new to me 811A modulator project

[N2DTS]

Great comments.
Some EQ within what the iron passes can make it sound very good.
Or you can eliminate all the iron in the path and get really clean..

[kb3ouk]

LOts of good things to consider. I'm starting to put everything back together and deciding what to keep and what not to keep. My amp I have has 4 output taps, 4, 8, and 16 ohms and a 70v tap. I'm not sure what the original speech amp used with this modulator was, there's just a 1/4 inch jack on the back that is the speech amp input, that goes straight to the driver transformer. The transformer is an Inca L-13, which I actually happened to find some information on. On page 22 of this PDF, it says this transformer was designed to go from the plate of a single 42, 45, 2A5, 46, 59, or 6F6 driver tube to class B grids.
http://dbase1.lapl.org/webpics/calindex/documents/15/522985.pdf

[N2DTS]

Maybe the 70 volt line will feed that transformer.
Single ended unbalanced step down most likely.
You could experiment.
Hammond also makes 8 ohm to 5000 ohm center tapped HIFI transformers (and others).
Back to back transformers is a silly way to do anything, output of a tube HIFI amp into another transformer seems the wrong way to go, but it can work.

[kb3ouk]

The Hammond transformers were what I was thinking about using. Like I've said earlier, the goal of this isn't to sound like a broadcast station, more of just to build something that sounds decent and get it on the air and use it and have some fun with it.

[N2DTS]

The easy (very) way to do it is the Hammond or other transformer driven by a 20 watt solid state hifi amp.
With zero bias triodes, it does not get much easier.
Sounds great as well.

[kb3ouk]

Biggest stumbling point I have found so far is trying to find tubes. I decided to go with 811As instead of the 809s I said about earlier, but finding a good pair of 811As that aren't chinese made for less than $100 or more is hard. Both tubes that came with the modulator are shot, the one's filament is completely missing, the other is broke on one side, half the filament lights up but the other half is dark.

[N2DTS]

Plenty around:

http://www.ebay.com/sch/i.html?_odkw=&_osacat=1500&_from=R40&_trksid=p2045573.m570.l1313.TR12.TRC2.A0.H0.X811.TRS0&_nkw=811&_sacat=1500

I have some Russian ones that still work after years.
The bases came unglued, but they still work.

572b's also work.

[VE3AJM]

The Hammond transformers were what I was thinking about using. Like I've said earlier, the goal of this isn't to sound like a broadcast station, more of just to build something that sounds decent and get it on the air and use it and have some fun with it.

Have a look at the Hammond 117F32 70v line transformer. It would work great.

Al VE3AJM

[kb3ouk]

What I want to do is use one of their push pull output transformers backwards by putting the primary side to the 811 grids and the secondary to my driver amplifer.

[kb3ouk]

What does everyone usually use for wire on something like this. I know about using copper core ignition wire for the HV (and already have a source for it), but what about the AC wiring and everything else? Some of the wiring was cloth coated and still in good shape, so I reused it, but most of it is in need of replacement.

[w1vtp]

My favorite combo was my Bogen M60 60 watt amplifier and I used a UTC CG50AX line to pp grid.  That was ~40 years ago.  Today I would insert some sort of line level EQ.  It worked really great with my pp 811A modulator.  The mod iron was a Thordason T11M77 300 watt unit.  The final was pp 813s running really conservatively.

I used separate HV ps' - the modulator @ 1250 VDC @ 300 ma  the RF Deck ran off a 1500 ps rated at 350 ma.  I didn't have the smarts back then to optimize it but it was a good start.  Look for a UTC CG59AX and use the 70 volt line output on that amp

Al

[VE3AJM]

What I want to do is use one of their push pull output transformers backwards by putting the primary side to the 811 grids and the secondary to my driver amplifer.

Yeah thats what many have done with the Hammond 117F32. The secondary 8 ohm side is connected as the primary, to the 8 ohm output of the amp. The primary now secondary ends are connected to the modulator grids. The CT is the 8w power tap. Ever hear Borgi K9YQQ? He sounds great using one to drive class B grids using a Crown amplifier.

Available from Mouser, DigiKey etc. They stock it and it is a modern transformer. Its about 1/3 the price of the Hammond 1600 series audio output transformers of a similar power rating for the same performance. Could buy you some 811As.

Of course there are other transformers that will work.

Al VE3AJM

[kb3ouk]

Ok, now that you explained how it works it makes sense. I even looked at the spec sheet but it didn't really make sense until now. I've heard Borgi's rig before, I believe its something like a pair of GI-7Bs modulated by a pair? I know it had a some kind of Russian tubes in it.

[VE3AJM]

Yes that's the tube I believe. I found some information about his transmitter online since. Nice work!

http://www.aimnhi.net/kj9t/K9YQQ_AM_XMTR/pres0005.html

I was previously skeptical of the back to back transformer arrangement too. But after hearing it being more in use on the airwaves now, it can sound very good.

Al VE3AJM

[N2DTS]

Yes, that looks very nice.

