Harvey Wells Modulation transformer 500 ohm second winding

[wa4fty]

Can someone tell me why and / or what the second winding on the Harvey Wells transmitter modulation transformer is for. It is shown on the schematic connected to terminals 6 and 7 and states 20 to 25 watts audio at 500 ohms. Was this for monitoring the modulator audio? There seems to be no reference about it other than what I stated above. I am curious as to why this feature is available. Perhaps the transformer had the additional winding and it was made available?
My reason for asking.....
I am building a matching transmitter using a 4-400 and will use the Harvey for drive. I am looking for about 200 AM watts. I will use the Harvey's 6L6's to drive a pair of 811's for audio.
I have already built the matching receiver. It can be seen on my QRZ page.
Oh and by the way I am in need of a modulation transformer rated 100 to 150 watts. If anyone has one available my email is:   WA4FTY@ATT.NET
My Thanks in advance, Wayne / Wa4fty

[KA2DZT]

That 500 ohm winding can be used to drive a class B modulator.  Typically a larger xmtr may be some distance from the HW xmtr.  Running a 500 ohm line from that winding to another 500ohm line to grid xfmr.  This xfmr will have a secondary with a center tap.

Fred

[wa4fty]

Fred, Thank you for the input!! I understand completely!! That is what I wanted to read! A pair of grid driven 811's will work fine. I am only curious. Did you find this information listed somewhere. Perhaps my search was incorrect. I was wandering why no reference is made in the service manual?
Thanks again

Wayne / Wa4fty

[w4bfs]

That 500 ohm winding can be used to drive a class B modulator.  Typically a larger xmtr may be some distance from the HW xmtr.  Running a 500 ohm line from that winding to another 500ohm line to grid xfmr.  This xfmr will have a secondary with a center tap.

Fred

hi Wayne .... just like Fred sez .... it is possible to use 2 chokes instead of a line to grid transformer UNLESS you have insufficient drive /  swing ....it all depends on:

20W into 500 Ohms --->  E2 = P x R = 10000 Vrms ---> E = SQ RT 10000 = 100 Vrms = 283Vpp = 141Vp  this is a little low for 811's, might be plenty for something else

[KA2DZT]

Fred, Thank you for the input!! I understand completely!! That is what I wanted to read! A pair of grid driven 811's will work fine. I am only curious. Did you find this information listed somewhere. Perhaps my search was incorrect. I was wandering why no reference is made in the service manual?
Thanks again

Wayne / Wa4fty

Hi Wayne and welcome to the Forum.

No, I didn't do any searches for the info.  The extra winding on that mod xfmr is not that uncommon.  There are other mod xfmrs that have similar type windings.

The winding can be used for anything, feedback, monitoring the audio or driving another modulator.  Driving another modulator will load the xfmr down (as Beefus outlined).  If you use it for feedback or a mod monitor you would NOT load it down with a 500 ohm load resistor because you would be robbing power that's needed to modulate the HW itself.

On second thought, a mod monitor would have to look at the modulated wave envelope, can't be done with that winding.  You could monitor the audio alone, but that's not monitoring the modulation of the class C stage.

Fred

[ae7db]

Wayne,

The copy of the manual that I have (I got it off the bama reflector, I think) does very briefly mention the use of the 500 ohm winding at the very end of the section on "Exciter Operation", which in my copy starts on page 13.

Good luck with the project.

Dean / AE7DB

[wa4fty]

Fellows, I really appreciate the input. Dean, I found the reference you pointed out. Thanks! Fred, thank you for the additional information. My goal is to give the Harvey a boost...RF AND AUDIO. You fellows helped a lot.
I now need to find a input matching transformer and a modulation transformer. If you fellows have a lead let me know.
Thanks again!!

Wayne / Wa4fty / wa4fty@att.net

[Opcom]

The 6BX7 has been used as a regulator and should be a fine 811 driver. Two have been used in in parallel in a commercial 200V@125mA power supply (ACDC Electronics, Burbank CA. model 125-200-1)

The cathode follower proposed will have only just less than unity gain, so, something has to be done anyway to step up the 500 Ohm winding's voltage.

The drive for 811's is <4W, and 25W is available.


How about a thought experiment on using the transformer alone with a low Z virtual CT. This avoids adding yet another transformer for input/step-up to the cathode follower or whatever:

So, fit this to the 500 Ohm winding.
Assume the max ratings to be used for the 811s.
811 Requirement: 150V peak grid to grid.
500 Ohm winding with 25W.
25W is 111V RMS @0.223A
158V is peak across the winding. grid to grid. Just barely enough.

