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Vertical 6EM7 Portable Amp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5134
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Author:  Geek [ 01 Jan 2017, 18:28 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

If you ever have the hankerin'... these little SET amps benefit from a CFB system where you draw the output cathode current through the secondary of the OPT - you also somewhat de-saturate the core and your F3 will probably go from just under 20Hz to 12-15Hz without the gain loss of gNFB (because there still is no gNFB) ;)

Cheers!

Author:  RossD [ 01 Jan 2017, 18:54 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Now I understand. Thanks.

Author:  Suncalc [ 06 Nov 2017, 21:52 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Just listening to this amp playing Pink Floyd's "The Great Gig In The Sky" off the "Dark Side Of The Moon" album. It sounds INCREDIBLE! This amp has really good bass response; tight and full. Just transitioned to "Speak To Me/Breathe" and the sound raises goose bumps on my arms.

There is NOTHING like an all triode signal chain!

Author:  Geek [ 07 Nov 2017, 03:15 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Suncalc wrote:
There is NOTHING like an all triode signal chain!


Amen! :smoking:

Author:  john55 [ 16 Jan 2018, 07:24 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

I have a power transformer that I bought to build the Lacewood V2. For some reason the secondary voltages are a little low. I'm reading 250-0-250 volts which should be okay to run a 6EM7 Vertical amp but the problem is only 5.6 volts for the heaters. It has a 5 volt winding (also running low) that I won't need, so my question is..... after connecting these two secondaries in series I measure 10 volts so I need to drop 3.7 volts.
Current draw for the heaters of two 6EM7s and one EZ81 = 2.85A
Volt drop = 3.7A x 1R =3.7 volts.
P = 3.7A squared x 1R = 13.69 watts.
So I have a couple of 1R 50W wirewound resistors. Could I use one for this?
I know it's not efficient and will generate a lot of heat, but is it safe? I like the idea of building another 6EM7.

Author:  john55 [ 21 Jan 2018, 12:31 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Well no input from anyone. Is this because the idea is ridiculous?

Author:  Suncalc [ 01 Feb 2018, 17:22 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

john55 wrote:
Is this because the idea is ridiculous?
No. I think people are a little reluctant to respond because we remember how much trouble you had with that transformer on the other project.

I think what you propose will work, but the design and build process will involve some iterations to get it right. I say this because that particular transformer doesn't seem to follow the normal rules and we don't really understand why. As they say on the edge of the map "Beware! Here there be Dragons!"

Go slow, prototype everything, and let us know how it goes.

Author:  Gio [ 01 Feb 2018, 20:44 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Hi John. Trying to drop that much constant heat across resistors inside an enclosure can be difficult. The heat buildup inside the chassis can become problematic with extended use. My suggestion is to use an oversized resistor as you noted, but to use one that will mount to the chassis or an external heatsink so you don't get heat buildup inside of the enclosure.

Author:  Suncalc [ 01 Feb 2018, 21:06 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

john55 wrote:
I have a couple of 1R 50W wirewound resistors
Be careful about power resistor dissipation ratings. See my write-up here: http://www.diyaudioprojects.com/Testeq/Dummy-Speaker-Load/

john55 wrote:
... after connecting these two secondaries in series I measure 10 volts so I need to drop 3.7 volts
Also remember that under load, the voltage will be lower. As I said, prototype first.

Author:  john55 [ 02 Feb 2018, 03:15 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Matt, Gio,
Thanks for your replies. Will prototype this weekend. I've already read your article on dummy speaker loads Matt, but I'll give it another read today. Gio, the resistors have their own heatsink which I'll bolt down with some thermal paste onto a chassis size piece of aluminium. Thanks again for the advice.

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