Ok, got some kind of plate modulation happening...

[VE3GZB]

Still not making any contacts though. Dropped by another ham's place today (VE3JNC) to buy myself a Heathkit SWR meter, then afterwards visited a friend's place, he and I went to school together and he's into electronics but never took it beyond hobby status.

He knew I got my license and so today he gave me what he thought would be a couple of funky gifts - an 815 and a salvaged power transformer from an old radio! He thought the 815 looked like it was from a satellite or something alien!

So I've put them into my transmitter. I'm using the 815 as a push-pull modulator with this salvaged power transformer (110V pri, 580V centre-tapped sec., using the C.T. sec on the 815, the pri. on the 6146). I have the screen R/C circuit, - 10uF||10k - tied to the plate B+ feed so the screen gets modulated too. B+ to the modulator and the 6146 is 500V

I'm watching my RF on the scope, modulation looks like about 70-80%, as long as I don't yell into the mic (which is an old 4" loudspeaker from a TV coupled to the audio via Coax).

The SWR meter shows I'm getting about 1.5:1 at 7140 and the series bulb in the 6146 B+ modulates with my voice.

I tried making contacts this afternoon but got nothing. I tried just 10 minutes ago for about 30-45 minutes, still nothing.

I'm hoping somehow I can get my hands on a proper powered AM boat anchor of a rig for cheap at this point now (I'm on a very strict budget).

Or would SSB be a better choice? I don't know!

Better modulation Xformer is on the way too, so I'm looking forward to getting that!
73s
geo
VE3GZB

[Ed/KB1HYS]

Are you using one B+ supply for the modulator and the final?
Sounds like an interesting lash-up. Can you post a schematic of the whole rig as it is now?

[VE3GZB]

Just take the "cold" end of the secondary (which feeds the RF PA) and tie it to the centre tap of the audio Push-pull primary. Make sure your RF is bypassed so your audio stage isn't subjected to any RF energy.

After all, as long as your RF PA tube and your audio tubes are rated to handle the same B+, there's no reason why they can't share the same DC source.

Decoupling is important, make sure you have RF and Audio decoupling in the right places.

With that said however, I haven't made contacts with it yet. Working on it!

73s,
geo

[VE3GZB]

Here's a photo of the rig.

Grey/black thing in the background is a 60Hz 115-585Vct Xformer I'm trying to use for plate modulation. The 815 does push-pull into it.

Still have made no contacts yet.

73s
geo

[Ed/KB1HYS]

I'm not sure, but I would think that a single 6146 in class C should give more than 1 watt carrier. I think you have enough there to get quite a bit more power out than you are now.  The 815 is rated for 54watts output, so you should have enough audio power to modulate a single 6146.

I would start with the RF section. Make sure you are biased for class C (since you are now plate modulating it) and getting a decent CW signal out of the single final tube.   with 500 volts on the plate, you need 250v on the screen and -150 on the grid.  the supply will need to be able to hold that voltage and put out 117mA.  This is for Plate modulated class C constant service.   

Get the final working on a single band (40 meters) as a CW rig.  After that adding what you have for audio is going to be a lot easier.

[VE3GZB]

Without a wattmeter at my disposal, I'm relying on other means to estimate the output power.

I put my scope probe (X10 range) across the antenna connector, tune everything up for the lowest SWR.

Just in carrier alone, I can see 80Vp-p RF being fed into the 50 Ohm Coax. Assuming the inverted V is tuned up correctly (best SWR I'm seeing is about 1.4:1), that would translate into (roughly) almost 16 watts carrier output.

Of course I'm not transmitting with the scope connected (don't want to risk anything happening to my scope).

B+ falls to 450V when I draw about 120-150mA max., so it's not terrible, not perfect.

Perhaps I'm just not modulating deep enough? I monitor modulation by forming a 1 turn pickup loop with the scope probe and just lay it near the Pi tuner coil.

Grid drive to the 815 (audio push-pull) is almost 90Vp-p from a triode phase-inverter (I'm using a 6BZ7 for that, not shown in the photo, it's under the chassis), and the audio is amplified by a pair of 6SK7s (under those grey shielding metal caps). Cathode bias on the 815 is a 330 Ohm 5W resistor, screen voltage is 300V, plate voltage is derived from the 6146's B+ (500V).

If I touch the grid lead of the first 6SK7 with my finger (not coupled to the antenna of course), the power Xformer I'm using for modulation makes a very strong buzz, indicating plenty of electrical activity happening (I'm not about to scope the HV business of the modulator, that's a sure way to blow the front-end of my scope).

I know the power Xformer's ratio from centre-tap to primary is 2.54:1. Maybe that's not a low enough ratio to excite the 115V winding that feeds the 6146.

And I am modulating screen drive as well, dropping the screen voltage via 10k in parallel with 10uF. That leaves me with about 180V on the screen.

[Ed/KB1HYS]

http://www.amwindow.org/tech/gif/tvtomod1.gif

Is this how you have the modulation transformer connected? 

