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Any one have good circuits to use with Toroidy's Toroidal CFB outputs?
Any one have good circuits to use with Toroidy's Toroidal CFB outputs?
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Old 13th February 2018, 02:17 PM   #1
Thinker1920 is offline Thinker1920  United States
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Any one have good circuits to use with Toroidy's Toroidal CFB outputs?
Default Any one have good circuits to use with Toroidy's Toroidal CFB outputs?

Hi Everyone,

I would like to build some mono block tube amps using Toroidy's Toroidal outputs that have the cathode feedback windings to run my Magnepans. If I'm using the currency converter correctly they seem to be a pretty good value being about $126 a piece being able to handle 80 watts and having that cathode feedback winding.

I'm just not sure how to incorporate the cathode feedback into the circuits I'm familiar with, I was hoping I would find someone with prior experience or some circuit suggestions.

I'm pretty fond of my 6336A SET amp and I found that Luxman made an amp using them in push pull that also used a cathode feedback winding but, I can't find the specs on the original outputs. It was the Luxman MQ-80 and the output was the OY-15-600P so I was wondering if I could use that circuit or maybe there's a better options out there.

Any thoughts or suggestions?
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Old 13th February 2018, 02:36 PM   #2
bandol83 is offline bandol83  France
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I think Norman Koren quad EL34 monoblock "TENA" ( The Emperor's New Amplifier ) is a very nice design for such transformer
He used Plitron PAT 4006 CFB toroidal transformer but they were very expensive and they are no more available anyway ! As a bonus Norman used a very clever auto bias design without processor.
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Old 13th February 2018, 02:54 PM   #3
Thinker1920 is offline Thinker1920  United States
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Any one have good circuits to use with Toroidy's Toroidal CFB outputs?
That's quite the design! But needing 8 kt88's seems pricey and it's pretty complex. I was thinking something a little more simple using just 2 output tubes.
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Old 13th February 2018, 06:46 PM   #4
Eli Duttman is offline Eli Duttman  United States
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That "iron" has a 5 Kohm primary, which is large for PP KT88s, but the 80 W. handling is reasonable for PP KT88s.

Tie the UL taps off, but use the CFB capability. Place the anode B+ above 500 V., to compensate for the 5 Kohm primary and the associated large step down ratio. Regulate g2 B+ at 350 V. Your choice between Williamson and Mullard small signal circuitry. Target 60 WPC, which will leave some magnetic headroom for the low freq. error error correction signal. Roll the response off, at the I/P, just below 20 Hz. to provide protection against core saturation.

Remember, the lowest note a "normal" double bass plays is 41 Hz. Big bass drums and 32 foot organ stops play in the 1st octave. Do the "Maggies" get down there to begin with?
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Old 13th February 2018, 11:40 PM   #5
Thinker1920 is offline Thinker1920  United States
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Any one have good circuits to use with Toroidy's Toroidal CFB outputs?
The Toroidy's come with several primary options, 2k, 4k, 5k, 6.6k, and, 8k. Curious why you wouldn't suggest using the the UL taps? I've heard a few people recommend Wiliiamson's circuit's, ElPasoTubeAmps mentions them a lot in his videos (I like his videos). I was thinking about going with their circuit that could be used with either the EL34's for 50 watts or KT88's for 60 watts, I just wasn't sure how to incorporate the CFB winding into the circuit. I'll see if I can attach the diagram.

How do I roll the response off, at the I/P, just below 20 Hz? Is there a formula like when you calculate grid stoppers?

The Magnepans are rated down to 35Hz.
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File Type: jpg Williamson High powered el34 or kt88 circuit.jpg (110.9 KB, 233 views)
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Old 14th February 2018, 12:56 AM   #6
multi is offline multi  Australia
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Default 6550 with 5687 driver

Depending on the amount of feedback you are going to need lots of drive.
If I was building this amp I would use solid state drive for the amp and the bias supply then no need for choke input.
I have used this circuit and 811a's with cathode feedback.
Have not tried the Toroidy transformer with cathode feedback, but have used several cathode feedback transformers C core for 811a DA-42's
The Toroidy transformers are excellent
Phil
Attached Files
File Type: pdf P1.pdf (234.9 KB, 49 views)
File Type: pdf P3.pdf (199.4 KB, 32 views)
File Type: pdf P2.pdf (264.9 KB, 26 views)
File Type: pdf P4.pdf (249.5 KB, 24 views)
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Old 14th February 2018, 01:22 AM   #7
Eli Duttman is offline Eli Duttman  United States
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You optimize for 1 O/P tube type. Otherwise you get something that sort of works, but ...

