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Vertical 6EM7 Portable Amp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5134
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Author:  john55 [ 21 Feb 2018, 02:18 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

The restaurant is tiny and we ran a 6EM7 with Fostex FF165WK 6.5" Full Range drivers and an active subwoofer for a couple of weeks. That was my first valve amp that I built last year. More than enough volume.
(See page 7 of this thread).

Author:  john55 [ 21 Feb 2018, 13:15 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

The 6EM7 is up and running and sounding beautiful. In the end I used a 1R2 50W wirewound to get the heater voltage to 6.1V under load.
I had to remove R1 (390 ohms) and have the rheostat full over (100 ohms) to get 265V and 251V.
The first stage of the 6EM7 voltages are: Vp 151V......Vk1.75V........Vp+k 151v (shouldn't Vp+k be a little lower?)
The second stage voltages are: Vp 252V.......Vk 32.5V..........Vp+k 215V
Would appreciate knowing if these are acceptable. The mains transformer is running very cool.

Author:  Suncalc [ 21 Feb 2018, 19:49 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

john55 wrote:
The first stage of the 6EM7 voltages are: Vp 151V......Vk1.75V........Vp+k 151v (shouldn't Vp+k be a little lower?)
The second stage voltages are: Vp 252V.......Vk 32.5V..........Vp+k 215V
Would appreciate knowing if these are acceptable.
John;

These voltages look great. Don't worry about the first stage voltages, there is a tonne of margin in the driver design. And don't worry about measuring plate to cathode. You are looking for a cathode voltage that's about 1% of the plate voltage. Just measure plate and cathode to ground separately and calculate the plate to cathode voltage. Second stage numbers are very acceptable as well.

Enjoy the amp.

Author:  john55 [ 23 Feb 2018, 05:42 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Today the 6EM7 is finally in its box. A bit over the top, but I had really good fun building it. You can see a short video of the amp opening and closing at ...... https://www.theleathershop.biz/wine-box-amp.html ...which is the website for my leather shop in Spain.
At the moment I'm running the amp with a pair of Mordaunt-Short speakers I had on hand. Looking forward to listening through my Fostex hybrids and active sub woofer at home.
I'm waiting for a switch and volume control knob to complete the amp and a momentary switch that will be activated by the lid movement so that it's impossible to close with the amp switched on.
Thanks go to Matt and mwhouston for your help.

Author:  mwhouston [ 23 Feb 2018, 15:01 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Compared to my "rustic" builds yours is Buckingham Palace. The boxes are nicer than mine which are cheap and light. I like the red bits and see you have added heat guard etc. for the boxes.

I used bigger tubes and found little heat radiated down. So, in my case, no problem with heat. Heat guard or not they are nice touches in your case. As you said though you are intending it for a commercial premises so a little upmarket visual tweaking is the way to go.

The Fostex speakers will allow better volume and with the addition of active sub you should get good volume and balanced sound. My big high efficiency15" Beyma concentrics play quite loud on just 2W the Fostex will be up there.

Excellent work so far look forward to the complete amp and speakers image. I hope this encourages others to do a wine box tube amp. I'm still thinking a dedicated thread for wine box amps may stimulate others to have a go.

Author:  john55 [ 22 Nov 2018, 12:13 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

A little advice please. I have a few 6EM7 tubes and a couple of what I think are suitable transformers spare. ( 550V (275-0-275) at 175mA center tapped, 6.3V at 4A ) I've already built two of Matts' 6EM7 Vertical Amps and would like to try something different.
I want to build a pair of monoblocks with a baxandall tone stack on each one. The Power Supply Transformers' 6.3v winding shouldn't have any problem with an EZ81, a 12AU7 and a 6EM7 but I'm not sure about the current draw for the HT.
So my question is..... Can I use Matts' 6EM7 Power Supply to run a monoblock with tone stack.
It screams at me that there will be no problem, if anything it's overkill, but I want to be sure.

Author:  Suncalc [ 22 Nov 2018, 14:01 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

john55 wrote:
Can I use Matts' 6EM7 Power Supply to run a monoblock with tone stack.
Yes.

But the more important question is how do you plan on compensating for the nominal 20dB loss in the tone stack? You will need an additional signal gain stage to get the signal back to nominal.

Personally I would recommend a 6C4 (i.e. 1/2 12AU7) preamp stage to drive the tone stack as it goes into the driver of the 6EM7. Move the volume control to the front of the gain stage and then up the grid resistance on the 6EM7 driver to 1MΩ. Oh, and use a 0.68µf coupling capacitor between the input gain stage and the tone stack.

Author:  john55 [ 23 Nov 2018, 03:01 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Thanks for the quick reply Matt.
My mistake, I should have said “revised line stage, tone stack preamp” not “tonestack”.
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.
By the way, looking forward to seeing your write up on Loudness Switches.

Author:  eshazen [ 27 Nov 2018, 06:14 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Here's my take on the 6EM7 amp. I'll use it for the sound on my home desktop computer.

I decided to go with a more conventional package and build a separate power supply. The 2nd stage of PS filtering (after the chokes) is in the amp chassis. I do intend to replace the messy power cable with something nicer, as well as install an A/B input rotary switch in the extra hole in the front panel.

The PS chassis is black walnut from some scraps I had around and the amp is western cedar from a different scrap pile.

Sounds very nice, though those old Radio Shack Minimus-7s I use on the test bench aren't the most efficient. I think my next project is a nice pair of horns to go with it.

Thanks for the original design!

Image

Author:  mwhouston [ 27 Nov 2018, 16:43 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Looks good and I have two pairs of those speakers. In one I replaced the woofer/mid with a much better 4" woofer/mid and upgraded the Xover. The other is empty waiting for some drivers. Locally I can buy 4" twin cones which are 94db efficient. Could work well with my low power tube amps.

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