Need help on a old circuit - early Lafayette amp

[solidjackson]

I'm not sure where exactly to put a topic like this, but this forum seems the closest to what I'm asking.


You may have seen this on the tube forum a couple of months ago. It's a "Auburn Amplifier" made by Wholesale Radio Supply. I researched the brand name and found that this company was the forerunner of Lafayette Electronics, but couldn't find a schematic in any books.

I eventually decided to draw a schematic.

It's probably pretty inaccurate, because I've never drawn one before. The transformer was pretty tricky because there were about 18 solder taps on the bottom (it's got a Victor logo on it, which is kind of cool). I hope you guys can read this.



I've replaced all the old electrolytics, but I'm left with about 3 wax-paper capacitors like the one in the middle of the picture. These don't have any value on them, but they have numbers like "85397", or "5957" printed on them. Does anyone have any idea what these are? Also, what size coil will I need to put in to replace the speaker's field coil? I wouldn't mind hunting down a field coil speaker, but it seems if most of them are proprietary to manufacturer.

[jeyurkon]

If they're not shorted try measuring their capacitance. They're probably really leaky, but hopefully still good enough to find the capacitance.

I don't think the values are critical, except for the tone control. You could try a 0.002uF for that one and see if you like the tone range.

I see that oldradioparts.com has catalogs from Wholsesale Radio Supply. I don't know if they would be of any help.

The choke could be a few henry. But, the field coil usually also has a significant series resistance. You might need to get that approximately correct. I'm also puzzled by the line going to the speaker, unless it's meant as some sort of hum bucker. I'm guessing that it's really just a jumper on the speaker plug to keep you from powering up the amp without a speaker.

John

[wa2ise]

The guys over in the "Tube audio" forum may be able to assist.

[Reece]

John beat me to it on the AC line to the speaker plug. That's what it is: there'd be a jumper in the plug to keep you from frying the tubes without the load of the speaker connected. The push-pull output transformer was on the speaker, too, and the screen line runs over there to it. Maybe studying some other push-pull output circuits would help to get a handle on it.
Reece

[solidjackson]

Thanks for the information.

This week I was able to talk to someone who's been around some of this old tube stuff, so I might get some help with the speaker. I'll take a look at some old push-pull schematics and see what I can find.