angela EL34 improvement and mods

Charles Wasserf · 17th January 2008, 08:02 AM

Some time ago I built this single ended amp using paralleled 6SL7's as drivers. The amp worked and it was quiet but sounded uninvolving and lacked dynamics. I tossed out the EL34's, replacing them with 807's. I employed an EL84 to regulate the screen and used plate to plate feedback like the rh design. Big improvement over the triode connected EL34's. But I want more. I would prefer to stay with octals but will do what ever is necessary to further improve things. Should I try connecting the 6SL7 in srpp or cascade or use 6SN7 or perhaps a ccs on the cathodes or led's. Help is needed gentlemen.
Charles

Tubes4e4 · 17th January 2008, 11:16 AM

Hi Charles,

starting with the original circuit, try to apply about 3dB of positive loop FB from the 125ESE secondary to the driver cathode. Yes, positive FB. I am not kidding you.

Tom

Tubes4e4 · 17th January 2008, 07:26 PM

Hi Charles again,

you may think that my previous answer is provocative or something, but it really is not. Let me explain.

As-is, the Angela circuit as-is measures astonishingly flat and with quite a nice monotonic falling THD distribution.

When you write that you found a (trioded) 6L6 in the very same circuit (w/o other changes) to sound better to your ears, this simply translates to considerably more H2 from the power stage (and somewhat less Po) at the resulting operation point for the 6L6, compared to the trioded EL34. The driver stage itself is faily linear and doesn´t add too much sonic imprint. All this can be derived from the loadlines and a few basic measurements.

So, if you found the trioded 6L6 to suit your sound preferences better than the original trioded EL34, some positive loop FB will even more "exaggerate" in this sonic direction. Be aware, though, that when doing so you deliberately create an "sound effect box", which will probably will sound gorgeous with, say, human solo voices, but will fail miserably on certain other music styles/programmes.

Tom

Charles Wasserf · 18th January 2008, 06:39 AM

Hi Tom,
I'm using some transformers other than the recommended 125ese.
So how do I implement 3dB positive feedback ?
I tried the 6L6, 5881 and KT66 in place of the EL34 and prefer the 807 to them all. I am using it as a tetrode with screen regulation as per jc morrison's article in Sound Practices. Max. screen voltage for 807 is 300V and I am somewhere around there with B+ at 390V
Regards,
charles

17th January 2008, 08:02 AM	#1
Charles Wasserf diyAudio Member Join Date: Mar 2005 Location: durban	angela EL34 improvement and mods Some time ago I built this single ended amp using paralleled 6SL7's as drivers. The amp worked and it was quiet but sounded uninvolving and lacked dynamics. I tossed out the EL34's, replacing them with 807's. I employed an EL84 to regulate the screen and used plate to plate feedback like the rh design. Big improvement over the triode connected EL34's. But I want more. I would prefer to stay with octals but will do what ever is necessary to further improve things. Should I try connecting the 6SL7 in srpp or cascade or use 6SN7 or perhaps a ccs on the cathodes or led's. Help is needed gentlemen. Charles

17th January 2008, 11:16 AM	#2
Tubes4e4 diyAudio Member Join Date: Jul 2005 Location: Leverkusen	Hi Charles, starting with the original circuit, try to apply about 3dB of positive loop FB from the 125ESE secondary to the driver cathode. Yes, positive FB. I am not kidding you. Tom __________________ If in doubt, just measure.

17th January 2008, 07:26 PM	#3
Tubes4e4 diyAudio Member Join Date: Jul 2005 Location: Leverkusen	Hi Charles again, you may think that my previous answer is provocative or something, but it really is not. Let me explain. As-is, the Angela circuit as-is measures astonishingly flat and with quite a nice monotonic falling THD distribution. When you write that you found a (trioded) 6L6 in the very same circuit (w/o other changes) to sound better to your ears, this simply translates to considerably more H2 from the power stage (and somewhat less Po) at the resulting operation point for the 6L6, compared to the trioded EL34. The driver stage itself is faily linear and doesn´t add too much sonic imprint. All this can be derived from the loadlines and a few basic measurements. So, if you found the trioded 6L6 to suit your sound preferences better than the original trioded EL34, some positive loop FB will even more "exaggerate" in this sonic direction. Be aware, though, that when doing so you deliberately create an "sound effect box", which will probably will sound gorgeous with, say, human solo voices, but will fail miserably on certain other music styles/programmes. Tom __________________ If in doubt, just measure.

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18th January 2008, 06:39 AM	#4
Charles Wasserf diyAudio Member Join Date: Mar 2005 Location: durban	Hi Tom, I'm using some transformers other than the recommended 125ese. So how do I implement 3dB positive feedback ? I tried the 6L6, 5881 and KT66 in place of the EL34 and prefer the 807 to them all. I am using it as a tetrode with screen regulation as per jc morrison's article in Sound Practices. Max. screen voltage for 807 is 300V and I am somewhere around there with B+ at 390V Regards, charles

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