Guitar Amp modulator

[W4RFM]

Well, last night I tried to make some sense of the modulator I have been building, it's the one from the Orr 1957 Handbook.  I built it like the plan first, then found out the 6B4G's I bought were no good, so I have been trying to substitute 6L6's in their place, with the appropriate circuit changes.  Apparently there is where my problem lies. I ended up with 150 mils of total plate current on the 6L6's, and decided I don't like the way this is going. According to my reading I have about 35 watts on the grids - all the time, volume up or down, So, I am going to build a Fender guitar amp to drive the 811's!
I will use a 6SN7 in place of the 12AY7 and a 6SL7 in place of the 12AX7, then the 6V6's in the driver position should give me plenty of drive for the 811 grids.  PLUS, I know this design works. Thanks the The Fender Amp Guide for publishing the drawing.

[WD5JKO]

Bob,

    Many of us make fun at the audio folks. I miss the posts of the late Fred Nachbaur on some of the audio forums. One of the bigger projects that Fred did was the Dogzill project. This was a lot more than just six 807's in push pull parallel class AB2 making over 200 watts RMS of clean sound.

    Take a look at the Dogzilla project:

http://www.dogstar.dantimax.dk/tubestuf/dzindex.htm

    There was a lot to take in, some innovative, some brilliant, and some just plain weird. I like the clipper circuit.

  I thought I would make this post to show off some of Fred's handiwork, and I wonder if some of the circuitry could find its way into an AM modulator.

Good luck with your project!
Jim
WD5JKO

[N0WEK]

Bob,

    Many of us make fun at the audio folks. I miss the posts of the late Fred Nachbaur on some of the audio forums. One of the bigger projects that Fred did was the Dogzill project. This was a lot more than just six 807's in push pull parallel class AB2 making over 200 watts RMS of clean sound.

    Take a look at the Dogzilla project:

http://www.dogstar.dantimax.dk/tubestuf/dzindex.htm

    There was a lot to take in, some innovative, some brilliant, and some just plain weird. I like the clipper circuit.

  I thought I would make this post to show off some of Fred's handiwork, and I wonder if some of the circuitry could find its way into an AM modulator.

Good luck with your project!
Jim
WD5JKO

Great website with lots of good stuff. Thanks!

I was going to ask you what happened to him but then found this...

http://www.noteworthysoftware.com/composer/usertips/bio.php?id=46

[WBear2GCR]

Whoa Cowboy!

There are no watts on the grids?? (except if driven into A2)

Let's not be too fast here.
If the only problem is making 6L6/6550s work here, that is very very easy.

Here's what you do:
Put a ~2kohm 2-5 watt resistor between the plate of the tube and the screen. Then wire it so that the connections are to the same elements as the 6B4s were - different pins now, and the plate connections go to the octals now.

Be careful that you get the pinouts correct.

You'll need to have a variable negative bias supply for the grids, set it as low as it goes to start, check that voltage without the tube in place.

Once you put the new tube(s) in the hole, measure the CURRENT via the cathode resistor(s) (I presume there are cathode resistors - if not PUT ONE THERE! A 10 ohm will do fine for measurement). Figure out via ohm's law in advance how much voltage you WANT to see in idle for each tube, and then adjust the bias voltage more toward + until you see that amount. Test the unit using a scope.

Before you put in the driver tubes, you need to check the input tubes and make certain that there is A) no DC on the grids of the 6L6s, and B) that there is the correct good looking waveform present at each grid, and that it looks clean on a scope.

Then it should work perfectly.

The guitar amp idea is not good at all.
Very bad in fact, because the guitar amp is designed to have WAY too much gain, and to distort.

                 _-_-bear

PS. The manual says that in A1 P-P at 270v B+, you get a resting current of 135ma., so you are not far off. If ur B+ is higher than that, you'd get a correspondingly higher current - adjust ur bias more negative. They call for -17.5 vdc bias.

If the tube is run in AB2 at 360vdc B+ then the bias is -22.5vdc, and the output is 47 watts, vs. 17.5 for the A1 P-P case.

You only need a 6V6 or 6W6 in that hole as a driver... the 6L6 has more power than you need, although it is ok to use it.

Check the voltages, do the "triode strap" do NOT run it with fixed or tapped screen bias in this circuit - you want to get the triode characteristics here, and you don't need the extra power out at all...

Oh, ur saying that you have 35watts of POWER available from the 6L6s?
Ignore that. You only care about the voltage swing, there is no power drawn by the load, unless the load draws power! As I said swap in a 6W6 or the slightly larger 6V6 and you'll have less quiescent (resting) current, and less power output available, same pinout. Done.

