OPT with secondary CT

[punkykatt]

Hello all,  I have a Hammond organ series K-100  power amp. The OPT has a secondary center tap. This is a first for me. Can someone here explain whats going on? And also what is the total speaker Z load on the secondarys of all three speakers? Does that 20uf cap change things? The schematic that came with the amp is way huge to post here, so I drew a section of the OPT. The amp is running 7591 power tubes cathode biased. The amp is rated 25 watts. I did a turns ratio test and came up with 5k primary with 4 ohm load or 10k primary with 8 ohm load.  Thanks Punky

[67polara]

Looks like 4 ohms to me.   Cap is used so only highs go to the 6 in speaker (tweeter)

[sluckey]

The grounded center tap is not being used for the speakers. Grounding the CT is necessary for the NFB circuit though. It provides a common ground reference point for the secondary which is important for the NFB to work properly.

[punkykatt]

Thanks sluckey. So I can use that OPT in a guitar amp build but dont ground the CT just ground the black wire correct?

[PRR]

Look at the impedance in the "power zone", 150Hz-700Hz.

The cap impedance is higher than 8 ohms up until 1,000Hz. So the six is (as polara said) just a tweeter, and hardly-any load at the lower frequencies.

The two twelves are parallel for nominal 8 ohms. "We know" these are woofers; because it is/as an organ, because there's two big cones for a mere 20W-30W, because there's a tweeter. Bass-optimized woofers with big/long coils have a rising impedance above 800Hz-1,500Hz. Combined with the cap and tweeter, the impedance is more-or-less 8 ohms from deep bass to quite high.

It's 8 ohm secondary.

When speaker leads go out of the box, you have to ground the speaker line somehow; specifically so when the OT gets a short from the 400V side to the speaker winding nobody has to die.

When speaker leads are run 300 feet side-by-side with microphone wires, the big speaker signal leaks onto the weak mike signal. Less if the mike line is balanced to ground. Even less if the speaker line is also balanced to ground. While "we never do that", when putting an organ and PA in an old church it was common to run ALL the wires together from altar to loft to cellar or wherever. Hence the balanced speaker output.

For Your Use: guitar speakers do not have to be balanced, and the standard jack "forces" one side groundy. Yup, lift the CT off ground, ground either side. The former CT is now a 2-Ohm tap, which may/may-not be worth bringing out.

You usually (not always!) want NFB. If you keep the Hammond driver and power-tube section intact (a good plan; Hammond was a musical dude), then you ground the black and _double_ the value of the resistor just off-screen to the left of "NFB". if you don't, the NFB doubles and the beast just might get unstable. Don't be fussy: the best NFB for organ is not best for most of the reasons we love tube guitar amps. You will probably want to go 2X to 10X higher than what Hammond used, after you try several speakers and all your tunes.

If you hack-up a new driver and power section, you will have to re-invent the NFB just like most from-scratch rigs. Smoke-test with no NFB. It should play, a bit raw and brash but no outright problem. Steal a NFB network from the nearest known-good plan to yours. Hook it up. Power-up with yout hand on the off switch. If it comes up in a big howl, shut-down and swap the plate leads. If it comes up "OK" but _more_ raw and brash, then you have Positive FB which will give trouble sooner or later; swap the plate leads. Ideally the NFB will "tame" the amp: a little less gain, less bass-slap, less shrill. Adjust the NFB values to taste.

[punkykatt]

Thank you PRR for that very informative post. I will print it out and keep it with that OPT for when I actually get to that project.  Thanks again Punky