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jeapel 1st March 2005 09:13 PM

SE amp like mcintosh ?
2 Attachment(s)

The vintage mcintosh amp use a separate winding on transfo output
to create a local cathode feedback.

But for a SE amp with normal transfo is it possible ? and a good idea ? to do that.


resident 1st March 2005 09:35 PM

I don't think it's correct the schem that you posted.I'm not sure though.But it isn't like McIntosh' unity coupled.
Anyway for a SE amp,you can't do it like McIntosh.But you can do it like Quad!
It's called CFB(cathode feedback).Do a google search to find the original Quad schem to see how it looks like.It's a PP design but...the half ... is a SE design! ;)

You need a separate winding for the cathode.The only disadvantage is that you will need more preamp gain to drive the output stage.But the main advantage is that with CFB you can drive loads down to 2-3ohms! :smash:

silurato 1st March 2005 09:56 PM



You need a separate winding for the cathode.
I saw a 807 PSE that works that way, and feedback was taken directly from the secondary winding... so, it works! :D

But it was backbiased... maybe Rk would cause some trouble, eating some of the NFB... :confused:

audiousername 1st March 2005 10:48 PM

The schematic you show seems to be a simple SEUL (single ended ultra-linear) amplifier. Since a triode does not have a screen grid, you cannot use it un UL connection this way. Have a look at Patrick Turner's 22W SEUL 13E1 amp

Yes, you can use cathode feedback taps on a single ended amp to good effect. Have a look at the "Scrapbox Challenge" amplifier in Valve Amplifiers if you have a copy.

Sch3mat1c 1st March 2005 10:51 PM

A minute amount of Rk will be taken up in the secondary resistance, negligible since the resistance is under an ohm. It will change the AC response a bit (forming a voltage divider with the cathode impedance, approx. 1/Gm), which is why a bypass cap is necessary.


audiousername 1st March 2005 10:57 PM

On second thoughts, I probably should add that using cathode feedback on the output stage of an amp by leaving Rk unbypassed isn't a great idea when using anything in fixed screen mode, because the valve will increase its effective anode resistace by Rk x m (which is large), so you end up driving the OPT with what is basically a CCS, which isn't all that great for bandwidth or the damping factor of the amp. Somewhat less of a problem with UL or triode connection, but CFB via taps on the transformer is a better solution, if you can get them since CFB this way reduces output impedance.

jeapel 2nd March 2005 12:45 AM

thanks for all your comments

if cathode feedback give better damping factor and better
transfo bandwidth why it s not popular on internet
SE amp

but a this point if i understand it s possible with
a normal transfo

silurato: have you a internet url for your 807 amp

audiousername 2nd March 2005 06:31 AM


Originally posted by jeapel
if cathode feedback give better damping factor and better
transfo bandwidth why it s not popular on internet
SE amp

Because people tend to rehash old desings without any creativity.

resident 2nd March 2005 07:19 AM

CFB amps require more difficult circuits.As I have posted previous they need more gain at the input stage to drive the output stage.
Notice that McIntosh is only 20W (or30?) class B with four stages!

planet10 2nd March 2005 07:44 AM

2 Attachment(s)

Originally posted by resident
the original Quad schem

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