The AM Forum

Here we go again.  So begins the journey to provide my 2x 813 transmitter with sound.  An Altec A-333 amplifier with an RCA 300 watt modulator driver and 2X 811A stage tacked on.  Got to be fun. The mod reactor is a broadcast unit of 70+ Hy's. The mod transformer is a Stancor 300 watt buggar, it and the reactor will be on another shelf.


(http://s20.postimg.org/lfud0hm4d/2015_12_22_19_05_07.jpg)

(http://s20.postimg.org/it5ns2g59/alta333a_RCA.gif)

Nice collection of clean parts.
You need another 811A.

T1 looks to be important since the 811's will draw grid current and present a variable load..
I think a step down ratio is often used.

Odd place to get the feedback from...

Brett, I ain't sure about that feedback connection. It "should" come off the 16 ohm secondary to do it the way Altec intended, (not having one anymore)  I hooked it to the lowest impedance secondary I could find.  I may move it after experimenting, or lose it completely and see what the resulting sound is like.
Here is the way RCA planned the 300 watt modulator using 807's.
(http://s20.postimg.org/ax4xoxd19/811a_RCA_Modulator.jpg)

The secondary of the driver transformer is not the same voltage level as the secondary of the original transformer designed to run speakers. You'll need to adjust and scale the values of the feedback loop.

It may not make sense to include the screen of the first tube in this, I'd go only cathode feedback in this scheme.
But ymmv...

My thought is to not include the secondary of the transformer at all, but to take the feedback before the primary or at the primary and run the secondary and the mod iron "open loop". The reason being that feedback around transformers, unless bandwidth limited tends to make for ringing and possible instability. The more feedback used, the greater the potential for instability and ringing when there is a transformer in the loop.


                 _-_-

When you put feedback around a bunch of transformers, you need to limit it otherwise the phase shift causes all sorts of problems.
You can put a good amount of neg feedback around the driver stage which helps I hear.
Easy to experiment with, and picking values and including a pot allows you to change the tone somewhat.
I once had a setup on my 32V3 off the secondary of the mod trans to an early stage, bypassed for highs because of the phase shift, and increasing the feedback cut the low frequencies and cleaned them up somewhat and reducing the feedback made the lows stronger.

I think some hifi tone controls used to work that way...

Well I have all the parts now, and they fit in the allocated space, so now we start burning fingers, melting solder, and hopefully getting something accomplished
(http://s20.postimg.org/9doi9rkcd/ALTEC_RCA_Mod.jpg)

Nice and clean, plenty of space, you have to love home brew...

Octals & big-pins.  Yum. ;)

73DG

What about all the goes inta's and goes outa's?

Nice...Painted chassis? or anodized?   

Painted, sand with 120 grit in an orbital hand sander, then coat with alum primer, then I use smoke gray from Dupli-color, auto parts store).

It's very beautiful!