new to me 811A modulator project

[kb3ouk]

the combined resistance of the resistors i used (big ceramic ones) plus the speaker all together equaled about 3700 ohms, which is about what i calculate the final will run at. The design i used was the the same thing that it was when i got it, that i posted before, just with a few modifications, mainly to the power supply. The only mods I did to the audio portion of the modulator were removing the cap across the primary that is in the original schematic, and changing the driver transformer out from the original Inca unit to a Hammond 70v matching transformer like what was suggested earlier on. As far as the power supply, I made it a solid state full wave rectifier, with a 10 henry filter choke and about 50 uf of filter. The original power supply had a two section filter with a 5R4 full wave rectifier.

[Opcom]

If you want a true test, then after using the resistors and determining the best load and if you can make your RF stage act like that load or close to it, then use the RF stage tube and HV supply as a variable current source and load (by controlling its bias) and draw the carrier level DC current through the secondary and test the frequency response and power bandwidth again. Then you can see what it will act like in use which may be quite different as in worse. If the difference is too great then you may want to limit the audio power and frequency response or to use a modulation reactor to avoid having DC current in the secondary at all.

[WBear2GCR]

I'm trying to explain that the combined impedance of the resistor + speaker is moot.
That's just not a proper way to determine anything.
Whomever suggested this, is barking up the wrong tree.

The method I explained is far better.

What Patrick said about running current through the secondary is quite valid as this will effect the
saturation of the core. (if you use my connection method to monitor, then you MUST use the cap, and rate it
according to the voltage supplied to the secondary.

Regardless, monitoring the audio by directly connecting a low Z speaker is simply not a good way.

I'm surprised that 3700ohms is proper, perhaps PER tube?? Seems a bit low, since 811a run at 850+ volts typically
want to see more like 8k-10kohms. But if the tube manual says it's ok, then fine. Seems low to me. You'll be shy power and monkey swing if the match is poor.

                     _-_-

PS. the URL for your link is NG - what was it supposed to show??

[Opcom]

If devlishly serious about a modulator, just get a 1570B and add the modulation transformer across the entire secondary of the amp's OPT. The amp is well tempered by its own OPT and feedback loops and the modulated load will be as well regulated. If the 175W amp has extra power not needed for the class C stage, swamp it if desired or to make match.

I do a similar thing with a 150W Stromberg Carlson AP-80 (PP-par beam tube amp) used to drive 3-500Z grids, which is a much nastier and less linear load than any class C stage. It works so well I have not bothered to put in the 1570B amp I had previously prepared for the job.

[kb3ouk]

That link was put in accidentally by me bumping the keyboard. No one suggested that I series up the resistors and speakers, I don't know where you got that from, I did that on my own because I needed a quick way to test the modulator and see how well and if it was working. If I wanted a hi-fi modulator, I would've built one and not even screwed around with this but I got what I wanted to experiment with, and built it as such. The whole purpose from the beginning was to build a communications quality modulator to use for a homebrew transmitter.

And regarding the impedances I used, the 3700 ohms is the calculated impedance of the 814 finals I will be using, the impedance ratio of the iron matches this to the impedance of the 811s, which according to the datasheet is 5100 ohms at 750 volts.

[N2DTS]

Its a good idea to build a hifi modulator even if you do not intend to run it that way.
It will be clean and sound good no matter how you run it.

[VE3AJM]

Sounds like you're making good progress with this 811a modulator project.

As you mentioned, and with what you're working with there, you don't want/require hifi? response from this project. Some of the hifi proponents here don't sound so great on the bands when I receive them. Weak signals with mushy bassy/no sibilance type audio response, that don't cut through very well. Globe Kings and B&W 5100s with stock audio sound fine. Listen to Russ WB3FAU or Frank K3SQP. The Globe King with the terrible 6L6 single-ended audio driver and over coupled final, and the 5100 with the 6146 modulators.

Hear you on 40m over the holidays.

Al VE3AJM

[WBear2GCR]

That link was put in accidentally by me bumping the keyboard. No one suggested that I series up the resistors and speakers, I don't know where you got that from, I did that on my own because I needed a quick way to test the modulator and see how well and if it was working. If I wanted a hi-fi modulator, I would've built one and not even screwed around with this but I got what I wanted to experiment with, and built it as such. The whole purpose from the beginning was to build a communications quality modulator to use for a homebrew transmitter.

And regarding the impedances I used, the 3700 ohms is the calculated impedance of the 814 finals I will be using, the impedance ratio of the iron matches this to the impedance of the 811s, which according to the datasheet is 5100 ohms at 750 volts.

Well, regardless, the method essentially does not work.

