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 Post subject: Re: 6EM7 power amp
PostPosted: 08 Nov 2011, 20:43 
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Location: Australia
I'll check the levels.

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 Post subject: Re: 6EM7 power amp
PostPosted: 09 Nov 2011, 00:17 
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Matt and fans: I re-checked the measurements:

Plate to cathode V= 234
Cathode R = 760
Votage drop across Rk = 34.7V

I think about 10.68W.

I'll back off the HT.

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 Post subject: Re: 6EM7 power amp
PostPosted: 09 Nov 2011, 03:11 
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I have replaced the 820 ohm limiting resistor in the PS with a 1.1K 10W R. Dissipation of the 6EM7 is back to 9.8W which is about where Matt suggested. Plate to cathode V = 220V, cathode drop = 34V, cathode R = 760 (2X 1.5K (carbon 1W) Rs in parrallel). There is a 53V drop across the limiting. This = about 48mA total draw/channel.

Though the underneath of the amp is realy shocking I have included a snap. You can see the small 5H chokes and the larger OPTs. The yellow X2 caps are also easy to see. Lots of twisted wire. Quiet amp.

Listening to a recent release of Liszt piano works. Excellent.
Attachment:
UnderBonnet.jpg


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 Post subject: Re: 6EM7 power amp
PostPosted: 09 Nov 2011, 06:29 
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The 6EM7, I am told, is not in production. So every new tube is NOS. I have found a supplier with a few in three different brands. I ordered two extra pairs. Does anyone know any good sources for these tubes? Some big companies who say they have them reply "out of stock".

This was my local suppliers response: "We have 10 of the Miniwatts, 4 AWV and 8 wafer based Sylvania."

What would you go for? I have the Sylvania at present.

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 Post subject: Re: 6EM7 power amp
PostPosted: 09 Nov 2011, 06:45 
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mwhouston: that looked like my 6L6 SIPP. Thought you had stolen my chassis. ;)

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 Post subject: Re: 6EM7 power amp
PostPosted: 09 Nov 2011, 12:20 
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Soundbrigade wrote:
mwhouston: that looked like my 6L6 SIPP. Thought you had stolen my chassis. ;)

Soundbrigade: put up an image.

The Vishays/Sprague poly caps should come today (4am in the morning now). Hopefully I can install them for a re-eval of the amp sound.

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 Post subject: Re: 6EM7 power amp
PostPosted: 10 Nov 2011, 07:25 
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Since dropping the HT back, where before there was a very slight hum in the left channel, now both channels are absolutely dead quiet. Nothing can be heard even when the vol. is up to 80% and your ear is hard against the speakers. Quietest amp I have ever built. Before this the Be Bamp was very quiet. And my speakers are very sensitive so any noise will come through. I think it is the dual filtering of the PS.

Also there now appears greater focus in the center but with still plenty coming from the left and right speakers.My new tubes which came today are AWV Super Radiotron (?) made in Holland. I only know of Phillips so this is a newy on me. NOS tubes in old style boxes. I would like another pair of Sylvanias.

Sprague polies also came. To be installed. Listend again to a new release of a classical piano CD (Liszt). This amp takes you there.

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 Post subject: Re: 6EM7 power amp
PostPosted: 11 Nov 2011, 01:13 
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Paris has polies: I just did a little open-heart surgery on Paris. I installed a two pole quad throw switch and some Vishay Sprague "orange drops" poly audio caps. With the switch in one position all signal caps (two per channel) are Sprague in the other position all signal caps are MKP X2 caps. Matt felt the X2s I had used in Paris, though a very good cap, may not be audio grade. I liked the sound and did want to lose it. But I agreed with Matt and in most other amps had used audio grade caps.

With the switch inplace I can switch between the two cap sets. I am yet to AB the two sets and may wait 'till I get another set of ears. I will not inform the other listeners of which caps they are listening to and see which they prefer. Since I have dropped the HT the amp runs cooler. Also now I have the AWV 6EM7 and Sylvanias I can roll between them. And 6 different preamps tubes and two signal capacitors sets and rectifier tubes. Bloody hell all I want to do is enjoy the music.

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 Post subject: Re: 6EM7 power amp
PostPosted: 11 Nov 2011, 23:55 
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Here are some test result-
@ 1W across 8ohms:
-3d points = 15Hz to 30kHz with pot at less than 30%
-3db points =15Hz to 16kHz with pot greater than 50% - OK Matt this is where you chime in. The pot is a 250K Alps feed from a 0.47uf cap from the 4S Universal. In normal use the Pot never exceeds about 20 to 30% of rotation.

Max output into 8ohms = 2.6W RMS before the 1K sine wave started to miss-shape. With 2Vpp input I can get 12.5Vpp across 8ohms. I think a gain of about 7db. Correct me if I'm wrong. Square wave at 1K is alright with a slight slope off. At 10K the leading edge is rounded.

I thought this was a quiet amp. With input shorted and pot closed only 2mV of noise (50z). With pot at 50% only 3mV of noise. That's quiet. Xtalk is about 26mV (crudely measured).
Attachment:
ParisIII.jpg


In this image you can see the Cap-set switch in the middle of the amp and Paris has been officially named with a name plate.


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 Post subject: Re: 6EM7 power amp
PostPosted: 12 Nov 2011, 20:56 
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Joined: 06 Apr 2009, 10:08
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Mark;

I'm a little concerned about your 16kHz top end with the volume potentiometer turned up. I am assuming that your input topology looks like this.
Attachment:
pic741.jpg
With this topology, the 6EM7 stage 1 input Miller capacitance of 92µµf, your lowest high end rolloff should be in the neighborhood of 26.5kHz. The rolloff plot looks like this.
Attachment:
Screen shot 2011-11-12 at 5.48.02 PM.png


If your high end rolloff frequency is really this low (16kHz) then I suspect additional parallel resistance in the 6EM7 Section 1 grid circuit. Is there a grid resistor on the input to section 1 of the 6EM7 in addition to the 250kΩ potentiometer? Also, did you implement a grid stopper resistor on the 6EM7 section1?

Otherwise I am at a loss for how the high end rolloff could have gotten so low.

Please let me know and I'll investigate further.


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