Single 6V6 Speech Amp/Driver Revisited

[W9ZSL]

Decision made.  I'm going with my original single 6V6 into a UTC S-8 speech amp / driver we have hashed out before.  Since I won't be driving 811A's and will modulate with a pair of 4-65A's with 2KV on the plates, and since I have a smaller 50 watt modulator in the works with 5933's, this schematic seems to be all I need for both.  Got the last parts I need to build it in the mail today.

You'll note the Triad A9-J input transformer which I proposed to use as a balanced line input.  It is 50/250/600 ohms into 85K.  I'd rather use it for an input transformer for a dynamic mic.  I also want to be able to use a crystal mic (EV 911).  So.  Rather than put the transformer into the 6SJ7 driver grid, I'd rather put it on the 6J5 tube.

So here's the question.  If I substituted the Triad for the xtal mic input, what changes would have to be made with the 47K and 1 Meg input to the mic pre-amp 6J5 to adapt it for use with both crystal and dynamic mics?  I don't think I'd need an extra stage of amplification for the dynamic mic with the Triad transformer considering the ratio.

As for the "Line Input" unbalanced would work.  I'm getting ready for the build and the basic schematic for an "all purpose" 6V6 single tube module makes sense especially since I've decided to not use 811A's in the modulator.

[w4bfs]

typical dynamic mic outputs are about 5 mV and xtal mic outputs around 100mV (ymmv )  I think that the selector switch may wind up feeding the 6J5 grid .... if you want a line level input then into the second stage as drawn ...hope this helps and the oscope will remove all doubt

[W9ZSL]

Would I be able to get away with putting the transformer in the grid circuit of the 6J5 for dynamic mics and switch between crystal mic or the transformer or would I need an extra stage of amplification for a dynamic mic?  That's the question.  Since the Triad is a "step-up" transformer, would the output be close to that of a crystal mic?  I'd rather not add an extra stage of amplification.  My power supply is good for 90 MA.

[KA2DZT]

I would switch the input xfmr into the grid of the 6J5 instead of the way you have it in your schematic.  Even though the xfmr is a step-up ratio I think you'll need the extra gain of the first stage (6J5).  I don't think you'll need another stage.

You may want to add a DPDT switch on the input side of the xfmr to be able to switch phases of the microphone.  The male voice has higher peaks in one direction than the other.  You'll want the higher peaks on the positive modulation.  Same is true for the crystal mike, but it's harder to switch phases on an unbalanced mike.  The phase can be switched in the modulator by switching the plate leads on the 4-65s or the grid leads to the 4-65s.  Once you have the correct phase for the crystal mike, the phase my be reversed for the dynamic mike.  Having a phase reversing switch for the dynamic makes things easy.  The switch should be mounted close to the xfmr, keeping the leads short.

Fred

[W9ZSL]

Actually the original diagram shows the transformer on the second stage because it was intended to be used for a balanced line input.  My plan is to change the position of the Triad A9-J to feed the 6J5 with a dynamic mic.  A switch would be added to select between the transformer and my EV 911 crystal mic.  My question is, will there be enough gain from the 6J5 to allow me to use a dynamic into the Triad iron or would I need another stage ahead of the 6J5?

[w4bfs]

looking at a gross gain budget:   dynamic mic (5mV) x 12 (root of turns ratio) x 20 (gain 6j5) x 100 (gain 6sj7) x .5 (mid div on gain pot) = 60V

this will drive the pants off a 6v6 .... no additional gan needed ....does this help ?

[W9ZSL]

Works for me!

[WBear2GCR]

suggest limiting the screen current of the 6V6 via  a series resistor...

[W9ZSL]

This is what I'm talking about.  Input would be controlled by a 3P3T rotary switch.  Note when the unbalanced line is switched in, both input and the output post coupling cap of the 6J5 mic pre-amp would be shorted to ground.  There would be a cap in the unbalanced line input in front of the 500K volume control.

[W9ZSL]

Bear, what value would you suggest for a screen resistor?

[W9ZSL]

I should have included the power supply schematic.  Transformer is 700 VCT @ 90 MA, choke is 14 Hy @ 75 MA and I substituted 2 x 10 MFD for the two 8 MFD's in the filter. The original diagram does NOT have a screen resistor for the 6V6.  The diagram provided includes modifications suggested by you folks.

[KA2DZT]

Bear, what value would you suggest for a screen resistor?

You can put a 100-200 ohm resistor in the screen line.  Not critical, anything in that range can't hurt.  Depends on how high the B+ is on the 6V6.  I'll check to see what the max voltage is for 6V6 screens, you may need a higher resistor.

Fred

[WBear2GCR]

It's the max screen current, not the max screen voltage.
The voltage is spec'd but in practice the goal is to not blow the screens.

For example 6146s are notorious for blowing screens and grids.
Just an example.

The max screen current is spec'd.

I'd actually *measure* the screen current via the vdrop across the resistor in actual use.
The way to go is to push the driver to clipping and see where that current ends up.
Peak current counts. Because the effect is to glow the screen white hot. This can be seen with
a magnifying glass for a close view on tubes where the innards are visible.

The 100-200 ohm value is likely going to be close, but if ur running 700+Vdc on the tube, it will need to
be higher.

The max power will decrease, so IF you need more max power, pick a higher power tube for that hole.
Which is easy enough to do for this pinout tube... 6L6, 6L6GC, 6550, etc all fit in the same hole (as you probably know already).

I've used up to 1kohm, on an audio circuit where the B+ was rather close to that 700Vdc value... NOS tubes will handle a bunch more overload than new mfr or modern import tubes.

Hope this answers...

[KA2DZT]

Thanks Bear,

I think his power supply is in the 300-350 volt range that's why I suggested 100-200 ohms.

Fred

[W9ZSL]

Right Fred.  The speech amp driver will be a separate module with its own supply; 350 DC or so, no load.  Both the 50 watt and 200 watt modulators will have separate supplies.  5933's in the 50 watt will operate at just short of 700 volts on the plates while the 200 watt will have a pair of 4-65A's at 2KV.  The one speech amp will drive either.

[N2DTS]

Are there any parts I might have that you need?
Always wanting to help anyone who is building something...

[W9ZSL]

I have everything for this project but may need some things when I get to the RF deck.  I may need a oil cap for the power supply.  Will keep you in mind!

[N2DTS]

What voltage/capacitance do you think you will need?

[W9ZSL]

Tried to respond via email N2DTS.  It didn't go through.

[N2DTS]

qrz is right, so I have no idea what is up.
You can pm me here...