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Modulator audio schematic




 
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N4LTA
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« on: June 30, 2020, 09:27:06 PM »

Here is a rough schematic of the 60 watt class AB1 modulator that I am building. it uses twin 807s running at 700 volts on the plate with a 300 volt regulated screen supply.
I hope to get 60 Watts to modulate a pair of 3C24 Eimac triodes on 75 meters.

Any errors or changes that need to be done would be greatly appreciated.

best 73

Pat
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KI4YAN
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« Reply #1 on: June 30, 2020, 11:25:06 PM »

Here are a few references for push-pull 807 audio designs.

http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0702.pdf

http://www.pmillett.com/807pp_amp.htm

https://www.bonavolta.ch/hobby/en/audio/807.htm


This one is from the 1954 ARRL Handbook:

http://viewoftheblue.com/misc/807_1.png and http://viewoftheblue.com/misc/807_2.png

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N4LTA
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« Reply #2 on: July 01, 2020, 10:47:24 AM »

Thanks for the info.

Pat
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K8DI
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« Reply #3 on: July 01, 2020, 06:09:54 PM »

Pat, reflecting on your schematic, a few things to ponder --

You probably should add screen resistors and bypass caps as seen in some of those references posted. Also, a few .01uf caps sprinkled about to keep stray RF from getting into things would be good practice.

That interstage transformer has dual secondaries. You can therefore have two bias pots and set each tube individually. Whether you do this or not, you need to AC ground the center tap/bias point with a bypass cap (like the cathode bypass on the preamp). The idea is to make sure the audio has "something to push against" rather than the other grid. In order to set bias as a tube current rather than a grid voltage, add a 1 ohm resistor in the cathode of each 807 -- then a meter set to millivolts will read milliamps when placed across it. The resistor can be a half-watter, even at tube melting plate current levels that's plenty (500mA=0.25 watt dissipated in that 1 ohm resistor).

That transformer has dual primaries. It would be relatively easy to use a dual section triode in front of it, and make the whole thing differential/balanced....If your audio chain is already balanced, this is nice. If you're saving space, 12ax7/au7 types come cheap, if you like old school, 6sn7..or even 6n7..

Like I said, stuff to ponder...

Ed



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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
N4LTA
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« Reply #4 on: July 01, 2020, 08:33:32 PM »

Ed - FB on the resistors and bypass caps for the screens.

The dual triode sounds interesting and I would like to have a balanced line input. Can you point me to an example schematic? Balancing each tube  with a separate bias pot sounds like a good idea and easy enough. I'll probably make a small bias PC board and will add another pot and bias output.

Also the ARRL article has a Ohmite Z50 chokes in each of the 807s grid circuit. I had about ten of them but they were destroyed in the fire. They look hard to get now. Wonder if a parasitic suppressor like a 47 ohm 2 watt carbon or ceramic comp resistor with 6 or 8 turns of wire would work. Or are they really required. I would assume that they are there as parasitic supressors?

I'm about to finally punch some metal in the apartment and make my wife mad. Can't wait to get back to the house and shop.

Thanks

Pat
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K8DI
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« Reply #5 on: July 02, 2020, 12:10:21 PM »

The dual triode sounds interesting and I would like to have a balanced line input. Can you point me to an example schematic? Balancing each tube  with a separate bias pot sounds like a good idea and easy enough. I'll probably make a small bias PC board and will add another pot and bias output.

Also the ARRL article has a Ohmite Z50 chokes in each of the 807s grid circuit. I had about ten of them but they were destroyed in the fire. They look hard to get now. Wonder if a parasitic suppressor like a 47 ohm 2 watt carbon or ceramic comp resistor with 6 or 8 turns of wire would work. Or are they really required. I would assume that they are there as parasitic supressors?

I have to find some of the circuits I've seen back..  In the mean time, here's a couple of mine to look at. The sketch is more what we're discussing here; the pdf is what I built last year. It works really well as a modulator, driving a Kenyon transformer...

I think the Z50 and the resistor/coil combos are supposed to serve as suppressors, along with plate parasitic suppressors, often seen in 807 audio amplifiers. The idea is valid, but I don't know that it's really necessary...it's common to have a resistor in series with the grid to swamp it; I'd try that and test first...

Ed


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* AM-40-Modulator-Section.pdf (27.31 KB - downloaded 24 times.)
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
N4LTA
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« Reply #6 on: July 02, 2020, 01:58:53 PM »

Ed

I really appreciate the drawing. I am changing the input to a balanced input and going with a 12AX7.