There is 25W of drive.
3.4W drive goes to the 811 grids.
About 20W goes to a center tapping divider for GND return. Should work with such a large swamping. Worth the experiment.
20W at 111V RMS =180mA
Grid to grid swamping resistor = 616 Ohms. Call it two 330 Ohm resistors with the midpoint grounded. 5W may do but 10W is proper for rag chews.

That resistance is very low as seen by the 811 grids in the circuit.
Regulation is likely good.

If drive is insufficient, raise the center tapped swamping resistor values to 470 or maybe 680 Ohms each, - to lighten the load on the H.W. modulator.
There may also be other ways to maximize the Harvey-Wells modulator plate swing if it is not pushing a full load.

[w4bfs]

The 6BX7 has been used as a regulator and should be a fine 811 driver. Two have been used in in parallel in a commercial 200V@125mA power supply (ACDC Electronics, Burbank CA. model 125-200-1)

The cathode follower proposed will have only just less than unity gain, so, something has to be done anyway to step up the 500 Ohm winding's voltage.

The drive for 811's is <4W, and 25W is available.


How about a thought experiment on using the transformer alone with a low Z virtual CT. This avoids adding yet another transformer for input/step-up to the cathode follower or whatever:

So, fit this to the 500 Ohm winding.
Assume the max ratings to be used for the 811s.
811 Requirement: 150V peak grid to grid.
500 Ohm winding with 25W.
25W is 111V RMS @0.223A
158V is peak across the winding. grid to grid. Just barely enough.

There is 25W of drive.
3.4W drive goes to the 811 grids.
About 20W goes to a center tapping divider for GND return. Should work with such a large swamping. Worth the experiment.
20W at 111V RMS =180mA
Grid to grid swamping resistor = 616 Ohms. Call it two 330 Ohm resistors with the midpoint grounded. 5W may do but 10W is proper for rag chews.

That resistance is very low as seen by the 811 grids in the circuit.
Regulation is likely good.

If drive is insufficient, raise the center tapped swamping resistor values to 470 or maybe 680 Ohms each, - to lighten the load on the H.W. modulator.
There may also be other ways to maximize the Harvey-Wells modulator plate swing if it is not pushing a full load.

I tried this once on a single ended 811-10 amplifier in class A2 a few years ago .... it worked fine until the drive swing went + enough to start drawing grid current ... with the grid rectifying the grid dc level started swinging negative so the stage 'soft' clipped before reaching desired power level

I think you will need an active device (follower) or a pair of chokes to ground .... the chokes need to have low winding resistance to prevent grid current from forming an additional bias voltage ... it is arguable that the chokes might generate distortion products same as a transformer passing the audio .... I need to think about this ....

[N3GTE]

In one of the H-W service notes it sez for the TBS-50A and D to gnd one leg of the 500 ohm output if you have an audio feedback problem.
Terry

[W9ZSL]

I have a Kenyon 500 ohm to any class B grids transformer.  It's a type T-261, NOS, in the box.  The rating is 7 watts.  I was going to use it to drive a pair of 811A's myself from my limiter amp.  I doubt I'll use it because I've scored a universal driver transformer for a modular speech amp and this iron is surplus.

[W3RSW]

Hi Wayne,

The Johnson Ranger also has an approximately 500 -0 -500 ohm (center tapped) output specifically for driving, say a PP pair of 811's or 810's, etc. as a higher power modulator.  Also the RF stages can be used specifically as the drivers, an exciter as we say, for a higher power class c final.  Change from internal modulation to external is done with a nine pin plug where external bias to the following amplifier stage big modulator tubes can be injected.

The Johnson Desk Kilowatt is designed to be preceded by a Ranger in this application.
The Ranger, if close enough to the following modulator stage doesn't need another matching transformer and can drive the higher power modulating grids directly.

Look those two AM rigs up if you haven't heard of them. You'll be delighted.

[wa4fty]

Fellows, MY THANKS TO YOU ALL.... I am absorbing all the information, input, and experience.

Wayne / Wa4fty

[Tim WA1HnyLR]

