Grid modulation on an 807?

[VE3GZB]

Hi

I'm still in the planning stages of my first transmitter (phone, AM, 40 and 80 meters, probably about 25-35 watts output). Oh, and I'm on a budget like zero dollars, just using whatever parts I have on hand from past salvages, etc...

In my original plans I was going to use a 6AG7 oscillator (haven't made up my mind whether to use VFO or Xtal, or a mix of both) driving an 807, then use Heising modulation from a 6AG7 driving a 6L6.

I've been told by an experienced Ham that a 6L6 wouldn't be man enough to fully plate modulate the 807 using Heising modulation. So I'm wondering if I could be successful if I use another 6AG7 (or some other tube) to grid modulate the 807?

Any experiences, thoughts, ideas on this to share?

Thanks & 73s,
geo

[w4bfs]

I have spent the last year plus basically answering the same question ...lotsa different ways possible ...lotsa fun trying to figure it out ...grid only modulation is simple but distortion prone ... for a single 807 a tv vert osc/output pair (6dr7, 6de7, 6ea7, etc) could do a low distortion cathode follower screen modulator ... maybe look at the Heath DX60 mods ...I found K4TAX's derived work from earlier work to be interesting and informative ... basically 807 and 6146 are repackaged 6L6 family tubes ... 73 ...John

[VA3AEX]

Geo, the WRL Globe Scout 65 used a 6L6 to modulate an 6146 using Heising Modulation for about the same output, so would think it would do fine with an 807: http://www.eht.com/oldradio/arrl/2006-11/Globe-65-article.htm

73  Alex

[K3ZS]

The Globe Chief used two 807's.    WRL had a screen modulator for it that used a 12AU7 driven by a 12AX7.   The 90 W input transmitter had to run about 30W output when using it, or 15W on 10M because of doubling in the final amp.

[w4bfs]

I had a qso with Tim, WA1HLR on 20 mtrs a while back ...he was using a modifiied Globe Scout .... some primo sounding AM !!! .... if you decide to go this route,be sure to look up his mods ...73 ... John

[Steve - WB3HUZ]

It doesn't matter what tube you use, straight up Heising modulation will not modulate the final to 100% if a common supply is used. You can use a bypassed dropping resistor in the B+ going to the final to get close to 100% but this reduces the RF output a little.

I'd go with Heising modulation over grid modulation. Tuning/adjustment is less critical. You should be able to run at least 50 watts input with the 807 and get 35-40 watts out. Even if you can't modulate 100%, this should be more effective than 15-20 watts out with grid modulation.

What are you going to use as the Heising (modulation) reactor? Low power modulation transformers are cheap. Then you could go with a push-pull modulator and have 100% modulation or more!

[VE3GZB]

I have a couple of old salvaged audio Xformers from a 50s chassis at my disposal. Everything I'm using is salvaged and on-budget (as close to zero dollars as possible, I spent $1.15 on wood today for this project, that was scrap wood from Home Despot).

The priority for me is the zero dollars budget. Second priority is getting on the air with a quality signal. I am very confident it can be done since it's been done in the past. The schematic is the thing to me. From what I see, the Globe Scout 65 looks very good!

Another friend of mine suggested I use screen modulation of the 807? Any thoughts on this as well?

Thanks & 73s
geo

[WU2D]

Hi Geo!

Wow lots of modulation ideas are coming out and with a low power rig, you can try almost anything. Do you remember the WS-19? That Wireless Set 19 was the first military transciever put into large production for the allies during WW2. It was deigned by a radio and TV manufacturer in England named PYE. Anyway they decided that an 807 should be conventionally control grid modulated by a small pentode tube. This saved on power, iron and complexity theoretically, so it made sense for a portable mobile radio where AM was only a short range mode.

