OK guys and gals, here's Norm's 6888 taken a few steps farther... First is basically the original circuit, with an added pre-amp, and the Pi net antenna matching network. With 10ft of wet noodle antenna(#20 or so wire), all three versions will transmit 100ft or more with ease.... YMMV

BTW... Many of the parts can be sourced from a old radio or your junk box... With some minor P/S additions a 6GY6 performs just as well as the 6888 or 7AK7...

http://amradio.freeiz.com/6c46888080910.jpg


Pictures:

http://mysite.verizon.net/am_radio/6888chassis.jpg
http://mysite.verizon.net/am_radio/6888chascab.jpg
http://mysite.verizon.net/am_radio/6888rear.jpg
http://mysite.verizon.net/am_radio/6888fan.jpg
http://mysite.verizon.net/am_radio/6888wiring.jpg
http://mysite.verizon.net/am_radio/6888o1.jpg (cab repaint, was cracked and in poor cond)

Next is very similar to above, but has a P/S added to operate the crystal osc, and uses a 6GY6 RF output. The 6GY6 does not pull enough cathode current to rely on its voltage to power the crystal, but makes just as much signal(and sounds every bit as good). Note B+ difference in this one and the above. It is higher because of less current draw. With this little increase in voltage, the 6GY6 performs virtually the same as the 6888... Also the 6GY6 requires less audio than the 6888 to achieve full modulation... In reality this one is the 6888 xmit with a 7 pin adapter for the octal socket.. Can use either tube with just one resistor change...

6GY6 xmitt with 6C4 pre-amp...


http://amradio.freeiz.com/6c46gy6080910.jpg


For those of you who do not have 1000Khz or some other common crystal frequency clear, this adjustable osc model may be the ticket. Uses a 12AU7 for the audio amp and osc... It does use a few more components, but if you need a non-std frequency this one does the job... Been using this one for a week or so, very happy with it's performance... Note it is running on less B+, but with the G2 supply resistor change, it's hard to distinguish a difference in signal from the higher B+ versions...

BTW this xmitt can be used with the 6888, see notes in the 6C4/6GY6 xmitt above...

Adjustable osc, pre-amped version of the 6GY6 xmitt...

http://amradio.freeiz.com/12au76gy62010.jpg


Pictures:
http://mysite.verizon.net/am_radio/12au76gy6a.jpg
http://mysite.verizon.net/am_radio/12au76gy6b.jpg
http://mysite.verizon.net/am_radio/12au76gy6e.jpg

http://mysite.verizon.net/am_radio/bluexmitt.jpg (repaint of second rough cabinate)


Lastly I'd like to thank Norm Leal, Curt Reed and Terry Judkins for answering about a 100 questions relating to transmitters in general... Without their help, not much chance these would have been successful...

6888

Tom

Looks good! I like em'

This looks great. How stable is the adjustable oscillator version? I don't have a local on 1000, but I do have ones at 980/1020 as well as 1230. Not to mention the 50Kwer on 710 just around the block. I like the flexibility of being able to tune the frequency. I'm thinking of building one of these, because my 6BM8 is all over the place - even slight changes in the antenna cause the frequency to change. Does this transmitter have the same issues?

Wha... huh? AAHHHH... I was takin' a nap...

Thanks guys, I appreciate the comments/interest....

Brian

No antenna movement doesn't affect the frequency, unlike the 6BM8 xmitt, the osc and RF out are two separate stages... Unless the 980/1020 frequencys are on top of you, I'd think 1000 would be OK... I like to stay 20Kc from a active station myself, but found by helping a local member, these xmitts aren't really bothered by a semi local we have on 1010. I'd have to guess the adj osc version drifts about 300/400 cps on warm up. It's not enough to detect by ear. I just tuned it after approx 10min and I can't tell any difference cold to warm... It is extremely important to use stable components in the osc... I've only used silver mica in all my builds, I don't consider a slight drift a problem... I'd be willing to bet most of these old radios drift far more than the xmitts..

