Thinking of building a low gain preamp

[kward]

I have always tried to steer away from preamps in general because I just can't find one that I think sounds good (meaning is neutral and doesn't suck the life out of the music). At least, with the preamps I've tried in the past---commercial units, kits, and some early attempts at home brew stuff, I have not been satisfied with any of them. The closest I have come to what works for me was a 6DJ8 gain stage coupled to a 6H30 cathode follower, using teflon output caps and oil power supply caps. But still even in that attempt, it just didn't sound as good as no preamp at all.

Especially with larger wattage push-pull type amps (like those using KT88s), it seems like a separate preamp must be necessary to take advantage of the full output power available in the power amp. So If I'm resigning myself to using a preamp, I might as well put my full effort into it.

I've mulled over the idea of using the 300B as a preamp tube for several years. I know this may be trying to pound a square peg into round hole, and wishing for a totally "neutral' 300B preamp may be an oxymoron. And yet going through my list of requirements, the 300B seems to meet these requirements.

Requirements are that I don't want a circuit with a lot of gain--eight to ten dB (2.5x to 3x) is ample. I don't want a circuit with a cathode follower, nor do I want a circuit with more than one stage, but it's still got to have usefully low output impedance. I think by picking the right operating point, putting the plate on a constant current source, and using DC filaments, I could get something that is silent, sounds pretty decent, measures well, and has usefully low output impedance, as well as offering the gain factor I'm looking for.

Is this a crazy idea? I've never used or designed a single thing around the 300B, so this would be a first, and a bit of a challenge for me.

What are the pros and cons of using the 300B this way (other than cost of the tube itself which is clearly stratospheric)? Has anyone undertaken a project like this?

I'd like to hear feedback from anyone who's either thought about a 300B preamp, or has built or used anything similar.

How about the 2A3 instead of the 300B? Other than the filament differences, I'm not sure what is different between these tubes (haven't done any research there yet). I've thought about the 12B4 as I understand is an indirectly heated cousin to the 2A3, but abandoned that idea because it's not available in new production.

I'm not married to the 300B or any particular DHT for that matter. I'm really looking for a solution that will be as neutral as possible and not suck the life out of the music, but will meet the requirements I have stated above. But, since the 300B is popular and well respected, I thought I'd start the discussion there.

 

[thorpej]

Well, the 300B certainly has a low enough plate resistance (~750 ohms) that you should be able to drive a tube amp with 100K or more input impedance. And the mu of the tube seems to fit your requirements at 3.9 (since you plan to use a CCS for the plate load, you should be able to achieve a stage gain close to that).

The 2A3 is similar in plate resistance (slightly higher at ~800 ohms, thus the mu is also a little higher at 4.2) and transconductance, but has a lower dissipation rating (15W vs 30W for the 300B). You only care about the dissipation rating for selection of the operating point, though.

It seems either would be equally suitable for your application (although that doesn't necessarily speak to their suitability in general to a small signal application ). I think the real trick is going to be finding the sweet spot in the operating point that produces the least distortion; you are not going to have enough stage gain to be able to apply any substantive negative feedback. But since you're going to be barely moving the needle on these tubes in terms of their driving signal, you might be in a very linear region no matter what your operating point.

 

[kward]

Well that brings up an interesting point (about the feedback). The other idea I'm mulling around is the use a medium mu triode for the first gain stage (probably a 12AU7 if I went that approach) and the 300B as the second gain stage, both configured in common cathode arrangement. Thus I would have a non-inverting output. In that scenario I was thinking of applying some loop feedback--not a lot, maybe 6 to 10 dB just to clean things up a bit and get the forward gain down to where I wanted it.

So that idea is not totally off the table I guess...

 

[Palustris]

It is possible to get astonishingly small H2 and H3 distortion figures for the 300B using the Triode / Pentode Loadline Simulator. I assumed a 5k ohm load and 350V B+ with 2Vrms input from the source.

 

[thorpej]

Well, if you had 2 gain stages, you could apply quite a bit of feedback, it seems... I was thinking you could do probably 6dB even with just a single 2A3 stage (assuming a plate load circuit that got you right to the mu of the tube).

With 2 inverting stages, you could feed back to the cathode of the first stage. With a single inverting stage, you'd have to feed back to the grid.

 

[thorpej]

It is possible to get astonishingly small H2 and H3 distortion figures for the 300B using the Triode / Pentode Loadline Simulator. I assumed a 5k ohm load and 350V B+ with 2Vrms input from the source.

