Re: VTTC modulator schematic question

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Sue Gaeta <sgsparky@xxxxxxxxxxx>

Hi Phil,

I was running the plates of the oscillator at 1000 volts at 200mA. I 
adjusted the modulation to peak at 200mA max also. I never really scoped 
around the modulator. I bought the thing at a hamfair. It was someones 
homebrew modulator, and it was missing the tubes and the mod transformer. 
The mod transformer was acquired separately. It was out of an ART 13 
tranmitter, and it is the worst mod transformer you can find for this type 
of work. It is made to reproduce telephone quality audio, and when you feed 
bass into it it just saturates. I was working with a transmitter guy at the 
time, so I had access to parts and these old moldy radio handbooks.

All I did was add a filter choke because the voltage was too high in the 
pre amp section, replaced the filter cap, and fed audio into the thing, and 
hoped for the best. It worked, so I didn't have to troubleshoot it, and 
therefore didn't get too familiar with the circuit.

It sez here in my tube manual for a class B modulator (which is what I got) 
that you typically run 185 peak AF volts from grid to grid when using 1000 
volts on the plate.

No, you can't use those 70 volt transformers, they will arc and smoke.
I used high level modulation, so the mod transformer must be able to handle 
your plate voltage. I would say it should be rated at 2500 volts. These 
things are tough to come by. You need to get in touch with the ham radio 
community, particularly the AM group.
If you go to <http://amfone.net/index.htm>http://amfone.net/index.htm and 
click on "Links", there are some transformer manufacturers listed there. 
Peter Dahl is the one that I have heard of people going to a lot for mod 
transformers, and plate transformers. As for the turns ratio, you go for 
impedance matching. In other words the primary impedance = driver voltage 
divided by driver current, secondary = plate voltage divided by plate 
current. Or better yet, just tell Peter Dahl that you want to plate 
modulate a pair of 811s, and he'll know what to do.

Ah yes, I never did find that old ARRL handbook. It must have been at the 
WBCQ transmitter site. I could tell from the tube lineup that the guy must 
have built the modulator right out of that old book! I have a 1945, 1952, 
and a 1960s ARRL handbook, but it doesn't have that exact circuit in them.

Sue

Tesla list <tesla@xxxxxxxxxx> wrote:
Original poster: Phil Rembold

Sue,

I know you've moved on to bigger and better things, looking in the
archives found a post you made that had a VTTC modulator schematic. No
grid bias resistors or bypass caps. This is sweet !!!

But I do have some questions about the driver and plate transformers :

For good modulation, how many mA of current runs through your 811 tubes ?
How many volts peek should the driver put on the grid of the 811 tubes ?
For the plate transformer, the wire gauge should be enough to handle
the current of the coil your driving. What primary / secondary turns
ratio is ideal ?
Did you "borrow" these transformers from old audio tube Amps ? Would a
70 Volt line transformer from Radio Shack work or do you think a
person could wind their own ?

Thanks for posting this design.

P hil TCBFW


----- Original Message -----
From: "Tesla list"
To:
Sent: Friday, May 30, 2003 8:38 PM
Subject: Re: Successful SSTC Audio Modulation - SWEET!

> Original poster: "S Gaeta by way of Terry Fritz "

>
> Hi,
>
> The tube coil schematic can be found at
> http://community.webshots.com/photo/8426665/48494920pFtYul
> and a simplified modulator schematic can be found at
> http://community.webshots.com/photo/8426665/48496190jiruxv
> The modulator is not my design. I found it in an old (before 1970) ARRL
> Handbook, but now I can't seem to find it again. When I do I will post it
> since there seems to be an interest.
>
> Sue