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Old 20th January 2008, 02:44 PM   #21
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There is a great deal of stuff going on in this thread!

As for the DC stage. It seems to me that this is a common situation where you become fixated on solving one problem, and in doing so ignore the design as a whole.

While there is nothing stopping you from going DC, it has a number of negative side effects. One, you are losing lots of your B+ potential because of having to get the output tube grid up to a value that matches the driver tube. This does two negative things - it lowers the voltage across the output tube, which lowers output power and IMHO, reduces the sound quality of the stage. I think tubes run at higher plate B+ tend to sound better than when they are run on the low end of their range. This applies to the driver as well. In order to keep its plate voltage down, you are going to have to drive it with a low B+ or use a plate resistor that will drop enough voltage... more than is considered "normally optimal."

All of this to remove one high quality paper in oil or metal film capacitor? I don't think I would go that route. It's very easy these days to get extremely high quality bypass caps. In the earlier days, not so much and poor use of lots of electrolytics are the reason most people think solid state sounds like garbage. But if you have SOURCE>DRIVER>OUTPUT with only 1 high quality capacitor in the signal path, then hey, you've done a great job. What's not to like about that? If you focus on the amp design as a whole and use quality parts, you'll likely end up with an amp that makes you go whoa!
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Old 20th January 2008, 05:47 PM   #22
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Quote:
By the way, was it you who did a PP-version of the RH84 ?
I like the idea behind that....
I have built a ECL82 based PP headphone amp based on the RH principle. It works very well and has one coupling cap.
I am currently working up to building a PP version with 807's with 6AU6 drivers. This will be direct coupled. In order to eliminate the coupling cap I will run the driver off of a -300V rail and couple its input through a phase splitting transformer. This is a definate possability for achieving a DC coupled SE amp without the heat waste. Any center tapped power transformer can be used to derive a negative rail as well as the more conventional +B, all that you need is a pair of silicon diode to form the other half of a bridge rectifier. The input transformer takes the place of a input cap in this case. It also brings other benefits to the party (ground loop braking) - the transformer need not be excessively expensive in this location.

I also built a parafeed version of the RH807, which I eventually broke down because something just didn't quite work with it. Mainly it was the bass that let it down, but I think there were phase issues as well at the top end. I'm hoping the PP version won't have the same issues.

Quote:
his does two negative things - it lowers the voltage across the output tube, which lowers output power and IMHO, reduces the sound quality of the stage.
My experience is somewhat the opposite. I found that some tubes sound better at lower voltage and higher current. I suppose this depends on the exact valve. I find the sound to be tighter and with more attack. If this is not what you want from your design then then I suppose its a negative. You might say it sounds more solid state.

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Old 20th January 2008, 07:21 PM   #23
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Quote:
Originally posted by Shoog
My experience is somewhat the opposite. I found that some tubes sound better at lower voltage and higher current. I suppose this depends on the exact valve. I find the sound to be tighter and with more attack. If this is not what you want from your design then then I suppose its a negative. You might say it sounds more solid state.
My best and favorite pieces over the years have turned out to be ones with high B+. Somewhere down the line I started to piece that together... and when designing and building my headphone amp I made it with testing tubes in mind. For instance, I made it so that you can run the amp mono with one channel driving the headphones, so you can compare the sound of a single output tube to a single piece of another type. This allows me to review tube types, even if I only have one piece. Anyway, on with the story... I also wanted to test my B+ theory... I used a 215V B+ and ran and listened to all the EL34, 6L6 and 6550 type tubes I had and made notes on the sound. I then switched to a 385V supply and re-biased to same dissipation and listened to all the tubes again. In every single instance, the amp sounded better with the higher B+ voltage on the tubes, all else being equal.

Now, I also wonder if we aren't on the same page... as I do agree, higher current usually sounds better. But, I think you are in general referring to the fact that many designs (especially with the high gain tubes) focus on using a large plate resistor to maximize gain and by doing that usually run the tube on a low current. I have no doubt that running a smaller plate R and increasing the current through the tube can be made to increase fidelity.

So, instead of being opposites, I think we are discussing two separate issues, and in some way are recommending high voltage and current to maximize sound quality. As for that, I'm all for it.
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Old 20th January 2008, 07:28 PM   #24
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I wouldn't disagree with anything you have said. My positive experience with low voltage high current is mainly derived from using 6080 at 100V 100mA rather than the more usual 250-300V and 40mA. I think we are talking sweet spots and the 6080 is sweet when run hot and with a lean voltage.

Of course transformers are a major deciding factor and a low ratio transformer has a whole heap of advantages over a high ratio one.

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Old 21st January 2008, 12:40 PM   #25
Empee is offline Empee  Netherlands
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Hi all,


@ shoog:
a PP version of the 807 is what Cycline3 described as

Quote:
Originally posted by Cycline3
an amp that makes you go whoa!


I thought of a interstage transformer to get rid of
the coupling capacitor but I found a decent transformer will
cost you more than just one decent cap...

But for a PP amp its a different story,
a good centre tapped interstage will save not only the coupling
caps, but saves a complete phase splitting stage !

Anybody on good (more than decent) interstage
transformers for a good (less than decent) price ?

Cheers,

Empee


<edit>
...ah...!
I see Nesha already suggested a coupling interstage transformer in post #17 of this thread... It went completely overlooked !

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Old 21st January 2008, 12:56 PM   #26
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The way I will do it is by putting the transformer on the front end. This will do the phase splitting - but also drive the two inputs of a LTP and so maximise gain. Because it is dealing with a small signal (2Vpp) it doesn't need a big core. A suitable transformer can be had from Farnell, manufactured by OEP and costing about 15.00 each. This is about the same as a decent coupling cap.
The main advantage for me is that simple cathode bias can be employed with the grids and cathodes referenced to a negative rail rather than GND.

The same transformer and negative rail arrangement could easily be implemented with an SE amp. It also can allow you to use a really big anode load resistor on the driver stage for more linear operation.
When i get home I will post up the schematic I am working on. Just use something based on half of it and all your SE problems are solved.

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Old 21st January 2008, 06:30 PM   #27
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Here;

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