Tubelab Simple SE & Edcor
I am in the middle of building my first amp..a Tubelab simple SE on a Hammond 17" chassis. It's looking so good I'm debating springing for the bigger Edcor CXSE OPT's (I have the smaller XSE's already 5K). I will be running the amp with the EL34's. My speakers are big (15") but relatively efficient Wharfedales from the mid 60's (15 ohm). Looking at it there are two potential transformers...5000 ohmsplate (5k) & 6500 ohms (6.5K). What's the difference & which is the most suited.
I have listened to a SimpleSE using the big Edcor OPT's and I liked the sound a lot, but those transformers are destined for a TubelabSE amp using 300B's or 307A's.
The impedance that you choose is a trade off. A lower impedance gives more power and more distortion, while a higher impedance reduces the distortion at the expense or power output. I have used 3K, 3.6K, 5K and 6K OPT's in SimpleSE's. I have come to the conclusion that the "optimum" impedance depends on two things. The impedance curves of your speakers and the B+ voltage that you are using.
If you have the impedance VS frequency curves for your speakers look at the area from 30 Hz to 2000 Hz. Does the curve spend a great deal of time below the "rated impedance" of the speaker? Are there any severe dips below the rated impedance anywhere (20 Hz to 20KHz) in the curve? If so you may want to consider a higher impedance OPT.
I can suggest the following:
If your B+ voltage is below 370 volts (a 650 VCT power transformer or lower) use a 3K OPT.
If your B+ is 370 to 400 a 3.6K OPT may be your best bet, but a 5K transformer will give a lower distortion with a slight loss in power.
If your B+ voltage is between 400 and 450 consider a 5K OPT.
If you really crank the B+ over 450 volts, consider a 6K or higher OPT.
I have a SimpleSE with 3K OPT's that were designed for 300B tubes. They work great on my small Yamaha speakers which are rated "8 OHMS" but the curve spends most of its time above the 8 ohm line, and ventures up to 20 ohms in the bass region where the distortion is more noticible. When I connect the same amp up to my big speakers (actual curve unknown) the bass is often distorted, so I connect the "8 ohm" speakers to the 4 ohm tap. This presents the tubes with a 6K ohm load and sounds great.
Without any knowledge of your sprakers, I would choose the 5K transformers if you are using the usual 375-0-375 (750 VCT) power transformer.
I will go 5K as the B+ will be around 450 & the power tx is the standard Hammond. No info on the speakers as all wharfedale records were destroyed in a fire. No doubt I will have a couple more questions when I get it close to finished.
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