I said full pentode, not UL, because a 5 K primary is incorrect for KT88s and a "tall" B+ rail is needed as compensation. If UL was tried with that "tall" rail, the screen grids would be subjected to excessive voltage. A 3.3 K primary is what you use with KT88s.

Buy the 6.6 Kohm primary model. Use 6L6GC or 7581A O/P tubes. Now, you can safely use both the UL taps and the CFB winding(s).
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Old 14th February 2018, 02:23 AM   #8
Wavebourn is offline Wavebourn  United States
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Any one have good circuits to use with Toroidy's Toroidal CFB outputs?
Quote:
Originally Posted by Thinker1920 View Post
I just wasn't sure how to incorporate the CFB winding into the circuit.
Here is how I did that: I grounded 4 Ohm tap instead of "zero ohm" tap, and took feedback from "zero ohm" and "16 Ohm" taps.
Of course, binding posts were not relabelled, so Zero ohm post is still zero ohm, despite between it and the ground is 4 Ohm output. The picture has a bit misleading marking of outputs.
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File Type: jpg Hercules-OS.jpg (414.1 KB, 211 views)
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Last edited by Wavebourn; 14th February 2018 at 02:27 AM.
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Old 14th February 2018, 10:50 AM   #9
Thinker1920 is offline Thinker1920  United States
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Any one have good circuits to use with Toroidy's Toroidal CFB outputs?
Thank you all for the info!

I find myself on the fence now in regards as to which output tube to go with. So I'm hoping you'll help me pick one. Most of the gear I've built and listened to till this point has been single ended cathode biased, 2A3, 300B, 6B4G, 6336A, EL84, 6L6GT, EL34, KT88. Of those I enjoyed the 2A3/6B4G, 6336A, and the EL84. I Found that the 2A3/6B4G had a warm tube sound and I enjoyed how it presented vocals, it reminded me a bit of the EL84 in pentode mode but, it went a step further. I found the 6336A and the EL84 in UL to both be more clean, the EL84 in UL being borderline dry but still pleasant. The 6336A I have isn't a normal cathode bias it's setup with a cathode current sink to prevent it from drifting, which I think helps its performance. (I'll attach a THD scan)

I did have a Yaqin PP EL34 amp in the past but, at least with the speakers I paired it with in the past I found it not really tubey at all, it was very clean sounding without much bottom end. I did repair a RCA BA-4C (1622/6L6) and I was surprised how well rounded it sounded, it wasn't bass heavy like I'd heard from 6L6's in single ended operation. I also had a Heathkit AA-151 which I liked but, it didn't have the power to drive the Magnepans :-(

So I'm just a little worried that I'll end up with a very dry or bottom end missing amp if I go with the wrong tube. I really do like the idea of trying a 6336A amp with just the exception of knowing that it wouldn't be able to put out as much power (40 watts) though when I use to have my metered gear I typically listened to the Magnepans around 2 watts with peaks into the 20 watt range so it might be adequate. I'd even thought about a PPP with EL84's but I'd probably need 6 per block and I've heard that they are trickier to design.

Thoughts?

Last edited by Thinker1920; 14th February 2018 at 10:55 AM.
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Old 14th February 2018, 01:51 PM   #10
Thinker1920 is offline Thinker1920  United States
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Any one have good circuits to use with Toroidy's Toroidal CFB outputs?
Quote:
Originally Posted by multi View Post
Depending on the amount of feedback you are going to need lots of drive.
If I was building this amp I would use solid state drive for the amp and the bias supply then no need for choke input.
I have used this circuit and 811a's with cathode feedback.
Have not tried the Toroidy transformer with cathode feedback, but have used several cathode feedback transformers C core for 811a DA-42's
The Toroidy transformers are excellent
Phil
I'm curious to hear about the specs of your 811a amp and your listening impressions.
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