The AM Forum

I am taking a break from restoring the Globe King, to build a 250 watt modulator. (Yes I am trying again !)
Anyway, this is the layout I am thinking about. trying to keep the input audio as far as possible away from the AC transformer for the low voltage.  Also, a given is to have a window in the front panel to watch the 811A's glow. Your opinions and criticism would be appreciated. Oh, the 8 ohm transformer can be switched in to replace the driver transformer, so's I can hear the low level (20 watt) stage of the modulator for testing and adjustments.
The low level audio stage is actually an Altec A-333 hi fi amp cloned.

Octals, yum!

Deliver me from PWM!

Besides, it doesn't start decent fires when it high orders anyway.

73DG

Except for the filaments 811s shouldn't be glowing too much.  I don't like the layout.  I would move the 811s back and put all the low level audio in the front of the chassis.  Move the choke and filament xfmrs to the back.  The 811s can be near the power stuff but the input xfmrs should be in front away from the choke and other xfmrs.  The driver can be next to the 811s.

These are my first thoughts with what you're building.  You'll get more opinions, so lets see what others think.

Fred

That is a lot of iron!
Go screen modulation and skip all that heavy stuff...

I often put the mod transformer on a shelf, or on the bottom of the cabinet along with big power transformers.
No way I am lifting and rack mounting anything medium power with all that iron on it.
In the past, I built an 811 modulator and put the tubes on a small sub chassis bolted to the top of the front panel and put glass above it so you could see the tubes but did not have to cut holes for windows.


Here is my 811 modulator and the mod iron.
Went screen modulation for now...

N2DTS I like that idea! Did you use borosilicate/pyrex or find it unneccessary? Was the glass drilled?

I prefer the heavy iron near the front panel so the leverage against the weight is better and less stress on the chassis -unless you got heavy steel.

A modulator project seen in an old handbook attached the modulation and filament transformers horizontally to the back of the front panel so no chassis was needed for a basic push pull stage with tubes also mounted horizontally.

The 811A plate shows a barely perceptible red color at maximum ICAS conditions. The 811 filament looks nice and is worth a view. If the modulator craps out, you can see right away if a filament is gone or of the tubes are going orange-hot due a shorted load or something.

Patrick,
I like the iron and high level plate modulation is good stuff, Loy Barton of Univ of Arkansas (and later at RCA) knew what he wanted when he developed it. I think your layout is a bit crowded though, maybe you could use a deeper chassis or put some of the smaller iron underneath? Firebottles near the window are nice, like watching fire in a wood stove or campfire! You are inspiring me to finish my dual 810 modulator, have all the parts, the chassis and the rack to hold it. Its got an RCA 250 watt mod. transformer and reactor from a BTA250M. Project got sidetracked over 5 years ago, after layout was complete and some holes made. But alas, I have too many hot irons already.

I have moved things around several times, trying to come up with an optimum plan.  I already plan the Hi voltage and mod reactor will be on the next chassis below, maybe I should move something there.  However, I do not want to open the door to oscillations due to long lengths of some connections. I will think on this over the weekend.
Thank you all very much, an unlimited set of eyes and brains certainly helps.
Bob :)

My suggestions:

move the regulator tube to the center, next to the LV xfmr.
move the mod iron back.
slide meter to one side or the other.
slide 811s closer to the mod iron (perhaps centered in the cabinet)

LV choke should be next to the LV xfmr.



I see no need to put the low level stuff at the front.
That's what shielded cable is for. Worst case put a preamp tube at the front, and put the rest to the rear.

The 811a should be xfmr or cathode follower driven. Apparently it is.
The driver xfmr should be next to the 811s. about where the LV DC choke is now.

They only need about 5-7watts (max) to drive. 6W6 are fine, 6V6 are more than needed but fine.

Not sure what the 8ohm transformer does?
Are you planning on external drive via a mic pre/power amp AND a mic input??

                          _-_-

Here is one modulator I did that way:

http://n2dts.smugmug.com/Ham-radio/i-z345zrf/A

Small chassis bolts to the top of the front panel and has a piece of regular glass in front of it.
Cost about $3.00 at the glass place, I just put electrical tape on the side edges for the screws to pull against, but the make U shaped edge stuff as well, in plastic or rubber.

A glass place will cut you anything you want and could likely drill holes in it.

The 811 modulator in the pic before has no window because they do not show color and I do not run them even remotely close to max power outputs.
Modulators run best at half power.

