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Old 30th November 2009, 09:55 PM   #1
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Question Question - Output Trafo with CFB

Hi, tube folks,

There are some output trafos with CFB (cathode feedback). CFB is described as 8% - 15% of primary.

Question: does this means 8% - 15% of
a) primary impedance
b) primary number of turns

Thanks in advance for any suggestion(s)
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Old 30th November 2009, 10:34 PM   #2
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Sorry, a bit of clarification:

a) primary inductance
b) primary impedance
c) primary number of turns
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Old 30th November 2009, 11:15 PM   #3
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I don't think there is any real standard for this specification. It could be primary impedance or percent of total primary turns. But very doubtful it's inductance. The only way to know for sure is to contact the manufacturer and ask. If you had one in hand, you could make a simple low voltage AC test measurement to find out which. But my gut feel leans toward total primary turns because that's how UL screen taps are usually specified.
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Last edited by HollowState; 30th November 2009 at 11:31 PM.
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Old 30th November 2009, 11:31 PM   #4
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hey-Hey!!!,
Dynaco advertised its CFB output by turns ratio. They claimed 15%, and it was just a bit over 17 if memory is still intact... The Chicago BO-6 is also advertised by turns ratio, and their BO-14 has no lableing as to its turns ratio( it is 10% by turns to the anode coil ). The a-a loading is also taken independantly of the CFB coil; the effective load is higher( 1.15^2*listed a-a load for the BO-6 for instance ).
cheers,
Douglas
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Old 30th November 2009, 11:59 PM   #5
pointy is offline pointy  United Kingdom
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it's turn in respect to the secondary winding (as a separate winding}

sorry dynoco but 7% would mean the amp needs less feed back
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Old 1st December 2009, 12:14 AM   #6
pointy is offline pointy  United Kingdom
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whoops............... i think that's of the second winding

it was used before the nfb was taken from the speaker out-put socket

diyers in the 50s would put their own on the out side of transformer with a bit of card and tape

now days i think its best to come off the speaker socket with a resistor and cap
even though this can change the impedance set-up.
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Old 1st December 2009, 12:41 AM   #7
JLH is offline JLH  United States
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CFB is usually specified as percent of primary turns. In other words, a percentage of primary voltage that gets fed back to the cathode. (10% CFB for a primary with 2000 turns of wire would have a completely separate winding of 200 turns of wire.) I love using CFB with Class A pentodes with separate G2 supplies and Class A2 triodes. To me CFB beats UL connect in pentodes every time. You can also get some very good dampening factors with high Mu transmitting triodes too. Electra-Print and Edcor are very willing to custom wind what ever you need.

Rgs, JLH
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Old 1st December 2009, 10:04 AM   #8
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Hey,
Have a pair of Quad IIs on the bench. They have 10% CFB. Together with light Schadeing and a pair of high-Gm pentodes as drivers and original KT66 this means we can skip global feedback alltogether.
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Old 1st December 2009, 03:12 PM   #9
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Quote:
Originally Posted by JLH View Post
To me CFB beats UL connect in pentodes every time.

Rgs, JLH
CFB is U-L as far as the g2 is concerned( with fixed g2 voltage ).
cheers,
Douglas
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Old 1st December 2009, 03:21 PM   #10
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Hey Douglas,

Think you will find this one interesting, written by D.T.N. Williamson and Peter J. Walker:

http://www.dc-daylight.ltd.uk/Valve-...perlatives.pdf

It might make you change your mind. If not, its anyway interesting reading from two gurus of that time.
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