• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

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
 

kevinkr

Administrator
Paid Member
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.)
 
Last edited:
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
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
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
 
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:

mwiebe

Member
2004-04-02 4:00 pm
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.
Matt
 
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.
 
The circuit shown in post #4 will NOT work with the CCS. The transformer has an impedance of 15 K, the CCS has an impedance of more than 1 Megohm. They are both in series, so all of your output voltage will appear across the CCS.

Either remove the CCS, replace it with a short or connect the CCS directly to the plate and use a high quality cap between the plate and the transformer.
 
tomchr: thanks for the book and software suggestions, I will definitely check them out. There are plenty of major holes in what I know that need to be filled in. Can you tell me why you dont think the ccs above the primary wont work? I just want to understand it a little better. Im using the OTs simply because I have them and havent built a transformer coupled line stage before. Thought this might be a good application and they kind of go with using a 1930s tube. Of course a ss ccs doesnt, but i like playing with little back to the future. For the record the best sounding line stage i have built so far is a 6SN7 Aikaido cathode follower buffer.

mwiebe: I will lower the grid leak to 225K, glad to hear the transformer is good. I have had really good luck with Edcor products. They are an amazing value.

funkytek: I just assumed AC filaments would be to noisy. Will have to do some more research.
 
The circuit shown in post #4 will NOT work with the CCS. The transformer has an impedance of 15 K, the CCS has an impedance of more than 1 Megohm. They are both in series, so all of your output voltage will appear across the CCS.

Either remove the CCS, replace it with a short or connect the CCS directly to the plate and use a high quality cap between the plate and the transformer.

There is one scenario where this can work, and that is to place a cap between the CCS output and circuit ground. To avoid interaction with the transformer this should be 10uF or more.. This will allow you to set a specific static (DC) operating point but otherwise acts like a conventional transformer coupled line stage. Use a smaller cap and the LF response may be peaked by resonating the primary with that capacitor.. (Not necessarily a good thing, but it can sometimes useful..)
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
Can you tell me why you dont think the ccs above the primary wont work?

You post and George's (tubelab) post may have crossed. But anyway. Here's an intuitive way of thinking about it:

A CCS has high output impedance. Its job is to keep the current constant regardless of what voltage is across it. Basically, as the anode voltage of the tube changes, the transformer primary will "ride the wave". As the anode voltage rises, the voltage across the CCS will be reduced by a similar amount, and the voltage across the primary of the transformer hardly changes. Hence, no change in voltage on the secondary side --> no sound output.

Adding a big cap from the CCS output to ground will reduce the AC impedance at that node to zero (close to anyway). So now for AC signals, the voltage stays put and you get the full anode voltage swing developed across the primary of the transformer.

Im using the OTs simply because I have them and havent built a transformer coupled line stage before.

Fair enough. The idea wasn't immediately intuitive to me, but that doesn't make it wrong. In that case, I'd probably build it like a power stage. Scrap the top resistor and the CCS. Connect the tube anode to the primary of the tranny and the B+ to the other side of the primary. Voila.

Then you have to figure out how to bias the darn thing. I'll leave that as an exercise for the reader... :) Actually, KevinK described it well, you just need to look up the terms and figure out what he meant. That's part of the learning experience... ;)

funkytek: I just assumed AC filaments would be to noisy. Will have to do some more research.

The 26 is a directly heated triode, right? In that case definitely go with the DC filaments. Definitely... Start with something basic like an LM317 (assuming it can supply the current).

~Tom
 
Last edited:
While you're here on DIYaudio, have a look at the thread '#26 preamp'. There are many design tips here, simple and advanced. Battery bias, cathode bias, even filament-current bias. The use of current-sources with the #26. The DIYers on this thread have made some fine preamps with this DHT, and shared their findings.

http://www.diyaudio.com/forums/tubes-valves/151421-26-pre-amp.html

The 26 filaments certainly need dc heating, or humm will be unbearable. The #26 is sensitive to tiny changes in the quality of the filament-supply, as you can see in the thread. Sadly, the 3-terminal IC regulators don't sound good in this position - they are too noisy, and their structure causes the anode-current to be influenced by the regulation of the filament current (transients & noise in a 1.05A current can do a lot of damage to a <1mA signal current!). My double-buffered solution for heating DHTs is used by some of the builders - still some kits for these available.
 
I've been playing around with DHT linestages for about 2 years. The best sounding tubes for me are 10y and 26. 01A is also nice, 31sp is OK and 46 is very neutral though without the magic of the 10y and 26 which are both a bit special. I like a transformer output myself. I use a Hammond 126C which is SE with good inductance of about 105H. This works beautifully with the 10Y and works well with the 46 - the 31 would also be possible and indeed the 71a or even a 2a3 I guess. It's 1:1 so you keep the gain which is important with DHTs since there's not much gain available.
With the 26 it needs more input inductance in the transformer - probably 150H or more, something like a LL1660/5mA. The 10K:10K 126C isn't comfortable with a 26 or a 01A.

So all this led me to use a 10y into a Hammond 126C. They match perfectly and sound very good, and the Hammond is cheap. Microphonics isn't an issue, which may be important.

I use filament bias, which is a pain to set up and not really recommended for a beginner. You could use a simpler circuit like grid bias with a 9v battery as Kevin suggests. This would need 180v of B+ feeding the 126C, and about 12mA which is inside the 15mA rating of the transformer. I'd use LCLC for DC filaments if you can. Hammond do suitable chokes - 159ZC will do. Use about 40,000uF of caps altogether. That's a way of getting good results with a predictable and relatively hum-free setup. The 10Y is more expensive to buy.

My "rationalised" version of my amp setup will be all in the amp - no line stage. Right now I'm planning 26 direct coupled into 10Y into the 126C. Then another 10Y with grid bias from a 9v battery on the secondary of the 126C, cap coupled into a 300b. A second 126C there might be an option but may be overkill. Still, they're cheap enough. All in all four stages and should be just enough for speakers around or over 90db sensitivity in smaller rooms.

Andy
 
Hi!

I'd use LCLC for DC filaments if you can. Hammond do suitable chokes - 159ZC will do.


Better yet LCL, keeps a nasty electrolytic away from the signal path. Lundahl offers nice filament chokes. A LL2733 gapped for 1A will have 600mH. This provides excellent isolation of the filament from the PSU. With a 40.000uF cap between the two Ls this will be hum free.

Best regards

Thomas
 
Hello Thomas!

The reason for using a 1:1 is purely gain - this would imply short cables and an input impedence of the next stage of 100k or over. Perfectly possible.

I know you use step-up transformers in your arrangement. That's a clever solution. Mine doesn't aspire to such sophistication, and just keeps it on a very basic and simple level - i.e. that you situate the line stage next to the amp.

Andy.