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Old 18th September 2006, 09:02 PM   #1
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Default Designing a preamp with two tubes

I beagan to redesign my two tube mic preamp that I built for recording. I have noticed ( and have always followed allong ) that many designs have the first tube and the second tube set up quite differently, plate voltage, plate resistor, cathode resistor and and current draw.
What are the reasons for this? I have some thoughts and would like to gain a more complete picture.

My main question is, I was thinking about setting up both tubes the same way. Will this work fine if the load lines are plotted well or are there reasons it would only be so so, like the second stage gets overloaded with too high a signal from the first or something like that. Thank you so much! Have a great day,

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Old 18th September 2006, 11:35 PM   #2
lineup is offline lineup  Sweden
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if you read some of these Tube Circuit of The Month
Tube CAD Journal
you will find a lot of good articles
and get some good basic knowledge in tube designs

I guess you mean that the second transistor
is a Cathode Follower ... has no upper anode resistor.
So you and some other intrested can start with reading
Easy Cathode Followers

.. and also see if find some good circuits
in the rest of Tube CAD Journal archives:


lineup thinks is one of world's best valve sites
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Old 18th September 2006, 11:53 PM   #3
lndm is offline lndm  Australia
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As lineup suggests, do you mean that the first stage is a common cathode stage and the second is a cathode follower?

To put it another way, if you connect the first stage so the large valued resistor is connected between the plate and supply, then the second stage is connected so the large valued resistor is connected between ground and cathode.

This configuration produces reduced distortion. It gives the first stage a light load to work into. Unless, however, this combination needs to drive a heavy load, I prefer to do away with the second half of it.

One advantage of using the pair is that the total current draw for them is reasonably constant, giving them some effective separation from the power supply. Not too bad a thing for a mic pre, but there are other options too.

If you feel like visiting, I feel it's worth a trawl through the older stuff; some gems there.
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Old 19th September 2006, 12:45 AM   #4
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Thank you for your imput! I am thinking of redesigning an two tube single ended mic preamp for recording. The one I built before is simular but not quite the same as the v72 ( I like traditional triode designs for home audio ).

The first tube draws .5ma and the second about 6.5. I was wondering if it could work well to have both tubes in a preamp like this biased and set up the same way with similar or the same plate voltages and amplification for each tube. I'll check out tube cad also, thanks. What do you think? Have a great day,

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Old 19th September 2006, 01:17 AM   #5
lndm is offline lndm  Australia
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Looking at that schematic, it appears that both stages are common cathode stages. The second one appears to be choke loaded. That is, the CKV72/1 takes the same role for the second stage as R16 does for the first.

The basic benefits of choke loading are an increased output swing, and a lower required B+.
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Old 19th September 2006, 05:54 AM   #6
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Could you help me personally, that would be great!

By the way I wrote down your explaination about how to caculate the gain of a tube stage from one of your posts, thank you for that.

My question is why do most people have the first tube in a mic preamp drawing much less current than the second one and how would it work to bias the two tubes the same way?

Also, if you are calculating the gain of a tube stage that is using an inductor to load it do you calculate it using the DCR of the coil, or is there a slightly different formula to use.

Thank you SY!!! Have a great day,

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Old 20th September 2006, 04:24 PM   #7
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Perhaps a bit off the subject, but check this out:

I was suprised how good it sounds. It has a softness, yet clarity that makes it sound a lot more expensive than it is (we A/B tested and compared to a bunch of expensive tube and solid state stuff in a couple of pro studios in Sweden, and it DOES sound expensive).

If you use it for complex/transient sounds like piano, distorted guitar, trumpet, snares, you'll find that nothing makes it bail out. I think this is due to better transient response than will be obtained with transformer coupled circuits, and perhaps very good IM figures (i never actualy measured it though).

I did replace the current source (the LM334) with an all-tube circuit, but it sounds excellent as is, and the modification has only minor impact. The primary reason for me was to cut off some noise.


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