new to me 811A modulator project

[RolandSWL]

Could the 105Vac tap on the power transformer be used to get a few more volts out of the secondary?

What could go wrong?

R...........

[N2DTS]

Why not?

That choke IS on the light side for 811's (320ma), but for less current you could insulate it from the chassis and run somewhat higher voltages.
It did? run at whatever voltage in the original setup...

Could the 105Vac tap on the power transformer be used to get a few more volts out of the secondary?

What could go wrong?

R...........

[steve_qix]

You should be able to get a good approximation of the Inca mod transformer's ratings.

We need to know these parameters:

1) dimensions L W H
2) Weight (this is important)
3) DC resistance of the primary and secondary windings
4) Inductance of both of the above windings
5) Any other observable data that you think may be useful.

With this, it should at least be possible to come with an educated guess on the power rating.  The resistance/inductance will be a clue into the current rating.  The voltage rating is a bit more sketchy, but if it has lots of taps and/or split primaries and/or secondaries, that will also be valuable.

From the picture, the Inca mod transformer looks reasonable for the job, but the numbers will tell all.

[kb3ouk]

You should be able to get a good approximation of the Inca mod transformer's ratings.

We need to know these parameters:

1) dimensions L W H
2) Weight (this is important)
3) DC resistance of the primary and secondary windings
4) Inductance of both of the above windings
5) Any other observable data that you think may be useful.

With this, it should at least be possible to come with an educated guess on the power rating.  The resistance/inductance will be a clue into the current rating.  The voltage rating is a bit more sketchy, but if it has lots of taps and/or split primaries and/or secondaries, that will also be valuable.

From the picture, the Inca mod transformer looks reasonable for the job, but the numbers will tell all.

I had a rather long reply typed up and then went to post it and lost it.

Anyway,

The case is 5.5 inches high by 6.5 inches long by 5.5 inches wide. It weighs 20 pounds. I have no way to measure inductance. The resistance of the primary is 124 ohms across terminals 2 and 4, and 172 ohms across 1 and 5. The first winding of the secondary is 97 ohms across terminals 6 and 9, 24 ohms across 6 and 10, and 73 ohms across 9 and 10. The other winding of the secondary is 129 ohms across terminals 8 and 11, 35 ohms across 7 and 11, and 94 ohms across 7 and 8. The first picture is the primary side and the second picture is the secondary side.

[KA2DZT]

The Inca is probably a 125 watt xfmr.  Most makers of the day made 125 watters as a standard catalog size.  Best to use the entire primary, forget the taps.  Put both secondaries in series and use the whole of both windings or move the B+ up one tap from the bottom end.  Only other way to connect the secondary is to put both windings in parallel but the step-down would probably be too much for most Class C finals.

You can study the xfmr until you're blue in the face but the bottom line is, it's the only mod xfmr you have and there are only a few ways to connect it up.

OTOH, if you have twenty other mod xfmrs to play with, keep experimenting.

Fred

[w4bfs]

Could the 105Vac tap on the power transformer be used to get a few more volts out of the secondary?

What could go wrong?

R...........

it could overheat

a simple test for a xfmr is to excite it as planned but without any load and see how much current is required just to energize the primary ... you can use a small resistance in series and measure the voltage drop then with Ohms law can calculate what is known as the magnetizing current .... this is a simplification for what is going on but yields useful info .... for example this xfmr you are interested in draws 50mA at 105V applied and after a half hour or so gets barely warm then you are sure that the xfmr is basically ok and can test at higher voltages .... so say the current draw doubles to 100mA at 117V applied .... the apparent power in the xfmr has increased from 5.5W to nearly 12W and the transformer runs a bit warmer but not hot

so where is this helpful .... it is an observed fact that electromechanical devices have a correlation between internal temperature and expected life ... it is basically stated that a 10 degree rise in temperature results in a halving of life expectancy ... the onset or critical temperature is dependent on materials and methods of construction

please run the test .... hv iron is on the endangered species list

[steve_qix]

Ok, good info.  Based on the weight and size alone, it's over 200 watts.    For instance, the UTC S22 is almost identical with respect to size and weight.  The S22 is a 250 watt transformer.

[N2DTS]

When you look at the size of some mod transformers, that one should do real well at 200 watts or below.

[K1JJ]

When you look at the size of some mod transformers, that one should do real well at 200 watts or below.

Yes!

I once did an experiment with my pair of 813s plate modulated by a pair of 813s.

