Combining step-start and Cutback taps?

[N8UH]

Hey All!

I have been making progress on the power supply/iron rack from my last thread. I'm trying to narrow down my step-start/protection schemes. I'll definitely have a glitch resistor, probably 10/10 or 15/10, and a homebrew HV fuse, using ideas I found here, consisting of a strand or 2 of coax braid in a glass tube filled with fine silica sand. Should work OK FB there.

I'm probably going to go with the traditional approach for step-start: A 10 ohm resistor will limit the start current, and after a second or so, a contactor will short the resistor out. I have some AB pneumatic time delay relays for timing, so I'm going to use what I have.

Now, the plate transformer has a cutback tap that will drop the secondary voltage to around 1200V from 2000V. With an extra relay or so, I could leverage this. So here are my questions for the gurus:

- Can I wire the neutral and cutback taps together and use a contactor on the neutral tap? With the contactor open, the neutral wire would be directly connected to the cutback tap. When the contactor closes, the cutback tap would be essentially shorted. Is this a workable and safe approach? I could add a On-Off-On switch that controls the neutral tap contactor to get the following:

Off > Step-start > full power for normal operation

Or:

Off > Step-start > cutback for tuning/adjustments

This would require, for timing, that the power supply is shut down so the timing relay can reset before switching it back on. Is there an elegant way to go from cutback to full, or full to cutback without having to power down the circuit? I would imagine that I could go from full to cutback by just throwing open the neutral tap contactor (if the above scheme works), and there wouldn't be any problem, as the bleeder and load would pull things down. But going from cutback to full would require another step-start, correct?

- Second question: on the topic of HV bleeders - This will be a shared mod/PA power supply. Since there will (Ideally) always be a load whenever the power supply is switched on to transmit, does the 10% of load current bleeder rule still apply, or does one use a lighter bleeder value? This is of course combined with a safety bleeder.

Thanks again!
TK

[KA2DZT]

Tim,

If you are using a tetrode for the PA you can reduce the screen voltage for tuning and leave the HV at full tilt.  That's what I do and it works FB.

I don't think switching the taps on the plate iron primary is a good idea while the xfmr is powered up.  I also would leave the neutral connected at all times.

You should have some bleeder current but remember that the PA and mod tubes keep some load on the PS.

I'll reread your comments to maybe add some other thoughts.  Although, I'm sure you'll get more comments from others.

Fred

[N8UH]

Thanks Fred.

I'll be using PP Triodes in the PA, so I'll have to take a more traditional approach. 

To clarify, the cutback tap is next to the neutral terminal on the plate transformer, something like this:

CB-0-208-2XX-2XX-2XX

To allow for different primary voltages while still providing a cutback.

As far as bleeders go, the PS will always have some load when it's on, being a shared supply... But at the same time as I understand it, there is still need for some bleeder current to keep the voltage regulation down during current swings? Not sure here...

TK

[W7TFO]

Hi Tim,

Here is a scheme I prefer:

1..Run enough bias on every stage to hold things stable without any drive.

2..Soft start from 500ms to 1 sec duration. 

3..PA HV stays energized for 5 minutes only on timer unless keyed up to reset it.  The start-up procedure is repeated after the elapsed time upon key-up command.

4..Key drive for transmit, and sequence relative to antenna changeover relay tally*.

5..Cutback tap used just for tuning or low-power op.

*..This is also vectored into master receiver mute.

This will give OK PTT action in a conversational mode, but not burn up a bunch of tubes in standby.  Also, you don't have a bunch of transients from switching B+ to deal with every time you hit the button.

73DG

[KA2DZT]

Tim,

Took a look at the primary terminal set-up.  You would switch the neutral from 0 to the CB terminal to reduce power.  But I wouldn't due that while the xfmr is powered up.

Fred

[WA1GFZ]

don't short windings or it will act like a shorted transformer....ka-bang

[N8UH]

Hi Tim,

Here is a scheme I prefer:

1..Run enough bias on every stage to hold things stable without any drive.

2..Soft start from 500ms to 1 sec duration. 

3..PA HV stays energized for 5 minutes only on timer unless keyed up to reset it.  The start-up procedure is repeated after the elapsed time upon key-up command.

4..Key drive for transmit, and sequence relative to antenna changeover relay tally*.

5..Cutback tap used just for tuning or low-power op.

*..This is also vectored into master receiver mute.

This will give OK PTT action in a conversational mode, but not burn up a bunch of tubes in standby.  Also, you don't have a bunch of transients from switching B+ to deal with every time you hit the button.

73DG

Interesting... So let me see if I have this straight...

• Bias voltage is always on. In a normal class-C stage for instance, normal bias would place the tube into cutoff , even with plate voltage applied.
• On the first PTT cycle, there is the HV step-start and a timer starts. HV remains on until the timer times out or it is manually switched off. Subsequent PTT cycles reset the timer, keeping the HV alive.
• During the first PTT cycle, and in subsequent cycles, there is a sequence: HV timer reset + Receiver mute + Antenna Relay > Key driver stages. Reverse on PTT off.

Sound about right? I might need to break out a micro-controller for this... 

TK

[KC2ZFA]

To step-start the high voltages (because I use choke input filters with 90 uF capacitance and 10H chokes) I followed the 'GFZ suggestion of using relays whose coils are across the primary of the mod and PA high-voltage transformers 1250 and 1500 V respectively)...I played with the resistors (one for each xfmr) until no thunk and no fuse blowing was obtained. I put a heavy relay on the mains of this supply.

Then I built a 24 volt version (cause I had such relays) of the simplified version of Don's relay sequencer (see below) given elsewhere by K6JEK (thanks!). I played with the time constants, and made R1 variable, to get what I feel is appropriate delays.

To xmit, single button on, I use three contacts on relay A to change over the antenna and mute the receiver, turn on the exciter and the mains relay mentioned above to step-start the hv's. On relay X I use two contacts to short the two cathode resistors (50K) on the PA and the modulator.

When single button off, X opens and then A opens. PTT for Dummies like me !!!