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 Post subject: Re: 6EM7 power amp
PostPosted: 12 Nov 2011, 22:21 
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Matt: Construction is as you have drawn it. There is no grid blocker or grid resistor for section 1 of the 6EM7. I was shocked at first when I tested the amp and checked the settings etc. I remember you mentioned once that amp performance can change depending on where the pot maybe. When I brought the pot down to where I use it in real live the HF performance doubled. Because there is a preamp installed gain is good and all my sources are about 2Vpp. So the pot will always be right down.

I have stated with a 2V input I get a 12.5V output but I may have had the pot turned right down so ignore that statement. I have one side of the two cap sets connected all the time. But I can't see how this could affect the cct.

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 Post subject: Re: 6EM7 power amp
PostPosted: 13 Nov 2011, 00:42 
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Matt: One other thing. The noise figures with input shorted: 2mV. Do you consider that low?

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 Post subject: Re: 6EM7 power amp
PostPosted: 13 Nov 2011, 12:26 
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mwhouston wrote:
I have one side of the two cap sets connected all the time. But I can't see how this could affect the cct.
All I can think is that the stray capacitance of the wiring is adding enough to the Miller capacitance to lower the high end rolloff. But, to get to 16.5kHz would require an additional 48µµf of stray capacitance.

The circuit for calculating high end roll off due to Miller capacitance is shown below.
Attachment:
pic742.jpg
Here Ro is the stage output impedance before the potentiometer (~11.5kΩ for the unbypassed 12AU7), Rpot is the total potentiometer value (250kΩ), Rl is the potentiometer resistance "below" the wiper setting, and Rh is the potentiometer resistance "above" the wiper setting. CM is the total effective Miller capacitance.

It could be that the extra capacitor hanging off the switch is causing enough stray capacitance to ground to cause the high end frequency loss. But without some investigation with the equipment, it's almost impossible to tell. What was the response before the capacitor mod?

Quote:
The noise figures with input shorted: 2mV. Do you consider that low?
If this is at the speaker output that's about -63dBW into 8Ω. Yes I would consider that very good for an amplifier of this type.


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 Post subject: Re: 6EM7 power amp
PostPosted: 13 Nov 2011, 15:58 
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Sorry Matt, no meaurements prior to the cap mod. It is the first time I have seen this and I offer no explanation. My wrining to the cap sets is short and open single strand wire wrap wire. The switch is largish. It could be in the switch. I have anorher switch but not sure my meter will go low enough to detect very small capacitance.

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 Post subject: Re: 6EM7 power amp
PostPosted: 18 Nov 2011, 21:46 
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I mentioned in an earlier post that the 6EM7 amp had 2mV of noise - 50hz. I just read an article in Sereophile where the Music Refernce RM-200 MKII has 0.4mV of noise. Now that's low. I'm still pleased with 2mV. Even with the 96db efficient Fostex FE206 horns I could not hear any hum, hiss or buzz. I had to check the amp was really on.

Last night the amp entertained the Opera group at a meeting where I showed Don Carlo over two DVDs on a new 46" LCD TV. The Oppo has two channel output and Paris drove my Osborn eclipse (91fb efficient) to good volume levels. I got two or three compliments about the amp one guy saying he thought it better than the 6AS7 SET and another felt the system took him the performance rather than brought the performance into the room.

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 Post subject: Re: 6EM7 power amp
PostPosted: 21 Nov 2011, 16:29 
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The my 6EM7, "Paris", got a couple of good comments from attendees at my Opera night a few days back. Qian, who I built the Siver Dragon 300B for, said it is my best amp to date and now wants to have listen at his place. Qian has a good ear and has been critical of some amps in the past so I know he is honest and up-front with his comments. He plays very loud large scale classical music so I know it will not be loud enough for him. But what he did like was the detail in mid-range and transparency of the amp. What I have found with the amp is on loud passages the inter-modulation distortion stays very low (make that in-audible). With my 3W 6AS7 SET you can hear the inter-mod in loud very dynamic passages. Paris doesn't sound like it does this??

One other feature I like about Paris is how dead quiet it is. But I have found on extremely bassy rock tracks it doesn't play them well at all. The bass goes to mush. But give me mid-range over bass any day.

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 Post subject: Re: 6EM7 power amp
PostPosted: 23 Nov 2011, 03:26 
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I took Paris around to Qian's. I have come home without it. Qian thinks Paris is better than the 300B Silver Dragon. I have the Silver Dragon back on the promise I build Qian a mono-block Paris. And he wants one with inter-stage transformers. Matt - help.

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 Post subject: Re: 6EM7 power amp
PostPosted: 23 Nov 2011, 06:14 
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I meant to put this in the last post but forgot. I asked Qian who heard Paris at my place what he liked about the amp. None of my other gear has changed for years so Qian knows my sound.

These are his words:

"Hi Mark,
I particularly like the sweetness, depth, three-demensional feeling and transparency of your new amp. The sound has subtle beauty, which gives the amp a distinct character. I hope it can produce the same sound on my system.

Cheers,

Qian"

I am to build Qian a mono-block version of Paris to swap with the Silver Dragon.

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 Post subject: Re: 6EM7 power amp
PostPosted: 24 Nov 2011, 01:32 
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I have used another dual/dissimilar triode quite happily in making a small power amp too: 13FM7. Schematic attached. I read with interest the discussion above regarding cap coupling vs. direct coupling, power supply issues, etc. This design takes a side step around the usual limitations by using a choke loaded driver tube, direct coupled and powered by a voltage doubler. The design was a cheapskate development from my first design using DC/choke loading, which I called "Free Lunch."
Attachment:
Small Saul.gif

You would not believe how small I was able to build this up! The chassis is maybe 10x12 inches, double decked with audio on top, power supply below. Solid state rectified to save space and money.

Comments welcome.


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 Post subject: Re: 6EM7 power amp
PostPosted: 24 Nov 2011, 12:23 
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There are a whole set of these dissimilar triodes that I like for small low power amps. The 6EM7/6EA7, 6CY7, 6EW7, 6FM7, etc. (and all their higher filament voltage equivalents). They are all dual triodes with one moderate to high µ signal section and one power section designed to drive magnetic deflection coils. The requirements to drive an output transformer are very similar.

This and the fact that there were millions of these tubes made and never used because of the rapid switch over to solid state televisions, means that they are cheap and plentiful. Just the situation required for building small inexpensive low power amps.

Tubegeek, do you have a picture of this amp? I'd love to see it.

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