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In Reply to: RE: Line output transformer for 76 linestage ... posted by KevinC on December 26, 2008 at 04:44:37
Thanks for those suggestions !!!
My question now then, is which is the better transformer ? The 76 will be running at 5ma current (should there be some headroom ? Will the 1680 saturate at 5ma ?), and according to the datasheets, has an Ra of 9500 ohms (can someone verify therefore, that 15k:600 would suffice ?).
Will you be terminating the secondary of the transformer with a 600 ohm resistor?
If you terminate the secondary with a 1200 ohm resistor the transformer will reflect 30K ohms to the tube's plate. And so on.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Mike,
An asylum member who goes by the moniker 'keto' has built a preamp using the 76 tube (great tube btw) and did indeed use a 15K:500 transformer with success. Running the 76 at 5mA should be fine. Try 160V on the plate and about 7V on the cathode (using a 1.5K cathode resistor). Depending on who you talk to, some may say that using the 76 and other high rp tubes with transformer coupling may be a slight compromise as it doesn't comply with that whole 3:1 ratio. But who came up with that ratio? I think a lot of this is one person's experience versus the next.
Designing a transformer with a 20K or higher primary and adequate high frequency performance is much more challenging which is why you don't see a lot of them.
Anand.
I can't tell you which will be better with a 76, because I haven't tried either with a 76. There is no issue with saturation of either transformer at your operating point, so that's just not a factor. There is no problem with using the 76 with either transformer. The implication you might see in print that you have to "match" the Rp of the tube precisely with the transformer impedance (e.g. 15K:600) is simply incorrect. A good transformer will "cover" a range of Rps. For the LL1680 and the LL1660, Lundahl recommends that one does not use a tube with an Rp of greater than 15Kohms unless one can compromise the high frequency response listed in the specs.
Kevin Carter
K&K Audio
www.kandkaudio.com
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