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Old 15th December 2010, 05:01 PM   #1
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Default Will This Work? 26 Linestage

Ive never worked with dhts before but thought I would try a fairly simple 26 line stage. Im fairly new to design period so there may be multiple errors here. I would appreciate any comments before I go ahead and attempt this. Will my grid biasing arrangement work? Am I connecting the OT properly based on loading the plate with a CCS? Anything else that jumps out that could be improved?

thanks!

26 ccs line.jpg
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Old 15th December 2010, 05:30 PM   #2
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That will not work. You need a parafeed cap. (DC block) either between the anode and trafo or between trafo and ground.

A look at the #26 data sheet indicates (IMO) you would be better off with 180 V. on the plate, -14.5 V on the grid, and IB = 6.2 mA. condition set. That gets the load impedance above 2X RP.
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Old 15th December 2010, 06:42 PM   #3
kevinkr is offline kevinkr  United States
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Definitely need to use a cap with the parafeed transformer as you have created a dc short across the 26 otherwise.

I would not recommend 180V based on my experience with the 26, but would recommend up to 150V at 6mA - 7mA.. At higher voltages IMHO they can sound a bit strained and service life isn't that good either.

I prefer fixed bias with these tubes, and have tried all sorts of approaches and prefer conventional grid bias with an input coupling cap. You can use a 9V battery to supply the bias in fact..

Something else to note is that this tube draws appreciable grid current and I would suggest keeping the dc resistance in the grid circuit to no more than 220K..

I'm not sure just how good that little Edcor transformer will be, winding a good 15K transformer is a bit difficult.

In terms of the CCS supply I would think about 225V of raw dc would be enough and would keep the dissipation down.

The 26 is extremely microphonic and is also susceptible to electro-static coupling between its plate and surrounding objects. As much as I like seeing the tubes mine have lived in reproduction WE shield cans like the ones used in WE 91A amplifiers for at least the past 4yrs. This greatly reduces electro-static coupling between them and nearby metal like ungrounded electronics chassis which may have AC currents flowing through them.. (Hum and lots of it..)

The tubes could be shock mounted on a separate little sub-chassis, or on a high mass chassis that is well decoupled from whatever it is sitting on.. Note that even with these precautions they are still going to be excited by external noises, but hopefully to a tolerable extent.

I have run 26 more or less continuously in my system in line stage use for nearly 10yrs and will continue to do so into the indefinite future. (I've been designing with the 26 for more than a decade, and have designed a number of modestly popular transformer coupled 26 line stages, as well as consulted on a number of choke, resistor or CCS loaded versions.)
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Last edited by kevinkr; 15th December 2010 at 06:48 PM.
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Old 15th December 2010, 08:13 PM   #4
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thanks for both your responses. If I keep this topology I will certainly add a coupling cap.

Also i will lower the B+ and raise the plate voltage to 150v.

As alternative to adding a the parafeed cap, would the attached work?

Are there other DHTs suitable for a line stage that dont have the extreme micro-phonics issues that it sounds like the 26 has?

Kevin I will have to look at your site for how to do fixed grid bias with a battery as I have no idea how to implement that.

26 ccs line 2.jpg
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Old 15th December 2010, 08:25 PM   #5
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BTW the edcor is a single ended, air gapped (i presume) output transformer so the dc shouldnt be a problem.
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Old 15th December 2010, 09:36 PM   #6
tomchr is offline tomchr  United States
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The topology you have chosen is that of a power amplifier. Not that there's anything wrong with that, it just opens up another can of worms -- magnetic components. For an output stage, this is a rather unconventional circuit and I highly doubt it'll work. In addition, the resistor on the "top" of your current mirror is not needed.

I suggest that you get your paws on a copy of Morgan Jones' book, "Valve Amplifiers". It's $50 well spent. Read through the chapters on how tubes work and how to design basic circuits with them. Then go back and look at Kevin's response above. It'll probably make much more sense then. In addition to books, there's also a bunch of information available on-line. Valve Wizard is a good place to start.

For the "will this work" kinds of questions, I prefer to use a circuit simulator. I use Orcad 9.3 Student Edition that's about 15 years old. You can still find copies of it floating around on the net. In addition, there's WinSpice, LTSpice, and others. There's also simulators for Mac if you're so inclined. You can learn a lot about how tubes work without ever touching a soldering iron or electrocuting yourself with the B+...

If I was to design a stage with the 26 (I have no experience with the tube), I'd probably follow post #3, AC couple on the input and output and go from there.

You could also google "26 preamp"..... I turned up a design on VT52.com (see the projects section). No idea if it's good or not, but it looks pretty fundamental.

~Tom
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Old 15th December 2010, 10:17 PM   #7
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Quote:
Originally Posted by leftovers53 View Post
BTW the edcor is a single ended, air gapped (i presume) output transformer so the dc shouldnt be a problem.
Yes, he is correct. This transformer is gapped for unbalanced DC. Although it can be run parafeed, it does not need to. The proposed topology (post #4) will work.
I am not certain of the current rating on that transformer but I am sure you have checked that out.

In fact, I have built such a beast, very similar to the circuit here. Don't worry that it looks like a power stage. Do remember to figure in the input Z of the next stage when simulating the load on the 26.

By the way, I found that I liked the sound better on AC than DC filaments.
If using DC, try a LCLC supply. I did try a 3Term based supply and that didn't sound so hot. YMMV of course.

If using AC, then try common mode choke. That often helps. Lots of material on that technique about, probably on this list somewhere.

BTW: I do not remember the operating point I used or I would state it here.
I do recall that I had to move it around some. There is a lot of variation in 26's, especially the globes I was using. Wonderful sounding though. Very rich.

Last edited by funkytek; 15th December 2010 at 10:20 PM.
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Old 15th December 2010, 10:19 PM   #8
mwiebe is offline mwiebe  United States
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I've used the GXSE5-600-5K for the past five months and it's a very nice output transformer. I get my GXSE5-600-15K this week and I'm looking forward to seeing how they compare.

You do not need the parafeed cap as you noted, since the GXSE5-600-15K is a series feed transformer. However your first schematic is set up as if it is parallel feed, which invited the you need a blocking cap comments. Your second schematic for the GXSE5-600-15K is better except eliminate the "CCS" between B+ and the transformer. With a series feed schematic B+ goes straight into the primary side, exiting directly to the plate. I would lower the grid leak and drop the stopper, but that's just personal. Should sound nice.
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Old 15th December 2010, 10:38 PM   #9
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It would be interesting to try it with and without the CCS. I am not familiar with the IXY types but sometimes an inductive load can be an issue. I used a VR tube shunt regulator on mine and that worked out just fine. Looked nice too.

Mine also used a ferrite bead on the grid instead of the stopper. But as mwiebe noted these are matters of taste and circumstances.
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Old 15th December 2010, 10:57 PM   #10
tomchr is offline tomchr  United States
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Without the CCS it should work just fine (assuming you implement bias at the grid or cathode).

But why use a transformer? I'm asking out of curiosity.

~Tom
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