FET Audio | Hi-End Audio & Electronics DIY Electronics & Repair

This is a new made preamp – Hybrid Balance Mu-Stage.  Dual triode medium gain subminiature tube like 6N16B-V, 6N18B-V, 6N28B-V, 6111, & 6021 etc… can be used.  The choice of submini tube is due to its low noise and good microphonic effect. 

Features: 

1. Hybrid (Mosfet + Tube) mu-stage design with full balance at input and output
2. Supply voltages: +128V, +59V, -69V and two 6.3V DC – all regulated supply
3. Tube biased at about 1.5mA at about 60V plate voltage
4. Mosfet IRF710 as Source Follower biased at about 7.5mA
5. Gain about mu of the tube, about 20 to 40 times
6. Non feedback design and thus tube may need to match for gain.
7. Tube can be replacable using IC sockets, the submini tube is first soldered on 8-pins leads and then inserted into IC socket.

FFT measurment:

FFT Balance Mu Preamp

Update new photo on Jan 3rd, 2011 …..

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For 9 pin tubes, I think Amperex 6DJ8 is very good performer in mu-stage pre-amp.  Thus many other triodes are evaluated in Tube post.  Now here is some photos of the mu-stage with subminiature tubes.  The way is to solder the small tube on the bottom of the tube socket and the two triodes in one tube is put in parallel.  Insulation shrink tubes is used to prevent short circuit of leads.

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I have purchased one pair Sylvania 6021 and 4 pcs Raytheon 6111 tubes (non marking type).  The pair of 6021 give very good matching for the gain but only 2 out of 6111 tube are with good gain match.  The worst 6111 tube is about 1 db lower gain than the other 3 pcs. 

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Note that the mu-stage is also upgraded with KBG and Telfon capacitors!

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Last update: 9th Jan, 2011

Tube evaluation summary in mu stage

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Last update: Dec 20, 2010:

Check the link for the evaluation of different tubes in the Mu-stage pre-amp.  Some are good and some are not that good due to microphonic problem.

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Last update: Dec 3rd, 2010:

For the pass month in Nov 2010, I acquired lots of tubes from ebay and namely 12BY7, 12GN7, 12HL7, 12AV7, 6BQ7A, 5670W, 6205, 6021, 6111, 6SN7GTB & 5U4GTB.  Mostly NOS but some are used tubes.  The experience on used tube is not that good and thus I only acquire NOS in the later part.  I think the tube can be used for mu-stage or pre-amp used.  The mini tubes is also good for phono stage or micphone pre-amp which will have smaller microphonic problem.

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