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Revamping A Modulator Input Circuit




 
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Author Topic: Revamping A Modulator Input Circuit  (Read 1577 times)
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N1BCG
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« Reply #25 on: April 16, 2018, 05:15:39 PM »

There's no question that this has been a fascinating project...thank you all!

I've been testing using Phil's original schematic (below) and have achieved the target 100V across a 100k load with a 300V supply @ 13mA, 5V of cathode resistor bias, and approximately 5.4VAC between the 6SN7 grids.

Is this fantastic or is there something I'm not seeing that's not right?


* Speech Amplifier 6SN7.jpg (70.13 KB, 694x366 - viewed 54 times.)
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PA0NVD
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« Reply #26 on: April 17, 2018, 01:30:53 PM »

Seems fantastic to me Grin
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DMOD
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« Reply #27 on: April 17, 2018, 08:03:30 PM »

Well, let's see. 5V Vk/470 RK ohms = 10.5 mA total circuit current for the two triodes which infers 5.3 mA plate current for each triode, which according to the tube curves is in the ballpark.

That would mean about 205 volts should be appearing on the plates and in a linear portion of the APC curves.

Now, 13mA total circuit current through 470 ohms = 6.11 volts Vk. 6.5 mA current through each RP yields a voltage drop of 117 volts, so each Vp should see (300 - 117) = ~ 183 Volts. This is also in the tube curve ballpark, and in a linear portion of the APC curves.

What are the actual plate voltages and component values now being used?



Phil

Addendum: May I suggest one more tweak to move the operation into an even more linear region:

 





* Speech Amplifier 6SN7 FLoating Secondary.pdf (37.78 KB - downloaded 14 times.)
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N1BCG
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« Reply #28 on: April 18, 2018, 12:36:42 PM »

Back in the saddle after a few busy days...

A lot was discovered by putting the circuit in the transmitter and running some real-world audio through it. First, and the most surprising, a *lot* less gain is needed. I was thinking 100V PtoP, but placing a VOM across the 6146 grids revealed that the needle wandered around the center of a 50V scale at full modulation. And that's using -6dBM of drive!  Padding, please!  Those four 6146s only need to produce 60-80 Watts of audio.

Other than that, things sounded pretty good. What's "real-world audio"? Andy Williams' "House Of Bamboo" for example. You all are too young to appreciate that ;-)


* IMG_7617.jpg (2547.57 KB, 3024x4032 - viewed 30 times.)
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PA0NVD
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« Reply #29 on: April 18, 2018, 01:23:34 PM »

Seem all correct to me, First of all, you need 50Vtt at each grid to get full max output, for 4 tubes 100W+ So if you need 60 - 80 Watts, approx 80Vp-p between the grids is sufficient which is approx 50V RMS. In addition, the driver design has about 100% signal margin to stay linear, so plenty of gain to drive the 6146 quad. I like the end result, you run the 6SN7 cool, long life and more reliable.
I advice to limit the drive in an audio stage directly after the mic., compressor limiter or so, Don't attenuate after the 6SN7 or lower its gain, the signal margin gives you superior audio quality.
Congrats with the result!!
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DMOD
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« Reply #30 on: April 18, 2018, 02:45:45 PM »

Ah, okay. A few clarifications...

This project is to effectively drive four 6146s where V1 and V2 are in parallel, V3 and V4 are in parallel, and those sets are in push pull. Thatís the modulator. The RF deck is four 6146s all in parallel driven by a Gates BC1G Oscillator/Buffer circuit...


You should be able to get approx. 240 Watts out to the antenna with a modulating power requirement of  ~ 130 watts for 100% modulation.

Neat project.


Phil
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N1BCG
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« Reply #31 on: April 18, 2018, 03:06:17 PM »

Congrats with the result!!

Hartelijk bedankt!

You should be able to get approx. 240 Watts out to the antenna with a modulating power requirement of  ~ 130 watts for 100% modulation.

That's where the power budget comes in as I'm now limited by the power supply. Are there any 6SN7 curves that permit a B+ of 150V? I'm running 250V now but I'd prefer to tap off the screen supply for simplicity. Remaining at 250V means adding a power divider from the 600 HVB+ or locating a second clamp tap for the 100W bleeder.
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DMOD
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« Reply #32 on: April 18, 2018, 03:25:24 PM »

Ho Hakiwisilaasamamo Waswasimamo

Here is another potential Power Supply using a Toroidal core transformer. I have used a number of these in various projects.