Yah, that was basically what I was thinking when I said:

But since you're going to be barely moving the needle on these tubes in terms of their driving signal, you might be in a very linear region no matter what your operating point.

 

[MannyE]

Interesting. I don't know a lot about the electronics of your project, but I've always seen tubes like the 12AX7 or AU7 as preamps and the 300B and 2A3 (2A3 is my favorite) have always been amplifier tubes.

Is your logic to get a tube capable of higher output and use it so it won't be "working hard" and not color the music? That's the idea I was getting from your post... in my non-electrical engineer layman's terms. Is that correct?

 

[thorpej]

Interesting. I don't know a lot about the electronics of your project, but I've always seen tubes like the 12AX7 or AU7 as preamps and the 300B and 2A3 (2A3 is my favorite) have always been amplifier tubes.

Is your logic to get a tube capable of higher output and use it so it won't be "working hard" and not color the music? That's the idea I was getting from your post... in my non-electrical engineer layman's terms. Is that correct?

Those small-signal triodes (12AX7, 12AU7, etc.) have a high plate resistance because they need to be able to perform voltage amplification. But they have a high output impedance, which puts some constraints on how they can be loaded.

Power triodes like the 300B have a low plate resistance because their primary function is to move lots of current. As such, they can drive difficult loads (such as a power amplifier with a low input impedance).

As far as "working hard" ... that doesn't really have much to do with it (or, if anything, the opposite is true).

 

[nerdorama]

Just be sure to carefully consider microphonics if using a DHT in a line stage. I recently considered buying a power amp from a friend that used a single 300B as the driver stage. It was quite microphonic in that use. I'm sure some shock mounting, etc. would go a long way to tame it. Have you looked at the Bartola Valves web page? He has some interesting projects for line stages, many with directly heated tubes. I'm building his 01a line stage now. Hope to have it completed by some time this week. The Russian 4P1L wired as a triode is also a great looking directly heated tube. Kevin Carter at K&K audio has some neat shock mount sockets for it.
http://www.bartola.co.uk/valves/

good luck with your project.
John

 

[kward]

Is your logic to get a tube capable of higher output and use it so it won't be "working hard" and not color the music?

Not really. Thorpej and Palustris got the essence of my reasonings--that is, low mu tube means low dynamic plate resistance which can allow low output impedance. Low output impedance is pretty much a requirement for a preamp so it can more easily drive different kinds of loads without being loaded down. Also the 300B is astonishingly linear so without much effort you have very low distortion.

They come with their drawbacks also as nerdorama pointed out (microphonic, sometimes noisy).

===

I'll take a look at that bartola site. I was not aware of it.

If I went with a two stage design, I was thinking of loading up the cathode circuit of the 12AU7 somewhat significantly (i.e., a good amount of degenerative feedback) to reduce the gain down by maybe 50%, then apply an additional 6 to 10 dB global feedback to get a net forward voltage gain of 3x. That would necessitate a negative supply as the first gain stage would now need a "long tail" so to speak. Or it would necessitate AC coupling in front and behind of the 12AU7. Or I could use a CCS in the cathode circuit with a small voltage negative supply. That actually might be a more precise approach and would (probably) allow DC coupling. Haven't given that too much thought yet, though.

 

[MannyE]

I see! Thank you for clarifying. It sounds like an interesting project. You may be on to something!

 

[MannyE]

By the way, Manley males a pre amplifier with 300B tubes... maybe you can get some ideas from it?

http://www.manley.com/products/view/neo3br

 

[thorpej]

Kevin, why are you so down on a cathode follower output? Just trying to understand your thinking here.

(I have a very simple line stage pre-amp that uses a ton of negative feedback + a cathode follower that I still haven't gotten around to building yet -- mostly because my garage is still full of construction materials and contractor tools ... slowest remodel project ever.)

 

[kward]

Specifically I don't want to use the 300B as a cathode follower. That seems like a waste of that tube for that purpose (at least so I think, I haven't built it so I'm speculating). Also because cathode followers offer 100% degenerative feedback, it seems cathode followers will siphon all of the uniqueness out of the tube being used. If I were to use a cathode follower, I think I would use a 6SN7 for that purpose.

