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 Post subject: Re: 6EM7 power amp
PostPosted: 06 Aug 2013, 07:25 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
The new 6EM7 on the Acacia chopping board is proving a headache. Another day on the drill press has got me closer to an assembly but just a few more holes to go. Next project is going to need only a soldering iron and pointy nose pliers. I'm over drilling holes.

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 Post subject: Re: 6EM7 power amp
PostPosted: 13 Aug 2013, 20:04 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
Allen, who I sold Paris to, tried a 12AX7 on the preamp section. I gave him a Australian made "Miniwatt" to try. Initially he liked the sound but after a day or so swapped it back for the Golden Dragon 12AU7 and thought it a much better sounding tube.

I have found the 12AX7 in the 4SU single stage preamps a bit aggressive sounding and almost a distorted sound. It may just have been my builds but I have noticed the same thing on two different 4SU preamps so far.

I have offered to swap the 12AX7 of Allen's for a different 12AU7. It will be interesting if the second U7 is better than the Dragon which I have always thought an impressive sounding driver\preamp tube.

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 Post subject: Re: 6EM7 power amp
PostPosted: 13 Aug 2013, 22:28 
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Joined: 04 Jun 2008, 20:59
Posts: 4192
Location: Arizona, USA
Hi Mark, Depending greatly on the actual circuit, swapping 12AU7s and 12AX7 can be a mixed bag. Guitar players do it all the time, but then they want distortion. The best operational points of the two types are quite different. In one case you can starve a tube (12AU7) in a circuit designed for the 12AX7, the flip side is true that you can run a 12AX7 above its best values when subbed for a 12AU7. I find that for audio us the 12AU7 really should run at about 5-8 ma and over 100volts on the anode. The 12AX7 at about 0.5 - 0.7 ma also at an anode voltage over 100. With the normal cathode resistor bias found in many circuits the resistance there will be way off the needed value to accommodate both types. Occasionally the circuit will have a bias arrangement that will work OK for both, but I have found very few of them. I would suggest your buddy try alternate versions of the tube the circuit was designed for. ECC802s and ECC82s for the 12AU7 and if it was designed for the 12AX7, then ECC803s, ECC83s and also 5751 both new and perhaps NOS Phillips JAN ones. I recommend not using the new JJ 5751 as they seem to behave differently in many circuits from the others. BTW I personally like the ECC types with spiral heaters. They seem to be both quieter and have a cleaner mid range response.

Good listening
Bruce

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 Post subject: Re: 6EM7 power amp
PostPosted: 14 Aug 2013, 05:07 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
Bruce, thanks for the great suggestions.

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 Post subject: Re: 6EM7 power amp
PostPosted: 24 Aug 2013, 03:26 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
Assembled, partly wired and looking good. The new 6EM7 amp is well under way. Probably about 4 to 5 hours of work to finalise the wiring. Problem solving at every solder joint, wire run and hardware mount has made this a slow-as-you-go build. More images and review to come when completed.

Those who know me know I name my amps. If I haven't stated it before this amp is called "Arabella". Strauss wrote an opera called Arabella. The amp looks like an Arabella.
Attachment:
Arabella_Assembled_I.jpg
Attachment:
Arabella_Assembled_II.jpg


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 Post subject: Re: 6EM7 power amp
PostPosted: 26 Aug 2013, 20:12 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
Funny what you stumble on in "da lab" when your not really looking for it. I was about to do some more work on the new 6EM7 amp and was thinking of purchasing some other tubes other than Sylvanias so I would have somthing to roll on completion. As I was putting the tubes back in the draw I noticed some AWA tube boxes. Taking a look at the ends I discovered they are 6EM7s. In fact I have two pairs?

What a delight. No need to order some extra tubes I have plenty and two different manufactures. Should be good.

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 Post subject: Re: 6EM7 power amp
PostPosted: 27 Aug 2013, 00:44 
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It lives! Finally got Arabella finished. It worked at first turn-on. I haven't lost my touch. I was beginning to dought myself.

I had to put in an 820ohm 10W limiting resistor after the choke and 100uf cap and before the next 100uf cap to get the voltage down to 260V. The amp draws 95mA as Matt said it would and sounds good. No hum, buzz or hiss. Nothing has caught fire yet so things are looking up. Now for a big listen. Wife's out tonight so a big listen is in order.

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 Post subject: Re: 6EM7 power amp
PostPosted: 27 Aug 2013, 08:07 
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Only had a few hours of listening but already convinced this is a good build. Basic sonics seem balanced if not on the light side. Soundstage is wide bit shallow. 100 hours from now this may all change. For the first time I am listening to AWA Holland made NOS 6EM7s. No special build here all very standard parts. Three heavy brass cones support the amp. To hide wiring under the board I made a light Al skirt which sits behind the front brass cone and between at the cones. The cones only project 2mm beyond the skirt. Wiring is tucked up behind the Al skirt.

I am using my VoxDeuX 2-ways speakers with stereo subbies (VoxITBassO). Good volume for the type of music I am listing to.
Attachment:
Arabella.jpg
Attachment:
Arabella_I.jpg
Attachment:
Arabella_II.jpg
Attachment:
Arabella_III.jpg
Attachment:
Arabella_IV.jpg


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 Post subject: Re: 6EM7 power amp
PostPosted: 27 Aug 2013, 09:48 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
Beautiful build Mark! Hope it sounds as good as it looks.

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 Post subject: Re: 6EM7 power amp
PostPosted: 27 Aug 2013, 13:49 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Wow, gorgeous!!!! :up:

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