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Old 16th December 2013, 06:55 AM   #1
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Default Fixed bias, cathode feedback?

I have a SE KT88/6550 amp loosely based on the tubelab simple amp. It uses a CCS loaded 12AT7 driving a fixed-bias KT88 tube @ 450v and around 60ma.

I have it built with a switch for triode/UL mode because with fixed bias I can swap in all sorts of tubes. ( I prefer the EL34 in triode; the 6550 in UL.)

A Question, I would like to try cathode feedback with the 6550 in UL. I have Hammond 1628SEA transformers with 0-4-8-16 ohm secondaries.

I would attach the 16 ohm secondary to the cathode of the output tube so that it is grounded through the common.

Will the standing current harm the secondary?

I this a bad idea because it will impose 60ma. current on the speaker voice coil?

I know tubelab cap couples the cathode feedback but that is in cathode bias.


I cant seem to wrap my head around the particulars...

Any guidance?
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Old 16th December 2013, 07:09 AM   #2
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Cathode lNFB is usually used with PP finals, so that there isn't that DC magnetization problem. With a SE OPT, you might try connecting it so the DC fields of the primary and secondary offset each other. Maybe that prevents core magnetization, or maybe not. YMMV.
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Old 16th December 2013, 07:14 AM   #3
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Thanks Miles. That was my thought as well. A bit of experimentation will be called for



Will the standing DC current be a problem for the speaker? this is the one that I am having trouble conceptualizing.....
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Old 16th December 2013, 08:28 AM   #4
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60mA DC through the Voicecoil is not so good i think.
But when you have separate sec. coils in your OT then you can perhaps take one coil for the feedback and the rest for the speaker.
Maybe you show us a little schematic how your OT looks.
Since you are from Felix Austria:
Ich glaub nicht, daß 60mA Gleichstrom gut für den LS ist, wenn Du aber,wie zum Beispiel bei Lundahl,separate secundär Wicklungen hast,könnte es gehen.
Best wishes,
Hilmar
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Old 16th December 2013, 09:24 AM   #5
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I have the Hammond 1628SEA outputs. I appears to be one secondary winding that is tapped.
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Old 16th December 2013, 09:30 AM   #6
SY is offline SY  United States
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What's the DCR of the secondary? You might want to calculate how the current divides between transformer and voice coil...
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Old 16th December 2013, 01:15 PM   #7
Ketje is offline Ketje  Belgium
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First,why fixed bias.Has only drawbacks for SE ,exept somewhat more Vb.
The 60mA in the secundary is almost completly shortcircuited by it's DC resistance.
Certanly not try to cancel the magnetic field with the current from the cathode !
If you do that the result is positif feedback and oscillation.
Don't forget,after that mod the input sensitivity of the amp will be smaller if there was no overall feedback.
Mona
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Old 16th December 2013, 01:26 PM   #8
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For example, if the 8 Ohm winding is 0.250 Ohms DCR and the 8 Ohm speaker is 5 Ohms DCR, the speaker will get about 3 mA, a 15 mV offset.

The DC current has only a small effect on core magnetization, since there aren't many turns. Cathode feedback effectively increases the primary impedance. New Z = (sqrt(Zin)+sqrt(Zout)^2
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Old 16th December 2013, 02:01 PM   #9
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I think it could work with some fiddling... an old post from tubelab:
"If the transformer has multiple impedance taps, you hook the speaker up where it belongs, and try each tap for CFB. Pick the one that works the best in your situation. If it happens to be the same one that the speaker is on, use it."

From here.
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Old 16th December 2013, 02:04 PM   #10
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Tom Bavis View Post
For example, if the 8 Ohm winding is 0.250 Ohms DCR and the 8 Ohm speaker is 5 Ohms DCR, the speaker will get about 3 mA, a 15 mV offset.

The DC current has only a small effect on core magnetization, since there aren't many turns. Cathode feedback effectively increases the primary impedance. New Z = (sqrt(Zin)+sqrt(Zout)^2
Indeed,looking from the tube you have anode-ACgrd-DCgnd-cahode.The first two is the primary and the later the speaker winding.Effectively puting the two in series.
Little bonus,if you don't have UL,now there is (somewhat)The interconnection ACgnd-DCgrd is also the ACgrd of the screengrid,puting it on a tap of the OPT.
Mona
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