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Vertical 6EM7 Portable Amp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5134
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Author:  john55 [ 12 Dec 2018, 12:45 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

I built some Fostex back loaded horns to go with the first 6EM7 I built. I also built some Fostex bass reflex speakers at the same time. If you're interested go to this link.... https://www.theleathershop.biz/class-a- ... ifier.html It's the website for my leather shop in Spain, but I used a couple of pages to put up the project for my friends in England to see. Both sets of speakers sounded really nice with the 6EM7. Also plenty of volume.

Author:  mwhouston [ 12 Dec 2018, 16:22 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

A better choice, after working with both, is the Markaudio 12P which is both inexpensive , better souding (more bass if you build thier deeper bass) and no shrillness.

https://www.madisoundspeakerstore.com/a ... ull-range/

Author:  john55 [ 12 Jan 2019, 13:37 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

As mentioned on the previous page, I want to build the 6EM7 as a pair of monoblocks with half of Matts' revised line stage, tone stack preamp in each one. I will use half a 12AU7 for the buffer and half a 12AT7 to gain back the 20 dB (and a bit more) lost in the tone stack.
The boxes are built from Iroko and are ready to be oiled. The top plates are cut to size. I've come up with a layout I like, but would like some input before I start drilling. I would like to have the input and output on the back of each monoblock, but they will be close to the rectifier valve. I think STP cable for the input? Do I need to shield the output? The three €1 coins show where the bass, treble and volume would be. The second monoblock would be a mirror of the first. The PS transformer is pretty close to the output transformer, is that a problem? Any advice would be great.

Author:  Suncalc [ 12 Jan 2019, 13:52 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

john55 wrote:
I will use half a 12AU7 for the buffer and half a 12AT7 to gain back the 20 dB (and a bit more) lost in the tone stack.
So why not just use the other half of the 12AU7 as the amplifier? This saves a tube and gets you a gain neutral tone stack and buffer. The 6EM7 amplifier itself is already very sensitive. I would think that gain wouldn't be a problem.

Author:  john55 [ 13 Jan 2019, 02:50 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

john55 wrote:

I used your circuit (two channels) in front of a lacewood V2 and loved it. So I intend to build a one channel version on each monoblock using half a 12AU7 for a gain stage and half as a buffer. I won't bother with a defeat switch this time as I never use it.

As you can see, that was the original idea, but I won't be able to "tube roll". For me, the 12AT7 isn't about gain, I just like the sound of it. Mind you, I could just be kidding myself as it was quite an expensive 12AT7 and I'd have to buy another one. I may just fit an extra valve base "in case", and at this stage take your advice and go for the one valve.

Author:  john55 [ 20 Jan 2019, 12:21 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Matt, I'm just about ready to wire the two 6EM7 monoblocks with bass, treble and volume for each one. I'm going for one 12AU7 per monoblock as you suggested. Could you please check these circuit diagrams of yours, that I've altered to show what I'm doing.

Author:  ILoveHiFi [ 20 Jan 2019, 18:05 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

You want 1M ohm to keep the grid ground, cathode or voltage refrenced, not in series with the gird.
In series with grid to help prevent high frequncy oscillation typically has a resistance value of k to 10ks of ohm, I don't know the orginal schematic but it probally should have its specifed grid resistor.

Not too much of a fuss to get perfect values but a value of 1.8-2.7k works well with most valves.

Author:  john55 [ 21 Jan 2019, 03:22 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

ILoveHiFi wrote:
You want 1M ohm to keep the grid ground, cathode or voltage refrenced, not in series with the gird.
In series with grid to help prevent high frequncy oscillation typically has a resistance value of k to 10ks of ohm, I don't know the orginal schematic but it probally should have its specifed grid resistor.

Not too much of a fuss to get perfect values but a value of 1.8-2.7k works well with most valves.


Thanks for the explanation, making more sense now. I'm removing the original 100k pot shown in (A). I assume (B) is correct now? Why would I not use a 100k resistor to ground? Why jump to 1M? Is it because the signal from the tone stack is different to say a signal from a CD player or iPod?

Author:  Suncalc [ 21 Jan 2019, 11:39 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

John;

An additional grid resistor is not required. The 250kΩ output resistor on the gain recovery stage will serve that purpose. The two stages could be coupled in the following manner:
Attachment:
Coupling.jpg

Please note that the Miller capacitance of the 6EM7 input stage is 162pf. If you do include a grid stop resistor on that stage, make sure to check the high frequency rolloff so you don't affect the upper end of the audio spectrum.

Author:  john55 [ 21 Jan 2019, 12:44 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Thanks Matt. All clear now.

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