Welcome! Need support, you got it. Or share you ideas and experiences.
Return to K&K Audio / Lundahl Transformers
98.206.37.100
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
My one reply has disappeared. (thanks for the encouragement though!)
Maybe I can rephrase my question more simply:
Which is worse for frequency response/sound on LL1682 transformers?
(a) 8 ohm speakers on the 5.5 ohm tap, or
(b) putting secondaries in series and using only 1/2 the secondary winding
Or are my concerns academic and it will sound find either way?
If no one has any input, I'll just build the 845 amp so I can swap both ways and I'll report back.
I would guess that 8 ohm speakers on the 5.5 ohm secondary would be preferred.
Kevin Carter
K&K Audio
www.kandkaudio.com
As I said in my first post, I was a bit concerned about the power loss when putting such a high load on the tube (but gaining lower distortion in the trade-off). I ran some numbers tonight and here's what I came up with:Power Out 3.16 W
into impedance 8 ohmsVoltage needed (at speaker) 5.03 V RMS
OPT pri 8200 ohms
Ratio 32.0Voltage Needed (at plate of 845): 160.97 V RMS
845 mu 5.5
845 Rp 1700
845 Gain 4.56Voltage Needed (at grid of 845): 35.34 V RMS = 99.96 V p-p
That means that with an op point of -50V bias, and an ~8k load, I can still get over 3 watts out. (FYI, the same calculations run on a 4100 ohm load yields 4.6 watts with the same -50V bias.) I was expecting more power loss, so that's a nice surprise.
Three watts is plenty for me, and I like the idea of the lower distortion. Looks like I'll go the 8K route, but I may still give a quick listen to the seriesed/4K configuration just to hear the difference.sonically.
Thanks, Tom
Edits: 10/17/09 10/17/09
Post a Followup:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2009, Audio Asylum, all rights reserved ]
[ Go Green ]
[ Take5 ]