Circuit problem on a modulator

[W9ZSL]

I debated using the Triad mic transformer with the 6J5 extra stage and setting things up for both crystal and dynamic mikes...or balanced / unbalanced input. Was curious to find out if you had enough gain using the UTC. Did you go with a capacitor or a choke input filter on the power supply? I brought up this exact topic a few years ago and the group suggested changing the .001 ufd cap in the feedback to 500 pf. I'll probably just can the feedback circuit completely. Keep us posted on the 4-125A amp. Like I said earlier, I have all the parts I need to build that unit you have in mind OR one using a single 813.

[W4RFM]

It appears I have plenty of gain using the A-11 step up audio transformer to the 6SJ7. I am keeping the 750K feedback resistor, and losing the .001 mfd.  The audio is hi fi, no kidding. 

[W4DNR]

Question :

On Bob's schematic, I see the 6V6 has the screen at plate voltage potential.
( no dropping resistor ).

If that is a No-No at RF, how does it work at audio ?

Is it just a characteristic of the 6V6 ?


Don W4DNR

[W9ZSL]

Bob, I'm going to keep the 6J5 and use it as an extra stage with a switch to take it out of the circuit and route the Triad to the 6SJ7. I have an NOS Electro Voice 911 along with dynamics so that will give me the capability to use either one.

[W4RFM]

Don, when I rebuilt the design, I incorporated a bigger tube, and adropping resistor so that now the plate of a 6L6GC (which could handle 450 vdc if it had to) gets 300 vdc , the screen gets 250, and the 6SJ7 stage get 105.

Everything runs cool to the touch after half a day of continuous duty, (well the tubes get warm, but never any red plates).

[DMOD]

Don, when I rebuilt the design, I incorporated a bigger tube, and adropping resistor so that now the plate of a 6L6GC (which could handle 450 vdc if it had to) gets 300 vdc , the screen gets 250, and the 6SJ7 stage get 105.

Everything runs cool to the touch after half a day of continuous duty, (well the tubes get warm, but never any red plates).

Bob, I was just about to ask what your final schematic looks like, but I take it from above you just changed the driver stage from the 6V6 to a 6L6 and gave the screen a dropping resistor?

Nice chassis work.  


Phil

[W4RFM]

Thanks Phil.  I like the stuff I build to look as good as possible, which sometimes leads me to "find a reason" to start over.  I love building, but I dont like paying for another Hammond chassis .

I have several friends whose work makes mine look like a beginner, but I keep on trying.

As for the schematic, you are correct, Lost the .001 in the feedback loop. lost the 6J5 mike pre-amp, changed to a 6L6GC tube, and played with the voltages hitting the various places.

[W9ZSL]

Busy weekend here. Finished the exciter shown on the left. Both units still need panel marking. Modulator has been on the back burner. Front panel for that is at a shop in town that specializes in metal work. They will cut the meter hole and powder-coat. The exciter front panel was powder-coated by them and it matches the front panel of the 807 amp on the right.

[Opcom]

Question :

On Bob's schematic, I see the 6V6 has the screen at plate voltage potential.
( no dropping resistor ).

If that is a No-No at RF, how does it work at audio ?

Is it just a characteristic of the 6V6 ?


Don W4DNR

It's done quite a bit for class A stuff and seems to be supported by the data sheet at least for 250V, but I'd also prefer a small resistor on the screen if the screen current is high. I guess measuring it would be the thing to do. Screen's rated 2W.

[W4RFM]

You might have missed my update on the second page of this thread, when I started over:
Replaced the 6V6 with a 6L6GC,                                 
it rests at 54mA /300 vdc, the screen draws a resting 2 mA at 250 vdc, the thing sounds great.

[Opcom]

Perfect!

[K8DI]

The units you show in the photos look great!  I'm building on my first transmitters, and I'd love to see your circuits. Would you mind sharing your schematics for the exciter and amp?

Ed, KB8TWH