Here is a different approach, I will move all the DC to another chassis 1250 and 450, and only leave the filament transformer on the same chassis for short leads, etc.
The 8 ohm transformer will be relay switched with the driver transformer, so I can tune the sound / adjust the modulator without having to cobble up a load for the modulation xfmr secondary.(http://)

Why?

Why move so much to a second chassis?

I'd use the real estate you have.

Looks like you've committed to the front panel already?
Also, you could reverse the left and right sides putting the mod iron behind the 811s.
Driver xfmr just to the left of the 811s (short leads are best) or under the chassis (better).

The fils on the 811 glow nicely.

For shielding you can put metal window screen (black preferably) behind the glass...

...I'm still unclear on how the 8ohm transformer is to be employed?

                       _-_-

Bob, check out my personal message.  Looks like we are in the process of building the same thing around the S-22 with 811A's.  So AM Fonies, you have two of us to nurse (lol)!

My design is simpler. S-22 center rear of the chassis, 811A's front and center.  Of course Bob already has a front panel cut and drilled.  I'm starting from scratch. I'm going with the speech amp driver on a separate chassis with its own power supply that can be switched to more than one modulator.  The 811A modulator chassis will have a switch to chose either the driver or a 600 ohm balanced source to grids. Not much different except he is going with a switchable 8 ohm XFMR for input and I'm opting for 600 without the driver on the same chassis.

Bob, I have another thread going RE: shielding HV leads.

I see that I am not explaining myself on the 8 ohm and driver transformer thing.  So here is a graphic.

I will relay substitute a standard 8 ohm transformer on the the output of the Altec design amplifier BEFORE it drives the 811A grids. Normally the relay will have the Thordarson driver transformer in the circuit.  You guys are trying to make this harder than it is, or am I :) Mod xfmr (on the circuit drawing) being the driver - not the actual modulation transformer. This is so I can listen to the actual amp without trying to match the 250 watt load of the s-22 transformer.

Bob, are you using a basic Hi-Fi amp as a driver?

I am building a clone of an Altec A-333 amplifier as the front end of my modulator, and adding two 811a's as the power. They will be powered by a separate 1250 vdc power supply on a chassis mounted just below the modulator chassis once installed in the rack. The Altec clone will be powered by a separate 450 vdc supply also on that chassis.  Putting them on a separate chassis lets me build this thing right without having to worry about real estate on the main MOD chassis.  I can use oversize chokes and such without worry. (And make it look better).

Bob, get in touch with me at W9ZSL@ yahoo.com.

Still lost on where this "8ohm" transformer fits in, and what you want to "listen to"??

Are you saying you want to be able to monitor the audio by putting an output transformer on the 6L6 drivers?
Or do you want to drive the 811a WITH an outboard audio amp??

If ur building a clone of an amp, the mod iron becomes the output transformer.

Space is nice, all that iron would take up a lot of space even under the chassis and leave little room for bleeder resistors, terminal strips, filter caps and so on.
Why not make a high voltage power supply that can be used for anything?
A 10 amp or so variac could make it very useful for any rig or modulator.

I do not understand the 8 ohm transformer, you could drive the grids of the 811's with one, and switch it in for outboard audio or out for an all tube on chassis speech amp.
Is that the plan?

Okay, one more time.  Normally when using the modulator for AM use, the 8 ohm transformer is electrically not in the circuit.  When I am setting up the front end of the amplifier, I want to be able to hear the sound of the amplifier, therefore I will operate a relay that will change out the driver transformer with the 8 ohm transformer.  At that time the Altec clone becomes a regular speaker feeding amplifier that I can adjust the E Q, COMPRESSION, ETC, without having to find a load for the mod transformer, sample the audio, etc. I realize this is unusual, but I want it easy to adjust.
Bear, The Altec amp clone will be just as it was designed to be right up to the output transformer.  The output will be able to switch from a standard 8 ohm output transformer, to a Thordarson driver transformer, that drives the grids of two 811A's which connect to a UTC S-22 modulation transformer. The set up will be, high level input into a 6SJ7, then a 6C5 then two 6L6GC's, THEN comes the switch transformer thing.

Gotcha.

I am sure you realize that the freq response of the old UTC mod iron will be somewhat less than the "8 ohm"
iron that you will be testing with.

If I were building it, I'd so some things to limit the passband of the circuit of the modulator, on the low end and on the high end, as compared to the hi-fi bandwidth of the Altec.

                              _-_-

That is a good suggestion, I take that seriously because I do not want to lose a mod iron due to excessive frequency travels. (Not that my voice can get that low anyway   ;), I will set up the mike processor a Symetrix 528 for a low freq cut off of 100 Hertz and the high end at about 8K. Should that be sufficient?  Also, I am losing the 8 ohm deal all together, I figure I can just tap off the secondary of the mod tranny with a big ol resistor and feed that to an amp, and monitor the real signal.

