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dsavitsk 1st September 2006 09:50 PM

71a Line Stage and Miller Capacitance
I am looking to put together a low gain preamp. I do want a little gain so a cathode follower is not going to work. One suggestion I came across was to use a 71a in a basic grounded cathode config (I think the original suggestion was to use a choke on the plate.)

However, another post suggested that miller capacitance could be a problem in this setup. Could someone give me a 5 line primer on what this means exactly and perhaps help me determine if this really is an issue.

Also, if anyone has other suggestion for a low gain preamp, I'd be interested in those too, but the 71a has me intrigued.



Eli Duttman 1st September 2006 10:43 PM

The Miller Effect amplifies a triode's grid to plate capacitance by mu. The Miller capacitance is part of the load the upstream circuitry has to drive.

Search this site and other sites for information on a 12B4 based line stage. You will get everything you want, without the problem of a LARGE Miller capacitance.

kevinkr 2nd September 2006 12:01 AM

Hi Dave,

The 71A has a cgp of 7pF and a mu of slightly less than 4 which would result in a miller capacitance of worst case about 28pF - in most reasonable designs with a 100K pot or less this is not going to cause much of a problem. (Output loading will reduce the effective mu making the miller cap even smaller.)

The 71A is a great sounding tube but will probably be a little microphonic in this application.

I published an article in PFO a while back about my 26 based transformer coupled line stage. There is information you might find useful in your quest for a 71 based line stage. You can get to it easily from my site. (below)

It's definitely do-able and if you are up to the challenge why be derivative. :D

Eusebius 2nd September 2006 11:32 AM

First, all the DHTs I have used sounded better than a 12b4, so although this is a nice tube and has many fans including me, for better sound why not go with a DHT as you suggest. I tried out a whole load of DHT tubes in a line stage, and the 71a came pretty far down the list. If you want low gain, you could try the 31 (mu 3.8). The 49 in triode (mu 4.7)is intriguing - didn't really set it up properly (lots of flying leads to a 4 pin socket) , very detailed but maybe a bit sharp. Frankly I prefer some of the other DHTs anyway to all the above, but they're all about mu=9.

Filament supply will have a huge effect on the sound. There was a thread here on 300b filament supplies which was a very interesting read. Basically the best sound is a current source of some description. the RonanReg, for instance was two voltage regs followed by a current source. A current mirror also sounds good. I tried common mode chokes at the end of current sources but the sound was worse. Interestingly CMCs improved the sound from plain voltage regs.

kevinkr 2nd September 2006 02:04 PM

Filament supply is critical to both the sound and the noise floor. A ronan type regulator would be an excellent choice, there are others particularly if you are willing to experiment.

I have used foil chokes with voltage regulators (not common mode) with good results (mainly for rfi reduction)

Good tubes to consider with transformer coupling which will give you generally less than 6dB of gain are the 30, 26, 12, 01A.

With chokes the 71A, 45, 6AS7GA are good choices.

Eusebius 2nd September 2006 02:42 PM

Good tubes to consider with transformer coupling which will give you generally less than 6dB of gain are the 30, 26, 12, 01A.>>

Yes - I agree completely with that. 30 is a leaner sound and very clear, 26 is warmer, richer and more lush, 01a is clear and neutral, 12a is similar. There's a family resemblance, with the 26 rather different in sound - warmer if you like that. All good. I found the 30 less microphonic than the 31, though others report the opposite. All my 31s would ring, though it dodn't come through the speakers. The 30 seemed a bit less prone to this.

I did also try the 6B4G (Sovtek) and 2a3 (plain chinese) in my line stage, but preferred the smaller tubes. But in my opinion even the 2a3 was better than a 12b4 or other indirectly heated triodes. Going back to indirectly heated tubes seems right away to make the sound "thicker" - only way I can describe it, that last bit of clarity just goes (I tried a 30 compared with a 12b4 with a friend and we both agreed). Unless you have just switched back from a DHT you don't notice the slight cloudiness of indirectly heated tubes. And let's face it, how many people have even heard DHT small tubes in line stages. I was happy for years until I did. The first time I heard a 26 line stage it was a shock - it's quite possible my draw literally dropped.

dsavitsk 2nd September 2006 05:53 PM

Thanks for all the suggestions. I do tihink the 71 is worth trying, but some of the others may get a round as well. If microphonics are an issue, does anyone have a good strategy to reduce them?