The existing mod transformer was a 60 pound old broadcast unit. It worked very FB and was hi-fi at whatever power level I wanted using 2200 volts. It was the only transformer in the chain and driven by the WA1GFZ MOSFET 11N90 audio driver into p-p 813s.

I wanted to see how an ART-13 mod transformer would work in its place, so I clip leaded it in. The ART-13 transformer weighs maybe 10 pounds? and can fit in the palm of your hand.  I was using a modulation choke in modified Heising, so the mod transformer carried only audio, no DC current.

I was amazed. That thing modulated a 300 watt carrier no problem. The low end was noticeably absent as expected due to the lack of mass. But it was reasonably clean above 200 Hz and the guys on the band thought it sounded like a DX-100  - not bad.

It continued to work FB for ten minutes... no problems.  This is a transformer that was built when men were men and transformers were designed to survive a war. (WWII)

The bottom line is if modified Heising is used and the transformer is of good quality, yours should hang in there.  Be sure to use spark gaps for protection and be VERY careful of unexpected audio surges.

T

[N2DTS]

I was kind of thinking of the typical 100 watt rig, or even the G76 mod iron.
70 watts carrier and plenty of modulation, running the rig 100 volts over normal, and the very small mod transformer never gave any problems.
I worked someone who ran one 200 volts over normal at 100 watts without issue.

[kb3ouk]

Yea that transformer is pretty heavy, its actually slightly heavier than the plate transformer is. If I put a modulation choke on it, finding one with enough inductance and be capable of handle the amount of current for the final I want to run could be difficult. Running an 814 at around 1100 volts 140 ma is going to have a plate impedance around 7800 ohms. I remember reading before to calculate the minimum inductance for the choke by multiplying the final's plate impedance by .0045, so that would be 35 henries. I could either find a single choke that was 35 to 40 henries or so or use two 20 henry chokes in series. Either has to be able to handle at least 150 ma (and thats right on the edge) at at least 2500 volts.

[N2DTS]

DC through the transformer is not going to hurt anything, its voltage spikes you have to watch out for.

1100 volts does not seem excessive.

I have had two mod transformers fail, both my fault, one was a 30K1 mod transformer when I did full power 20 hz testing, before I knew better.
The 2nd one shorted when it had voltage only on one side of it (rf side) because the modulator was not turned on.

Never had one fail in normal use at below max ratings.

[kb3ouk]

So proper transmit/receive sequencing might be a better idea? Would it be better to key the high voltage on the final and modulators (it appears that the transmitter this modulator came from was designed to do that with the external relay connection on the primary side of the plate iron) or to leave the high voltage on and bias the tubes on/off to go from transmit to receive (using a resistor on the center tap of the filament transformer, and probably fixed bias on the final or a cathode resistor there too).

[N2DTS]

Both ways work.
The voltage can go way up with no load if you leave it on, and can cause noise on RX if you have any current flow.

[K1JJ]

So proper transmit/receive sequencing might be a better idea? Would it be better to key the high voltage on the final and modulators (it appears that the transmitter this modulator came from was designed to do that with the external relay connection on the primary side of the plate iron) or to leave the high voltage on and bias the tubes on/off to go from transmit to receive (using a resistor on the center tap of the filament transformer, and probably fixed bias on the final or a cathode resistor there too).

Hi Shelby,

Take a look through pages 4 to 9 of this thread to get an idea of the problems and solutions to keying up and un-keying a big plate modulated rig.

http://amfone.net/Amforum/index.php?topic=33816.75

Yes, a sequencer is a great idea.  I sequence/ key the high voltage transformer's 240VAC primary and the cathode (fil xfmr center taps of the RF and modulator tubes)  of my 4-1000A plated modulated rig.... as well as the screens, RF drive, antenna, and a few other things.

I also use Hall effect fault / shutdown circuitry that monitors current levels on the tube's various elements.

Another good idea discussed in this thread is putting a power resistor across the mod iron output that is open when transmitting and during un-key it loads the modulation output to quench any arcs. (relay controlled)   Whatever it takes to have the rig come up quietly with no arcs and then un-keys without arcing, audio pops on the air, etc. Smooth as glass.

Below is the sequencer I built that uses variable delays for key up and independent decays for un-key. It all still works FB.

W1VD / Jay makes a nice, simple sequencer board that could probably do the job for you.
This will help preserve your mod iron.  Getting a handle on keying transients is part of the shakedown process of building a big rig.

T


Below: The second picture is the simpler pre-assembled four stage sequencer board that you can purchase.

