Speech Amp / Driver

[W9ZSL]

We've been here before regarding using a single 6V6 and a UTC S-8 to drive a P/P pair of 811A's.  There was a lot of discussion as to whether the S-8 had a high enough rating to do the job.  Meanwhile, I've come across a Stancor A-4416 which is 3-10K CT primary, 40 MA / Side 5:1 Pri to 1/2 Sec ratio.  There is a speech amp driver in the 1949 ARRL handbook where this iron is used with a 6SN7 as a driver.  The modulator described there is a 6L6 job.

My modulator will be a pair of 811A's.  Given the wide primary impedance of the 4416, what if I were to use a pair of 6SN7's paralleling the triodes in each tube externally on the sockets, and then putting 2 + 2 triodes in P/P?  It would be like using a pair of 6B4G's.  I could then drive those with a 6C4 through the UTC S-8.  If you checked the 1957 handbook page 256 you would see what I mean.

Of course I could use a pair of 6B4G's, but hey are expensive as hull and I have several 6SN7's.  The driver in the handbook with the 6B4G's would have more than enough poo to drive the 811A's, but if a pair of 6SN7's could do the job operating as 4 triodes, why not try it?

[w4bfs]

there is merit in your premise ... airc a pair of 811 will need 5 or 6 W of drive as a minimum with 10W being a better number .... calculate the min impedance from peak swing and drive power ... these numbers are generally just a few hundred ohms .... reflect this impedance back to the primary by the square of the turns ratio .... this is generally a few k Ohm .... whatever you choose for the driver tube will need to be able to deliver req'd power at that load Z ... I would breadboard your parallel push-pull 6sn7 driver into a substitute resistive load and measure performance ... I suspect it may be a bit light ... something a bit bigger such as a triode connected 807 could fit the bill ....73

[N2DTS]

Is there a reason why you want to do it the hard and low fidelity way?
The more iron in the path, the more phase shift and frequency response issues.

What is the going price for 6b4's?

[W9ZSL]

6B4's are going for around $100 a pair. They can drive a pair of 811A's no sweat. It's just a thought right now.  I know the more iron the worse the fidelity.  I've been in the sound recording business since 1966.

Here is the schematic.  The 6B4's would be replaced with 6SN7's with each tube's triodes wired in parallel on the socket.  The output iron would be a Stancor A-4416.  I'd increase the value of the coupling caps above .003.

[Opcom]

Instead of two 6SN7 or screen grid tubes, why not consider the 6N7, a common-cathode zero bias dual triode, will do 10W to an 8K p-p load @300V and drive those 811's for cheap. I'm surprised it is often overlooked, but is designed for ease in this application.

Two in parallel are not necessary but can be used with no fuss for a little extra current on them 811 grids, drop the maul! Matching is not necessary with them but a 33 Ohm cathode resistor can be used in each one. It only takes about 1/2 Watt to drive a push pull 6N7 all the way.

Drive the push pull 6N7 with the smaller transformer and the 6V6 in triode connection or better yet with a second 6N7 as stated in the datasheets.

There are many useful 6N7 circuits, including driving one set up for zero bias push pull with one set up for class A. The datasheets spec this. There is also a RC amplifier data for it.

I bought 10 on ebay for $4 each. They come also in GT style but the metal might be better for in an RF area. Great tube, the 'phools don't care for them, and cheap. Tons of circuits show up in google images for 6N7 amplifier.


-Opcom, the 6N7 ambassador

[N2DTS]

So you do not want to drive the 811 grids with an 8 ohm amplifier and want to go old buzzard.
I think I have a bunch of 6b4's, I will have to check when I get home.
Got anything to trade?

Can anything be done with 6080's?
I have a lot of those.

[W9ZSL]

That 6N7 set up looks really interesting!  I think I actually have some of those and the circuit is way simpler than anything else I've considered.  Questions.  The input shows a mic transformer, but since this is for mobile use, I'm assuming the battery shown can be eliminated with the filaments powered off the 6.3 winding and can a dynamic mic be used?  That R1 variable would seem to be unnecessary if the PS was similar to the one shown in my 6B4 schematic.  For all that matter, it seems to me a single 6N7 could be used in place of a pair of 6B4's?

[VE3LYX]

I use a 6bl7. 6SN7 pinout (BBD.) Sweep tube power. If using minatures I use 12bh7.
don

[W9ZSL]

Looks like there are a few better choices for those output tubes other than the 6B4G.  After checking out the specs, the 6N7 looks like a good bet!  The Stancor A-4416 will handle 80 MA total on the primary and it appears a single 6N7 in class B would pull around 70 MA with 300 V on the plates for a solid output of 10 watts.  Using the original diagram with the 12AX7 and 6C4 into the UTC S-8 followed by a single 6N7 operating P/P rather than a pair of 6B4's would probably work just fine.

[w4bfs]

use as low of a plate resistance as possible in class B driver service to minimize distortion generation ...

