Bauer 707 500W mode

[w9jsw]

Can anyone explain how a Bauer 707 is wired for 500W mode. I have heard that they use dropping resistors (Yuk!) but cannot find anywhere in the documentation of how it is implemented. 

Also seems that they keep the modulator running on 3kv even if the final is running 2130V or 1500V. The AB1 modulator driver is right out of the Orr Eimac care and feeding book.

TIA,
John

[KA3EKH]

It’s a common thing to use dropping resistors in the HV line to the PA on old tube transmitters, most often have a relay so you would go between Day (high) and Night (low) power at the push of a switch. Most often a voltage divider would be included with a relay that would reduce the audio input for when you switched to night mode. Real inefficient being you were still drawing about the same power and just dissipating lots of energy in heat. The old Gates BC series had a huge wire would pot that was in the HV line so you were able to adjust power up and down a hundred watts or so.
Back when I was running an RCA on 160 what I did was remove all the low power junk and installed a 240-volt autotransformer to feed the HV transformer and that was way more efficient. Another trick was during conversion and tune up ran the HV transformer on 120 instead of 240 until I had all the output tank worked out.

[W7TFO]

Ham rules are rules, but BC transmitters were happiest running at the top power they were designed for.

Many stations had two transmitters, one for day, the other for night power just for that reason since half the day was on either one.

73DG

[K8DI]

When I look at 707 manuals, I note nothing about a 500w mode, just 250 and 1000 watts. In that, I note different parts as “power determining parts” that would have to be swapped to get the desired level, not a switch from low to high.   It’s possible they did not design it to operate at two levels..

If you look at manuals/schematics for the RCA BTA-1M and the 1R series, you’ll see the earlier 1M used a resistor bank to waste the power. The 1R was hailed as efficient as it switched primary taps to choose two power levels. Maybe you can get some ideas there.  I have become pretty familiar with the 1R if you want to delve into that one.

Ed

[w9jsw]

Think I might use a big powerstat variac like my 813 rig (see it on QRZ)

The 707 brochure shows 3 power levels, any 2 are switched. Some manuals show 3 sets of performance specs. The schematic shows the center tap of the plate trans provides 1500v to a relay for 250w mode.

The reasoning for the question is that I am acquiring the iron from a Bauer for my 4-400 homebrew rig.

[K8DI]

Think I might use a big powerstat variac like my 813 rig (see it on QRZ)

The 707 brochure shows 3 power levels, any 2 are switched. Some manuals show 3 sets of performance specs. The schematic shows the center tap of the plate trans provides 1500v to a relay for 250w mode.

Must be a different manual than the one I have downloaded!

Be it contractors or a variac, adjusting the transformed ac is the reasonable way to select a power level.  Electricity must’ve been pretty cheap to waste it off in resistors, all night, every night….

Not sure (because I have a simpler version’s schematic) what all the Bauer did when switching.  I can offer that my RCA switches HV for both finals and modulators, the screen resistor for each, the modulator bias, the audio drive level, and the mod monitor output level all have a pot that gets bypassed with relays for the power change.  Notable is that they’re all normal relays, driven by one latching relay, so it stays in whatever mode it was in when shut off.

Ed

[w9jsw]

Just to put this to bed,

Brochure has the perf numbers. I think the plate current in the 500w mode is a bit low. Wonder if they re-dipped the final when going to low power - probably not!

Third pic has the power resistors, I believe. They appear to be large black rectangles on the side of the enclosure above the transformers. See if you agree.

In the attached schematic snip, K-4 is the low power relay. I found a different version of the manual that has the same schematic but has the schematic part numbers listed for the 500W dropping resistors (but not the Values!). The kit assembly manual from steampowerradio.com has a had written note that states the low power board is furnished assembled. Probably had separate install instructions. I am sure the 500W mode is a one off config as most of the stations would have used 1KW/250W mode.

I hope to achieve this -
Plate current
430ma@2000V nominal (650W) - this is what Bruce W2XR runs nominally.
550ma@2000V max (825w)
550ma@2500V possible (1000W) with Screen Dropping R change

These are all assuming ~75% efficiency which I think is low.

John

[W7TFO]

As a matter of note, The Cetec group bought out Bauer.

They went on building them (with slight modifications) under the Sparta moniker.

Manuals for them should easier to source, as to being more recent.

73DG

[KD6VXI]

Think I might use a big powerstat variac like my 813 rig (see it on QRZ)

The 707 brochure shows 3 power levels, any 2 are switched. Some manuals show 3 sets of performance specs. The schematic shows the center tap of the plate trans provides 1500v to a relay for 250w mode.

Electricity must’ve been pretty cheap to waste it off in resistors, all night, every night….

Ed

Some of them where run on diesel generators.  They like a constant load level of 50 pct or better.  Performance drops when run at a lower load.... C and cylinder wash down becomes a problem as well.  This gets diesel in the oil and well......

Our diesels at work require an oil change weekly.  My LPG generator is 3 times a year.  It runs 24 x 7 x 365, excluding downtime of a half day to change plugs, etc.

This is one reason nobody cared about wasted energy back then. 

--Shane
WP2ASS / ex KD6VXI

[w9jsw]

Keep looking long enough and the mouse finds the cheese...

In the snip from a Bauer/Sparta manual is Note 3 - which is the wiring for the transmitter in the 1000W/500W mode. A pair of 1500 ohm 200W resistors, one fixed and one variable with a relay to short them out of the circuit.

John