Nothing really new here, this is in response to the 12AU7 getting pricey, so I did a little rework of my VFO 12AU7-6GY6 Tx using tubes that can be sourced more cheaply... In fact if a 2.7 ohm 2w resister is added in series with pin #4, a 5BK7 can be used and those along with the 6DT6 are only a buck at some suppliers...

Anyway here's the schematic, audio is as good as any of the previous designs, just uses mostly unloved tubes...

http://amradio.freeiz.com/transmitter/6bk76dt6.jpg


Tom

Hi Tom,

Very nice mods. Now I know what you're really up to, is trying to drive the price up on all those surplus TV toobs.

Kevin

Hey Kev

Dang I've been found out already...

The 6BK7 is actually a better choice than a 12AU7, as it has a higher gain so provides more audio drive, otherwise specs are close enough no circuit mods were necessary(other than heater rewiring)... It's actually close to a 12AV7 that also performed nicely, but is generally another audiophile tube... The little known 6211 is basically a 12AU7 with higher gain would be a good choice, but I've seen a pair of clear tops listed at $395.00 on ebay...

Tom

Very nice transmitter.

abcvacuumtubes has the 6211 for 3 bucks. It's mu is similar to the 12AU7 but I think the 5963 is the 'computer' version. Plate curves are closer anyway.

The 6BK7 has a 450mA heater and to get the 'one buck' 5BK7 it's 600mA. At that level a number of 'dollar days' Triode-Pentode pairs become viable with triode mu ranging from 20 to 100 using the pentode (or tetrode in a few cases) as the osc. Of course, if one needed gobs of gain that could be reversed using the triode as osc and the (sharp cutoff) pentode for audio.

For example, a 6CM8 has a 100 mu triode, pentode, and a 450mA heater. The 6KT8 also has a 100 mu triode, pentode, but with 600mA heater. The 6EU8 had a 40 mu triode, pentode, and 450mA heater.

Both the 6CL8 and 6CQ8 have a 40 mu triode, tetrode, and 450 mA heater.

Of course, the pentode/tetrode could also be operated triode mode which, I think, usually comes out 30-40ish mu.

The 6BQ7/6BZ7 is another TV tuner tube like the 6BK7, but with a 400mA heater. There are 'one buck' 5V (actually 5.6V) versions of those too with 450mA heaters vs 600 for the 5BK7 (which is actually 4.7V). Same basing.

I suspect the reason you saw, 'back in the day', so may 12AU7s in these roles is because they were cheap. But with audiophiles figuring there must be majic in the bottle that doesn't hold true anymore.

A 6BQ7 could no doubt used and I've tried it along with a couple others, one being 6DJ8 but we won't go there... The 6BQ7 wasn't quite "happy" in the circuit as is, was a bit heavy on current draw... Plate on the audio section was dropping approx 100v across the 82K supply resistor, would no doubt be fine with a bias tweak...

EDIT

I rechecked the specs and a 6BQ7 should have subbed without issue, hummmmm, so I checked what I thought a NOS tube but it was a used RCA in a RCA box(most used are at least a different brand)... The tube was then tested on my B&K 707 and the shorts indicator lit up like the proverbial Christmas tree, apparently the tube was a issue... I'm yet to try a truly NOS 6BQ7 in the circuit...

Tom

Tom,
I am not familiar with an AA5 oscillator coil since I don't have any AA5s laying around. Could you measure the inductance of L1 and maybe I can wind one. I am guessing that since the oscillator is a hartley design, the tap on L1 is probably about 10-20% up from ground? The 68pf feedback cap C6 could always be changed out for a mica padder to provide a variable frequency control using a fixed inductor. Also, could L3 be a molded inductor? Or, what do you recommend? One last dumb question. Do you have a favorite used tube dealer and who might that be?
Jerry W5JH

Advertise in the classified section and I can almost guarantee that you will get what you need for the very best price.

viewforum.php?f=15

HI Jerry

I checked a osc coil like I have in another transmitter and it measured 168uh & 6uh, so apparently the tap is quite a bit less than 10%... That said, back in '06 the members here on ARF were building a screen modulated 6BM8 Tx and we were winding a 100 turn coil with a 25 turn overlay... That coil performed very well, so I have a feeling construction can vary considerably... That transmitter was a modulated osc and tended to drift with any disturbance of the antenna, not the case with this one...

