike KO6NM has converted more of these for hamming than anyone else.  Contact him for the best way of conversion.

73DG

There is a lengthy discussion about converting a Collins 820D-2 "The Rock" 1 kW AM broadcat transmitter from 5-500As to 4-400As that I starterd here:

http://amfone.net/Amforum/index.php?topic=27701.0

I spent quite a bit of time on this and would be happy to provide any additional insight and help with the conversion.  If optimum performance is desired, it is not as simple as just dropping in the 4-400s and lowering the filament voltage.

I would also be interested in seeing some audio performance measurements for the McMartin BA-1K at 250 W.  My expectation is that it will not be that great since there isn't much modulator voltage swing available with the reduced plate voltage.

Brian, WB6QED

Thanks for the link,  Brian.
I don't know why I didn't think to do a search first.   I have to remember that I'm not the first person on the forum to convert one of these heavyweights.
That answered allot of questions.

Many Thanks,
Pete

Brian,

I just read through your Collins 820D-2 thread for the first time.

That was an excellent journal showing hard work and advanced measuring techniques.  The ending THD numbers from 20 HZ to 10 KHZ were outstanding.

Interesting that in the end, you went to the original 5-500s and saw the best results... :)

I'll bet you are getting pristine audio reports on the air with it...

T

Steve,

When I was working on the conversion to 4-400As on my 820D-2, I researched a bunch of other 1 kW AM broadcast transmitters that used 4-400As (or 4-500As) for the PA tubes and the modulators.  What I learned is that the McMartin BA-1K is unique.  I believe that it is the only 1 kW plate modulated AM broadcast transmitter that used tetrodes that cut the high voltage back on both the modulators and the finals for low power operation.  Every other plate modulated 1 kW AM broadcast transmitter that uses tetrodes keeps the B+ at roughly 3 kV on the modulators whether it is at high power or low power and only the finals have reduced plate voltage.

The reason that the McMartin BA-1K interests me is that it has essentially the same circuit design as the Collins 820D-2 with one big difference.  The 820D-2 uses 5-500A pentodes!  For 250 watt operation, the B+ is dropped to roughly 1500 V while the modulator screen voltage remains at 750 V.  As I understand it, with a tetrode you really can't swing below the screen voltage without having some rather nasty things happen while this isn't an issue with a pentode.  To 100% modulate the final, you need to be able to double the plate voltage and with a typical modulation transformer there just isn't quite enough swing to do it at low power with a tetrode.  My hypothesis is that McMartin more or less stole the Collins design but they used the wrong tubes!

With my 820D-2, I found that when operating at 250W and using the 4-400As, the THD+N started to really take off at about 65% modulation.  This is what ultimately led me back to the 5-500As.  I was able to achieve good performance with the 4-400As at 1 kW, but not at 250 W.  I suspect the reason that Collins switched to 5-500As when they introduced the 820D series was for power consumption savings at low power.  For local stations that operated at 1 kW day and 250 W night, cutting back the modulator plate voltage would have represented an improvement in overall efficiency.

If you do get a chance to do some distortion measurements with the BA-1K at low power, I would be very interested to know the results!

Brian, WB6QED

I'd love to own a Power Rock. I was corrected in my belief that they were hard to tune and fussy. After studying the manual, it is an engineering beauty.

So good in fact that it gives a modern PWM transmitter a run for the money.  It sounds quite spectacular!
I was able to successfully prove that the engineers at Collins Radio who designed the 820D-2 actually knew what they were doing!  ;)

I am finishing up the restoration of a 1 kW pulse width modulated Collins 828C-1 Power Rock One which shares much in common with its big brother the 5 kW Collins 828E-1 Power Rock and is quite different from the 820D-2 The Rock which preceeded it.  The 828C-1 uses three 3-500Zs rather than four 5-500As.  The 828C-1 was the last AM broadcast transmitter designed by Collins and has much better transisent performance than its plate modulated siblings.

