No, normal out. Just realised that haven't measured the bias voltage properly as I did it from ground to the top of the LED array. Will need to add 1V
Have used Pete Millett's interface, but my Win7/EMU-202 don't get well at all. What I was more interested in was to identify the type/brands of tubes with the lowest distortion in relation to the rest.
cheers,
ale
Yesterday I bought some UX-201A and CX-301A tubes on Ebay and want to build a DHT preamp. I know that some of you have used 01A tubes and it’s obvious that it’s characteristics differ from 26. Could some of you give me an advise what a good starting point is for tube settings: –Vg, Ia and Va?
I am going to use a CCS anode load and battery grid bias or LED cathode bias (I have not decided which yet).
Peter
I am going to use a CCS anode load and battery grid bias or LED cathode bias (I have not decided which yet).
Peter
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The max anode voltage is 135v, and the data sheet suggests -9v and 3mA for that. Since that fits a 9v battery nicely for grid bias, I'd start there.
Filament is 250ma at 5v so you need some kind of DC supply per tube. Rod Coleman's boards are very popular, but also consider choke input. A hammond 159T choke is 2.5H at 300mA which would be well worth putting in the circuit. You could follow that with a 158T which is 1 Henry at 300mA. That would be a nice solution. Keep all filament chokes way out of the signal chassis in another box, because they can be a source of hum.
I like two 156C anode chokes in series as an anode load. I also like filament bias, which works nicely with choke input. Avoids a cap on the input. As you can see, I find Hammond is a cost effective source of components for these kind of preamps. I'm in the OK and use Bluebell Audio who are good to deal with.
andy
Filament is 250ma at 5v so you need some kind of DC supply per tube. Rod Coleman's boards are very popular, but also consider choke input. A hammond 159T choke is 2.5H at 300mA which would be well worth putting in the circuit. You could follow that with a 158T which is 1 Henry at 300mA. That would be a nice solution. Keep all filament chokes way out of the signal chassis in another box, because they can be a source of hum.
I like two 156C anode chokes in series as an anode load. I also like filament bias, which works nicely with choke input. Avoids a cap on the input. As you can see, I find Hammond is a cost effective source of components for these kind of preamps. I'm in the OK and use Bluebell Audio who are good to deal with.
andy
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Hi Andy, thank you for the reply. I will start with –9V and 3 mA!
I have already decided to put the transformers including rectifiers and first filter caps in a separate box.
The problem with chokes is that they are rather big, so I think I will use Rod’s solution, or something similar. I am handy with transistors (and prefer them over Mosfets in many applications) and not afraid of using them in combination with tubes, even when these tubes are from the mid twenties
.
I have once used Lundahl LL1668 anode chokes in a 6N6p preamp. It sounded good, but not as good as a really good transistor CCS.
Peter
I have already decided to put the transformers including rectifiers and first filter caps in a separate box.
The problem with chokes is that they are rather big, so I think I will use Rod’s solution, or something similar. I am handy with transistors (and prefer them over Mosfets in many applications) and not afraid of using them in combination with tubes, even when these tubes are from the mid twenties
.I have once used Lundahl LL1668 anode chokes in a 6N6p preamp. It sounded good, but not as good as a really good transistor CCS.
Peter
Hi Peter, for the low current filament regulators (up to 2,5A), you'll be pleased to hear that BJT pass transistors are used, instead of FETs! They do have very useful advantages, including lowest Cob, and highest gm.
Please send some email via the board, if you would like the PDF application notes, etc.
Please send some email via the board, if you would like the PDF application notes, etc.
As Rod knows (he's very patient) I'm pretty hopeless with solid state technology and despite the size of chokes, they have this useful habit of only having two connections and just sitting there forever with no blue smoke coming out of them. I also think they sound divine, which is fortunate given my knowledge of solid state.....
andy
andy
My first version 26 pre amp
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An externally hosted image should be here but it was not working when we last tested it.
I use this, too, to get electrostatic screening into tight corners.
Chomerics tape can make a reliably conductive joint - the adhesive is also conductive.
It's fiercely expensive,though:
CHOMERICS|CCH-18-301-0100|TAPE, ADHESIVE, COPPER, 25.4MM | Farnell United Kingdom
26's to spare?
Hey, does any kind soul have a couple of pairs of various decent 26's to spare? Or perhaps know of somewhere I can get them for a decent price? (Besides E-BAY. Been watching that for a while.).
I will be happy to pay for them (via Paypal). I really, really want to hear for myself this "magic" so many talk about. You can PM me if you'd rather not post on the forum.
Hey, does any kind soul have a couple of pairs of various decent 26's to spare? Or perhaps know of somewhere I can get them for a decent price? (Besides E-BAY. Been watching that for a while.).
I will be happy to pay for them (via Paypal). I really, really want to hear for myself this "magic" so many talk about. You can PM me if you'd rather not post on the forum.
BTW, a lot of people have said they are not mad about the sound of LED biasing. I wasn't either, until I tried bypassing the LED string with quite large value low ESR caps. WOW! Different beast. It worked on IDHT's, and even better on 4P1L DHT's too. The new polymer caps are good for this. I used Oscon SEP's 330uF 16v. It seems the better the quality of the valve, the more you have to gain by doing this. I would love to hear the outcome if someone could try it on a 26 preamp or gainstage. You can also bypass the bypass caps with good foil caps for yet another step up in sound. Really worth a try IMHO.
Regards,
William.
Regards,
William.
andyjevans have some ST type for sale
http://www.diyaudio.com/forums/swap-meet/194952-fs-6sn7-jan-black-glass-tubes-dhts.html
inside the star #26
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Hi!
I guess the 26 has been loaded by just about everything possible: transformers, chokes, CCS, triodes and pentodes for SRPP, mu-follower, mu stage etc.
For me as preamp output only transformer coupling with step down ratio really makes sense with a 26. All else will lead to a too high output impedance for a preamp.
Different story if this is a stage within say a power amp, with controlled, not to low impedance to drive and no cable lengths. I would still prefer transformer coupling in this case but could also live with choke loading. All the other schemes are not really to my taste. Have not tried them all with a 26 though (but with other tubes)
Best regards
Thomas
I guess the 26 has been loaded by just about everything possible: transformers, chokes, CCS, triodes and pentodes for SRPP, mu-follower, mu stage etc.
For me as preamp output only transformer coupling with step down ratio really makes sense with a 26. All else will lead to a too high output impedance for a preamp.
Different story if this is a stage within say a power amp, with controlled, not to low impedance to drive and no cable lengths. I would still prefer transformer coupling in this case but could also live with choke loading. All the other schemes are not really to my taste. Have not tried them all with a 26 though (but with other tubes)
Best regards
Thomas
As a long time proponent and user of the 26 I am always amazed that such a crude device performs and sounds as good as it does..
(I designed and built my first 26 based transformer coupled line stage 11yrs ago..)
In terms of reducing hum pick up with the 26, 01, 12, etc in line stage applications I have found it mandatory to use electro-static shielding cans around the tubes. This of course hides them, but also effectively prevents electro-static coupling between the tube's plate and surrounding metal objects potentially with stray AC currents on them. You can find knock offs of the Western Electric style cans on eBay from time to time, that's where I got mine.