18Watt.com

I have a couple OT's from a junked Conn 720 Church organ that I want to re-use. It was a 100 watt chassis with 2 pair of 7027A's and a pair of 7868's. The 7027A OT p/n's are K70272/3946401 and should be good for a 40 watt 6L6 build. The OT's are multi-tap and have black, white, yellow, and green wires. Before I look for a signal generator to test it with I thought I'd ask if someone knew what the color code might be. Could be 2/4/8 or 4/8/16 ohm. I want to try it for a 5E7 type build. So which color wire is which? Thanks.

Mark

White, yellow and green are typical standard colors, but you can't trust them for old iron. It is so easy to test for turns ratio that any builder using recylced iron should spend the five minutes required to get the measurement. To do this find a low voltage a/c supply. An a/c wall wart is good, if you can find one, but I use an old transformer's filament winding.

From primary to secondary, you are getting step-down, and often 6.3v applied to the primary is not enough to get readings on the secondary. Sometimes it works and it is a bit less work if you are able to get it done this way.

If you can't get readings going from primary to secondary, just go the other way. So, you make three tests, one leg of the supply to the common and the other in sequence to the white, yellow, and green. Test the supply voltage under load (it drops when connected to the tranny) and then clip your voltage meter onto the primary (outer legs; ignor the CT). Write down the results, do the math and then you know the turns ratio.

From there assume the 7027 is 6K primary and the 7868 is 6.6K primary. Divide the assumed primary (i.e., 6K) by the square of the turns ratio, and you will know what the secondary rating is. For example, you find a turns ratio of 27.5:1 on one of the secondaries. 27.5^2=~750. 6000/750=8ohms.

Once you get the hang of this, you will want to use salvaged iron in all your projects. I got hold of a very nice pull from a Scott for my TMB18W. The price beat the pants off of new brand name stuff and the quality is realy there.

Since you are new at this, look for turns ratios of roughly 19, 27 and 39 on the 6K tranny and 20, 29, and 40 on the 6K6 tranny. You get a bit of slack in this, so don't be concerned about precision. There is an inverse relationship: the lower turns ratios are for the higher ohms secondary.

I'd like to know what you find, so please post results and good luck.

I don't know if the manufacturer could help but they might have records going back to the first week of '64, you never know.

http://www.foster-transformer.com/

Thanks for the info. I have AC wall warts at work and several power supplies. I'll test this Monday. I did send an e-mail to Foster this morning. I'll see if they respond.

Mark wrote:Thanks for the info. I have AC wall warts at work and several power supplies. I'll test this Monday. I did send an e-mail to Foster this morning. I'll see if they respond.

I didn't realize it is a Foster. I once got a very nice reply from an engineer on an unknown tranny. I would never have figured it out. Good luck.

I remembered I have a Variac at home so here are the numbers. Primary reisitance 65 and 68 or 133 total ohms, secondary w/black common are Y=.5, W=.6, G=.7 ohms. With 20vac on the primary side I get about .65, .95, and 1.25 volts. The calculated turns ratios are 32,22, and 16. If I assume I need 4200 Load Resistance for a pair of 6L6's then 32x32x4=4096. Yellow it is. Should be fine. Thanks.