The Inductors can be sourced from Mouser or one of the other suppliers... L2 is Mouser 434-23-102J, L3 is 434-23-331J also a 470uh could be used, especially if you want to use a freq between 1150 & 1300Khz... At 1000Kc or lower a 1mh can be used... Till recently I've mostly used some 500uh surplus coils I had on hand, removing windings to reduce inductance as needed...

I've bought tubes from ARF member Genoo and have bought the $1 specials from ESRC, Stan has a minimum 10 piece order on those dollar tubes but my last order was 75 12CS6... AES in AZ also can supply some tubes for as little as .50(5HZ6 for example)...

Here's a link to the original build of this Tx(bottom of my first post), using a 12AU7 & 6GY6, a under chassis picture is included... If you'd prefer a 6GY6 or 6HZ6 can be used, refer to notes on that schematic for proper bias...

viewtopic.php?t=68236&highlight=6888+6gy6


Tom

Tom,
Thanks for the info. I have been planning to build a small AM transmitter forever. This design looks simple enough and I like the fact that it has a VFO instead of being rock bound. I am afraid that I maybe breaking some rule here but I was planning to wind the RF chokes on FT50-43 cores or use molded inductors. The Mouser part numbers help a lot. I'll ask on the buy/sell forum to see if anyone has an AA5 oscillator coil I can buy off of them. If not, I'll just wind one on the FT50-43 core. The other thing I have trouble finding is power transformers. I end up buying the 110/220V to 12VCT transformers from Marlin P. Jones and then connecting them back to back. It works but eats up 2 transformers. Also, do you get any hum off of a 1/2 wave rectifier power supply? A bridge circuit (4x1N4007's) doesn't cost all that much. Again, thanks for all the info.
Jerry W5JH

There are osc coils on Ebay from time to time but the only reliable source I know of is AES. They have a semi clone of the Miller 'universal' 70-OSC, their part number P-C70-OSC. Ironically, oldradioparts.com has a few of them for less than AES charges, but they also have a $20 minimum order. They do have a lot of other coils (and other parts) too, though. The coils page is http://www.oldradioparts.com/2a6fla.txt
I use a W04M. Convenient and rather inexpensive.

As for the transformer, I haven't tried this because I use UHF converters, which include a pretty nice little transformer, but try using one of the 110/220 V windings for AC in and the other for your B+. I'm not sure how the voltage comes out but it should be better than back to back. Secondary for heater.

Oops. That might do it, all right

Actually, I kind of like the Triode-Pentode pair idea since there's no real need to tie up a dual triode for an osc, you can get more gain for 'cheap', and nothing special, trivial though it is, needed for the heater.

Might as well use them 'cheap TV tubes' since the audiophiles don't think they're good for anything.

Nothing wrong with a pentode osc, is there?

I don't know if anyone has used the 43 composition ferrite for these coils or not, Bob Weaver recommends the 61 for his toroidal multi-tap antenna matching coils... With six taps between roughly 250 & 850uh and when mounted on a selector switch, it's easy to match the antenna at about any freq you'd want to use... Maybe Bob will pop in here and provide a link to his handiwork...

The back to back transformer design works fine, I did so on a build using a pair of 6.3v CRT heater transformers from early '70s solid state portable TVs... The $3.95 MPJA units are cheap enough that cost isn't much consideration(I see they are out of stock though)...

The half wave PS isn't a issue, measured with the scope there is zero AC at the second cap... I do have one Tx with a full wave, but only used that one because the transformer has a CT...

Tom

The pentode osc will give higher gain, which if too high causes difficulty achieving full modulation on the mixer/PA out, I've seen a response on the scope similar to that of the remote cutoff pentagrids(which are poor modulators)... For best results, osc drive should be limited to no more than 12-15v PP, the 6BK7/6DT6 Tx has approx 5.5v drive into the PA... If I were going to use a type with a pentode and triode, I'd probably use the pentode as audio amp and triode as osc...

This is straying a bit and almost resembles audiophoolery, but these little Tx can and will give a different audio freq curve dependent on type of drive signal... When driven directly from one of the 1Mhz clock osc, the heptode 12CS6 sounds rather flat on the sq wave compared to a similar voltage sine, which is why the choke is specified in the osc coupling of the 12AV6/12CS6 Tx...


