Ever since I saw the plans for a transmitter using a dual control pentode I knew I had to build one. I settled on the 6GY6 version with the 6BK7 preamp/tunable oscillator version. I decided to build mine inside an old computer power supply. I did take quite a bit of liberty with the audio caps, increasing them in size to pass 20Hz as I wanted this to double as a piece of test equipment to check frequency response of various radios. I find it actually passes down to 15Hz fairly well. C2 was increased from .01 to .05; C3 was increased from 4.7 to 100uF as that's what I had on hand. Calculations showed I could have gotten away with 68uF. C4 also took a whopping jump from .05 to 1.0uF. The "chassis" is a piece of double-sided circuit board.

Because this is a tube project I decided I also wanted a tube rectifier so I got rid of the sand state diode and used a 6X4. I put .001 from cathode to each plate. Power supply caps were 22u input (tube rectifiers are a bit more limited in what they can handle for input caps and 47u was kept as the output section. If I had to do it over I'd probably have used 33u input and 68u output, but these values still worked. Transformer is a Hammond 261G6. It's overkill for this projecxt but it was the only one in the plate voltage range which had the filament current I needed.

Photos and commentary-

Continued from the above post- photos and commentary.

That's a great build, nice attention to circuit detail.

Reason I didn't show 6BK7 shield, program didn't include one in the dual triode choices. I've seen 12AV7 with shield inside but not connected as pin 9 is heater CT. Other than shield & heater configuration, 6BK7 & 12AV7 are same tube.

I happened to be thumbing through a GE tube manual while I was constructing it and it showed the shield on the base diagram and when I looked between the sections of the actual tube, sure enough there it was which is why I knew to change it. Not a biggie, though, as I know I've overlooked small stuff like that myself. It's a great design otherwise and sure beats every other "part-15" transmitter I've ever built.

The FCC has a part-15 band in a section of the 22m band (13.5MHz region) where you can run a whole watt. I'd love to build a companion transmitter for that frequency. I'd be more willing to opt for some sort of frequency control up there since a free-running oscillator tends to wander around. My build above drifts +/- 500Hz even after it's warmed up though it does stay centered on where it's tuned.

Scott Todd

Very nice build.

Thanks for the kind words. I wanted to do something creative and that seemed to be the most compact way of doing it. I do like the ones where the tubes are standing out in the open too, though.

I ran it again this afternoon, and as I was doing so I dug out an old Dorrough 310 audio processor. While it's not an antique per se, in terms of radio technology it is still considered pretty primitive with early ICs and discrete components. I did it because the transmitter really does need some way of controlling modulation to prevent carrier pinch-off on negative peaks, otherwise you have to run the modulation down a bit so as not to run the risk. It's pretty obvious when it does that. Now it's blowing a hole in the dial when I have it on. This transmitter is a lot of FUN!

Scott Todd

With it used as test equipment, a enclosed design is no doubt the best.

I'm running a 6GY6 built in RCA UHF converter, BUT because it now has a octal to 7 pin adaptor to use 6888, I can't get it back into it's case. I should install octal socket, but works so well in it's duct tape & bailing wire configuration, I'm almost afraid to mess with it.

For fun factor, thank Norm Leal for designing the original 6888 this transmitter is based upon. You've no doubt improved upon original, my skill set is mostly trial & error. One of these days I'll probably use your cap values.

I use Stereo tool as audio processor, good at maintaining constant levels. Over the last dozen years I've sent four or five Win XP computers to their grave that ran 24/7 as audio servers. Overall had more computer failure than xmitter.

Very nice design and construction. Would you be willing to share the schematic for this? Thank you. Mike

If you can stand yet one more update on this thing, just out of curiosity I wanted to try adding a 0A2 VR tube to see if it would improve hum and drift, and it did both rather well. I can keep the frequency within +/- 250Hz once it's warmed up, which isn't bad for a free-running oscillator. From the second cap of the power supply I added a 390 ohm resistor to pin 1 of the VR tube ( I think it should probably have been a 430 or maybe even 470 as the tube get's really bright for half a second before the rest of the tubes conduct enough) and used the internal feed through jumper at pin 5 to the rest of the transmitter; grounded pin 7 (one of three tied to the cathode.) This addition really was a kluge- I had to solder the tabs of the tube socket to the inside angle of the L, but it's mechanically stable and there was room for it.

Before I forget, for low frequency response down to 20Hz the formula is C [in Farads]= 1/(10xR). The R [in ohms] can be your cathode resistor, the screen dropping resistor for pentodes, or the grid resistor of the following stage.

Here's a link to the schematic, Mike. This is the unaltered original, though. I haven't drawn up a revised one with all my mods, but from my previous posts you should be able figure it out. I used a center tapped power transformer for the 6X4 rectifier tube.

viewtopic.php?f=12&t=296962

Scott Todd

PS- does anyone on the forum still have some 6888s to sell? I'd like to duplicate just the output stage and use it with a BE CQUAM AM stereo generator. The L-R modulated RF carrier I'm going to feed into the first control grid and modulate the second control with the L+R audio.

Hi all. I have done exactly that . I have a BE AX10 stereo generator and have fed it to my 6888 TX. It works quite well after the setup. Make sure the plate circuit is tuned up before you adjust the rest. Also you should use the zener circuit in the cathode even though you are not using an oscillator powered from thereas it keeps the bias stable to hold down on IPM. I also raised the screen filter to a 1UF for the same reason. The envelope output is quite robust and you may have to use an isolation transformer such as an A20 or A21 here. It will sound good. I take it you have a second DAP310 with stereo jumper. I use other processing but you do need good control of those peaks. And you may not need much of a preamp as the BE can drive it at +10 easily so you can drive the 6888 without a preamp if you wish. Enjoy it.