It did not turn out to be so easy and the final circuit which went into production was quite complex. Most of the complexity came when they tried to stabilize the RF drive level (essential for low distortion control grid modulation) over a wide frequency range and two bands - and then be able to switch to efficient class C for CW. With Control Grid Modulation you also have to run high voltage on the final 2X the peak RF voltage and don't expect too much efficiency - but the frequency response will be GRRReat. After all this is how they modulated TV transmitters!

The final circuit demanded a fancy pentode buffer outfitted with true RF feedback with a 6H6 diode. So the moral is that you can fully modulate an 807 with a 6SK7 and a carbon microphone, but it will be tricky.

By the way, the 807 has a kink in the current characteristic of the screen grid as it near zero volts so be careful with screen modulation.

How about one of those cathode modulators with some triode connected 6L6's in parrallel?

Mike WU2D

[VE3GZB]

I've never heard of that transmitter but it sounds neat! Is this the schematic of it? If so, it looks very involved!

http://home.clara.net/rod.beavon/WS19_cct_Atx.gif

I thought about Cathode modulation......I have 2 salvaged small output Xformers from a 50s AM/FM entertainment unit (you know, those old console TV/record players). Maybe I could put the "speaker" side of the Xformer into the Cathode circuit of the 807 (of course with RF filtration) and modulate it that way?

How much power would I need for Cathode modulation of the 807? I only have the one 6L6 (I have several 6AG7's on the other hand) so however I use it, it has to be used cleverly.

Or would Screen modulation work alright (because it doesn't require heavy current and I could use the 6AG7 for that)?

Oh, yes I know about the Screen kinks, thanks..I'd have to put limiting resistance into the Screen circuit to guard it from overcurrent!

[VE3GZB]

How can I find out what modifications WA1HLR did to his Globe Scout?

Thanks

[Steve - WB3HUZ]

Dunno about Tron, but some mods for the Scout here.


http://www.amwindow.org/tech/htm/gscout.htm

[WU2D]

Geo,

I think you are on the right track with screen modulation and a 6L6.

The WS-19 looks alien and complex because it achieves single tuning for TX and RX with tracking and offset, shared TX-RX heterodyne oscillators and IF stages. It is a true single conversion transceiver. These features are not seen in US designs until Collins first XCVR, the ARC-2 and the  KWM-2, which came 10-20 years later.

The 1937 design WS-19 is also complex because it is essentially the first COTS (commercial off the shelf) military radio design. COTS  employs complexity with high number of stages and component count of low to modest quality TV/radio grade parts, to achieve high performance rather than the usual MIL SPEC design pholosophy which calls for low parts count and very high quality custom components and rigid construction to achieve performance. In effect, this is the first RICEBOX.

Mike WU2D

[w4bfs]

Tim described what he did to his Globe Scout over the air ... I will do my best to recollect properly

100% is not possible with single supply Heising as Steve has noted without pa stage power reduction ...Tim addressed this by using a transformer ...the neatest thing is this .... the transformer is wired with the secondary out of phase with the primary ... think about it ...this places the dc currents of the pa and the single ended modulator in opposition and greatly reduces core saturation ... well it definately works as it sounds marvelous, even with Tim's voice ... oops did I SAY THAT? ...73...John

[VE3GZB]

What's the ratio of the modulation transformer he's using? I'm surprised anyone can still find such things as a relatively low power modulation transformer these days! Sounds like he has his theory right though.

I'll probably end up doing some kind of modulation involving maybe a mix of plate and screen modulation together (well that is if I have the time to devote to getting the operating levels all worked out in my spare time, which dwindles with kids!)....or if I have no time, then I'll try to get either Heising modulation OR screen modulation to work, whichever gives me the best results at the lowest cost.

[w4bfs]

well Geo, I don't know but with some info from you and the RCA rx tube manual, we can figure it out ... Tim also recommended a 6550 or (I assume) at least a 7027 as the modulator .... at least a 6L6GC with its 32 watt dissipation as a fallback ... what are the pa operation conditions?   John