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Old 29th September 2012, 10:25 PM   #2041
Magz is online now Magz  United States
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Quote:
Originally Posted by Magz View Post
Well, I finished up my 26 preamp last night and man, does it sound great! I'm running the 26 directly into the AVC in a parafeed arrangement using the mu output of the plate ccs. Square waves look excellent at all volume settings, frequency response extends below 10Hz (the limit of my equipment), and even with only 3 hours running time it sounds clear, punchy and warm with rock-solid bass even at lower volumes. Unfortunately I had fly to Florida for work today, until next weekend, so details, pics, etc. will have to wait.

The AVC is the Bent Audio TAP, with full remote functionality, stacked 3x3 for about 150H inductance. Using LL1676 transformers on input for galvanic isolation, phase inversion and gain cut 2:1.

Stay tuned, more details in a week.
Quick clarification. The AVC's are functioning as both output autoformers and volume controls. Since I didn't need the LL1676's as output transformers as I originally intended to use them, I put them on the preamp input as noted above. It's totally silent, no hum, buzz, or hiss. I'm very happy with the sound so far.

I'm using battery grid bias so that I can run the autoformer return back to the tube cathode since it is at ground potential. The autoformer cannot handle the DC which would be present in a cathode biasing arrangement. I've found from experience that returning the current to the cathode in a parafeed setup sounds better than returning it to ground.

Last edited by Magz; 29th September 2012 at 10:33 PM.
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Old 30th September 2012, 09:16 AM   #2042
H.Michi is offline H.Michi  Japan
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Originally Posted by andyjevans View Post
Yum yum, that's a tasty transformer. Feeling jealous here - was it expensive?
Yum Yum, good taste is always cost high!
Following OPTs available used Finemet core.

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Noguchi : FM20K600CT FM24K600CT
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Old 2nd October 2012, 11:06 AM   #2043
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Beautiful construction Magz.
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Old 2nd October 2012, 04:07 PM   #2044
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What would be a good transformer readily available to buy online to start building one of these pre-amps?

There are so many schematics, and I am still trying to pick one. I have a friend I can call out for help and advice and of course all 205 pages of this amazing "thread", though this seems much to informative to be just a thread...

Thanks

Mark

P.S I haven't gone thru all of the pages yet, but I am working on it.
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Old 2nd October 2012, 04:09 PM   #2045
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one more question, wouldn't balanced outputs help with noise in general?
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Old 2nd October 2012, 05:34 PM   #2046
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Hi!

Quote:
Originally Posted by markc2 View Post
one more question, wouldn't balanced outputs help with noise in general?
Balanced outputs would help to reduce noise pick up in the interconnect from preamp to power amp, but only if the power amp has a balanced input. With an output transformer you can have both. PN me if you are interested to give the Lundahl LL2745 a try, which has been developed for the 26

Best regards

Thomas
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Old 3rd October 2012, 11:07 AM   #2047
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Hello Mark! Welcome to the thread.

Thomas is one of the Big Daddies in this and he's a safe bet to sort you out - he started us all off with filament bias, and what a pleasure that has been. You'll need a couple of Rod Coleman's boards for the supply, and they are cheap and very effective and come with comprehensive instructions, including filament bias use.

Thanks to this thread, we've evolved this design into a real building option which can scare off multi thousand dollar commercial products. We've got quite a big pool of users now.

I remember reading in a post at least 5 years ago that "the 26 was the king of preamp tubes" and now we have the proof!

There's a rogue break-off group including myself who've gone over to the 4P1L, and most of the theory and practice applies to that as well, including filament bias. I use a Lundahl LL1660/18mA for that, which I got from Thomas. He's the guy for the transformers.

Andy
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Old 4th October 2012, 08:27 AM   #2048
mogliaa is online now mogliaa  United Kingdom
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Hi Mark,
I recently built my third incarnation of the 26 preamp using the LL2745 provided by Thomas. I'm very happy so far with its sound and overall tone. Starving filaments slightly can eliminate microphonics (if you have any globe ones) and also rubber suspension on the sockets.
Rod Coleman boards are invaluable if you are looking to have a hum free preamp using DC and avoid capacitors using filament bias.
I don't have my latest diagram at hand as an away from home. Can post it when back if there is any interest
Thanks
Ale
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Old 4th October 2012, 08:36 AM   #2049
mogliaa is online now mogliaa  United Kingdom
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Quote:
Originally Posted by andyjevans View Post

There's a rogue break-off group including myself who've gone over to the 4P1L, and most of the theory and practice applies to that as well, including filament bias. I use a Lundahl LL1660/18mA for that, which I got from Thomas. He's the guy for the transformers.

Andy
Yes, and me being a promoter of a great tube blamed to be very microphonic. 4p1L as a preamp is fantastic if properly used. Needs care on microphonics though. Filaments in parallel and starved at 500mA and easier for filament bias. 4P1L doesn't like currents below 20mA (check curves) so either the Hammond gapped for 30mA or the LL1660/20mA should work best. I just ordered from Thomas an LL1660/20mA so will report results soon
There is a multi-tapped secondary OT from Pieter which can provide dual function (OT and Volume control) in one. Apparently has had great feedback.
Thomas, can you share your experience with The tribute's OT?
Thanks
Ale
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Old 4th October 2012, 08:49 AM   #2050
massimo is offline massimo  Italy
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Originally Posted by mogliaa View Post
I don't have my latest diagram at hand as an away from home. Can post it when back if there is any interest
Yes please!
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