Two of the HV secondaries are paralleled to provide 1.24 A.

Phil - AC0OB

* 4X4 PS with 50k 100W Resistor.pdf (49.27 KB - downloaded 23 times.)
* 4X4 HV PS For CLark.pdf (54.35 KB - downloaded 19 times.)
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« Reply #33 on: April 18, 2018, 09:23:26 PM »

Should be able to run the speech amp off the 600V supply through a simple dropping resistor and a cap.  Another 15ma load on the supply shouldn't be much of a issue.
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« Reply #34 on: April 18, 2018, 10:48:55 PM »

Should be able to run the speech amp off the 600V supply through a simple dropping resistor and a cap.  Another 15ma load on the supply shouldn't be much of a issue.

But what do you do on KeyUp when the supply goes to 725V? Do you let the 300V rail go to 425V and have the tubes heat up?

We can't neglect the effect of the higher voltage on KeyUp.

One can regulate it with 0A2's or zeners.



Phil - AC0OB


* Series Gas Tube Regulator.pdf (41.77 KB - downloaded 13 times.)
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« Reply #35 on: April 18, 2018, 11:38:14 PM »

He runs the tubes at 5 mA each, 425 V Vd shouldn't be a problem.
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« Reply #36 on: April 19, 2018, 12:04:47 AM »

I was going to suggest a couple of 150 volt gas regulators in series.  In my HB xmtr the 300 volt supply is keyed along with the 600 volt supply.  My xmtr is designed in a way that only allows the voltages to rise less than 10% on key up.
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« Reply #37 on: April 19, 2018, 12:15:36 AM »

He runs the tubes at 5 mA each, 425 V Vd shouldn't be a problem.


Quote from: N1BCG
I've been testing using Phil's original schematic (below) and have achieved the target 100V across a 100k load with a 300V supply @ 13mA, 5V of cathode resistor bias, and approximately 5.4VAC between the 6SN7 grids.

By my accounting he's running each tube at 6.5 mA. I later suggested a move to 7.5 mA for each tube, for more linearity, by decreasing the cathode resistor to 330 ohms.

But that was not the point. The point is you have to consider no load voltages when working with power supplies and branch voltage rails.


Phil
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« Reply #38 on: April 19, 2018, 11:18:28 AM »

I agree that 7,5 mA will be a little more linear, that's about the current I suggested in the very first post. When the 600 Volts jumps to 725 with key-up, the Vd for the 6SN7 will jump from 300  to approx 380V, when you use a series dropping resistor, not to 425V, Seems no problem to me.
But off course, the solution with a stabilizer will be the cleanest and prevents plops due to charging/de-charging coupling cap's during the jump which I doubt are noticeable because the 6SN7 drives a balanced amp. I should just leave it as is, but that is my personal opinion, KISS
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« Reply #39 on: April 20, 2018, 12:25:06 AM »

The secondary of the input transformer needs to loaded to a point in order to flatten out the response otherwise the lows roll off at 100Hz and the highs show an upward curve past 16kHz.

I have found this true in experimental breadboarding. At one point I thought a transformer didn't care about being loaded, i.e. driving no grid current, why not just consider the ratio? No load is easier on it than a full or double load, right?

But as it turned out, the frequency performance was seriously affected if the load was not reasonable to the transformer's design. Alas, one of the imperfections of a 'perfect' transformer, and I mean good UTC parts not a $2 unit.

Transformer experts please comment as to why loading is important.
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« Reply #40 on: April 20, 2018, 01:13:51 AM »

May have more to do to the load on the secondary reflecting a load back to the primary and the source.
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« Reply #41 on: April 20, 2018, 01:55:59 AM »

The secondary of the input transformer needs to loaded to a point in order to flatten out the response otherwise the lows roll off at 100Hz and the highs show an upward curve past 16kHz.

I have found this true in experimental breadboarding. At one point I thought a transformer didn't care about being loaded, i.e. driving no grid current, why not just consider the ratio? No load is easier on it than a full or double load, right?

But as it turned out, the frequency performance was seriously affected if the load was not reasonable to the transformer's design. Alas, one of the imperfections of a 'perfect' transformer, and I mean good UTC parts not a $2 unit.

Transformer experts please comment as to why loading is important.

http://www.kandkaudio.com/wp-content/uploads/PSW_WhitePaper_Download_Chapter_6.pdf

See Figures 1,2 and 3.