That's been the frustrating part of playing with my 6BM8. You want to experiment with the antenna, but everything I do affects the tuning. With the crowded dial we have here, it makes it tough. Especially since the trimmer cap is mounted under-chassis, and can only be tuned when I have the chassis out of the case.

Mike's going to let me test-drive one of his 6888 transmitters. If it works as well as everyone says it does, then I'll be putting myself on the list for the next batch of parts.

Thanks for all the R & D.

This looks really nice! I think I might make this my first real DIY electronics project (I have done repairs, just never built things from scratch yet). I do have a couple questions though:
1) What in the world is a "mmf"? Is that millifarads? I can't find any references to tell me what exactly that means...
2) In regards to the first schematic, do L1 and L2 have to be hand-wound, or can I go buy some coils instead? I think I might not be so good at the hand-winding, but Fair Radio is in my city so I can go get the coils. If I can just buy them, is there a particular type I should look for? I've seen things like pi wound, coated, etc.

Thanks!

-Dan

One way to get much more stability in the varible freq version is to change the L to C ratio of the oscillator circuit, where, in place of 68pf you use 680pf. That way any delta C due to loading or what ever is a smaller part pf the total osc capacitance.

It would make the osc coil less handy to find and may require that the osc sig sent to the output stage be stepped up.

But over all, keeping the osc cap as large a you can will always improve stability

Just a thought.

rcw

Hi Dan

MMF = micro-microfarad(old school term) which is same as PF...

The osc coil is from a junk Japanese AA5 radio, the others are no real problem to wind... Are also available from AES and likely Radio Daze...

The cap RCWade mentions was also part of the original circuit in the radio(but I do have a new one in it so possibly the original was a different value)... Something to check on...

Tom

mmF is micromicrofarads which is the same as picofarads. It's older terminology.

1000 mmf = 1000 pf = 1 nf.

We used to call mmf, mickey-mikes, as we now call pf, puffs....

I have know idea what any slang word is fo nf...... nuffs?

So, more "C" and less "L" gives stability, and more "L" and less "C" gives greater signal. http://antiqueradios.com/forums/viewtop ... 74ede188ac


On Brian's 6BM8 transmitter he used a 300+ uF var cap. He had drifting problems just walking around anywhere near the antenna. I, on the other hand, same transmitter (he copied mine) except I used a small 25uF var cap, had very good stability and excellent range. This is contradictory to large "C" small "L".

I'm confused. I don't know all the formulas and theory, but I know my transmitter covered almost a block. Remember I'm talking Syl's 6BM8 variable frequency transmitter.

The thing to remember, the more isolation between the osc and the antenna, the better your stability.

High 'C' oscillators are always more stable, if the capacitors are of equal quality.

Junk doesn't work.

I have found, in many small signal xmitters I have designed, mostly at VHF, that the best stability against movement of the antenna, is to use an oscillator-doubler, where the oscillator operates at 1/2 the output frequency.

The use of a Vacker modified Colpitts, lightly coupled grid circuit, to a plate tuned second harmonic output tank fairly well isolated the oscillator from the antenna.

I must admit, that most of the small signal xmitters i have designed since the early 1960's have been solid state.

But the same basic principles still apply:

Keep the oscillator from 'seeing' the antenna.

Don't let the oscillator know if the supply voltage changes.

Try to keep the modulation % constant vs. xmitter tuning.

"Keep the ball low"

Hi Mike

Therein lies the difference, I believe RCW is referring to C6 in the osc circuit(12AU7/6GY6 xmitt) and you are referring to the antenna matching cap...

The more L, less C seemed to hold true till I tried Bob Weaver's multi tap toroid(nice) in the Pi Net... With it, I get more signal at 1500Kc using the next to lowest coil tap(approx 350uh)... As I increase L it takes less C to match the antenna, BUT signal(observed with the scope)is defiantly less... Of course I understand there is a limit to L vs freq relationship, just turned out to be less than I thought...


Tom

Why don't you flip th trimmer towards the chassis make an access hole big enough for a plastic trimmer screwdriver?
Eliminates flipping it over for adjustment.


Jason

5 year old thread.

Larry

Yeah Brian just sort of disappeared, have wondered what happened to him...

Tom