 

PPS. You'll likely want to change T3, the B+ for the front end to a lower voltage so that the pentode type tubes (strapped for triode) can be used in PP class A within their proper ratings. The 6A3/6B4 type tubes want the 350vdc B+ but only make about 5 watts each... whereas the newer 6L6 family makes more power, and wants a lower B+ in class A... it's more of a class B tube, whereas the 2A3/6A3/6B4 tubes are designed as Class A tubes. So you adjust the operating voltages accordingly. 

[Opcom]

Bob,

    Many of us make fun at the audio folks. I miss the posts of the late Fred Nachbaur on some of the audio forums. One of the bigger projects that Fred did was the Dogzill project. This was a lot more than just six 807's in push pull parallel class AB2 making over 200 watts RMS of clean sound.

    Take a look at the Dogzilla project:

http://www.dogstar.dantimax.dk/tubestuf/dzindex.htm

    There was a lot to take in, some innovative, some brilliant, and some just plain weird. I like the clipper circuit.

  I thought I would make this post to show off some of Fred's handiwork, and I wonder if some of the circuitry could find its way into an AM modulator.

Good luck with your project!
Jim
WD5JKO

Storm The Castle!

[K4TLJ]

Here is an amp I threw together... maybe it could give you some ideas. It was quite a learning experience. Please excuse the hokie schematics done with 'Paint'. That was a learning experience too. 

http://home.comcast.net/~suptjud/HomemadeHiFi.htm

[flintstone mop]

I'll have to agree with BEAR. There's a little more to this than hooking audio from one device to another.
One way to cheat that I have done is to get an output transformer from a dead audio amp, preferably a transformer with 6L6's in push-pull.
Hook your geetar amp to the 8 ohm tap on this now-to-be-driver-transformer and the wires that were connected to the output tubes connect to the grids of the "modulator" (811's). What is the bias on 811's???  -22.5 VDC for class B? The center tap would be for the bias voltage to get the modulator the class of operation you desire.
A pair of 811's could modulate a pair of 4-400's fairly well.
I know that the audio folks will want you to bias for AB1 or Class A. But B'cast stations used class B amplifiers for modulation with great success.

Fred

[flintstone mop]

There are super suggestions on another thread down about "driving 811's"

http://amfone.net/Amforum/index.php?topic=18508.0

K1JJ has a great solution

Fred

[W4RFM]

I guess I did not fully explain myself.
What I did was to substitute the guitar amp, up to the output transformer, and there connect in the UTC S-9 driver transformer, which in turn will drive the 811 grids.  I did this over the weekend, and it works great.  In fact, I used clip leads to substitute a regular speaker output transformer and it drove a 12" bookshelf speaker very well.  So, in reality, I am using the Fender up to the output, there it gets the UTC S-9 driver iron, then the 811's. I have a UTC Ouncer input transformer, that will take 600 ohm at line level and connect to the volume control before the first stage of the 6SL7.
As to a question about watts?, the "handbook" says you need a 10 to 12 watt audio amp if you are going to do the back to back transformer trick with 811a's, so why not just build the 12 - 15 watt amp and change the iron?

[WBear2GCR]

...ummm, they call that a "modulator" circuit?

That's what the Orr circuit is... an amp with an IT (interstage transformer) instead of an output transformer... (wish someone had explained it that way when I was still a novice    )

...again the 6L6s or 6W6s will work fine in that circuit, as I described.

The guitar amp has suffcient gain to run a high-Z mic in... not sure if the Orr design has a mic "preamp" stage or not. Easy enough to add one more tube for that if you needed to - I looked at it over the weekend, but I didn't even look at the input end, was looking at the driver tubes since you mentioned that...

                   _-_-bear

[flintstone mop]

Hello
Somethng hit me in the head on this thread and it's not making sense.
Are you sure that this "interstage transformer" will handle the high voltage and audio coming from your 6L6's???
I cannot find a UTC S-9 specs
Usually WB3HUZ has listings for transformers on the "Window" and UTC doesn't have anything below S-14. An S-14 can handle 10 watts of audio. The higher the S- number the more audio power.
My suggestion earlier is a lot easier. Get an output tranny from a dead amplifier and hook it up backwards to the grids of the 811's. Keeping the guitar amp intact.
Am I on the right track???
Thanks
Fred

[WBear2GCR]

I suppose that depends on the "guitar amp" in question.
A 200watt Marshall  or a Fender Twin would be a bit foolish in that application.
A small "practice amp" sized unit might make sense from a power point of view, but the cost of today's decent name brand guitar amps is silly high, and kind of a waste in that application.

Then too, taking an existing amp, with output iron, then taking a second output iron to reverse the voltage step down is a bit wasteful of iron... but if you are cobbin' something together, I suppose it is "legal."

I don't know what the S-9 UTC is offhand, but you only need a couple of watts to drive 811s into grid current, so that's not too hard to manage.

Using a solid state amp with a reverse driven transformer is at least in my mind a different thing than taking a tube amp and dong the same trick, that doubled up iron thing bothers me - even though it can work.

