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6EM7 power amp
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Author:  mwhouston [ 02 Nov 2011, 07:13 ]
Post subject:  Re: 6EM7 power amp

Purdy, purdy, purdy. If this ain't the purdiest amp you have every seen. As stated on Green Acres: "about as pretty as a pimple on a pound of pickled pork".

It's not wired unrer the bonnet but it is assembled. In the next four days this purdy little thing should be up and singing. I usually adorn the top plate with sparkly hardware. In this case... not needed. Yes the base is a little big. For once I will not be bitch'n I'm cramped for space. On the back there are two switchable inputs to this true intergrated. The input will go direct to the preamp (4S Universal). A 250K Alphs blue velvet will feed the first stage of the power amp after the preamp section. I get anoyed when some amps claim to be intergrated when the include a pot. For me a real intergrated amp has a preamp section. This does.

I think some may have questioned my sanity when the saw the colour of the plates. I hope now you can see what I had in my mind when I chose this colour. I could see it would look like this assembled. It looks just as good in real life. I don't care if it sonds bad (lol). But it will not!

I have called the amp "Paris" - retro-thermionic "Paris"
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Author:  mwhouston [ 04 Nov 2011, 05:26 ]
Post subject:  Re: 6EM7 power amp

Some good news some different news: Firstly I have the dog trained in tube amp construction. She is a bit messy Miss Paris but knows her way around a schematic.

Now for the different news: I just got a paying job to buid a Le Pacific phono stage for MC. I only got all the parts today so strange I "got the call". I will put the "Paris" amp on hold while I 'tend the paying job. Check real soon.
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Author:  mwhouston [ 05 Nov 2011, 18:50 ]
Post subject:  Re: 6EM7 power amp

I have decided to press on and finish the amp. I stayed up late and finished the PSs. Under a 6.6K load each PS delivered 250V. That would be about 38mA. I think Matt wants 45mA at 260V into the second Triode of the 6EM7. I will wait until the amp is complete but what I may have to do is to lower the limiting resistor in the PS. It is currently 820 ohms. There appears to be more drop in the rest of the PS than originally thought. T

he amp will probably function OK with a 10V drop on the main HT but I know Matt likes to get the best out of the tubes so a little tweaking may be required when complete. Easily enough done my just lowering the limiting R.

Author:  Suncalc [ 05 Nov 2011, 19:08 ]
Post subject:  Re: 6EM7 power amp

Have you got a PS schematic at which we can look? :geek:

Author:  mwhouston [ 05 Nov 2011, 23:17 ]
Post subject:  Re: 6EM7 power amp

Amp finis. Yes Paris lives. Worked first time. No buzz or hum just the smell of burning new tubes and some hot 2W Rs cooking.

The final voltages are a little down @ 250/240V. One side higher then the other by 10V. I'll let it burn in then tweak the 820 R limiting resistor to make volatges 260/250 and balanced. One side is powering the 4S MKIV Universal preamp but by tweaking the limiting R I can bring it all into line.

The PS filtering looks like this 10uf-5H choke-47uf-820 ohm R-100uf-2.2K-47uf. The B+ which feeds the OPT comes off the 100uf cap. I used bi-polars across the power triode - 100uf/100V. Hope these dont's short like the last lot.

How does the amp sound. FABULOUS! And it will get better. Better than my 3W 6AS7 but it is early days. Loud as heck!!! on my 91db efficient Eclipse. Nice having the preamp built into the power amp. On set of inter-connects not required. Out of the CD player and TT into the amp direct. I used MKP X2s caps in the two inter-stages. I think they sound fine for cheapies.

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Author:  mwhouston [ 06 Nov 2011, 00:19 ]
Post subject:  Re: 6EM7 power amp

Suncalc wrote:
Have you got a PS schematic at which we can look? :geek:

Matt I will get you a schematic when I have come down and stop enjoying this great little amp.

Author:  Gio [ 06 Nov 2011, 11:01 ]
Post subject:  Re: 6EM7 power amp

Great looking build Mark.
Cheers

Author:  Suncalc [ 06 Nov 2011, 11:52 ]
Post subject:  Re: 6EM7 power amp

It looks great Mark. I do love to see new designs come to life.

What tube do you have in the preamp socket? And by that I mean, what's your preamp gain? And is the preamp stage cathode bypassed or not? Having the 4S Universal in the amp means just swapping preamp tubes depending on the nature of the source; 12AX7 or 12AT7 for lower voltage sources and 12AU7 for higher voltage sources. Cool.

Author:  mwhouston [ 06 Nov 2011, 16:03 ]
Post subject:  Re: 6EM7 power amp

Suncalc wrote:
It looks great Mark. I do love to see new designs come to life.

What tube do you have in the preamp socket? And by that I mean, what's your preamp gain? And is the preamp stage cathode bypassed or not? Having the 4S Universal in the amp means just swapping preamp tubes depending on the nature of the source; 12AX7 or 12AT7 for lower voltage sources and 12AU7 for higher voltage sources. Cool.

Matt: not sure what is in the preamp socket at the moment. I had the tube marked but the markings has rubbed off. I think either a 12AT7 ot 12AY7. It sounds good at the moment so no hurry to change just yet. I settled for a Gold Lion 12AX7 in the stand-alone Universal and probably that is what I will go back to in Paris after a bit of rolling. Just waiting to get some more hours on the amp. Already clocked up 10 hours in one go.

The preamp stage is NOT cathode by-passed. I thought about this when building the amp. What I found was the Universal sounded better with un-bypassed cathode cap. There is a lot of gain so at this point it will remain un-bypassed. I felt the bass to be not as wegithy as the 6AS7 but after more consideration I think it is just tighter. The 6AS7 tune with 1.7K impedance OPTs may have been "fluffing" the bass up. The 6EM7 seems to have better bass control. Very pleased with the amp at this point. What do you think of using MKP X2 caps for the two inter-stage caps?

Author:  Suncalc [ 06 Nov 2011, 19:43 ]
Post subject:  Re: 6EM7 power amp

mwhouston wrote:
What do you think of using MKP X2 caps for the two inter-stage caps?
Actually I view this the same way that I view using motor run caps in a power supply filter. (See those comments here.) If they work out (and it sounds like they have) then that's great. But I don't typically use components in "off design" applications if I can avoid it. Let me explain.

MPK X2 caps are thin film capacitors designed for EMI suppression while connected directly to the mains. As such, they are designed for several safety related characteristics. Generally in compliance with the standards IEC 60384-14 or UL 1283 / UL1414. The Vishay application note on the subject lists the main requirements of these types of capacitors as the following:

• Excellent capacitive filter: Low inductance and equivalent series resistance are preferred
• Withstanding pulse loads: Uncontrolled mains switching must be sustained
• Continuous biased by the mains voltage: A powerful energy supply is always available
• Withstanding surge voltages: High energy surge voltages could destroy the capacitors
• Safe end of life behavior

One thing you will not generally find on the list is low noise or high fidelity audio performance. Now, by their very nature, well made thin film capacitors tend to have good audio performance. However, other design features may compromise that performance while meeting the primary requirements for the part. And the builder really has no way to determine what design trades were made in order to meet the primary requirements.

As always, I'm not saying that they can not be used or that they won't sometimes give good performance as coupling capacitors, it's just that they are not designed for this type of use. As such, it's a case of let the builder beware.

Just my opinion. :2c:

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