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6EM7 power amp
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Author:  Les [ 20 Oct 2011, 08:02 ]
Post subject:  Re: 6EM7 power amp

Hey Mark!
I kinda like the layout with both tubes side-by-side, but I agree with the layout with the rectifier back with the PT would be more advantageous in keeping the AC away from signal lines.
:2c:

Author:  rock4016 [ 20 Oct 2011, 13:02 ]
Post subject:  Re: 6EM7 power amp

I'm no EE, but I would put the 5Y3 in the back with the power transformer. :2c:

Author:  Gio [ 20 Oct 2011, 20:27 ]
Post subject:  Re: 6EM7 power amp

I'll also suggest putting the PT and rectifier at the rear and away from the audio tube.

Cheers

Author:  Suncalc [ 20 Oct 2011, 21:15 ]
Post subject:  Re: 6EM7 power amp

Mark;

I guess I agree with the crowd, I'd put the 5Y3 at the back with the power transformer. Best to keep all that nasty AC away from the audio tube.

I like the button base 6EM7. I have a couple of GEs in my stash but they have the typical plastic sleeve octal base. The pair of these side by side should look pretty sharp. I look forward to your updates.

Oh one more thing. According to RCA, the 6EA7 is an acceptable substitute for the 6EM7 (the 6EA7 can take a higher pulsed plate voltage and is slightly different internally) so you could even do some limited "tube rolling" with these little amps. :D

Author:  mwhouston [ 21 Oct 2011, 00:00 ]
Post subject:  Re: 6EM7 power amp

@Matt: Thanks for the comments. AC on the heater (6EM7) is OK? That's a question. If the power tranni doesn't have CT 6.3V I'll do the 100omh to ground both sides trick. I haven't marked the cast Al chassis yet BUT I would like to put the power cord (fixed), small fuse holder, speaker binders, RCA (input) and power switch all at the back. I may not have enough room. I can use an internal fuse if need be. I'll see how it goes.

Author:  Suncalc [ 21 Oct 2011, 10:08 ]
Post subject:  Re: 6EM7 power amp

mwhouston wrote:
AC on the heater (6EM7) is OK?
AC heaters should be ok. I would make sure you use twisted pair and that the leads are well dressed within the chassis. In fact, going to TSP might be a good idea. But I don't think you should have any trouble with AC bleed inside the tube.

Author:  mwhouston [ 21 Oct 2011, 15:46 ]
Post subject:  Re: 6EM7 power amp

Suncalc wrote:
mwhouston wrote:
AC on the heater (6EM7) is OK?
AC heaters should be ok. I would make sure you use twisted pair and that the leads are well dressed within the chassis. In fact, going to TSP might be a good idea. But I don't think you should have any trouble with AC bleed inside the tube.

TSP? You've got me here.

Author:  Suncalc [ 21 Oct 2011, 16:03 ]
Post subject:  Re: 6EM7 power amp

Sorry about that. :blush: Industry terms...

TSP - Twisted Shielded Pair
STP - Shielded Twisted Pair
UTP - Unshielded Twisted pair

TSP and STP are used interchangeably depending on the source.

What I meant was a twisted pair with a shield around it. One end of the shield tied to the chassis ground and the other end open.

Author:  mwhouston [ 21 Oct 2011, 16:33 ]
Post subject:  Re: 6EM7 power amp

Suncalc wrote:
Sorry about that. :blush: Industry terms...

TSP - Twisted Shielded Pair
STP - Shielded Twisted Pair
UTP - Unshielded Twisted pair

TSP and STP are used interchangeably depending on the source.

What I meant was a twisted pair with a shield around it. One end of the shield tied to the chassis ground and the other end open.

Wiring for the heaters??

Author:  Suncalc [ 22 Oct 2011, 12:53 ]
Post subject:  Re: 6EM7 power amp

mwhouston wrote:
Wiring for the heaters??
That's what I was thinking. The 6EM7 has a heater current of 0.925A (or 1.05A if you insert a 6EA7) which means that the heater leads will have a healthy magnetic field. As such, twisted heater feeds are a minimum requirement.

Now simply twisted leads hold the equal and opposite currents very close to each other which limits the magnetic field largely to the region directly around the wires. This practice helps prevent inductive coupling to other elements in the enclosure. This is due to Maxwell's magnetic field equation ( ∇×H = J + ∂D/∂t ). However, this practice does nothing to prevent electric field coupling. In order to prevent the changing electric field from affecting other items inside the box, you need to include a Faraday shield. Now I grant you that the electric field from the heater wires is small (17.8v peak). However, given how tight everything is in such a small chassis I was just suggesting it as an added measure of protection.

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