Also will change the bias to your circuit.

Information like this is what I love about this board. Continuing to improve the old vintage circuits.

Thanks and best 73

Pat
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K8DI
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« Reply #7 on: July 02, 2020, 02:22:05 PM »

My sketch should have the bias pot wiper to the transformer and the end now on the transformer connected to the bias supply — as drawn turning the bias to zero shorts the bias supply/burns up the pot. You’d likely never turn it there but best to not have it be possible...

Ed
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« Reply #8 on: July 02, 2020, 02:33:16 PM »

Just looked and the Hammond 124B doesn't have a dual secondary. Any ideas where I might find a transformer that does?

Thanks

Pat
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K8DI
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« Reply #9 on: July 02, 2020, 02:46:06 PM »

That’s what I get for clicking on the spec sheet link on the Hammond website...and only looking at the picture of the dual dual example, which is not any of them...

There’s the others in the series but they have plain cores vs the Nickel in the ‘B version

Ed
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N4LTA
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« Reply #10 on: July 02, 2020, 08:32:39 PM »

I'll find something. I have an offer on an EBAY transformer

Pat
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« Reply #11 on: July 04, 2020, 01:30:49 AM »

Noticing the parasitic comments, In the RCA MI-12188A 70W theater amp that runs a quad of them in almost class A at the ragged edge and often shows color on them idling, the plate stopper is 47 or 56 Ohms, the screen stopper is 100 Ohms, and the grid stopper is 1000 Ohms. All are 1/2 Watt carbon resistors. It's a very simple arrangement and those amps are squirrely on peaks, especially near clipping, without them.
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N4LTA
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« Reply #12 on: July 04, 2020, 04:57:49 PM »

I'll probably use the stoppers as a safeguard.

I ordered a couple of Hammond 124 E  driver transformers from Hammond and should get them late next week.

Pat
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WBear2GCR
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Brrrr- it's cold in the shack! Fire up the BIG RIG


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« Reply #13 on: July 04, 2020, 05:43:16 PM »

Minor stuff...

The line level input pot would be better at 50kohm or 100kohm.
I'd also drop a resistor to ground from the grid of the input tube for
when the pot decides to open... 1meg will do fine.

807s *really* like regulated screen voltages. Also the screen
voltage - if you test it, actually has an optimum voltage for best
performance. (Thus the regulation comment). But a tight supply
for the screen is good, and don't just guess at the screen voltage.

The series screen resistor is important to LIMIT the max screen current.
Choose wisely.
The 807 is robust, but too much screen current will shorten the life of the tube.

Grid stoppers, always a good plan.

Some schemes involve driving the screens + grids, tied, or in proportion to get a
triode like result. Other schemes driver the screens, not the grids...

EDIT: Just read the RCA Hamtips? Real winner there... also could be set up for
cathode follower drive. @750v they're calling for 125watts! Zowie! It's a modified
triode connection with the drive to the screen and to the grid via a resistor.

                 _-_-bear

Depending on what the driver tube is doing, IF the 807s go into Class 2, you may
find it good to have a resistive load on the secondary of the driver transformer, and
enough current available in the driver tube. Once it flips from Class 1 to Class 2 current
is being drawn and the impedance the transformer sees goes from extremely high to
not high at all. So unloaded condition to loaded.
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N4LTA
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« Reply #14 on: July 16, 2020, 06:22:04 PM »

Getting around to wiring things up. I will use a 12AU7 instead of the 12AX7 since it' s triodes are similar to the 6C4 that I originally used.

I hope to start testing in a few days and have a 6500 ohm 100 watt resistor that I plan to use for testing. I'll mount it to a large heat sink.

The 807s are closer together than I planned but I don't think this will cause problems. I am shooting for 55-60 watts and they are capable of twice that.

Hope to crank up the power supply and see where the voltage goes. I have several bleeder resistor values to try. I have found that the Hammond chokes swing a bit at low current, so hopefully I can use larger bleeders and save heat and transformer capacity.

Pat
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« Reply #15 on: July 17, 2020, 11:53:50 AM »

I would like to add a gain control to the balanced input.

Is this the best way to do it?

The two pots are ganged

Thanks

Pat
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KI4YAN
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« Reply #16 on: July 17, 2020, 02:37:38 PM »

How much gain do you have in the next stage? Might be better off to put the 12AU7's in as cathode followers, basically a power attenuator at that point. Then adjust up the gain of the next stage.

If the stage gain is a little lacking in the next stage, then what you have will do a good job to add extra gain.
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