Collins Transmitters

[WB6QED]

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....I am driving the TX with a bamboo radio and the audio with an external  70V PA amp

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

[WB6QED]

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.

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As a sidenote, the 5/10 kW 820E/F which was superceded by the 5 kW 828E-1 Power Rock in 1977 was only capable of +/-100% modulation when it was introduced in the mid-1960s.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

[flintstone mop]

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.

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

[John K5PRO]

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.

[WB6QED]

Did you move it up to 160 or 80 meters yet?

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.

What did you do for the transistor driver in your Power Pebble?

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

[John K5PRO]

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.

[flintstone mop]

Did you move it up to 160 or 80 meters yet?

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.

What did you do for the transistor driver in your Power Pebble?

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

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