[KC9LKE]

I have had two mod transformers fail, both my fault, one was a 30K1 mod transformer when I did full power 20 hz testing, before I knew better.

This has been mentioned before but I can’t find a full explanation or how to
find the mod transformer frequency response without damaging it.
Anyone care to explain?
Best regards
Ted / KC9LKE

[w4bfs]

I'll take a crack at it ....

to really understand what is going on in a transformer you need an accurate at least third order model ...higher orders are more accurate .... the problem is that the vendor of the named transformer is not around to help provide this information ...so we will need to set up some tests to help determine some of these parameters

this is not easy but is very interesting and will help knowledge of electronics grow...

if I could schematic capture, I would put in the model now...

now break it down to the low freq , mid freq, and high freq models and note how which parameters change in importance...

all of the models I have seen do not account for when the transformer is driven past ratings into the non-linear regions ... this is not easy and will challenge your reasoning ...

it would be easy just to give up and do something else .... just how curious are you ?

[steve_qix]

The modified Heising my very well buy you nothing other than more weight in the modulator !   That transformer is MORE than likely an Amateur modulation transformer which is designed for DC.  If you're not exceeding the DC current rating, it's should be all OK.  That's a pretty good size mod transformer.

Sequencing - THIS IS IMPORTANT !!!!!!!     You need a 2 stage sequencer for this rig.

Keying - my suggestion having built literally hundreds of transmitters, tube and solid state? -> Key the H.V. supply on and off.  Safe, reliable, no guesswork.  Use a step start and your rectifiers and other components will not be subject to any current spikes.

Here is a sequencer I use at Rattlesnake Island.  Also is used by Wayne WA1SSJ and a few others.

[kb3ouk]

Does the adjustable hold and delay just change the amount of time that particular sequence stays in that state? Does this need to be fine tuned in any particular way?

[N2DTS]

I do not sequence anything other then the step start.
Most ham transmitters had no complex sequencing.

Its not a bad idea, but it adds complexity and failure points.

[steve_qix]

Does the adjustable hold and delay just change the amount of time that particular sequence stays in that state? Does this need to be fine tuned in any particular way?

Yes, that's it.  The RX hold time determines how long, once you go from tx to rx, the system will actually delay before releasing the antenna change over relay, shut down the RF drive (or whatever you have controlled by the RX side of the sequence).

The TX delay is how long, when you from from rx to tx that the system will wait before engaging the H.V. power supplies, and anything else you have controlled by the TX side of the sequencer.

There is no fine adjustment.  Just do it by ear.  I usually do a 1/4 to 1/3 second delay on the TX delay, and about 1/4 second on the RX hold time.  Adjustment is strictly by ear.


Just as a note, I built my first sequencer in 1975 and am still using THIS SAME UNIT (amazing !). 

[steve_qix]

The thing is, most transmitters don't actually shut down instantaneously - they take some finite number of milliseconds to really put out 0.

I like to give it at least 100ms to 200ms to "settle" to 0 output.  That's the receive delay in the sequencer.

A lot of times you will see arcing at the antenna relay, hear big pops in the receiver, etc.  when going from transmit to receive.  These are good indicators that you have a sequencing problem.

Coming up is similar.  Drivers, relays and other things do not settle instantly - they take some finite time to do so.  That's where the transmit delay in a sequence comes in.  This gives everything a chance to stabilized, and then the H.V. is keyed.

[K1JJ]

Yes, sequencing is important.

The dead giveaway on my 4-1000A rig was arcing at the spark gaps on the modulation transformer during un-key. It was nasty.  There was obviously a lot of audio power that needed to be dumped. Usually this power is dumped into the RF final and out to the antenna in a brief instant. This means kill the HV, kill the audio input, hold the antenna relay closed and keep the RF deck energized a little longer than the mod deck - this  usually works to quench the audio arc and dissipate the power.  (Along with the relay switched power resistor across the mod sec as I mentioned before)    

And as already mentioned, pops in the receiver during un-key are a tell tale sign of RF wanting somewhere to go. I have actually blown out the front end of an FRG-7 receiver as a result of this antenna relay arcing... poor timing.

Some rigs are easy to tame and some are a real bear.

This is why I made all six sequencer relays variable with independent key delays and un-key delays. I was able to actually adjust the pots on the 12 delays and see the arcing, popping and farting all go away to perfection once the timing was optimized.


The ultimate arc acid test, if you feel lucky, is to sock an atomic yallo into the mic while at the same time rapidly keying the rig in staccato. All of my rigs will do it, but it took some work, especially with plate modulated rigs with big audio power looking for a place to go.  I have done it and have seen several  others do it on the air on a dare. It WILL break parts if things are not dialed in right.


