bias - keyed or on all the time?

[W2INR]

I have finally completed my new HB rig and I have it operating on the 75-80 meter band at this point. Planning on having this rig run 160 -75 meter.

what is your preference: Keying the modulator bias with the plates or having the bias always on.

Tnx

G

[N8ETQ]

always on.

[WBear2GCR]

been a busy bee, there Gary!
that's really clean, nice work!
Do we get a wide shot/front panel shot too?

Pleading ignorance, I guess. What do you mean by "keying the modulator bias with the plates"??
In this neck of the woods, bias is applied between the grid and cathode.

I've seen schemes that have two levels of negative bias voltage, switched by a relay between standby
and operate. The higher bias voltage (more negative) to push the tubes into low idle current. The other
scheme drops a large(r) cathode resistor in, achieving the same effect, again with a relay.

Are you talking about keying the B+ to the modulator tubes??

[W1AEX]

Gary,

With my HB 4-400 X 833's my modulator bias was always on. Never had any problems with it. Hope to catch you on sometime!

Rob

[W2INR]

Thanks for the replies,

Trying to get my radio mojo back. I could not remember how I handled the bias on my Hb rigs in the past.

Rob I should be on this weekend if all goes well. I have been testing down around 3680.

Thanks Bear - I have bee working on a station over the last year, slowly coming together.  In regard to the bias I was talking about the modulator bias and whether most have it on or switched with the plate.

Thanks

G

[Pete, WA2CWA]

Gary: I don't see any advantage to switching the modulator bias. Most designs I've seen have it on all the time.

[K1JJ]

what is your preference: Keying the modulator bias with the plates or having the bias always on.

Hi Big G -

Broadcash style layout...  833As in parallel modulated by a pair... nice.


Bias: There's many ways to key the rig. For many years I have placed a 50K resistor in the fil CT of the final and the modulator and keyed these resistors on and off using separate contacts. (PTT) This completely cuts off each stage.

I also break the audio signal going into the modulator just before unkey to eliminate pops and noises.

In addition, though not required, I key the HV power supply on and off using a step start that uses a 10 ohm 200 watt resistor in the HV AC primary. I just like to have the HV off to save bleeder power and for more safety.

All of this needs to be sequenced in the proper order with delays. The RF drive and antenna relay may need sequencing too, of course.

BTW, if you get any EXCESSIVE mod transformer or Heising reactor spark gap arcing during key or unkey, I solved it using a 200W 5K power resistor across the gaps that gets opened and closed to suck down the stray/unused RF and magnetic audio power during unkey. This requires a HV or vacuum relay in series with the 5K. The description is in my 4-1000A project thread if interested.  Works like a charm to completely snuff any PTT-related arcs/sparks.

Maybe your appearance back on with the new rig will get me motivated to flip the switches on too... :-)

T

[N8ETQ]

      Can't recall where I first saw it, but
I utilize a keying scheme that opens the ground
of the Zener or whatever regulator you use
allowing the bias to soar..  Works well.

      Probably more important with Tetrodes
like 4CX???, or 4X1's. 


/Dan

[W2INR]

 Good Morning all

Thank you all for the input. I now have some great ideas to tackle the low level power supplies and keying circuits.


The rig is the combination of two broadcast rigs, an RCA and  Gates. The plan is to operate it from 1800 to 4 megs.  I decide to bring the rig up on 75 hoping that once I am complete 75 -80 meter band 160 should not be much of an problem - - update to follow. The rig is being built in three phases, 75m then 160m and band switching then the panel work. Should be complete by fall.


Dan -I have seen that diagram you are talking about sweet and simple I must look into that.


T - Wow it has been at least 5 years, good to hear from.  Your ideas brought to light some areas I had not considered. thanks OM


Pete  WHO?   Good to hear from you Pete, hope all is well with you and family.


Back to the rig

Catch you on the airwaves

G