Alternative to James OPTs

[sorenj07]

I've recently gotten a very reasonable quote from Edcor for some 3K/8R 15W SE transformers so I'll probably go with that. There's a $20 designing fee but after that, each one is $60 a pop I assume this is at standard 20Hz-20KHz bandwidth. This might be the way to go!

 

[Kyle K]

"Let me get this straight, do I just connect the (-) of the cap to the white lead of the secondary and still have the yellow (8 ohms) (+)and black (-) leads going to the speakers?"

That's correct. Using the white lead for the feedback connection maximizes the amount of feedback. Keeping the speaker connected to the yellow lead preserves the 5K primary setup. You can also try connecting the (-) of the cap to the other taps for reduced feedback.

quote:
Then try the white secondary to drop the primary to 2.5K. You might think that's a little low for the 6EM7's 750 ohm plate resistance, but the cathode feedback will compensate for it and you will get a little more power. You only get 5K by the turns ratio anyway - inductance isn't increased to support the higher impedance.

"Now in this test both the cap (-) and the speaker (+) will be connected together. Is this correct?"

Yes, that's correct.

"Question... Is this negative or positive feedback? I always thought that we would use the black wire for feedback to the cathode for negative feedback (sorry I do not know what the difference is). I think one will add and the other would subtract from the outputs????"

There's an explanation in Claus Byrith's article on Lundahl's website:

http://www.lundahl.se/claus_b_se.html

With the Hammond 125ESE the connection works correctly this way. With Edcor's XSE series the connections for feedback and grounding of the secondaries have to be reversed.

Byrith actually runs the cathode current through the secondary, and I can say that it works well that way, but causes a small amount of DC offset on the speaker terminals. Connecting just the cap will not do that.

The only down side that I could hear using cathode feedback was that when driven to clipping, it clipped harder, so you will have to see if that's a problem for you. It only happened when played very loudly with someone's bass demonstration CD. We were running the 6EM7 much harder than you are, about 240V plate to cathode at 40mA.

 

[Kyle K]

"I've recently gotten a very reasonable quote from Edcor for some 3K/8R 15W SE transformers so I'll probably go with that. There's a $20 designing fee but after that, each one is $60 a pop I assume this is at standard 20Hz-20KHz bandwidth. This might be the way to go!"

Periodically some smaller, cheaper Transcendars come up on ebay. There's a pair available now - 3K:8 ohms, 100mA, 20H, rated for 10W. Buy it now price is $90/pair. No srceen tap though. Do a search for seller "ggaet".

Does anyone have any measurement data on the Edcors? I tried emailing them to ask about primary DC resistance, DC current ratings and primary inductance but got no response.

 

[RockysDad]

Cool! Let me try this and I'll report back with my results. Mahalo plenty...

Sorry, for hijacking this thread....

As for the custom made Edcors, for another $20 might as well get the CXSE 25 Watt series. They look pretty cool. Nice color too. But then this comes back to what the James XFMR cost. We've then come full circle...

 

[sorenj07]

$20 x 2 = $40 for extra power, weight, and chassis space that I'd rather save. to be honest, i'm beginning to like dealing with Edcor, because they always come through and have reasonable prices. if anything, it's kind of a loyalty thing that has me pursuing this company.

 

[RockysDad]

For cathode feedback try the following:

You don't need 1200uF. The 47uF specified in the original schematic is sufficient to get 1.2Hz at -3dB. I've found that smaller film caps sound much better than large value electrolytics - especially in the bass. You could use a 30-47uF @ 250-400V Solen or similar. Not too costly.

Just connect one lead of the cathode bypass cap to the cathode (pin 3) and the other lead to the white secondary lead of the 125ESE. The connection to ground is now through the OPT secondary. If you are using electrolytics, observe the polarity. I assume you are using the yellow secondary to give you 5K:8 ohms. Try it that way first and see how it sounds. Then try the white secondary to drop the primary to 2.5K. You might think that's a little low for the 6EM7's 750 ohm plate resistance, but the cathode feedback will compensate for it and you will get a little more power. You only get 5K by the turns ratio anyway - inductance isn't increased to support the higher impedance.

I tried this over the weekend and came to the conclusion that its better to get the higher priced transformers. You get what you pay for. It seems that I lost some of the magic of SE when feedback is used. The BW seems to widen a bit but then I lost the airiness that SE provides. The vocals sounded rather flat vs. floating in air. Again, its a tradeoff I guess. So its back to saving all my pennies to get a pair of higher end trannies from James. KyleKs link to the lower end Transenders looks promising also... Thanks again for all your help...