new iron for original SEX

[najo49]

Hi, What new output trannys available from magneQuest for original SEX mono amps with 6dn 7s. I plan on useing the dn7s,and expermenting with zero feedback. also going for better sound. Thanks in advance.

[Doc B.]

TFA-204s. That what I have on the original "original S.E.X. amps", the first pair of MQ transformers I ever got from Mike. Of course as our original kit those might actually be worth some money some day, so in the long term it might behoove you not to modify them.

[najo49]

They are very valuable to me already! I bought them for 200. and think I got a great deal. I am only interested in removing the feed back which is one resistor and a cap. I could always remove the mag. ot .,and restore them to original very quickly. Could you tell me about the iron that is on them now. Im listening to them now,with great satisfaction, on my altec Lansing horns and 16 ohm 15 inch driver,in an open baffel configuration. over 100db I think.

[Doc B.]

The original output transformers were something I picked up surplus. Don't know too much about them except that they had a rather low turns ratio. Installing the TFA-204s will require adding a bunch of new holes, that is why I was suggesting it might not be the best thing to do in terms of collector value. That of course assumes that they have any at all, which I don't know. If you got them for $200, they obviously aren't going for a ton on ebay. I would suggest looking thru the VALVE articles, as there are other ways to mod them besides the TFA-204s. PJ cooked up a parafeed mod, and I came up with a local feedback circuit at the cathode of the output tubes that worked quite well.

[Paul Joppa]

IIRC - and it's been a long time! - the original design had feedback in order to extend the frequency response, which would otherwise roll off at a few kHz - losing about two octaves of treble. The loop also had a capacitor to provide some equalization. There was also a 1000 ohm resistor in series with the output transformer to increase the impedance seen by the tubes; this resistor also reduces the output power capability - it was rated at 2 watts output. The following notes may help identify the version of your amp:

My personal amps were rev. 1.1, manual dated 1996. The photos in that manual do not show mounting holes for the TFA-204, but my chassis plates do have those holes. I have the original power transformer (copper flux band, 1.25" lamination stack). Later models has a transformer by deYoung, with a flux band and 1.5" stack. I think the last versions had the PGP8.1 Magnequest power transformer, which had no flux band and used solder lugs rather than flying leads.

The original output transformer was labeled 430V039C10 first line, and 668 6746 second line. It was a TV picture tube transformer. Mounting centers 3 1/8" apart, lam stack 3/4".

My manual has six pages of modification ideas - it really was an Experimenter's amp! The feedback loop was normally eliminated when using a better transformer such as the TFA-204.

[najo49]

Thank you, both for taking the time to give me this information! I have now picked thru the Valve mags and the schamatics. And have identified the transformers. being new to the forum I expected to be ignored for a while ,not the case! Thanks again.