[AMRadio] I've decided (retry)

Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Tue Aug 7 13:41:07 EDT 2007

The 4-125 will do about 150 watts out at 1500 volts,
maybe a little more if you boost the current up
from 150 ma. Not sure about your modulation peaks holding up
at lower voltage more current.

The 4-125 likes 2500 volts and will give 300 watts out
at that voltage, with good modulation.

At 1500 volts and 150 ma, you get 10,000 ohms, on
the high side but not unreasonable.
power input will be 225 watts in, so you need between 125 
to 155 watts of audio power to modulate the rig.
811A's do 340 watts a pair, you do not need or want 4. 

I think the 811A's will be around 12,000 ohms?
I forget, if so, the 1:1 mod iron might work well and
you could try both mod transformers.
I am not big on roller inductors unless its ganged to the plate
tuning cap and tracks frequency at a fixed Q.
That is hard to do homebrew.

You will need to find  good spots on each band  and frequency 
and mark it, as with a roller inductor, the tank circuit Q can vary 
all over the place, from a harmonic generating low, to a very
lossy high.

I think the rig will take about 10 watts of drive with a tuned
grid input. 
You can wind grid coils on pvc, use coil stock (better),
for each band, each with a link, and switch them in.

One rotary switch will do coils and links, a cap with a vernier
drive to tune it, the vernier drive makes it tune nice and
has a built in scale for logging.
Drive it with the rice box (use something else as a receiver)
and have digital readout, stable and accurate.
Later, you could build a homebrew exciter if you wish.

Since I am meter mad, I meter everything, and in order to get
things to match, I used cheap radio shack meters and built
my own shunts. I don't think radio shack sells meters anymore...

For bias, a fixed supply off the filament works well, use a 6
volt to 120 volt transformer, the 6 volt side hooks to the filament
supply and the 120 volt side gets rectified and filtered.
If the filaments light on the tube, you have bias.
I add a diode and a grid leak resistor to allow the bias 
to track the grid drive.
Most times I use a BIG wire wound pot so I can adjust it.
More resistance, more drive power needed, but more beyond cutoff
you go which gives better modulation (up to a point).

If you build things big enough, you could use a 
4-125, 4-250 or a 4-400.
Filament current goes up with the 4-250/400, and the
plate voltage can go WAY up to 4000 volts.

You also need a screen supply, and a way to modulate it,
I like a fixed supply, plus a dropping resistor that drops
100 to 300 volts, and a choke to allow the screen to self
modulate. I also add an overload relay to all screen supplies.

The resistor helps protect the screen a lot also, more current flow
means more voltage drop through the resistor, which helps the screen
self regulate somewhat, without it, screen current changes a lot
with loading and so on.
Most tubes don't tolerate much screen overload at all.

If pi net output, you need a tuning and loading cap, the
tuning cap should be good for over twice the plate voltage,
four times is better. You can take the DC off it to help.

The 4- series tubes does not require neutralization.
(nor does the 813).

Hope this helps.



> -----Original Message-----
> From: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Geoff/W5OMR
> Sent: Tuesday, August 07, 2007 10:13 AM
> To: Discussion of AM Radio in the Amateur Service
> Subject: [AMRadio] I've decided (retry)
> (Don't know if this made it to the list, the first time I 
> sent it.  I'm 
> -hoping- to have fixed a few gmail problems)
> My homebrew project.
> A single 4-125 modulated by a pair of 811A's.  (might use 4)
> 1500v on the final, 1250v on the modulators (0 bias).
> I've got a multi-match 300w Thordarson modulation transformer 
> or an RCA 
> 1:1 @ 5,500 ohm modulation transformer to choose from,
> 811 sockets,
> a nice 'Christmas tree' RF choke,
> filament transformers (6.6 (adj) @ 18amp)
>  a roller inductor,
> a BC-610 tuning capacitor for a pi-network output,
> and a 4-125 socket coming.
> Can you think of anything else?  (got meters)
> What I need to do yet is build a bias supply for the final and a 
> grid-input circuit.  (I'll probably use a Viking II VFO until I get a 
> digital vfo built up. )
> I thought about just using my Kenwood TS-450S as the VFO, RF 
> exciter and 
> receiver, and might do that for 'testing'.
> I'm soliciting ideas on grid inputs on a 4-125.
> Thoughts?
> -- 
> Driving your AM Rig without a scope, is like driving your car 
> at night, 
> without headlights. (K4KYV)
> -- 
> 73 = Best Regards,
> -Geoff/W5OMR
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