T

[steve_qix]

Here is the full-size schematic of the sequencer (this is .pdf, which is nice - can be resized, etc).

http://www.classeradio.com/2_stage_sequencer.pdf

Feel free to pass this around.  The circuit has been built many times.

[WBear2GCR]

In a QSO a few days back "tron" WA1HenryLellaaRrrr mentioned that Inca was a pre WWII mfr.

My 2 cents:

Try the existing interstage xfmr. You can test that fairly simply - but you'd want to know what input Z it is
"expecting" first. That can be determined by the put 10vac on the primary and measure the voltage on the secondary test. This tells the voltage ratio, aka "turns ratio". Which when plugged into the formula, and using
a calculator to do the sq rt or the squaring (depending on which way you use the formula) will tell you the impedance *ratio*. Knowing that you are driving grids of an 811a (for the original) once can then surmise
what the likely load side Z was, and so determine the primary side. The transformer either wanted to see a tube's
plate or maybe a low Z (~600ohm) source.

Knowing this info, you can put a test load on the secondary (the approximate resistance representing the presumed load Z the transformer was designed for) and then apply the requisite voltage on the primary - a fraction of the operating voltage is ok for getting in the 'ballpark" - and see what the frequency limits are. Especially the HF side, this will be somewhat of a valid test. On the LF side, you can expect a bit less, especially if the primary is supposed to sit on the plate of a tube and pass DC. If you wanted to get "fine" on this test, you'd put a cap to couple the swept audio signal to the xfmr, and put DC through the primary via a current limiting resistor to simulate the current through the driver tube.

Also, beware, driving a transformer from a low Z source (like an audio amp) usually produces a better response curve than when run off the plate of a tube (high Z source). But if it drops like a stone on either end of the spectrum, then you know that for sure.

The same thing can be done with the mod iron itself, although keep in mind that all transformers perform somewhat better at lower signal levels than near max or "peak" signal levels, and also with less DC rather than more.

A great way to drive 811 or similar tubes can be found by copying the Altec 1570B driver stage. It is perfect for this application and super easy to implement. In fact the secondary of the existing driver transformer can
be perfectly applied as the P-P cathode driver choke for this circuit. The limitation on that is the power/current rating of the iron. But it would likely support a pair of 3-6watt tubes, no problem. The great part of this is that using this method the frequency response of what is now a CT choke is irrelevant. In fact a transformer with "bad" HF response is just fine, and will have no lack of HF response as a cathode choke. Plus you could apply bias through the CT, need be.

The schematic can be found online.

I'd say that if you could boost the HV up to about 900-1000vdc you'd be in a good place to do ~150watts rms.

But if all you need is 100watts for you project then the 800vdc is more than enough, the tubes will loaf. Zero bias ought to be fine.

This method adds one stage to the speech amp, that being the actual driver, in this case a cathode follower. So, the voltage swing/gain must come before the follower. But for the 811s, you can look up the drive requirements in the tube manual, and not that much voltage needs to be swung, so nothing exotic is needed at all.

You might try the lower voltage primary taps, and see what the voltages look like.
The single rectifier in the schematic is rather anemic for anything of this power level. The 1570b, rated at about 125watts uses 4 dual section tube rectifiers (I forget what number, maybe 5U4) in parallel.

I'd go solid state and avoid the problems, just use a method for slow turn on of the HV on the 811s.
There are ways to do this and not need a sequencer for *this particular job*.

One of the worries about a restrictive frequency response due to limitations of iron in the signal path comes when
anyone tries to wrap feedback around the iron - the feedback tries to boost the extremes of response, which of course can't really be done without basically adding a boatload of EQ (what the feedback does has this effect). This extra feedback comes with issues of phase shift, and the resulting frequency response appears to be "flat", but in terms of transient response and phase response, it is often horrid and creates all sorts of *icky* things that can be seen using scopes and other test gear, and often sounds not so great.

But feedback is another issue, not actually mentioned WRT this modulator.

Generally speaking, I think anyone using transformers for audio is in safe waters as long as they do not try to extend the usable frequency extremes, and stay within those boundaries. In practical terms, trying to get a flat response to 20kHz (for example) and down to 20Hz from iron that does 80Hz to 6.5kHz flat is likely not going to go well, imho. So, the trick to making this sort of iron sound any good is to not "excite" it outside its passband.

Well, thanks for reading...

                 _-_-