[Gito]

Hi

A 500 volt plate voltage and loaded to 100ma can give about 70% X 500 X 0.1 A =35 watt output for a 6146 RF watt output .
and this is important Geo knowing RF transmitter impedance ,that is 500 : 0.1A= 5000 Ohm.
Now an class AB2, 815 push-pull amplifier with 500 vdc on plate has  6000 ohm impedance output(from the data book).
Why are these two impedance important .
To modulate the 6146 transmitter We must have the right turn ratio of the Modulator transformer and it depends on this two impedance.

The impedance ratio is 6000 : 5000= 1.2 So we need a modulation transformer with 1.1 to 1 ratio ( the root of 1.2 is around 1.1
if you used a 110 v transformer and 580 v center tap seconder,as a modulator transformer.that is a 1 to 5 ratio .
Since You used the sec as the modulator part its 5 to 1 ratio
So if the modulator tube gives 500 v peak to peak on the Mod.Transformer You got only 100 v peak to peak on RF tube,so it's only 20% modulated
With the right turn ratio 1.1 to 1 You can get about 440 v peak to peak on the Rf tube ,nearly 88% modulation assuming the plate voltage of the RF tube is 500 VDC

Gito.

 

[VE3GZB]

http://www.amwindow.org/tech/gif/tvtomod1.gif

Is this how you have the modulation transformer connected? 

No, I have the secondary B+ tied to the push-pull B+. But that's not a bad hookup either, I should try it!t

[The Slab Bacon]

You can just use the secondary winding alone. Just tie the center tap to your B+ supply, one end lead to one of the 815 plates, the other end lead to the other 815 plate AND the bottom of the plate choke. This makes it into an autotransformer. If you need MORE modulating voltage put the primary winding in series and in phase with the lead on its way to the plate choke..

FWIW, an 815 is a pair of 2E26s in one bottle. Adjust your operating parameters accordingly. (with 500v on the plates and tickle them into grid current and a single 815 can make 50w of audio)

                                                               The Slab Bacon

[N2DTS]

I found ups would ship the boxes for a reasonable price, so on the way is a vm2 120 watt mod transformer, a bunch of 807's, 1625's, and some 6146's, some oil filled caps, a power transformer, a nice driver transformer, a bunch of resisors and caps, relays, sockets, etc.

Its supposed to arrive Wed, but goes in the trash if they have any trouble delivering it...

Brett

[Gito]

Hi ED

It's a great input to me when you Attached Tvtomod1.Gif.and after browsing I found this article about "TV power to Mod transformer" it never cross my mind to used a power transformer as a modulator transformer hooked that way.
But you must be care full ,for the phasing voltage of this windings.

Thanks

Gito

[ke7trp]

Why not tune your Reciever into the same Freq and monitor the audio to make sure you are clear? 

I think you will have a rough time at 7140 for AM.  Most AM I hear is up at 7293 now on the 40 meter band. Maybe thats just my location on the west side of USA.

Clark

[VE3GZB]

I do keep the receiver tuned to the transmit freq., but the close proximity of both Rx and Tx to one another causes the front end of the Rx to overload (it's a tube Rx, no damages of course), so whatever I monitor is very distorted.

If I disconnect the antenna lead from the Rx and monitor things that way, then I sound much better - but of course there is a troublesome delay of reconnecting things in order to tell if someone heard my "CQ" - so I usually leave the Rx connected.

I use a knife switch to toggle the antenna between Rx and Tx BTW.

I have some proper mod. Xformers on the way now apparently, so I'll probably not have to monkey around with an ill-fitting Xformer for much longer!

73s
geo
VE3GZB

[N2DTS]

You could build a simple detector like what is in a crystal radio with a little headphone amp so you can hear yourself while transmitting, with a relay to switch between rx and tx audio for headphone use.
That helps detect defects while you are on the air.

Nothing like making a long transmission with some problem that makes you uninteligable.
A scope is also good, to monitor the rx and tx modulation waveforms.

Something to get in the future, but many guys never get that stuff, or dont use it, I was listening (trying to) on 80 meters tonight and heard w4auv working someone  with a good carrier and about 30% modulation.

One of the keys to AM is to put as much audio on the carrier as you can without overmodulating, compression helps a lot.
The sdr display shows a dramatic difference in the audio on something like an AM broadcast station and most ham rigs, the broadcast stations have big sidebands, many ham signals have almost none.

Brett
 

[VE3GZB]

I've asked a neighbour to sign for the packages for me today.

The big package is enroute, I checked the status a moment ago.

The smaller package is being held in "exception" form something called UPS internal activity. Are they raiding the package?

Who knows!

I found ups would ship the boxes for a reasonable price, so on the way is a vm2 120 watt mod transformer, a bunch of 807's, 1625's, and some 6146's, some oil filled caps, a power transformer, a nice driver transformer, a bunch of resisors and caps, relays, sockets, etc.

Its supposed to arrive Wed, but goes in the trash if they have any trouble delivering it...

Brett