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Hi All,
A couple of years back I bought a pair of LL1682 transformers for use in a low-powered low-current 845 amplifier driving Altec 755As. It was an odd project, but came out beautifully. I ran the metal plate 845s at 450V and 50mA with a bias around -45 if I recall, driven by EF37 pentodes similar to a WE91A circuit. The Altec speakers were 4 ohms, so the normally 5.5k:5ohm LL1682s presented ~4.4K load to the 845s. The sound was surprising - the bass was really strong and powerful (kind of the opposite of what I expected). If the amp had a flaw, it was a little less delicate than I had hoped for, but that could have been due to the whimpy pentode driver tube, but all-in-all it was a very nice sounding amp.
Since then everything has changed, and I now have an 8 ohm system. I would like to put these transformers to use again. I have two potential uses in mind, but due to the 8ohm requirement, both uses 'tweak' the ideal situation for the transformer and cause it to be used in less-than ideal ways. Which do you think will have the least impact on the sonics?
Option 1: I really enjoyed the sound of low-powered 845, though I would probably do without the low-current pentode input stage this time. Now, with 8 ohm speakers, choosing the load is more complicated. The LL1682 is a 16+16:1+1 transformer. With secondaries paralleled, it presents a ratio of 32:1. With an 8 ohm secondary load, there would be an ~8.2K primary load presented (8 ohms * 32^2 = 8192 ohms). The amp might have put out around 4 watts before, so doubling the load to 8Kish would reduce the power significantly. I know that people 'misload' transformers all of the time, but as I understand it (from the math presented by Mike L. a long time ago on the SP mailing list) if there is a direction to err, ratio down. (In other words, make a 10K transformer 5K but putting a 16ohm speaker on an 8ohm tap, but not the other way around.) But maybe this is the least problematic use?
Option 2: The high load presented Option 1 gave me an idea for an alternate use. I have some 10/VT-25 tubes, and have wanted to build a nice amp with them for ages. Although the textbooks indicate that a SE 10 amp should have a 10K load, it's really not ideal. At that operating point, they have a 5K Rp, and a load of 2xRp is a bit light. You'd really rather see a 15K load (3*Rp). If 10k SE transformers are hard to make with decent bandwith, a 15K transformer would be really tricky. However, a paralleled 10 amp would only need a 7.5K load with about 40mA of current. So, using the above 8.2K load connection on the LL1682 into an 8ohm load, I could build a paralleled 10 amp with maybe 2 watts output. Still light on power, but it would be a 10 amp which would be cool.
Option 3: Not entirely ready to give up on the 845 plan, I did some calculations and came up with a 4W 845 option, but maybe at too much of a performance cost. If you wire the LL1682 secondaries in series you have a 16:1 transformer. When loaded with 8 ohms would present a 2k load to the 845, which is too low. However, if you call the secondary 16 ohms, then the CT of the seriesed winding is an 8ohm tap. Using a 16 ohm speaker on the whole winding, or an 8 ohm speaker on the center tap would reflect a 4K primary impedance to the 845. But I don't really know how bad a hit will the HF response take by connecting the secondaries in series. Does anyone have an idea? Of course, if I choose this option and it sounds like poop, I can always wire it as in option 1, but I would appreciate any thoughts or math you wish to weigh in with to help me predict in advance.
Thanks for your thoughts and time!
Tom
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