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RE: How to use the 4 ohm tap in the SEX amp.

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In Reply to: RE: How to use the 4 ohm tap in the SEX amp. posted by TerryG on June 03, 2007 at 13:10:30

Strip the gray (4 ohm) wire 1/4 inch. Unsolder the white (8 ohm) wire from T5U, and replace it with the gray (4-ohm) wire. Be sure to leave the black wire from the primary side in T5U. Solder. Insulate the end of the white (8 ohm) wire and bundle it just like the gray wire used to be. You can clip the exposed wire portion, or tape it with electrician's tape, or slip a bit of heat-shrink over it.

Do the same on the other side, at T25U.

This will reduce the headphone output 3dB, but is very easy and reliable. It is in theory possible to use the 8 ohm tap for the headphones to retain the full voltage, while using the 4 ohm tap on the speakers, but the re-wiring gets pretty complicated, and most 'phones are too sensitive anyhow.

Follow Ups:

RE: How to use the 4 ohm tap in the SEX amp. - TerryG 20:17:46 06/03/07 (2)
Great!! Thanks so much, for some reason that is not in the manual, I can't find it anywhere, it says it is possible, but doesn't say how. My speakers have three 16 ohm drivers (essentially 5 ohms), so I would think the amp will like if I use the 4 ohm tap much better.

Why as an upgrade there is so much iron to replace, these OPT's seem to give out plenty of bass, why change them out? Also why the chokes, I don't have any hum or anything, they seem to be fine? I can see how the C4S boards will be a good thing though, and I see some caps that it would be good to upgrade, but I just don't see why the iron upgrade.

Thanks again!!

Terry
- RE: How to use the 4 ohm tap in the SEX amp. - Paul Joppa 09:56:44 06/04/07 (1)
  The stock transformer is indeed an over-achiever, sounding much better than one has any right to expect at the price point. We studied quite a large number of these inexpensive units before we found this one. But it cannot incorporate any of the superior materials or workmanship that you get from a premium manufacturer like Magnequest. And those factors do pay off in audible and measurable ways.
  
  The upgrade iron provides lower distortion at both the bass and treble ends of the spectrum, and more extended bass as well. There are too many effects going on to detail all of them in a forum post, but I'll mention two in the bass as examples:
  
  1) The stock transformer has about 100 henries of inductance at saturation, which is plenty for bass extension. (The 30 henry plate choke has a reactance equal to the load impedance of 8000 ohms at 42Hz; the 50 henry upgrade takes it to 25Hz.) But at small signal levels, 20dB or more below saturation, the inductance drops to 10 henries. The frequency response at these small signal levels is OK (it's set by the 2000 ohm plate resistance) but the load line becomes severely tilted and ovalized, increasing the tube's generated distortion. The upgrade transformer uses much better core materials, and has about 5 times as much inductance, as well as having lower losses. Both effects translate to less magnetic distortion.
  
  2) The stock transformer saturates at about 40Hz at 2 watts, so it cannot provide full power below that frequency, and distortion is elevated below about 80Hz at full power. The upgrade will saturate at approximately 24Hz at 2 watts.
  
  These are of course merely technical explanations. What counts is what you hear; most people who have heard the difference - including me - have found it significantly better.
  - RE: How to use the 4 ohm tap in the SEX amp. - TerryG 01:20:38 06/05/07 (0)
    The more I study this schematic the more inovative it is in its design approach. What is amazing to me is, it still maintains a fairly simple circuit approach. This could quite possibly be the best amplifier Bottlehead sells. At normal listening levels I am peaking at 1/2 watt, so I have plenty of headroom and wattage. I would like more refinement, and clarity though. One thing is for certain there is not even the slightest taint of harshness you have with solid state, it is as smooth as silk.
    
    It looks like the iron upgrade will give me the lower level detail I crave.
    
    Thanks for the explination.
    
    TerryG

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