The speaker wants a low Z source, especially on bass.
That is the thrust of my suggestion. My suggestion was to permit you to know how to find out what your modulator is doing.

Dunno about the load Z that the 814s present, but from what you just said the load you use on the 811s should be 5100 ohms IF you were using the resistors in place of the transformer, which is what your text seemed to say.
If it was on the secondary, then fine. But still you want to use a divider to drive another amplifier or similar to "listen" to your modulators output.

I would expect the speaker to distort badly running it from such a high Z source.

                _-_-bear

[kb3ouk]

I don't know what is so hard to understand about what I am doing with the resistor/speaker hookup. The transformer is in the circuit, hooked up exactly like I would use it with an RF deck, except instead of an RF deck across the secondary, there are a pair of resistors to absorb most of the audio power, and the speaker to reproduce sound. All I was concerned about was that the modulator did actually make power, pass audio, and didn't have any power supply hum. It met all those criteria on that quick and dirty test, the real test will be when I get the RF deck built and actually try it out under real world conditions.

Sounds like you're making good progress with this 811a modulator project.

As you mentioned, and with what you're working with there, you don't want/require hifi? response from this project. Some of the hifi proponents here don't sound so great on the bands when I receive them. Weak signals with mushy bassy/no sibilance type audio response, that don't cut through very well. Globe Kings and B&W 5100s with stock audio sound fine. Listen to Russ WB3FAU or Frank K3SQP. The Globe King with the terrible 6L6 single-ended audio driver and over coupled final, and the 5100 with the 6146 modulators.

Hear you on 40m over the holidays.

Al VE3AJM

I've noticed that too, a lot of bone stock rigs sound much better than the modified ones do. Sure the modified ones sound good too, when the bands are in great shape, but when conditions start to go south, the stock ones seem to have the ability to cut through the crud a lot better.

[N2DTS]

I have no high frequency hearing left, so most people sound stuffy and I enjoy d104 audio into a 1 or 2 meg resistor.
The thing is, if the modulator will pass 50 Hz it will not distort at 300 Hz.

Nothing says you have to run something to 20 Hz but if it passes 20 Hz, then 200 Hz is going to be clean and look like a sine wave.

We have all heard rigs that sound great with a D104 plugged into them, and some that sound bad, why not sound good?

I do not think you can tell what something is going to sound like till its in use.
DC through the mod iron, plate and screen bypass caps, even tank circuit Q and bandwidth might come into it, every rig I have sounds different..

[WBear2GCR]

I don't know what is so hard to understand about what I am doing with the resistor/speaker hookup. The transformer is in the circuit, hooked up exactly like I would use it with an RF deck, except instead of an RF deck across the secondary, there are a pair of resistors to absorb most of the audio power, and the speaker to reproduce sound. All I was concerned about was that the modulator did actually make power, pass audio, and didn't have any power supply hum. It met all those criteria on that quick and dirty test, the real test will be when I get the RF deck built and actually try it out under real world conditions.

It's easy enough to understand, but you complained about bass distortion...
...I said your test is not valid to see if you have a clean output or not, especially bass.

What I suggested does give you a pretty valid test. Of course weirdness in the plate of the RF output
tubes could cause unforeseen issues, but probably none will emerge.

Sorry that we're not quite on the same frequency.

[W4RFM]

Bear, a little side question, what is the value of that cathode choke on the Altec 1570B circuit. I was going to build that once upon a time, but could not find that information.

Now...back to the thread. 

[WBear2GCR]

The choke is rather non-critical since its role is to look like a high-Z at audio freqs, but it does serve to provide a DCR for "self bias" (if that is used) or a pass through for a bias voltage. You can see that as you go lower in frequency the inductive reactance present equates to an equivalent resistance. So, you can figure out the minimum required inductance based entirely on the equivalent resistance value needed to make the follower work "properly". The minimum value is going to be the DCR, but that is somewhere around 1Hz.

In practice the primary of a more or less random 5-10watt output transformer will have all the required parameters. Since it is a choke, generally speaking the quality of the transformer which would matter when used as a transformer (the HF performance especially would be of interest) makes little difference. But similarly to how the transformer fares at LF, where the inductance dictates the LF ability, it does as well here - but the impedance at the cathode is low vs. the impedance of the plate, which makes the inductance matter in similar but not quite the same way.

Also, since the "choke" is inexpensive, you can simply throw something in the hole and see how well it works at LF, and if it is ok then keep it, if not up the power handling (core size) and/or inductance.

It's neat that way.

The other aspect is that as one goes higher in freq the reactance increases, making the follower work closer to "ideal" - there are circuits that use resistors that once can compare with.

                    _-_-