Other reasons (less scientific):

1. I already have a 6SN7-based cathode follower type preamp. Sounds "reasonably" good, but doesn't sound fantastic. (It's a Dennis Had kit design under the Audio Electronic Supply brand).
2. "Everyone" does cathode followers. I was looking for something a little more unique or maybe boutique.

So to be precise, yes, this presents another option-- two stages with 300B front and 6SN7 cathode follower. But I don't see how radically better this would be over just a single 300B stage. The 6SN7 cathode follower would be adding very little not already provided by the 300B stage, I think.

Summarizing topology choices discussed so far:

1. Single stage. 300B. CCS plate load. No feedback. Inverted output. Hardest to design (and sound good).
2. Dual stage. 12AU7 or 6SN7 frontend, 300B output, both tubes wired in common cathode mode. Some degenerative feedback applied in first stage. Some global feedback applied. Non inverted output. Perhaps easiest to design and have sound good, because global feedback covers up potential lack of design optimization.
3. Dual stage. 300B frontend and 6SN7 cathode follower. No feedback (probably). Inverting output. Same design difficulty as 1. Questionable utility of the cathode follower given the use of the 300B

 

[marantzfan]

Have you looked at the Bottlehead BeePre? I own one and love it. You may be able to glean some design ideas from it.

 

[thorpej]

Specifically I don't want to use the 300B as a cathode follower. That seems like a waste of that tube for that purpose (at least so I think, I haven't built it so I'm speculating). Also because cathode followers offer 100% degenerative feedback, it seems cathode followers will siphon all of the uniqueness out of the tube being used. If I were to use a cathode follower, I think I would use a 6SN7 for that purpose.

Oh, yah, seems a waste to use a cathode follower after a 300B -- it would only drop the output impedance to somewhere between 350-450 ohms (depending on what sort of tube you used), which isn't really that much better than 300B's plate resistance to begin with. It seems like the easiest choice to make an "interesting use of 300B" pre-amp is option #1.

One other question I would have with that option, though, is what effect does the CCS plate load have on output impedance. Normally with a fully-bypassed triode gain stage, the output impedance is the plate resistance in parallel to the plate load resistor... so with the "infinite" plate load resistance presented by the CCS, I suppose that it's effectively just the plate resistance.

Also because cathode followers offer 100% degenerative feedback, it seems cathode followers will siphon all of the uniqueness out of the tube being used.

Well, I always though if it as "siphoning out all of the uniqueness of the tube being used as the cathode follower".

On the face if it, it actually seems like option #1 is actually the most straightforward. I think the biggest question is what do you use as the CCS? Do you go the easy route with the IXYS part, or do you use something like a 6V6?

 

[kward]

Have you looked at the Bottlehead BeePre?

I am aware of the BeePree, at least to the extent that it exists. Same for the Manley and Transcendent Sound. I've never heard any of these nor studied them in any detail. I understand Bottlehead owners love their BeePre's. .

so with the "infinite" plate load resistance presented by the CCS, I suppose that it's effectively just the plate resistance.

Yes this is my understanding too.

Merlin's book "Designing High-Fidelity Tube Preamps" has a whole chapter devoted to CCSs. Excellent read. I will probably go with either the dual BJT approach or the dual MOSFET approach. Leaning toward the daul BJT approach for no particular reason. Its supersonic performance is not quite as good as the dual MOSFET approach, but I don't think that matters down in the audio range. Might matter more if global feedback is used.

Also I was contemplating using a regulated B+ supply (either shunt regulated or active regulated), in addition to the CCS. That may not be necessary, I don't know. I haven't given the power supply any thought at all yet (except deciding I should use DC on the filaments to make it as quiet as possible).

Well, I always though if it as "siphoning out all of the uniqueness of the tube being used as the cathode follower".

Exactly how I see it, too. Same reason why opamps make good buffers.

 

[nerdorama]

Something else to consider for a DHT is a good filament regulator. If you haven't already seen it the Coleman regulator is a very highly regarded filament regulator for directly heated tubes.
http://lyrima.co.uk/heatv7pdf/
http://www.bartola.co.uk/valves/dht-filament-regulator/

It started here: http://www.diyaudio.com/forums/tubes-valves/38248-dht-heater.html

John

 

[kward]

Interesting. I'll give that some review also. Thanks!

 

[gadget73]

Another random thought for the low impedance anode follower might be a 6AS7 or 6080. Its a dual direct heat triode meant as a voltage regulator. They're also relatively cheap. 280 ohms of plate resistance also makes it much lower than a 300B.