Or take a small output audio transformer from a radio with an big resistor in series and a capacitor to block the DC. Connect this to the modulated DC and ground and you have a suitable audio monitor output for driving a loudspeaker.

Martin

I like this layout and panel and the transformer switching so the driver can be listened to. How about dual purpose the selectable 8 Ohm output as also the audio amp for receive using a nice speaker? It looks like the iron is fine for that and plenty of power from those driver tubes.

That makes sense, it is not BC iron. Thanks to LTSpice it should be easy to insert such a filter anywhere.
Isn't it good practice to set the bandwidth in the voltage amplifier stages before the driver?

Progress. Also put in the diagram for the modulator.

Looks good,  schematic shows the CT of filament xfmr connected to the cathodes of the driver tubes.  Why??  doesn't look right.

Fred

Bob, I've had another thread going about a speech amp/driver.  Since we're both building basically the same modulator around the S-22, I Googled the schematic and it looks like it would work with my project. What are you using for driver iron? Seems to me 6L6's could be replaced with 6V6's but other than that, it's pretty sweet.  I'm also wondering about the 4700 resistor in the feedback loop considering the original Altec took the feedback from the 16 ohm winding of the output transformer.

Here is the complete diagram of Bob's driver and YES, that filament xfmr center tap to the 6L6 cathodes is correct.  See the note under the specs.

-Most of you folks know more about circuit theory then me, but I assumed that was some sort of bias for the 6L6's.
I have built this amp twice before for hi fi use, and it works well, so my plan is to just change the output transformer, cut the voltage back a little and let it drive those 811a bottles.
Mike,
 I thought I would try to find an appropriate place to get that feedback.  I have also tried that circuit without feedback, and it still sounds good.

The push-pull 6L6 tubes derive their bias (25 volts, in this case) across the cathode series resistor.  In audio amplifiers, it is common practice to elevate the center tap of the 6.3 volt filament winding to a positive potential, to reduce hum.  The net effect is to raise the filament of the microphone preamplifier stage positive with respect to the cathode.  This prevents the cathode from attracting electrons emitted from the filament. 

Think of the filament and cathode of the 6SJ7 as a diode.  If the cathode is negative with respect to the filament, then current will not flow from the filament to the cathode, thus reducing hum modulation from the 6.3 volt AC heater supply.

Bob, I thought that might be the case because even though the feedback resistor taps into a higher secondary impedance, the power output is the same regardless of the actual load be it a speaker or a grid.  I think we are basically building the same thing.  That Altec clone looks like a great driver to me.  I looked it up and have the complete schematic.  The cool thing is the power supply tap for a mic preamp.  I have the diagram for an Ampex MX-10 preamp with an input transformer to match.  Rock and roll!

If you want a better distortion performance without additional cost and can supply 5A for the pair's heaters, it could be as well to sub in 6CD6's for the 6L6's.

The regulation over the half cycle of audio at the 811 grids will be better because 6CD6s will deliver much higher current at low plate voltages and allow more plate voltage swing because they need only 150V on the screens.
6L6GC G1=0, G2=300, Eb=150: Ib=240mA
6CD6 G1=0, G2=150, Eb=150: Ib=475mA

The bias voltage will be in the same basic range and fixed bias will help make a few more peak watts in the driver if you have the time to do it.

The overall FB resistor 4700 Ohms, it may be worthwhile to experiment with a 10 turn pot and use the most NFB possible before instability or lack of gain comes in. It will help the regulation.

In case of instability or problems due to the combination of variable impedance of the 811 grid interacting with the driver transformer characteristics, the feedback could be taken from the 6L6 plate before the transformer.
  With or without blocking cap depending on how it might affect the first tube's cathode bias,
  Using much larger R like 200K bypassed with 20-80pF to tailor the HF response) before the transformer.
example: http://www.montagar.com/~patj/rcami-12188a_03.gif

Sweeping the audio using a resistive load (instead of the grids) can help set up the response and the gain. Response should be flat at least an octave above and below the bandwidth you want. Then verify it's still good enough with the 811's in place. :)

You may find these useful. From RCA Ham Tips.

Steve, thank you.  That is excellent information. I knew I would eventually cut back the B+ since only 10-12 watts of drive is necessary.  I realize I can jut lower the input gain but I would rather kept the noise figure in check as well.  This is good stuff, thanks again.

Good luck with it Bob. Hope to hear it on the air in the future.