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Fuling 2nd August 2006 10:06 PM

Taking CFB to the extremes...?
Good evening (almost midnight here)!

After playing around with Mosfets and (gasp) chip amps for a while I feel that itīs time to return to the real stuff, tubes that is.
I do already have more tube amps than Iīll ever need, but nothing that matches my needs right now: a four channel amp.
Of course I could just use two of my other amps but hey, letīs not mute my enthusiam...

What I need is something to drive a pair of FE167Eīs in vented boxes from 100Hz and up and something else to feed the SS8545-based subs below 100Hz.

Iīve already come to the conclusion that I donīt need much power, so a four channel SE amp would be quite nice.
Plan A was to use 6AV5GA, but theyīre not readily available here.
Plan B is to but my box of NOS 807`s to use.
Maybe not the most exotic choice, but theyīve been around forever and nobody seems to complain about them...

>100Hz: Triode wired 807 with 125CSE, cathode feedback from the unused 16 ohm tap.

<100Hz: UL 807 with OPT from an butchered Edison 60, 33% CFB.
This is the interresting part, these OPTs has four primary sections which can be used in many different configurations.
My idea is to wire three sections in series to achieve roughly 5,6k/8ohm, and the transformer will also be connected in split load mode where 2/3 of the primary is on the plate and 1/3 on the cathode. By connecting the screen grids to B+ I also achieve 33% UL operation.
The driver stage will have to deliver almost 100Vrms, not an easy task but a choke loaded EC86 should manage it.


Giaime 3rd August 2006 09:27 AM

Hello Fuling,

interesting use of the Hammond OPT.

But I would say that 100Vrms drive requirement scares me. It's difficult to archieve with under 1% distortion.

You have to try breadboard it and let us know how it goes. I think that partial feedback on the 807, or better, feedback from 807 plate to driver tube cathode would be better. Usually CFB sounds better than the other options, but overdoing it requires an hefty driver.

Btw, EC86 is a very nice tube :)

Fuling 3rd August 2006 11:47 AM

According to Steve ( the Hammond 125CSE responds very well to CFB, and I have a pair of CSEs left over from some project that never got finished.
Theyīre not big, but definitely big enough to deliver a couple of watts from 100Hz and up.

The Edison OPTs are much bigger, about the size of a 100VA power transformer.
If the 33% CFB idea fails Iīll just go back to "normal" CFB using the secodary winding and connect the screen grid to a 33% tap at the primary.

EC86 is my standard choice whenever I need plenty of gain, swing and drive capability. 100Vrms seems to be no problem with two Hammond 156C chokes as plate load and 200V plate voltage.

Fuling 3rd August 2006 10:35 PM

2 Attachment(s)
14 hours ago, this project only existed in my head.
Things has happened during the day:

Fuling 3rd August 2006 10:37 PM

2 Attachment(s)
And a view of the OPTs:

Fuling 31st August 2006 12:48 PM

Itīs beena while since I updated this thread, so here we go:

The amp is almost finished, whatīs left is to design some kind of cover for the OPTs and replace the HV transformer with one that does not hum. A 240VAC 0,9A toroid is on order.

Except the mechanical buzz from the transformer this amp is dead quiet, even with my 94dB speakers!
This is thanks to the capacitance multiplier PSU (B+) and CLC-filtered DC for the heaters.
The heater PSU employs one 1,1mH 10A choke and four 27000uF caps...


Bass channels: 25Hz - 35kHz -1dB @ 5Wrms
(Some visible distortion below 30Hz, probably due to lack of primary inductance)

Treble channels: 30Hz - 50kHz -1dB @ 1,35Wrms.
(Not bad for such cheap OPTs!)

The output power is slightly lower than expected, but when my new PT arrives I can probably squeeze out a few % more.
Currently the B+ is 10-15% too low.
An 250VAC 200VA transformer would be perfect, but such things doesnīt seem to exist (at least not for any reasonable sum of money).

I had a fair deal of luck when I built this amp, when I connect it to my speakers and to my active crossovers the response is almost perfectly flat without adjusting the active crossovers level controls!
Just luck, I did no calculations on this...

It sounds pretty darn good to, good punch and control in the bass channels and sweet highs from the treble channels!

As soon as my new PT arrives I will take some pictures of the setup and from inside the chassis, right now itīs busy playing music...

DerFly 31st August 2006 12:56 PM

Hejsan Fuling,

Well, I've got a couple of 807's sitting around. I haven't yet finished my first tube amp project, just thinking though ...

Circlotron 31st August 2006 01:19 PM

Re: Taking CFB to the extremes...?

Originally posted by Fuling
This is the interresting part, these OPTs has four primary sections which can be used in many different configurations.
Are the four sections identical? I know a good setup you can do if they are.

Fuling 31st August 2006 06:33 PM

Derfly: Hejsan!

Put those 807s to use, ever though they are cheap and not very exclusive they do sound good enough to deserve some attention.
I really like the trioded 807 + 125CSE + cathode feedback combo, with a little more voltage on the plate 2-3W shouldnīt be any problems to achieve. The screen grids are rated for 300V only, but Iīve seen several schematics that pushes this a little bit.

Circlotron: As far as I understand the four sections should be identical, each with a 8,7:1 ratio to the full secondary.
Iīm not planning to change anything, but please let me see what you have in mind!

Fuling 14th September 2006 04:05 PM

2 Attachment(s)
Some pictures taken today, the amp is almost finished:

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