Brian, WB6QED

I look back at the time that I actually had two broadcast transmitters in one shack for granted, and let them all go in my foolish decision to move to Pa. and shed the entire herd. The profit from the Oxon Hill QTH would have paid for a nice shack away from the house to re-build what I had. It might have cost another $1500 to have the moving company get those units out of the basement to the present QTH.
I did own Jay's ,N3WWL, Raytheon,,,,,,but it's not the same as having the box in the shack. When it is remoted 75 feet away and you do not feel it humming in the same room with you.
Fred

Fred...I agree that it is the most enjoyable to operate when you are sitting in the room with the big box transmitter....I have one TX that has such a powerful and noisy fan that I am mounting the fan outside the shack in its own enclosure so I can stand to be in the same room with the transmitter...

Brian, I put the BA-1K on the antenna yesterday on the 500watt tap running about 400 watts of carrier....The scope shows that the positive peaks are about 90% and The neg is hitting the base line with a few sparklies....If I run the audio up to 100% positive, the neg pinches badly....The reports on 75 meters from a local and a distant station were good.
audio was said to be OK...I am driving the TX with a bamboo radio and the audio with an external  70V PA amp...I haven't got the test gear at this location yet for any THD tests ...I'm gonna take a break from this project for a while...I'll keep you posted on any test results when I get back to this  one.....Steve

Steve,

I never did any tests with my 820D-2 running at 500 watts, but I would anticipate that the modulation capability would be a bit better than it is at 250 watts because of the greater voltage differential between the modulator B+ and the the 750V modulator screen supply.  At 500 watts, the B+ should be in the neighborhood of 2250 volts which gives a 1500 volt B+ to screen voltage differential which is quite a bit larger than the 750 volt differential at 250 watts.  It sounds like you are running out of positive peak capability around 90% which is in line with what I would expect for 500W operation using the tetrodes.

You might want to get a limiter in front of that 70V PA amp.  ;)

Have fun with your BA-1K!

Brian, WB6QED

Fred,

The Collins 20V was designed before broadcasters started to push the positive peak modulation beyond 100%.  I have a friend who has a restored 20V-3 and it won't do more than about 105% positive peak modulation at 1 kW RF output.  It modulates like crazy at 250 watts because only the PAs get cut back while the modulators continue to run at full output.

The 820D-2 was the first Collins 1 kW AM broadcast transmitter designed to have +125% peak modulation capability.

As a sidenote, the 5/10 kW 820E/F which was superceded by the 5 kW 828E-1 Power Rock in 1977 was only capable of +/-100% modulation when it was introduced in the mid-1960s.  In the early 1970s, Collins changed the transmitter design and also offered a modification kit that consisted of a new modulation transformer to enable the transmitter to modulate to +125% at full power.

Brian, WB6QED

Good info
Thanks Brian..I never knew that

WB6QED, What did you do for the transistor driver in your Power Pebble? Did you move it up to 160 or 80 meters yet? That little bipolar amp is stressed at even 1600 KHz, according to Sellmeyer, one of the engineers at Collins in the late 70s.

No.  It is currently tuned to 1140 kHz.  I haven't applied audio yet, and I still need to run through the factory test proceedure to make sure that it meets original specifications.
I used 2N6547s.  I know of a Continental Pebble that has been successfully moved up to 75 meters using 2N6547s instead of 2N6575s for the RF driver transisitors.  There are a few other changes that also need to be made to get it up to 75 meters.

Brian, WB6QED

Mine (power pebble) had  a shorted damper diode, when i got it from Wyoming. That caused an immediate breaker trip at first turn on. Continental wanted over $500 for the diode, it was a Semtech flat black plate thing with fins. I replaced it with a string of fast recovery door-knob diodes, that I found surplus. They were made by HVCA (High Voltage Components Assoc, a Dean industry). A stack of them gave 15 kV PRV and were easy to mount, screwed them together with all thread studs. Less than 10% of the new diode cost.

The Power Rock had improved audio performance over The Rock, as it had no mod iron and no filter choke in the power supply to wobble voltage around with modulation (due to LC). But it was more complicated electronics and much higher B+ inside (8500V).  Someone (at least 1) has died in a Power Rock accidental electrocution. Behind that PA door lies some really scary stuff. All BC transmitters can kill though.

Definitely something to consider with the newer design transmitters and PDM. Not so easy to move in Freq., just change some RF parts to get to the ham bands.
Fred