[WD8BIL]

Replace T1 with a phase inverter.

http://www.amwindow.org/tech/htm/rangmod.htm

[W9ZSL]

Then there's always this one.  Difference would be substituting one 6N7 for the pair of 6B4's.  This uses 811A's in the modulator.  Any suggestions for better frequency response like changing the value of the .01 caps in the inverter output?  The mic pre-amp cap as well?

[N2DTS]

You can make all the electrolytic caps bigger, and remove the one in the mic preamp cathode.

[N2DTS]

I have three 6B4's.

[W9ZSL]

N2DTS, what would you want for them?  I have a lot of parts.  You mentioned swapping.  Anything special you can use?
Mike

[N2DTS]

I will test them and let you know if I come up with anything to swap for or a low price...

[w4bfs]

Then there's always this one.  Difference would be substituting one 6N7 for the pair of 6B4's.  This uses 811A's in the modulator.  Any suggestions for better frequency response like changing the value of the .01 caps in the inverter output?  The mic pre-amp cap as well?

imo this is a strange design ... the push pull 6N7 is class a and can be done with a 12AT7 since each half only has 1.5 mA current available ... this cannot provide but a tiny drive current to the 6B4s following which are self-biased into class ab with cathode bias .... I don't believe it works but to a point without clipping the waveform, which may be the designers intention ... reminds me of a military way of doing things

[VE3AJM]

Good deal with the 6B4 tubes.

Here's another 6B4 driver circuit using a 12AX7 and 6C4.

Al VE3AJM

[W9ZSL]

Yeah Al, that's the same diagram I attached early-on in the discussion when I suggested using a pair of 6SN7's with plates and grids of each tube tied together running in P/P with the other tube.  Since 6B4's are basically two plates and two grids tied together internally, I thought I could accomplish the same thing with 6SN7's with the connections external on the sockets.

After that we discussed substituting a phase inverter in place of the inter-stage iron.  I have the NOS UTC S-8 which would perfectly for that purpose.  That's where the last diagram with the 6N7's came in.  Back to square one?

[N2DTS]

Are you really going to build something?
And use it on what?

I ask because I have a UTC LS something or other, push pull plates to push pull grids at about the 20 watt level I have no use for.
I will get the number when I get home.
It would be nice if someone actually used this stuff...

[VE3AJM]

Hi Mike

Okay on your original circuit. The 6N7 phase inverter looks cool eliminating the interstage xfmr. Any of those circuits will work. Fun to experiment. I believe the external Johnson speech amp for the Desk KW used a PP 6B4 audio driver. The 250-33?

Build one of them up and try it out with the 811a modulator.

Al VE3AJM

[W9ZSL]

I've been planning on building an AM / CW / SSB (?) amp since forever and have been collecting parts just as long.  Then I came across a UTC S-22 mod iron so the final design is being based around it.  The speech amp / driver will be separate.  The mod deck shows the filament xfmr on the left and a 500 ohm line to class B grids Kenyon on the right as an alternative switchable input choice in addition to the modular driver.  That along with switching and a meter will be the only components on the mod deck. The modulator power supply will deliver either 1250 or 1000 VDC @ 300 MA under load. The 811A's will be toward the front and the iron in back.  Filament iron will probably be under the chassis on the right with the Kenyon top left. The SS rectifiers will be mounted directly on the chassis eliminating that assembly with the insulators which originally came from a Gates BC-1T tranny.  The RF power supply is similar also using Thordarson iron.  Both supplies will be on the bottom of the rack.

Mike

[W9ZSL]

FB on the UTC iron Brett!  Bob, W4RFM, is working on a modulator built around the UTC S-22 with a driver based on an Altec A-333A power amp using a pair of 6L6's.  The output is around 20 watts so a driver iron of that rating would be perfect.

I dug up the full schematic for the Altec and it looks like a solution for my driver as well.  Exclusive of the output transformer, the amp is basically flat 20-20 KHz.

Of course, this doesn't include a mic pre-amp, but that is fine because I have that covered.  Looks like this is the way to go.  Problem solved.

Won't need the 6B4G's, but thanks for the info!

[N2DTS]

The 6b4's tested good, the trans is an LS 22.

Push pull plates to push pull grids, split primary and split secondary.
30K ohms plate to plate primary, 80K secondary, +28 dbm.
20 to 20K Hz.

[W9ZSL]

Brett, I'm familiar with that iron.  I just wonder about matching.  I've decided not to go with the 6B4G's because the Altec A-333A amp Bob (W4RFM) is building using a pair of 6L6's makes a lot more sense.  That amp is rated at 20 watts, more than enough to drive a pair of 811A's.

Here is my problem.  The LS-22 would handle the power and the frequency response is out of the ballpark.  But would it match for the application?  I've attached the driver schematic.  Secondary would just feed a pair of 811A grids with a plate voltage of 1250 DC max.  Little or no bias.

Next problem would be the price.

If I can build a relatively hi-fi modulator around a UTC S-22, FB.  I've been planning this project for over 50 years.  A few days more won't matter.