Originally posted by kevinkr
[B]Good tubes to consider with transformer coupling which will give you generally less than 6dB of gain are the 30, 26, 12, 01A.
Can you suggest a transformer? I have had trouble finding a transformer to use for a line stage.


Originally posted by kevinkr With chokes the 71A, 45, 6AS7GA are good choices.
Hmmm. I had a run in with the 6080/6as7 in a line stage a while back and found it to be a very noisy tube. I tried a ton of different tubes and different setups and could not get it quiet.

The 45 looks great too, but is a little too expensive for now.

Eusebius 2nd September 2006 09:38 PM

To reduce microphonics do stuff like the following:
a) Enclose the tubes, either in the chassis or some kind of tube over them
b) Try O rings
c) Make the chassis as heavy and dead as possible - I got good results with mounting the sockets straight into 4mm alu plate. The chassis should not ring when tapped.
d) If you want to be extra careful, try something like this - worked for me. Mount the two tube sockets on a rectangular 6 mm teflon sheet. You will secure this with two holes drilled across the middle, and on the two ends put rubber under the teflon, using cut pieces of a normal stationary type eraser. Put a couple of solid bolts through the bottom of the chassis and tighten with nuts on the inside of the chassis, so the bolt ends are sticking up. These go through the holes in the teflon, but cut the holes bigger so you can put a rubber grommet in each, so the bolts don't touch the teflon. Put larger rubber grommets top and bottom of the teflon and finally bolt the lot down so the teflon goes down and is held at the two ends by the rubber pieces. The teflon is non-resonant - you could use some other material - and doesn't touch the chassis because of the rubber grommets.
e) Put the chassis on some kind of soft rubber feet or other dampening material like sponge.

kevinkr 2nd September 2006 10:22 PM

Hi Dave,
Both Magnequest and James make suitable transformers. I'm using vintage HA-133 which have gotten a bit ridiculous in cost lately.

15K:600 ohms will give you 4dB of gain with the 26 or 01 and a source Z around 300 ohms. See the table in my article for further details. Note that I don't normally terminate into 600 ohms, but more like 10K - 100K so effectively the limiting factor on gain is the mu divided by the transformer ratio.

It's important to make sure the transformer has relatively high primary inductance to assure low distortion at low frequencies. I'd say something like 150H (xl = 18.85K ohms @ 20Hz) as an absolute minimum, this is critical with dhts with an rp of about 8K as is the case with the 26 and 01, somewhat less critical with the 12A as it's rp is about 5K..

A transformer can be used here too, in this case something on the order of 5K ought to suffice. Here you will need about 50H for good low end performance, and you will get right around unity gain. Choke loading seems like a good choice here and will give some gain as well. 100H chokes are quite practical at the currents needed here and a lot of the problems of using transformers just go away, at the expense of considerably higher source z, (basically rp) and increased noise/microphonics due to the missing attenuation from the plate to line transformer.

Pretty much everyone around here (Northern New England Tube Group or NNETG as we are known) has built a 12B4 except me, and I have a/b'd many of these extensively and just don't need to go down that road.

I have heard one 12B4 pre-amp I really liked and it used a high transconductance pentode as the upper device in a dissimilar mu follower confiuration. Quite unusual, and the best sounding 12B4 design I have heard to date.

mach1 3rd September 2006 02:12 AM


I would strongly urge you to use a separate filament transformer when using DHTs. On my 1LE3 pre, which uses a ronan reg for LT, the amount of hum, ringing and hash that came through when using a common transformer was unbelievable.

Using a separate LT transformer has dropped the amount of garbage considerably, but it still quite audible - especially an amplified version of the high pitched buzzing sound that emanates periodically from transformers.


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