DIY Audio Projects Forum

6EM7 power amp
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Author:  mwhouston [ 24 Nov 2011, 16:10 ]
Post subject:  Re: 6EM7 power amp

I'm going to get the chance to re-visit the 6EM7 with a mono-block build. I'm hoping to make each amp quite small and build the whole thing on some cast Al cases. This time no built on preamp so rec tube and 6EM7 on each chassis.

The guy who swapped my 6EM7 for his high-end 300B (Silver Dragon) emails me every day to rave about the amp. As I have said in the past the mid-range is rediculous. Also supprising is how loud the amp plays with no sign of strain. Matt may be closer to the mark than he thinks when he says these tubes are designed to work with inductive ccts. It may well be the secret of the tube/amps success.

Author:  HT Performance [ 24 Nov 2011, 17:29 ]
Post subject:  Re: 6EM7 power amp

It´s impressive the way you talk about the Paris!

I´m glad the project worked out so well, and exceeded your expectations. :P

But, what about the 300B´s?

I have always heard that they are the audiophile´s favorite tube.

Could you please describe both of them with some simple words?

I never heard any of those, but I confess that i´m very tempted for the Paris! :)


Author:  mwhouston [ 24 Nov 2011, 23:08 ]
Post subject:  Re: 6EM7 power amp

HT Performance: The way I would compare Paris with Silver Dragon is Paris is a specialist amp. 80% of its perfermance is in its mid-range rendering. It exceeds all amps I have heard anywhere with its mid-range detail and transparancy. 12% goes to treble which is very extended and sweet with no brightness or hashness at all. 8% goes to bass. My speakers are not bass weighted but I feel balanced. In a bassier system Paris' bass performance would be considered right. In my system it sounds a little bass shy. A subby woud work well here.

Silver Dragon is a first-rate all-rounder. Bass, mids and treble get equal attention. But where The Silver Dragon exceeds is it handles all three areas extremely well. It has weight is all these areas and it grabs your attention right from the first bar of any piece. You know you are in for something very special. Great bass, detailed mids and crisp extended highs. A REAL tube amp. The sound is bolted between the speakers and projected with authority into the room.

Paris takes you to the performnce, Silver Dragon brings it to you. Paris lets you hear "stuff" you didn't know was on the CD, Silver Dragon presents 98% of the sound recorded to you. Paris keeps the speakers very much in your focus, Silver Dragon hides them with a big solid sound.

Bang for buck, Paris could eat many Dargons and not get its fill. For a few hundred dollars Paris is audio crime. Go for dual mono-construction and build in your preamp. Or mono blocks. I will post full schematic soon.

Stay tuned soon I will soon start a pair of Paris mono-blocks.

Author:  tubegeek [ 25 Nov 2011, 01:07 ]
Post subject:  Re: 6EM7 power amp

Suncalc wrote:
There are a whole set of these dissimilar triodes that I like for small low power amps. The 6EM7/6EA7, 6CY7, 6EW7, 6FM7, etc. (and all their higher filament voltage equivalents). They are all dual triodes with one moderate to high µ signal section and one power section designed to drive magnetic deflection coils. The requirements to drive an output transformer are very similar.

EXACTLY!!! The power sections have a lovely low Rp and they drive a transformer very well. Plenty of gain from the high-µ section.

I always thought this would be exactly what I'd use if I ever wanted to build a 5.1 setup for home theater. One tube per channel, nice and simple.

Suncalc wrote:
This and the fact that there were millions of these tubes made and never used because of the rapid switch over to solid state televisions, means that they are cheap and plentiful. Just the situation required for building small inexpensive low power amps.

I find, in general, that the 12 pin compactron sockets for these types are more expensive than the tubes!

If you get adventurous enough to use the 13 or 15V versions the tubes cost practically nothing - no one wants to use these voltages even though they are just as easy to derive as 6V or 12V. The power transformer I used was running well under its current spec so it provided a fair amount more than 12.6 V which is exactly what I was looking for. The power transformers look identical but one is 120:12 and the other is flipped to give 12:240 into the voltage doubler. Apex Jr. had a boatload of these super-cheap which made it VERY easy to build these amps cheaply. A smaller 12:240 would be just the ticket for making a little preamp or whatever that could be supplied by a 12VAC wall wart, if you like that idea. The ones you see here are surely overkill.

Suncalc wrote:
Tubegeek, do you have a picture of this amp? I'd love to see it.

Why indeed I do!

The one in the photos has some botched metalwork which is why I still have it. The 2nd one I built has mesh around the top deck and was recently removed from service at a local restaurant - after 8 years of playing all day every day, it outlasted the restaurant. The owner brought it home and for all I know it's still rockin'.

Unit #2 got the better Hammond single ended output transformers as spec'ed in the design, the ones in the pictures are universal "experimenter" SE/PP if I'm not mistaken. Size is 8"D x 12" W x 3.5" H to the top deck, 5.5" H to the tops of the tubes.



You can see the two back to back power transformers to the left, with the 6 filter electrolytic caps in front of them and to the right.
The little circuit board has the divider which keeps the filament supply biased up (I'm not sure why there are two sets of resistors?) and then in front of that is the L channel output transformer. On the right side is the R channel output transformer and both the loading chokes snuggled in close to each other.
When I built the 2nd one it was considerably neater than this one!

(one of the things I find cool about these choke-loaded DC-coupled amps is that there's almost nothing inside there but iron.)

Author:  rock4016 [ 25 Nov 2011, 07:54 ]
Post subject:  Re: 6EM7 power amp

Bottlehead has been very successful offering kits for 6DN7 parafeed se amps. They can also run a 6EM7 with a few changes. Magnequest also makes upgraded iron for them.

Author:  mwhouston [ 25 Nov 2011, 08:57 ]
Post subject:  Re: 6EM7 power amp

Tubegeek: the Hammond universal OPTS, are they any good. They are nice and small. Are they bad or just OK?

Author:  tubegeek [ 25 Nov 2011, 18:12 ]
Post subject:  Re: 6EM7 power amp

They are better suited to push pull amps - the single ended (gapped) versions are far better for SE use. Makes sense!

The SE versions are actually pretty decent for SE amps, for the money.

Author:  mwhouston [ 25 Nov 2011, 20:54 ]
Post subject:  Re: 6EM7 power amp

Are you saying that the universal SE Hammond OPTs are OK or do Hammond only make PP OPTs in the Universal series. I'm looking for a couple of tiny SE OPTs.

Author:  tubegeek [ 25 Nov 2011, 22:39 ]
Post subject:  Re: 6EM7 power amp


Originally, the Hammond "experimenter" transformers were configured as dual-purpose for either push-pull or single-ended use. They weren't really optimal for single-ended use because they had no (or perhaps just a very small?) air gap to prevent core saturation by DC current. These are numbers 125A, 125B, etc.

Then they added some more transformers in that series which have larger cores and air gaps: 125ASE, 125BSE, etc. These are definitely more suitable for single-ended amps, although they are not as small as the previous models, but that's by design and you can't have everything.

The Angela Instruments web site has a lot of good detailed information on the various Hammond transformers, which they sell by the boatload.

Author:  Suncalc [ 25 Nov 2011, 22:45 ]
Post subject:  Re: 6EM7 power amp


All the Hammond universal SE transformers (p/n 125xSE) are limited to 100Hz-15kHz response. They are small (at least the low power ones). Here's the table from their catalog.

I would recommend that you stick with an Edcor XSE or 10W GXSE if you can fit it in. And the XSEs are at least rated 70Hz -18kHz. And they're cheaper.

Just my thoughts. :2c:

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