[kb3ouk]

Right now I'm waiting for some parts to get here so I can get the power supply section rebuilt, but I have decided to make a slight change in the plans for this modulator. I'm going with a pair of 814s for my RF deck instead of a single one, running at about 900 volts 240 ma off of a seperate power supply. I figure this modulator should still provide plenty of audio (since the 811A spec sheets give ratings for CCS service at 750 volts with something like 170 watts of audio), but the question is should I stick with running the RF deck straight through the mod iron secondary, or switch to modified heising to keep the current off, since it is higher than the 140 ma I originally planned on. I found a Millen 90800 exciter on ebay that I just couldn't pass up for the price (it was in great shape but is missing the tubes and coils) that I plan on using to drive the RF deck. I was thinking about using my icom 718 to drive it originally but the more I thought about it the more I wanted to make this into more of a standalone transmitter. The RF deck is just going to be a monoband 80 meter one, I also want to try adding 3rd harmonic resonators to the output network to try to get the efficiency up some, I figure at the voltage and current I'm running, if I can get it up to about 85% to 90% efficiency, I can get somewhere around 190 watts out of the pair of 814s. But that's a ways off, I'm still gathering parts for that.

[N2DTS]

You can always add the hy zing circuit later.
Much depends on the mod transfotmer.
I have some nice RCA ones that sounded better without the hysing.
I had a 50 henry Peter Dahl choke and tried various size caps and could get it to sound different but not better with various size caps.
I just ran without it and sold the choke.

Some iron sounds poor stock (cvm-5 was poor).
The Thordarsons sounded quite good to me.
Maybe try it the easy way first.

And I find most tubes work better at higher voltages and less current.
 

[WBear2GCR]

Dunno, the audio power amp to drive a reversed (small) output iron to drive grids of 811a tubes is a bit overkill, although it can be a fast-er way to get the job done. Using a larger solid state amp to run a backwards output iron to directly modulate the RF stage is a different deal, I expect.

The iron that works with the tubes you listed, including the 45 is pretty basic. Instead of a 45 you could use a triode strapped 6W4 or a 6V6, assuming you want to do it cheaply. Both tubes can be had for almost nothing by today's standards.

And, again, give some thought to that cathode choke loading method, and download the Altec 1570b schematic, you can basically just copy it. You get almost DC to light frequency response from the driver this way. Assuming I have it on my drive, I'll attach it to this post. Feel free to ask any questions about how to apply it. Btw, the Altec uses a triode strapped 6W4 (which is basically a 1/2 output power 6V6).

The 811s like a bit more than 750vdc, and I'd encourage you to get that voltage up to about 850-900vdc or so, if it is practical given what you have on hand for power iron.

                     _-_-bear

[kb3ouk]

I should be able to get over 800 volts out of it using the 105 volt tap on the primary of the plate iron.

[WBear2GCR]

I'd not worry too much about going from 750vdc to 800vdc. Although I guess a little more is a little more.

Consider the basic circuit idea though.

             _-_-

[KA2DZT]

I should be able to get over 800 volts out of it using the 105 volt tap on the primary of the plate iron.

Running too high a voltage on the primary is not a good idea.  You could saturate the core and over heat the xfmr.

Fred

[kb3ouk]

I should be able to get over 800 volts out of it using the 105 volt tap on the primary of the plate iron.

Running too high a voltage on the primary is not a good idea.  You could saturate the core and over heat the xfmr.

Fred

I wasn't planning on anyway, the way I see it is what real difference is it going to make going from 750 to 850 volts? My line voltage here is high enough that the 125 volt tap is what I'll have to use on that transformer.

[WBear2GCR]

If you take a look at the suggested operating points vs. B+ voltage in the RCA manual (again easy enough to find online), you can see what the effect(s) of (in this case) going from 750vdc up to ~900vdc will have, especially on the max output. But also on the suggested load Z.

A bunch depends on how much power you will actually need based on the carrier your finals can muster.

                         _-_-

[kb3ouk]

Well I finally finished it on Saturday. I got some big resistors to use as a test load today and was able to do a working test with it, using the resistors in series with a speaker to get the impedance up to something the mod iron could handle. The power supply puts out about 790 volts at idle and drops to 760 volts at full power (around 300 ma, it has the capability to go higher as I discovered during testing but thats all the higher my meter goes). The tubes idle at about 30 or 40 ma with no audio. Everything appears to be running cool. The power supply voltage doesn't sag under 760 volts, which I think is pretty good. At least to my ears, it sounds pretty good, the low end gets a little distorted but the midrange and upper end doesn't sound too bad, nothing that a little touch of eq couldn't smooth out. The real test will be once I get the RF deck built and try it out on the air. I have the power supply for that and the exciter started. I'll try to get some pictures of the finished product over the weekend.

[WBear2GCR]

which schematic/design??

perhaps you might put up the schematic with parts values...

A sign gen with a scope will give you an idea of the raw freq response (important) and using
the square wave you can see the general balance of HF to LF (by the tilt) and that will also show
the bandwidth via the shape of the square wave (1kHz. is a good place to start).

x10 probe on the scope is a good plan.

It might be prudent to adjust the coupling cap values to limit the LF below some reasonable
frequency so that you do not "crunch" the mod iron.

The mod iron is not set up to run speakers... using "resistors" in series with a speaker probably is not
a great idea. What you need is a load that is approximately the same as the Z that the modulator will see in operation, which is HIGH. So about 100 watts of resistors there, maybe 7-10kohm I'd guess. A scope on that, and then perhaps a BIG resistance value divider, something like >100kohm with something like 1kohm in series, taking the 1kohm at the GROUND side and using the junction with a coupling cap (like 1ufd or so) into an audio power amp... then you could listen to it. You'd want to set divider such that the output level at the divider junction is about 1vac, but a bit less and a bit more (like 4 volts ac) when the modulator is a full grunt is about right for driving the input of
a power amp. Or you can run this same thing into a mixer with headphones.

Put a 100kohm or so resistor to ground on the output side of the coupling cap... just in case any DC potential somehow build up.

This would give you something reasonable to hear...

                     _-_-

                      _-_-