In regard to using the Harvey Wells modulator as an audio driver for a pair of 811s FORGET IT! unless you want to rework the whole modulator.  6L6 tetrodes make very poor audio drivers unless a good amount of negative feedback is wrapped around them. The best form of a driver transformer is use of one transformer NOT two transformers back to back. Direct coupled cathode follower or solid state methods are the best. The usual 25 watt PA or monaural high figh audio amplifier is an easy way to drive the 811s. If you are insistent on the use of the TBS 50 as a complete exciter, look for a modulation transformer from a Ranger or Viking I or II. A mod transformer from a Valiant has an even better step down turns ratio to drive a pair of 811s. The secondaries of these transformers are center tapped for the grid return. There is an engineering hack that could be used with the mod transformer in the Harvey Wells . Use the secondary that originally modulated the 807 PA tube. Take a small power transformer or push pull audio output transformer. Parallel the secondary winding of the mod transformer in the Harvey Wells with either the secondary of the small power transformer  or the primary winding of the non discript audio outpoot transformer. This is assuming that there is a center tap on the winding in either case. This creates a center tap for the grid return for the 811s . This is a better method than back to back transformer to affect an impedance match. Another hack to use. If you come up with a 25 watt solid state PA amp. take two identical 12 volt 1 amp filament transformers . Connect each 12 volt winding to the direct outpoot of the PA amplifier ( not using any internal 70 volt transformer). Each 120 volt winding will drive each 811. Make sure that the transformers are wired so that when a AC meter is placed from grid to grid of the 811 modulators that there will be audio voltage present. Incorrect phasing will result in either push push or pull pull operation of the mod tubes.         
Good luck ,DE Tim WA1HnyLR

[WD5JKO]

  Glad the Tron weighed in...

   There is one way to use two transformers without all the pitfalls of phase shift, load regulation, and peaky frequency response. That is to use each as a center tapped choke. The link below is to a proven circuit designed from John WA5BXO:

http://www.qsl.net/wa5bxo/driver1.html

   This circuit was used for many years by W5OMR on his "Titanic" rig. This was a pair of 250TH's modulated by a pair of 100TH's. The circuit from John makes +/- DC directly off the AC line. I think I'd toss in an isolation transformer instead. Geoff had some of the best audio in the fifth call area, both in depth, and sound.

   I often listen to 3890 while doing the morning rituals. I use a portable Sony receiver. There are often roundtables of 3 or more, and one person always is clear as a whistle, and maintains a high density of modulation. The rest of the guys averaging 30% modulation if that are simply not heard when the S/N ratio is poor. This guy no stronger then the others has audio heads and shoulders above the rest. Some would say this is rude. To me my goal is to put some audio on the carrier...not tickle it a little.

   So back to point, John WA5BXO makes many valid points concerning the driving of a class B modulator. It doesn't matter which way you get there, the important thing is that you do indeed get there. Driving the varying load of a class b grid is non trivial, and attempts should be made so that the driver does not Peter out half way there.

Jim
Wd5JKO

[W3RSW]

The Johnson Ranger does have a tertiary feedback winding from its mod transformer back to the speech amp stages for the express purpose of taming the beam power tetrodes in normal service --or if used as a driving stage for a following, bigger modulator.

The Ranger au natural or as an exciter wasn't meant to be a HIFi transmitter.   The tertiary winding feedback circuit was not intended to be a bandwidth flattener but was intended to reduce the effective plate resistance of the beam tetrode tubes to that of low mu triodes.  The Ranger modulator has sufficient power to overcome impedance mismatches in its intended applications, within communication-grade frequency limits.  Virtually any ARRL handbook of the 50's and '60's has a good Modulation chapter explaining all this and with practical circuits along with examples built.

I don't have the schematic or manual for the Harvey Wells Bandmaster Senior handy, but there may be more direction in addition to the other fine ideas presented as to using that 500 ohm output.  I'll see what I can find to add.

[wa4fty]

Well Guys, It seems I started a series of most interesting brain storms!! I have read and continue to read all with the utmost interest!! The wheel of AM modulation need not be reinvented but the various methods of success are fascinating!! I have gone in all directions reviewing the attached recommended ways and means. Every time I return to the site I am enlightened even further. When my endeavor comes to a final conclusion I owe all of you my gratitude! Thank you!!!

Wayne /  Wa4fty

[w4bfs]

Wayne ... heard you fb on old friends net for about 5 min this am .... nice going on the cv4 score ... looks like a plate modder is coming up for the HW ... there have been a few remarks here about cv series mod xfmrs having talkback issues .... I don't know if the cv4 was one of these ....

while you are in the planning stage, please consider using a tx sequencer in your design to avoid tesla coil effects in your new mod iron

[wa4fty]

John, Thanks for the comments. Sorry about being late getting back to the sight. Been doing other stuff. One might say Honey-Dew stuff.
I really do not know if this transformer has lamination talk or not. It is round with flange for mounting. Not sure how the transformer is mounted internally. I do however need some clarification. Can you explain ....sequencer in design to avoid tesla coil effects?
I know about counter EMF. No load on secondary and so forth. I certainly do not want to ruin a good transformer.

Wayne / WA4FTY

[w4bfs]

several 'am fonies' (myself included)  had an ongoing discussion about arc suppression and sequencing

http://amfone.net/Amforum/index.php?topic=33816.msg269418#msg269418

its lengthy and demonstrates the nature of a true evolution in technology just as Tesla had his long ago