Tom

Flipperhome,
I had never noticed but the MPJ transformers do have dual independent 110V primaries. I was assuming that the input 110/220 would be the same winding and therefore not offer isolation. However, according to their schematic, only one transformer will be needed to create the 150VDC power supply! Thanks for mentioning this. I would have never looked without your suggestion!
Jerry W5JH

I should have been paying closer attention when checking out those MJPA xformers...

My guess is you'll need to de-rate the transformer approx 50% if only one primary winding is used, on the 1A units load will probably at max with 500ma on each leg of the 12v secondary... With less than the full 500ma load, there would be current available on the second primary... I'd think it'd handle a 6BK7 & 6DT6 plus have 20-25ma or so avail on the second primary...


Tom

I'm mostly a plagiarist when it comes to osc 'design' but are you sure it works that way? My understating is the sine positive peaks are going to clamp at onset of positive grid drive so 'more gain', which I think is actually gm, not 'mu', in this case, would cause more negative bias, wouldn't it? I'm not sure 'pentode mode' really matters much here but it could be strapped triode mode if it does. Or divide it down. Or take it from a small plate load. Seems to me there's a number of ways to skin that cat.

But pentodes aren't known for terrific audio linearity, without feedback, and that much gain isn't needed there.

It's not that I have anything against twin triodes but, for the cost of 'common' ones, the triode-pentode pair has more triode mu choices, such as the 'sort of like' 12AX7 ones.

Speaking of 'cheap', though, there is a 6AU7 at 4 bucks. And then the more commonly kown 7AU7 at 3 bucks.

That's an interesting observation but I need to chew on that a bit. I mean, I can certainly see how a square would splatter RF harmonics all over the place but it isn't clear to me why that would affect audio.

I got it from someone else and glad to pass it on.
You'll probably need to experiment a little because I doubt it'll do quite at much DC out as a 'proper' 110V secondary would because the second primary won't have an 'overwind' to compensate for loading droop. But back to back is normally worse (depending on loading) because one of the overwinds is running 'backwards'.

Well I'll have a hard time keeping up with you when discussing tube theory but can talk real world results...

I tried a few different types in another VFO transmitter I have that has a octal socket wired for 8R/8AD(6SA7 & 6SA7GT), 7S/7AC(6K6, 6L6 etc) & 6Q(6J5, 6C5 etc)...

I measured signal from plate that is fed B+ through a 150uh coil and cathode(tap on coil), results were interesting... Voltages listed are PP, as measured on scope, screen supply through 10K, B+ at 160v... Plate measurement was coupled using .001 cap, no other circuit coupling...

...... Plate ........ Cathode

6SA7.. 98v ........... 4.2v
6C5.. 290v .......... 11v
6J5.. 320v .......... 20v
6G6.. 350v .......... 10v
6L6.. 380v .......... 21v
6Y6.. 425v(est)..... 25v(with probe at 10x and voltage set to 5v[max], plate reading off scale)

I measured the 6C5 & 6J5 twice to be sure cathode readings were correct, still scratching my head on the results...

When driving the output with signal from the plate, it will be deep into class C territory, the tubes generally perform best with zero bias, grounded cathode...



I also fired off my signal gen that will produce saw, sine, triangle, and square waves, the results are interesting but difficult to explain...

Using 6v PP into a 6888, cathode bias varied considerably, which would explain the difference in perceived audio... Still even correcting bias, it modulated poorly on the sawtooth wave... Tubes were biased for best modulation linearity on sine wave prior to measurement...

Cathode bias voltage measured with saw, 5v... sine, 4.2v, triangle, 3.5v... Sq, 3.9v...

I observed similar results with other types, some more sensitive to the various signals than others...

Tom

Inductance is proportional to the square of number of turns. So, if you take the square root of these values, you'll get the approximate turns ratio for the tap. Square root of 168 is 13, and square root of 6 is 2.45. So the tap ratio is 2.45/13 or 19% which sounds reasonable.



POP!

I don't think I'd ever given more than a brief description, so I took a picture. Please excuse the dust.


It's an FT114A-61 ferrite toroid with about 75 turns of wire. Wire size isn't critical; I used #24. There are taps at 45, 52, 58, 64 and 70 turns. It's mounted on the back of a Radio Shack 6 position rotary switch. The taps line up fairly well with the switch lugs.

Type 43 ferrite doesn't appear to be recommended at these frequencies except for use in broadband RF transformers. Maybe okay for RF chokes as long as the DC current is kept low.