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I was discussing new projects for amps/preamps with a buddy, and the subject of low mu tubes for use in a preamp came up. He said he had made 12B4 preamps in the past with excellent results, so I suggested a preamp with 6AH4 tubes.
Well, he put a prototype together, based on the DIY 12B4 circuit, with a 5U4/5V4 rectifier. Well, the results far exceed expectations.
The 6AH4 is one dynamite tube for preamp applications. Easily one of the best preamp tubes I've ever heard. The low mu tube approach seems to be the way to go. I've got it mated to a pair of 120 watt mono block amps with 12SN7 input and a quad of 1625 power tubes.
Honestly, I don't know why anyone would bother with 6DJ8/6H30 tubes (with all the associated issues) when there are tubes like the 6AH4 readily available.
I'm currently having a high end version of a 6AH4 preamp, based largely on the Thomas Mayer circuit (with a few small differences). Should be finished in a couple of weeks. Will post some pics once completed.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
Follow Ups:
good to hear of their sonics. I have underway a balanced output pre built around them. LTP with SE input to drive an active cross over. high voltage, and high current and a lot of heat made from nearly 20 Watts dissipated in their resistive loads. AC heaters to start with, but room and voltage to try an L-C filtered DC supply...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Lots of good feedback.
The 6CK4 is another good candidate for a line stage. I just really liked the specs of the 6AH4, and the sonic results were even better than I was hoping for.
The 26 tube looks interesting. May have to look int that. For now, will be anxiously awaiting the high end finished 6AH4 preamp.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
You're right about the 6CK4...but its heater current is a wee bit higher. I don't recall if its base is the same as the 6AH4, but it can't be a very big job to change it so as to try the 6CK4 in a 6AH4 circuit( and what I have in mind will be able to accomodate that ).
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I've used the 6AH4 in a couple of builds - a great tube!
It sounds best at higher currents...but, it was too noisy with AC on the filaments.
Have never tried the 6CK4 - anyone have some recommended op points?
Jim
I'm running a pair of 6ck4s as output tubes (SE) in a junk-box amp. They sound pretty good, and are very simple to implement and drive. I'm running them at about 250v and 40ma, they bias up similar to a 46. Somewhere i read that they can take higher voltages easily, but I have yet to try this.
Jim, could you define, 'higher current' please?
depending on the response, it may be I will go towards an OPT design from Intact. The little LTP 6S4P-EV rig I have built around BOH-2's is quite nice, but with 6AH4's I would not need the step-down. Using an OPT would also make the B+ significantly lower; right now I have in mind B+ OTO 3x idling plate voltage with resistive loads. With the prototype circuit, it was more like 4x with 12B4's.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
180 vpk, drawing approx 24 mA - bias -15 v.
Each plate loaded with paralleled pair of 5k6 12 W Mills resistors.Cap-coupled output using a pair of nickel grid chokes...input uses 50k Goldpoint attenuators.
Edits: 04/02/14 04/03/14
That is do-able..:) I had in mind 10k late loads, and about 30 mA current and comparable plate voltage. Takes a near 500V supply( I have an L-C set up that delivers about 480 so I think I'll keep using it ). Will likely run the heaters on DC between ground and -6.3 negative and use that for the tail CCS as well. That will keep CCS dissipation manageable...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Hi!
I am glad that you got such good results with the 6AH4. It is a great tube and easy to use since it can be just AC heated in a line stage. A similar alternative would be the 6CK4 but I have not used that one yet.
Best regards
Thomas
On the next thread up we have details of a 26 preamp, and I hate to say this but in my experience it's a better tube in all respects, especially with filament bias.
I know Thomas Mayer's work and it's beyond excellent - and ironically he was the person that developed filament bias for tubes like the 26 and his own favourite the 801/10Y. Now that's another marvellous preamp tube.
I have a lot of respect for the 6AH4 - in fact I ended up giving all my stock to Thomas. I was using it as an output tube where it does a good job, until I discovered the 4P1L which obliterated my previous thinking.
The 6AH4 is a good tube, no mistake. It's just that some of the directly heated tubes like 26, 10Y, 46, 4P1L are better in line and driver stages. More clarity and tone and a little bit of magic. That's my take on it, and I haven't used indirectly heated tubes in about 3 years for that reason. So clearly I'm biased!!!
Thinking ahead to another project. How is the sound of the 4P tube as compared to the 26 tube? I wanted you to build me one but I guess life got in the way. So be it! Now that I found a builder here in the states that I am very pleased with(and will know more once I get it here)I might, after a little while ask him if he's interested in doing another project.
Hi there - indeed, life got in the way - I've been busy.
Anyway, the 4P1L is microphonic. No getting away from that. So you should warn whoever's building your project that they need to take special measures. In no particular order:
- starve the filements. I use them in parallel at 1.9v
- don't even think of using a chassis that isn't massive and sonically dead. I'm talking 4mm aluminium here. Anything less and you're asking for trouble.
- reckon on feeding the 4P1L with a 2v signal. Less than a volt and you're in trouble.
- wrap plumbers tape several times round the bottle and fasten with electrical tape.
- consider some kind of rubber decoupling of the sockets (but bolt the sockets to something solid before decoupling it). I actually haven't needed this.
Read up on all the DIY Audio threads about the 4P1L, Especially the line stage thread.
Good luck! The 4P1L is a real performer.
Is that a typo?
Seems like a lot to drive a line stage.
Freo
The secret is out 8^D
I like them at about 15mA
with a 9V NiMh on the cathode
and choke or transformer load.
DanL
Thanks!
The 6H30 and 6DJ8 are good when you need more gain.
Other tubes will offer lower output impedance.
It's apples and oranges, but I agree that your approach is a good one for a line stage.
I agree with your point about gain. Since the subject is a line stage, gain is not much of an issue.
I personally have developed a dislike for most frame grid tubes in general, and 6DJ8/6922/6H30 tubes specifically. I think they are too prone to microphonics, noise, overpriced, and can sound steely/strident. I think there are a lot of better alternatives out there for DIY applications than the frame grid tubes. To me, the main purpose of DIY is to use tubes that are high quality and reasonable cost, which rules out most current production tubes. Why pay a lot of money for a current 6L6 when a 1625 is MUCH better made, still available, and MUCH cheaper? The same logic applies to small signal tubes as well.
I remember reading in Stereophile years back a running debate about the suitability of the 6DJ8 for (commercial) audio applications. I believe David Manley was against it's use, and I agree with that assessment.
Having said all that, I do have an old Wavelength Audio Cosine DAC I kept that uses a single 6GM8. It's still very musical for CD playback.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
In the last six months, I have bought plenty of Amperex 6DJ8's for less than $5 each... suitable tubes are still around.
They do sound like crap if you don't have stoppers on the grids and plates though.
For an RIAA preamp, I would take them over a 12AX7 any day, though I do think there are other options worth investigating.
Might I ask what size grid stoppers do you find necessary? Mandatory? I've been running a 6kn8/6dj8, DAC/line stage for a few years without any grid stoppers... thanks CB. JH
For plate stoppers, I find that 50-100 Ohms works fine (carbon film or carbon comp).
For grid stoppers, I tend to go between 200 and 1K on a 6DJ8. (Sometimes higher on other tubes)
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