Seems like the OP (original poster) had built up the Bill Orr '57 circuit already, so dumping in a different driver tube is simple enough... even converting to cathode drive isn't too difficult either.

I'd go with the right circuit for the application and use a "modulator" for the rig long term, no matter what one uses for "testing".


So, I'm quite confused here... anyone else?

                _-_-bear

[W4RFM]

Well maybe I can help.  The UTC S-9 is rated at 20 watts.  Orr called for it in his "General Purpose Modulator" and the 2 X 811a modulator in his 1956 and 1957 handbooks.  All I did was to change to a simple fixed self biasing amplifier design, with available tubes, that will faithfully produce up to 12 watts of power. (Again using the ARRL handbook suggestion of that amount of power to drive 2 X 811a's).  If I dont need it, I can vary the amount of screen voltage on the 6V6's and get the proper amount of power to the grids.  The tube manual however says I need 3.5 watts of drive to get 200+ watts out of the 811's with 1250 volts on the plates, so I have more than enough power. I guess the ARRL was thinking their would be a lot of power wasted in the double iron.
I think we have all over thought this project of mine, I know I have.  I just wanted to do it right.

[WBear2GCR]

I have to admit I am completely confused and lost.   

Did you actually build a version of the Orr design on a chassis?

Did that have a problem?

Are you using a guitar amp now as the modulator?

Or, if what ur using now is built up from parts and used the 6V6s (per an earlier suggestion) did you figure out the biasing and current draw alright?

        _-_-bear

[Steve - WB3HUZ]

He's using the guitar amp to drive a pair of 811s. The 811s are the modulator tubes.

[WBear2GCR]

what happened to the Orr design that was drawing "too much" current with the 6L6s then?

           _-_-bear

[W4RFM]

Okay let me try again.
First I built the Orr design that calls for 2 x 6B4G's driving the 2 x 811's. I had bad tubes, so I looked for something else to use as drivers.  I re wired the SAME chassis for 6L6GC's.  It worked finally, with way too much power.  So I then rewired the same chassis for a Fender guitar design that uses 2 x 6V6's and delivers about 12 watts of power.  NOW, the Fender design called for min. tubes, my chassis is already punched for octals so, I changed the 12AY7 to a 6SN7, and the 12AX7 to a 6SL7. Then I placed 2 x 6V6's in a self biasing design, and it works.  I have included a pot to lower the screens on the 6V6's to reduce the power to any level, as the tube manual says I only need 4 watts max.

Their is no guitar amp near my 811's.  I used the Fender DESIGN to make a modulator front end.  Here is an actual picture of the amp, I have not wired in the B+ for the 811's yet.

[Steve - WB3HUZ]

Yes, but have you played a guitar through it yet? 

[Opcom]

That is still a very nice looking modulator with the 6V6's. The driver plate and screen voltages you are running are more suited to them anyway. I'm glad it's working. Once it's final, can you post a schematic?

[W4RFM]

I wil post a schematic as soon as I get one together, and while I haven't played guitar thry it yet, I did build a Fender Bassman amp from scratch and play my old Precision thru it (but that's off the subject). Thanks guys, I appreciate the input.

[WBear2GCR]

Well, that's it's a long walk around the block when ur only trying to go next door? 

I admit that I am a bit thick and slow at times. But it seems to me that you ended up with a pair of 6V6s, and a whole lot of rewiring, when all you needed to do to the original design was to put 6V6s in there, and maybe to be simpler strap them for triode, to eliminate the screen bias issue entirely? Thus making the Orr design work as designed.

No matter, it looks nice, and as long as it works ok, it is ok.

                _-_-bear

[W4RFM]

You are exactly right, when I learned the 6B4G's were shot, I should have done the 6V6's instead of the 6L6's and wired them triode. However, for what ever reason, I find that I enjoy working at the bench and finding out what will work. I have a new modulation transformer to install now instead of the ART-13, Dan in PA. one of the regulars here, sent me a UTC CVM-3 that will now improve the fidelity, and let me re-wire some more.  I love this stuff!

[flintstone mop]

Yes, but have you played a guitar through it yet? 

LOL
Might be the source for the music heard on 3885, Steve

Fred

[W3FJJ]

Yes, but have you played a guitar through it yet? 

One time I plugged my  electric guitar into the mic jack on my unmodified
scratchy apache, and played it into  a dummy load (of course)
It had coolest fuzzy distorted clipped overdrive sound.  <all in a good way.
Vintage tone, that I think alot of guitarist would die for....

[W3SLK]

Chuck said:

One time I plugged my  electric guitar into the mic jack on my unmodified
scratchy apache, and played it into  a dummy load (of course)
It had coolest fuzzy distorted clipped overdrive sound.  <all in a good way.
Vintage tone, that I think alot of guitarist would die for....

It was those EL34's that added to the tessitura