On Thu, 14 Jun 2012 22:23:29 -0700 (PDT), Patrick Turner
wrote:
On Monday, 11 June 2012 03:39:00 UTC+10, flipper wrote:
Okay, so I like to make up cutesy names 
I wanted to do 'something' with a 6ME8 so I tried using it for an AM
transmitter and the first prototype works rather well. Bandwidth is
'too much', less than 1 dB down at 18 KHz, but we'll worry about that
later.
This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2
is a UHF converter coil scramble rewound, so that was 'free'. Power
supply is from the same converter and if the remaining issues get
worked out it'll probably end up in that cabinet as well (but where to
put the air variable?).
The schematic is rough, but should be rather self explanatory, and the
ganged tuning hasn't been fleshed out yet. I'm still using separate
caps while jiggling things around.
Orphaned web page, not yet ready for prime time, has a recording of it
playing through a table radio.
http://flipperhome.dyndns.org/Beamus.htm
I couldn't download Fliper's page above on his transmitter last week
Sorry about that. Every once in a while the servers is offline for
various reasons.
but can now, and the circuit looks well.
Thanks.
Must say I'm rather pleased with the deep mod and excellent bandwidth
using only two 1 buck tubes.
Possibly some improvement could be to used cathode followers to drive each beam delctor plate.
Well, as is, there isn't a signal to put into a CF for the other
deflector plate. They're being driven single ended.
I've considered that driving the deflectors with a PP transformer
might improve RF harmonics, and maybe a CF to drive it's primary, but
I think it might be possible to simply wind a PP secondary on the osc
coil. However, I'm not convinced it's worth the added complexity.
If he has 35mW and say 50Vrms anode signal at antenna output, then
0.035 = 50 x 50 / RL so RL = 71kohms. This is rather high but the tuned tank circuit probably has such a high Z at Fo.
That's 80 volts peak to peak (Vpp) so RMS is 28 V. Guesstimating
1.25mA gives the 35 mW figure and resonant impedance around 22.4 k but
I think the current guess was a tad high so impedance is probably a
bit higher.
The comments made about coils are of interest, and it should make no difference what type of core is used, because 270uH is easy to achieve with many recipes.
I don't know why you say that. It isn't a matter of 'achieving' 250
uH, at least on an LCR meter, it's how it behaves at that frequency.
For example, I talked to Bob Weaver about that and he confirms
'switching power supply' ferrite cores (which I also tried) soak up RF
like, his words, "a sponge." Now, I don't know whether the UHF coil is
doing the same thing or exhibiting some other 'problem' at MW
frequencies but the volts weren't there.
Might perform better if I had some Litz wire but I'm using a ferrite
antenna for the coil now and AES has them for 3 bucks so it isn't
worth spending moolah on Litz when buying the thing 'ready made' is
just as cheap.
But in the best old radios the 455kHz and MW RF coils are pie wound windings using litz wire and large Al cans to keep the Q high, ie, RF resistance low. In an AM receiver I built from scratch in 1999, I made copper cans for two RF input coils and I wound solid wire on 50mm long x 10mm dia pieces of ferrite rod normally used for ferrite rod antennas. The Q became higher than using air cored windings on say 20mm PVC pipes. In my receiver, the Q was so high the BW was less than 15kHz, giving slight cutting of sidebands and thus reducing possible audio BW, so I used TWO such coils, and a 3 gang cap, with 2 gands devoted to the the two RF coild and the other to the oscillator. To overcome the reduction of BW, I used antenna input to a tap on one coil, then a resistor from top of coil to top of next coil and then I tuned the coils
so they were "stagger tuned", ie, slightly apart at the low end of the band where the LC Q is higher than at above say 1.2MHz. All that worked well, but compact fluorescent lamps and other junk creates a rectified version of incoming RF and I've had to change to a long rod ferrite antenna with a shielded coil and very short wires to keep out electrostatic portion of RF wave.
In my RF generator which I could use as transmitter, I get the same very nice AM wave on the CRO as Flipper's site shows. I have a 6BX6 for the RF output tube, and the coil is a hand wound thing on ferrite with 3 layers of not very tightly wound windings of solid wire, and with PVC tape used between layers. The self capacitance of the coil must be kept low if you wish to be able to tune the tank to as high as possible, say 1,750kHz.
Many old radios went from 550kHz to maybe 1,550kHz, a 3 fold F increase. Say you have 20pF for coil self C, then if the cap gang = 365pF max, then total max C = 385pF. Say the coil turns or ferrite position adjusted for Fo = 530kHz, then L must be 234uH. Say the C gang min C = 25pF, and you have 20pF self C
then Cmin total = 45pF which gives Fo = 1,551kHz, and not as high as anyone should accept. If C minimum could be reduced to 22.5pF, Fo = 2,194kHz, but you will never see this, and besides, one has to be able to use a trimmer cap to set the top of the F range. So the less stray C or coil self C, the better.
I don't really care what the 'top end' is as this isn't to 'test'
radios, its a broadcaster, and being able to find a 'quiet spot' on
the dial is the only reason for tuning.
The 'problem', so to speak, isn't the local osc but the plate tank
because a 10 ft wet noodle wire antenna comes in at 30 pF-35 pF before
you even get to cap minimum, stray, and coil self capacitance.
I'm not sure yet but may go to another slug coil on the plate because
even though less inductance lowers voltage swing, hence generated
power, that may be better than cap tuning because, in that case, most
of the circulating current ends up in the tuning cap rather than the
antenna.
On the other hand, I'm getting sufficient range even with closed
plates so it may be a case of "don't fix it if it ain't broke."
Btw, I've tweaked/padded values around the existing coils and caps to
avoid the irritation of being able to 'dial' a frequency you can't
peak on the antenna tank so the tuning range is now narrowed down to
roughly 600 KHz to 1100 KHz. So, now, if you can dial it you can peak
it. Basically I put 100 pF in parallel with the osc cap and lowered
the coil to 190 uH.
I'm listening to it at about 680 KHz as I type.
Keep the coil cans as big as possible, and never use iron cans.
Patrick Turner.