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 Post subject: Re: 6EM7 power amp
PostPosted: 13 Oct 2011, 23:28 
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rock4016 wrote:
In effect you are saying that the coupling cap presuming the proper value is chosen, has little or no effect on the sound of the amp.
I believe you have misunderstood my position. I never said that the coupling capacitor has no effect on the sound of the amp. Nor did I indicate that all coupling capacitors were equal in effect. I agree that different capacitors will impart different overall sounds to an amplifier. The choice of component values, characteristics, and quality are all part of the design process.

However, one does not simply remove the coupling cap from an amplifier design to arrive at a direct coupled amplifier. The topologies are different. I am suggesting that the sonic differences between capacitor coupled amps and direct coupled amps (and transformer coupled amps for that matter) arise from the fundamental differences in topologies and operation; not simply from the presence or absence of a single capacitor.

Your statement that "... even the very best caps that I can afford (V-Caps) still don't sound as good as no cap at all" is still, no matter how fervently you may hold that position, just your opinion; and other's will have their own opinions. Also, I fully grant that your opinion has just as much merit as mine in the overall discussion. However, our opinions are just that, opinions and not objective fact. All that I said was that, in my opinion, the impacts to the design in choosing the direct coupled topology, for the removal of a coupling capacitor, were not warranted. And I still believe this to be the case. You are, of course, free to disagree. This is one of the true benefits of a free and open discourse.

As to the efficiency, you are correct that I usually don't care much about overall power added efficiency in a tube amplifier. However, my comments were in the context of a specific power supply topology where inherent limitations exist. As such, they should be evaluated in that light.

As for biasing methods, again these are my opinions. I realize that there are many who feel that the "more direct" approach to biasing is better, but this topic has been argued to death on these forums and I am reluctant to restart the shooting war again here. The methods may be sufficient to meet requirements in some designs, I simply don't like them because of the other restrictions they impose upon the design process. As I said "I'm not a fan", not that these methods are inherently evil. ;)

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 Post subject: Re: 6EM7 power amp
PostPosted: 14 Oct 2011, 02:18 
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Joined: 18 Jun 2010, 12:46
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Location: Northern VA
I hope that the real benefit of open discussion is that someone will experiment and decide for themselves what they prefer, that in my opinion is the essence of DIY.

I never suggested or implied that I could remove a coupling capacitor from an existing design without making any other changes to the design, but I have done experiments with SE amps where I could remove the cap and make the necessary power supply and output stage changes in a short time. Short enough to remember differences that I had heard with the cap in place.


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 Post subject: Re: 6EM7 power amp
PostPosted: 14 Oct 2011, 06:08 
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@rock4016: I would be the very first to say "thank God someone posted a different schematic". How stagnant and dead would DIY audio be if there was only one schematic for every type, amp topology and design. I who have hundreds of dollars worth of expensive caps would love to build gear where they are not required (you never have the right value or voltage rating).

From what I am understanding about direct coupling, for the sake of one inter-stage cap, some of the other issues are not worth the hassle. I understand that caps are about the worst component you can place in the signal path (inter-stage transformers are far better) but good caps have minimal impact. One inter-stage cap will not make your $200 speakers sound better or worse.

So I'm going with the cap and will select a cap I think suits the amp. Let's all now move forward. This thread is about building a 6EM7 amp. Maybe how stages are coupled could be a separate thread.

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 Post subject: Re: 6EM7 power amp
PostPosted: 15 Oct 2011, 07:41 
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This guy is serious, no a super-duper-serial builder. Check out his whole site but work your way down to his 6EM7 amp using trannies backwards. http://www.wdox.com/

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 Post subject: Re: 6EM7 power amp
PostPosted: 17 Oct 2011, 02:42 
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I wanted to build this amp on one of my largest cast Al chassis but I just can't hide the open frame OPTs. Some would say leave them and leave them bare but aesthetically I just can't do it. I'm now down to a couple of other ways to assemble the amp and what will be used as a chassis. I have a couple of small timber based chassis which I have used extensively in the past. My retro-thermionic Pinot KiSharn used one of these. The other is steel cooking pans which I used with mu Guitar Puppy. I can encase the OPTs and mount them on top of the steel chassis. Yet to decide. Balance of parts are still not here.

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 Post subject: Re: 6EM7 power amp
PostPosted: 17 Oct 2011, 18:51 
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My tubes all came today. Two Sylvania 6EM7 (USA). I didn't know Sylvania were making tubes. Then are these NOS?? Also in the box the 5Y3GT Sovteks. Ceramic sokcets and the additional 5H choke, now I have the lot.

I have to build the amp on a wide wooden based chassis because the 15W OPTs won't fit in the low profile timber bases I have. I want to use these OPTs. After building the Silver Dragon, with oversize OPTs to handle the current and being impressed by the bass extension and weight of that amp I want to give this little amp every chance for good bass. Not only that I'm trying to use up parts I have. I have a number of small 50mA 550V centre tapped Hammond trannies but I also have some bigger trannies. I wouldlike to use the smaller power trannies.

I was thinking on using two of the smaller trannies beause each channel requires ~50mA. But also each channel only requires 260V. I guess the question is will one Hammond 370BX rated 50mA at 550V (CT) be enough to power two channels? Or do I need to use both trannies? Would some one let me know please.

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 Post subject: Re: 6EM7 power amp
PostPosted: 17 Oct 2011, 21:20 
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mwhouston wrote:
I guess the question is will one Hammond 370BX rated 50mA at 550V (CT) be enough to power two channels? Or do I need to use both trannies?
Yes, you'll need both transformers. One will not supply the necessary current. I would be tempted to build them as mono-blocks. One transformer, one 5Y3, one choke, one 6EM7, and one o/p transformer per channel.

My guess is that the 370BX with a 5Y3 rectifier is going to generate about 300v at 47mA draw at the input to the filter. This means that you're going to need an 850Ω/2W dropping resistor for each mono-block power supply to get back down to 260V. This is just a quick guess, I haven't actually run the detailed numbers.

Sounds interesting. :2c:

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 Post subject: Re: 6EM7 power amp
PostPosted: 18 Oct 2011, 03:38 
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Matt thanks again. I'm going to run through a few assembly scenarios tomorrow and see what fits with what I'v got.

Thanks again.

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 Post subject: Re: 6EM7 power amp
PostPosted: 18 Oct 2011, 15:28 
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Joined: 04 Jun 2008, 20:59
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Hi Everyone, A most interesting discussion. I see no need to comment on direct coupled vs cap or transformer coupled stages, there are enough opinions to go around on the merits and shortcomings of each. I use all three in various designs. One thing that I note that is easy to fix that I did not see clearly covered for many of the less techie folks on the forum. This is the effect of the high cathode voltage on the heater to cathode circuit and how you resolved it. The schematic showing the tap off the cathode circuit of the output tube that has to go to the heater supply will do the job fine, as long as you don't ground any part of the heater circuit. That is key, as I have seen folks do just that which often results in rather spectacular failures.

I like new or clever ideas in designs,

Good listening
Bruce

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 Post subject: Re: 6EM7 power amp
PostPosted: 20 Oct 2011, 01:58 
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Lots of snaps few words. Please comment on the layout. Open frame OPT in small enclosure. 5H choke fits in the base. I like the rec. tube at the back - all the AC away from the amp tube. The paint scheem maybe hammer tone but then??
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6EM7LayoutII_I.jpg
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6EM7LayoutII_III.jpg


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