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 Post subject: Re: 6EM7 power amp
PostPosted: 10 Jan 2012, 01:25 
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Joined: 17 Feb 2010, 20:10
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I'm not stating that the EDCORs have problems: they are a great little unit for the price. However using CFB lowers distortion and increases their bandwidth. I also use this on my 7082 amp with Bogen opts to great effect. These opts have multiple output taps, so I wired in a rotary switch to allow variable feedback. There is a tangible improvement as you go from 4ohms to 8 to 16 (optimum). At the next tap (32 ohms I think) the sound deteriorates.

The point is - this mod only takes minutes and is easily reversible. Instead of theorising and speculating try it!


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 Post subject: Re: 6EM7 power amp
PostPosted: 10 Jan 2012, 02:50 
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Joined: 06 Jun 2008, 18:23
Posts: 5301
Location: Australia
Suncalc wrote:
Hmmm... 5.8vpeak of feedback against a 34vpeak swing isn't much (-15dBv) but I guess every little bit helps.

What is the problem with the edcors you'd be attempting to fix with this little LFB loop? Is it just an issue with the XSEs?

Matt: I'm not aware of any problem with Edcor OPTS. Bruce is an Edcor user aficionado and maybe he can enlighten us. But regardless of OPT issues adding some global NFB will alter the performance of the amp. I see Matt you have done the math and said the NFB is very low. Again I would like to hear Bruce's comment of the low level of NFB.

At this point I'm trying trying to avoid NFB particularly GNFB.

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 Post subject: Re: 6EM7 power amp
PostPosted: 10 Jan 2012, 14:14 
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Joined: 18 Jun 2010, 12:46
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Location: Northern VA
Using cathode feedback on the XSE will alter the performance of the amp, you'll have more power, lower distortion and better bandwidth. It might be worth trying. It's easy to reverse if you don't like the sound.


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 Post subject: Re: 6EM7 power amp
PostPosted: 10 Jan 2012, 17:41 
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rock4016 wrote:
Using cathode feedback on the XSE will alter the performance of the amp, you'll have more power, lower distortion and better bandwidth. It might be worth trying. It's easy to reverse if you don't like the sound.

Just to get it real clear can someone please post a drawing of the wiring for this.

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 Post subject: Re: 6EM7 power amp
PostPosted: 10 Jan 2012, 19:56 
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Location: Northern VA
If you look at V11 in this Tubelab schematic, you'll see a cap that goes from the top of the cathode resistor to the neg connection on the speaker output.
http://www.tubelab.com/Simple_SE_schematic.htm


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 Post subject: Re: 6EM7 power amp
PostPosted: 10 Jan 2012, 22:32 
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Re-iterating this is NOT global feedback.


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 Post subject: Re: 6EM7 power amp
PostPosted: 10 Jan 2012, 22:59 
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rock4016 wrote:
... you'll have more power, ...
Just to be clear, feedback (local or global) does not provide any more power. It only reduces overall distortion and gain. Greater input voltage is needed to attain the same power.

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 Post subject: Re: 6EM7 power amp
PostPosted: 11 Jan 2012, 00:29 
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Joined: 17 Feb 2010, 20:10
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Finally found it. This is the original DIYAUDIO post that got me trying cathode feedback on the Edcor and several other OPTs. George was certainly on the right track.

Posted by tubelab.com on 03-13-2006 01:20 PM:
'Now back to the 6AV5 in SE mode. I decided to perform some basic measurements on the Edcor - 6AV5 combination. I discovered (while playing around with the 6EM7) that the Edcor transformer really likes cathode feedback, so I tested that too. I set an arbitrary 5% test limit for distortion.

In triode strapped mode I got 4.75 watts at 5% with 300 volts and 60 mA. Adding cathode feedback allows 8 watts at 5% using the same voltage and current. Distortion at all power levels is reduced considerably.

In UL mode I got 6.3 watts at 5% with 300 volts and 60 mA. Cathode feedback allows 9.0 watts without adjusting the power supply! This is like adding a second tube.

I have not listened to this yet, but I will soon. It has been my experience that when cathode feedback works, It sounds good. It tightens up the bass without killing the sound. '


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 Post subject: Re: 6EM7 power amp
PostPosted: 11 Jan 2012, 03:02 
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Suncalc wrote:
rock4016 wrote:
... you'll have more power, ...
Just to be clear, feedback (local or global) does not provide any more power. It only reduces overall distortion and gain. Greater input voltage is needed to attain the same power.

NFB generally reduces gain, adds bandwidth and reduces some types of distortion. BUT there is no free luch in amp tweaking. GNFB "can" reduce soundstage, reduce amp speed, cloudy a transparent amp, reduce dynamics, reduce realism (organics).

Shaken but not stirred.

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 Post subject: Re: 6EM7 power amp
PostPosted: 11 Jan 2012, 07:50 
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Joined: 18 Jun 2010, 12:46
Posts: 284
Location: Northern VA
Suncalc wrote:
rock4016 wrote:
... you'll have more power, ...
Just to be clear, feedback (local or global) does not provide any more power. It only reduces overall distortion and gain. Greater input voltage is needed to attain the same power.



It provides more usable power, since it lowers distortion, just to be clear.


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