807 E-Linear

Fuling · 8th January 2007, 07:56 PM

A while ago I promised my brother to help him build a tube amp.
Since we were on a budget, we decided to use those 18 bucks Edcor OPT´s and a pair of 807´s.
After a few detours I finally got a prototype running yesterday,
but to my disappointment it didn´t sound as good as I had expected. The output tubes were running in UL mode with CFB from the OPT secondary, and obviously this wasn´t enough to get the output impedance low enough.

Since my brothers speakers are quite insensitive triode connection was never an option and I didn´t want to surrender to global NFB, I came up with the (seemingly) great idea to connect the input tubes plate resistors to the UL taps rather than to B+. I believe this is called E-linear. I´ve read that this topology works best with high impedance driver stages, so I removed the cathode bypass caps for the input stage.

The bass response immediately got much better, not "floppy" as it was before.
This drawing shows how the amp is wired today:

Any obvious mistakes here?
The amp sounds really good, but I haven´t done any measuerements yet.

Bandersnatch · 9th January 2007, 12:57 AM

Hey-Hey!!!,
Looks good to me. I prefer pentodes for the input stage...actually cascodes but your ears are the final arbiter on that one. The pentode plate returns more NFB as it doesn't really care what the voltage is( like a triode ).

It is indeed called E-Linear. That circuit is different enough to deserve its own name.
cheers,
Douglas

arnoldc · 9th January 2007, 05:33 AM

Is this topology specifically made for plate loaded input tubes?

What if I have a CCS on the input tube plate? Will that make a difference?

Sheldon · 9th January 2007, 05:39 AM

If you're in to mode of experimentation, here's another partial feedback approach you might want to look at: http://www.tubeaudio.8m.com/807/807.html
If you search under RH807, you'll find some examples.

Sheldon

Fuling · 9th January 2007, 08:44 AM

What I´ve come up with here seems to be a mix between Pete Milletts E-linear amp and Alex´s RH807.
Cost and simplicity has high priority, that´s why I use a dual triode on the input. (Well, actually it´s because I didn´t plan to use this kind of feedback from the beginning).
Can I expect huge improvements by changing to pentodes (6AU6 or something)?

I´m going to run some measurements on the prototype tonight, I´m not sure that my driver stage is capable of pushing the output tubes to full power.

janneman · 9th January 2007, 10:16 AM

Quote:

Originally posted by Fuling
A while ago I promised my brother to help him build a tube amp.
Since we were on a budget, we decided to use those 18 bucks Edcor OPT´s and a pair of 807´s.
After a few detours I finally got a prototype running yesterday,
but to my disappointment it didn´t sound as good as I had expected. The output tubes were running in UL mode with CFB from the OPT secondary, and obviously this wasn´t enough to get the output impedance low enough.

Since my brothers speakers are quite insensitive triode connection was never an option and I didn´t want to surrender to global NFB, I came up with the (seemingly) great idea to connect the input tubes plate resistors to the UL taps rather than to B+. I believe this is called E-linear. I´ve read that this topology works best with high impedance driver stages, so I removed the cathode bypass caps for the input stage.

The bass response immediately got much better, not "floppy" as it was before.
This drawing shows how the amp is wired today:

Any obvious mistakes here?
The amp sounds really good, but I haven´t done any measuerements yet.

I think if you decouple the 10 ohms cathode resistor you double the output power.

Jan Didden

Fuling · 9th January 2007, 11:50 AM

The 10 ohm resistor is only there to measure the voltage drop (bias current) across, it shouldn´t affect the output power...?

janneman · 9th January 2007, 11:56 AM

Quote:

Originally posted by Fuling
The 10 ohm resistor is only there to measure the voltage drop (bias current) across, it shouldn´t affect the output power...?

The 10 ohms doesn't know this...

Fact is that the signal voltage across the 10 ohms is about equal (even more) then across the speaker load because they carry the same signal current. So you lose half your power. Just put a cap across it, you still have the DC bias but now all the signal is across the speaker.

Jan Didden

Circlotron · 9th January 2007, 12:26 PM

The 8 ohm winding is coupled to the primary winding so the bottom of the 10R resistor is kind of bootstrapped. In any case, the current available from the transformer secondary is [cathode current variation] x [turns ratio].

Fuling · 9th January 2007, 12:45 PM

I agree with Circlotron, the 10 ohm resistor is in series with the primary winding rather than with the secondary winding.
It drops about 0,65VDC at idle and perhaps 1Vpp at max output power, a very small fraction of the total plate swing.

9th January 2007, 12:57 AM	#2
Bandersnatch diyAudio Member Join Date: Jul 2003 Location: Ann Arbor, MI	Hey-Hey!!!, Looks good to me. I prefer pentodes for the input stage...actually cascodes but your ears are the final arbiter on that one. The pentode plate returns more NFB as it doesn't really care what the voltage is( like a triode ). It is indeed called E-Linear. That circuit is different enough to deserve its own name. cheers, Douglas __________________ the Tnuctipun will return

9th January 2007, 12:26 PM	#9
Circlotron diyAudio Member Join Date: Jun 2002 Location: Melbourne, Australia	nahhhh.... The 8 ohm winding is coupled to the primary winding so the bottom of the 10R resistor is kind of bootstrapped. In any case, the current available from the transformer secondary is [cathode current variation] x [turns ratio]. __________________ Best-ever T/S parameter spreadsheet. http://www.diyaudio.com/forums/multi...tml#post353269

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9th January 2007, 05:33 AM	#3
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	Is this topology specifically made for plate loaded input tubes? What if I have a CCS on the input tube plate? Will that make a difference?

9th January 2007, 05:39 AM	#4
Sheldon diyAudio Member Join Date: Dec 2003 Location: San Diego	If you're in to mode of experimentation, here's another partial feedback approach you might want to look at: http://www.tubeaudio.8m.com/807/807.html If you search under RH807, you'll find some examples. Sheldon

9th January 2007, 08:44 AM	#5
Fuling diyAudio Member Join Date: Aug 2001 Location: Sweden (Mora)	What I´ve come up with here seems to be a mix between Pete Milletts E-linear amp and Alex´s RH807. Cost and simplicity has high priority, that´s why I use a dual triode on the input. (Well, actually it´s because I didn´t plan to use this kind of feedback from the beginning). Can I expect huge improvements by changing to pentodes (6AU6 or something)? I´m going to run some measurements on the prototype tonight, I´m not sure that my driver stage is capable of pushing the output tubes to full power.

9th January 2007, 11:50 AM	#7
Fuling diyAudio Member Join Date: Aug 2001 Location: Sweden (Mora)	The 10 ohm resistor is only there to measure the voltage drop (bias current) across, it shouldn´t affect the output power...?

9th January 2007, 12:45 PM	#10
Fuling diyAudio Member Join Date: Aug 2001 Location: Sweden (Mora)	I agree with Circlotron, the 10 ohm resistor is in series with the primary winding rather than with the secondary winding. It drops about 0,65VDC at idle and perhaps 1Vpp at max output power, a very small fraction of the total plate swing.

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