The Transformers- more than meets the eye

[Pabloaguas]

Hi Everyone,

I have found this forum to be very helpful for my ametuer tube amp musings. Much thanks.

Can anyone help me with OPT calculations? There must be an equation for this somewhere, but I can't seem to find it.

I'm trying to calculate the primary impedance and how it varies based on the speaker impedance. My speakers are 6-ohm Audio Note "K". So for most applications with 6-ohm speakers, there is no 6-ohm tap, and therefore I would choose either the 8-ohm or 4-ohm secondary. So wondering how to calculate the variation on how this affects the primary winding.

For example, if I were to purchase a Hashimoto HC-507U or H-207U, and use the 5k primary tap, and hooked my 6-ohm speaker to the 8-ohm tap, then what would the primary impedance actually be? Same as if I use the 4-ohm tap with my 6-ohm speaker, what would the primary be? And if I use the 7k primary and 8-ohm tap with 6-ohm speaker, what would the primary impedance be?

Maybe Peter Q will chime in- I believe that he recommends to just use the 8-ohm tap for his speakers across all applications. However, with my current transformers, which are clearly not the right ones for SE KT88 because they are only 2.6k (Audio Note T-150), the 4-ohm tap sounds better because it would seem to raise the primary impedance when tapped with 6-ohm speakers. Am I thinking this correctly?

So- if someone can provide an equation of this it would be much appreciated!

Or, just the following 3 calculations would be appreciated:

A. 5k primary, 8-ohm tap secondary with 6-ohm speaker

B. 5k primary, 4-ohm tap secondary with 6-ohm speaker

C. 7k primary, 8-ohm tap secondary with 6-ohm speaker

(For this application, I'm looking at using either of the abovementioned Hasimoto transformers in UL mode, and my output tube is GEC KT88 equivalent with cathode bias and some global negative feedback, so thinking I want to be somewhere in the 5k-6k range for the primary of my OPT)

Thanks everyone

BTW, will be selling a used pair of AN T150 OPTs after making the switch to something more appropriate for the KT88

 

[zackthedog]

Practically speaking, most speakers represent a varying load depending on frequency, so trying to anticipate how a given speaker will affect the reflected load while playing music is kind of a wild goose chase. The best way to choose an output transformer is to consult the tube data and choose the recommended load for your preferred operating point and power requirements. Then connect your speakers to the closest (or in your case, recommeneded) tap and assume the best. ;-) Also, I'm not sure why you think your output transformers are not appropriate. Pete Millett uses a 2.5K load for his E-Linear SE 6550 amp. 5K seems to be more common in many designs, and I imagine that's meant to reduce distortion to some extent. But there's nothing wrong with a 2.5K load as far as I can tell.

 

[TriodeLuvr]

A. 5k primary, 8-ohm tap secondary with 6-ohm speaker
B. 5k primary, 4-ohm tap secondary with 6-ohm speaker
C. 7k primary, 8-ohm tap secondary with 6-ohm speaker

They're directly proportional. Just apply the difference in load Z to the primary. For example, in (A), the secondary load is being reduced to 75% of the rating. Applying that to the primary yields a reflected impedance of 3750Ω. Alternately, divide the primary Z by the secondary Z (in this case 5,000 ÷ 8 = 625) and multiply that by the new load. As @zackthedog said, speaker impedance is all over the place, so it's futile to rely on this type of calculation.

Jack

 

[gadget73]

I do the divide and multiply thing but it all comes out the same.

Also something worth mentioning, things without a center tap on the secondary are rarely optimally placed. If you have a 4, 8, and 16 ohm tap, the 4 ohm is at 50% of the winding, and that tends to be a fairly accurate placement. The 8 ohm is 70.7% ideally, and its rarely ideal. The 16 ohm is 100% of the winding, so that is what it is.

If its a 4 and an 8, the 4 ohm is usually going to be off by some amount from optimal for the same reason.

that plus the speaker impedance curve thing tends to make a lot of this a situation of taking your best guess when designing it, then hooking speakers up and deciding for yourself which tap it sounds best on. I have a set of 6 ohm Klipsch and some amps they like the 4 ohm, others they like the 8 ohm.