Effectively you are broad-banding it by loading it.

But having to put a very low resistance load across an audio transformer secondary indicates to me that the transformer quality is questionable and that it has excessive internal capacitance.

The reason I use these transformers:

https://www.edcorusa.com/wsm_series

in the 600 ohm to 600 ohm (1:1) series is that they have very low internal capacitances, center taps, and good frequency response without having to load them excessively.

Another approach is to do something like this for a transformer without a centertap;



Phil



* Speech Amplifier 6SN7 Floating Secondary CT Bias.pdf (37.64 KB - downloaded 14 times.)
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« Reply #42 on: April 20, 2018, 08:51:54 AM »

It is indeed an old problem of spread inductance and capacitance. In the years 20 Philips did wind tube coupling transformers with resistance wire to flatten the resonance!!
I always load the sec until the high resonance is hardly noticeable, not more. If you want more low, lower the DRIVE impedance to the prim. The lowsof a correct loaded transformer roll off due o the inductance of the prim which you can overcome by driving at a lower impedance.
I suppose you are not looking for highs up to 20 kHz. otherwise you need more expensive transformers.
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N1BCG
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« Reply #43 on: April 20, 2018, 05:24:35 PM »

When Murphy strikes, he's calculated and effective...

I had the new 6SN7 driver all wired up in the modulator chassis and was giving the transmitter a dynamic test when I noticed how thin and tinny the audio was. Hyellified, I started testing audio at each stage after the processor. It didn't take long to find out that my *prized* 600:15k input transformer now had an open secondary.

So, that's that until I can locate replacement iron. Fortunately, it's small iron...
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« Reply #44 on: April 20, 2018, 06:13:14 PM »

When Murphy strikes, he's calculated and effective...

I had the new 6SN7 driver all wired up in the modulator chassis and was giving the transmitter a dynamic test when I noticed how thin and tinny the audio was. Hyellified, I started testing audio at each stage after the processor. It didn't take long to find out that my *prized* 600:15k input transformer now had an open secondary.

So, that's that until I can locate replacement iron. Fortunately, it's small iron...

Ok now I understand why you were getting such a high gain and had to pad the secondary. I recall you mentioning two different input transformers but I thought you were using the 600:500 ohm transformer.

You had a 5:1 step-up situation. +8dbm in would have resulted in 10 volts at the grids of the 6SN7.

That 15k secondary would have a lot of distributed capacitance and inductance.


Phil

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« Reply #45 on: April 21, 2018, 10:44:42 AM »

Some ideas
Many Philips radios from the years 50 / 60 have small AF output transformers that have a negative feedback winding for approx 500 Ohms There you have 500 - 7 kOhms. I used these, worked very nice
Microphone transformers, sometimes in the base of CB table mic's
I may have one for you, but my stuff is still in my sea container until my house is ready. So that is the least probable at short notice
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N1BCG
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« Reply #46 on: April 21, 2018, 12:07:41 PM »

Nico, bedankt! I found and installed another with reasonably good specs when loaded carefully. I also discovered that the power transformer secondary has an unused center tap, so with the full-wave bridge, that would make a useful 300V source.

Love those Philips radios, and Grundigs and Lowe Optas... heel goed!

My next task is to determine why adding negative feedback causes the screens to go into super-conduction regardless of polarity. Supersonics?

Troubleshooting homebrew equipment designed and built 50 years ago can be quite a challenge!
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« Reply #47 on: April 21, 2018, 12:14:03 PM »


...My next task is to determine why adding negative feedback causes the screens to go into super-conduction regardless of polarity. Supersonics?

Troubleshooting homebrew equipment designed and built 50 years ago can be quite a challenge...

Drawings, schematics? Modulation section? Speech Amplifier?


Phil - AC0OB
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« Reply #48 on: April 21, 2018, 05:14:29 PM »



My next task is to determine why adding negative feedback causes the screens to go into super-conduction regardless of polarity. Supersonics?

May be RF oscillation
That's the reason I don't like neg. feedback, especially when there are inductors in the circuit. Just get rid of decoupling caps across cathode resistors, that gives you a stable and nice negative feedback per stage. I never used neg. feedback in my HI-FI tube amps other than that.
Fine to hear that you found an other iron.  Cheesy
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« Reply #49 on: April 21, 2018, 06:50:15 PM »

when you use the center tap of a full bridge loaded transformer to have a lower DC voltage, DON'T forget to put a diode in series with the center tap!!
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