The AM Forum

While locked in a small apartment waiting for my shop rebuild, I thought I might build a small, quality audio modulator.

Thinking I can improve the audio by using a capacitor and reactor to keep the DC off the modulation transformer winding, I am assuming that the inductance of the reactor needs to be equal to or more than the modulation transformer inductance with rated DC applied. More inductance would improve low frequency response?

First - is this a correct assumption?

Can the true inductance at rated DC be measured by applying a DC current to the winding along with an AC signal and measuring the AC voltage? Would it be better to add a capacitor in series and find the resonance frequency, and calculate inductance.

Thanks

Pat Bunn
N4LTA

hi Pat. ... a figure of merit for inductance reduction has not been mentioned in the literature Have read ... radio lore holds that it will reduce inductance up to 10 times with less reduction depending on inductor construction ... this is the same difference between regular chokes and swinging chokes .... swinging chokes reduce inductance with increased dc current flow with something like 5 to 1 being a norm.... the swinging choke not having a magnetic gap while the non swinging choke has one

interestingly I don't recall that the transformer models show this effect but it has been a while ... 73.  John

Here is an old post from Patrick Turner on the still going rec.audio.tubes group from 2008:

https://tinyurl.com/yaoj4a7m

Pat used to really get long winded!

Here is a simpler thread by Matt Warren:

https://tinyurl.com/ycdwblzl

Jim
Wd5JKO

Thanks John and Jim

Am I close in estimating the required inductance for the choke  to be roughly the same as the inductance of the Mod transformer under working DC current? I have seen that the modulation reactor needs to be 8 H /1000 ohm of modulation resistance but that makes for a physically large reactor compared to a small 30 watt modulation transformer.

Pat
N4LTA

I would say yes - that is that low freq content will roll off and phase shift with comparative inductances - more inductance is better

as others have said most any inductor can be used for a reactor - I would add with min inductance met - watch the swingers

at the 30 W level I would assume a fairly low current value for typical hollow state modulators - the current ability dictates the size for equal inductance

I have a 30H 110 mA that isn't a swinger. I'm thinking that will work. Also have a 20H at 150mA but that seems a bit low. It will be fun to play with. The Mod transformer is a Kenyon 493 rted at 40 watts.

I thought about using an 807 as the class C final but a while back I had a heck of a time taming a pair on 75 meters. Lots of parasitics. Maybe better with a 6146 running CCS at 450 volts or so.

Any other tube candidates for a 50 watt input class C telephony tube?  I didn't see many searching around some old ARRL handbooks from the 50s.

Thanks

BTW - The power transformer that you sent me looks like it made it through the fire.

Pat
N4LTA

Hi Pat,
I think that’s the same mod transformer I used in my first home brew rig last year, a single 807. I run 600v at 110mA cathode current for about 44w rf on 40m. Eighty is less, my guess is that has to do with output network and driver more than anything.  As a first try, I’m ok with it, but there are of course twenty things I’d do differently.  One tube I played with and rejected due to being unable to tame parasitics was an 809 triode; my 807 is stable and was stable even on a breadboard with wires outed everywhere and no shielding. Maybe we can have opposite luck :-)

Ed

I always thought it'd be cool to build up an 829B. Have a couple here, so maybe soon.

Phil

I have a bunch of new 807s and 1625s. Maybe I should try again?

My trouble may have come from paralleling them?

I had an 829 but the fire may have gotton it.

Pat
N4LTA

I think my firs try will be an 807. I found some 829s but the plate current will be too high for my choke (and my modulation transformer.

Pat
N4LTA

Yeah, I didn't look too carefully at something spec-wise. O.K., I know the B+ will be a little higher, but what about a 3C24? Just trying to throw some workable, but not so common ideas out there. 

That tube (3C24) fits the bill nicely with a plate current of 60 mA at 1KV. I think the modulation transformer can match that modulation resistance also at 16,600 ohms to 8000 ohms for the two 807s.

I may try to find a couple to play with.

Ah! someone else with a Kenyon that might be of the same series. I have a T495 here that I haven't had luck finding connection details on. Do you have any details on the T495?

And since I'm packing things up to move, I'm finding a lot of cool stuff collected. I'm really tempted to try building another hollowstate rig again.

There’s a link on the left side of this page under Tech, On The AM Window, to Kenyon and other mod transformer specs and connections. It includes the T495.

Ed

Any other tube candidates for a 50 watt input class C telephony tube?

After becoming enlightened by this group I recommend the 4D32 tube. Will do 50 watts without breaking a sweat - all day, and at 600 VDC.
With 807's or 6146's you could keep everything in the lower voltage realm.

I have a couple 3C24s and it is a very "cool" looking tube. Specs show it at 1KV and u p at less than 75 mA. I read that it lights up like a light bulb.

A couple running at 750- 800 volts at 125 mA might be interesting at about 100 watts input or a single running at 900 volts at 70 ma at 63  watts input.

The twin 807  modulator is good for about 40 watts max with the Kenyon 493 - should be ok with one tube. May a bit larger modulation transformer but getting very hard to find.

A 60 watt transformer would be ideal. A CVM 2?   Any out there or are they unobtainium?

Pat

A pair of 807s should be able to hit 70+ watts if you have sufficient plate voltage and drive them into AB2. The 493 is rated for 40 watts, but remember it was designed to carry the plate current. If you are using a choke and a cap to keep the dc off of it, you’ll probably have some saturation headroom...

Ed

Hi Pat,

I once built a single 24G (3C24 equivalent)  plate modulated by a pair.  It was an amazing tube. I found I could get out 90 watts carrier and 150% modulation peaks when it was driven past the grid current recommendation. They are honking triodes for their day.  I had 1800V on the single one IIRC.  Just to see what it would do.

I found it hard to find good ones without gas. Most lit up blue when first tried. Maybe there is a good procedure to bring them up.

I am tempted to make a pissweaker rig using them but maybe reduced down to 30 watts or so.  You will have to neutralize the final.  Really good-looking and fun tubes. The plate looks so tiny for what it can do.


Tom, K1JJ

 

I have never built a Triode output tube transmitter. Thinking seriously about using two 3C24s in parallel and trying it. Any sugested circuits or guides. Also how to neutralize it?

Thanks


Pat
N4LTA