TAB-Funkenwerk U47 with C3G (Triode strapped) - Gearspace.com
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TAB-Funkenwerk U47 with C3G (Triode strapped)
Old 28th May 2020
Here for the gear
🎧 5 years
TAB-Funkenwerk U47 with C3G (Triode strapped)

Has anyone experimented with using a C3G Triode Strapped in a U47 circuit? Modifications to the socket assembly, bias circuit, voltage divider array, and B+ Would be necessary but possibly worth it?

Check out these specs:


Ri of 2.3K when Triode strapped! That would make a good match for a BV8 style XMFR.

I know Ronin Applied Sciences used this tube in their Pegasus mic but wired as a pentode.

Any thoughts or ideas would be appreciated.
Old 29th May 2020
Lives for gear
gyraf's Avatar
🎧 15 years
It's completely unpredictable how well a tube - that is not made for the purpose - will behave as microphone front end. This because the parameter most important - behavior at very-high grid impedance - is undefined.

So try it, preferably with 5-10 tubes from same batch, then report back here

I have considered trying the same, just haven't gotten around to it yet..

/Jakob E.
Old 30th August 2020
Here for the gear
🎧 5 years
TAB-Funkenwerk U47 with C3G (Triode strapped)

I built a test circuit using Oliver's Alt Circuit values and added a 80pF cap for the capsule sim. I am using the TAB PSU running at 120v - 6.3v. I triode strapped the C3G using this circuit and it came up fine. Im getting around 14DBv amplification at the xmfr secondaries unloaded with little to no distortion visible on the scope.


Anode Voltage WRT Cathode: 78.4v
Cathode Voltage WRT PSU ground: 1.1v
Grid Voltage WRT Cathode: -1.3v

Im kind of amazed this didnt require any tweaking of the resistor values to get a stable bias. The circuit remains stable all the way up to 2v P2P from the signal gen (inserted at the capsule capacitor). Testing using a 0.77v sine wave at the "capsule" I get 3.2v on the scope which should be more than enough output. I even did a square wave which showed a very clean rise and cutoff.

Anything im missing here, this seemed a bit too easy.
Old 31st August 2020
Lives for gear
gyraf's Avatar
🎧 15 years
Tubes are forgiving creatures

You'll want to test at much lower levels - perhaps 1/10 of what you ran above

And you are supplying the input signal through the 60-80pF capacitor?

Quantifiable: Look out for low-frequency response, and sub-frequency noise. Noise in general off course. Tube microphonics (it's a frame-grid tube iirc?).

Then listen and compare to a good microphone tube, get e.g. a known-good-sounding 6072. There's things going on in a mic that can be hard to measure or even quantify - although most of this is probably in transformer geometry and core material

And as mentioned earlier - even though you get one tube sounding good, this really dosen't tell us much about how other tubes of the same type will behave in that circuit. In this case there's no way around trying a handful of tubes to see if they behave well "in general"

/Jakob E.
Old 31st August 2020
Here for the gear
🎧 5 years
Thanks for the response Jakob,

You mentioned testing at lower levels, my signal gen only goes down to 0.2v. I could always add a voltage divider before the "Capsule" capacitor to get a lower signal. How much amplification should I expect in this circuit unloaded? Ive been googling trying to find acceptable output ranges for U47ish microphones but I have not found any.

I would love to test with more C3G tubes but at $100 USD per tube that is a lot of money for a one-off build.
Old 5th September 2020
Here for the gear
🎧 5 years
rebuilt my test jig yesterday using the BV08C and tested Freq. Response (I used 0.07v this time) and found a flat response all the way up to 100k! I was getting a strong AC Hum which I was concerned about on the jig but I assumed this was due to no chassis grounding.

Because my tests were looking good I decided to finalize my build in the mic body. After some problem solving in regards to capsule wiring I was able to get the mic up and running in cardioid-only mode. It is extremely quite, almost as quiet as my Bock 195 FET mic. I used Mundorf MCAP Alum-Oil PP caps for C1 and C3. C2 was a VCAP TFTP cap. My capsule is a Nuemann K49 replacement from Sennheiser. Over all I am satisfied with everything but there are some sound qualities I was expecting that were not present in this mic. I am going to let it burn-in for a couple hours and check it again.

The sensitivity is unbelievable, way more than any other mic in my locker and the mid range is sweet and forward. The low end I would describe as "natural" but not unflattering.

I will post a follow up next week once the mic has had some time to burn-in after which I will do a comparison between a couple mics using my studio monitors. I realize most of this is anecdotal at this point. I do plan on sending this off for Anechoic testing at some point and I will post results here as well.
Old 15th September 2020
Here for the gear
🎧 5 years
TAB-Funkenwerk U47 with C3G (Triode strapped)

Now that this build is done I thought it might be helpful to list some of the issues I ran into in case anyone else wants to try this tube.

-The C3g Tube uses a Loctal socket so you will need to make your own socket suspension assembly. I did this by purchasing Tab's 47VF-TSA and replacing the VF14 socket with a Amphenol 78S8L LOCTAL snap ring mount tube socket I found on etsy. You will need to drill 3 very small holes in the new socket (be very carful not to interfere with the socket pins) in order to mount the rubber belt. I also recommend grounding the center pin to help reduce hum.

-The C3g tube is much larger than a EF800. The Long body U47 will barley have enough room to fit this tube. I loosened the brackets that hold the tube socket assembly to allow a little bit of movement to accommodate for this. I also installed a Duende Criatura [27 - 32 mm] support ring and glued the foam ring included with the kit to the exterior. This helped keep the tube vertical and reduced some microphonics.

-The C3g has slightly higher current and filament requirements than the EF800. I recommend rebuilding the bias circuit (R2-R3) to accommodate this. I used a 470R for R2 and a 100R for R3 which still allowed for a -1.1 bias point while delivering a full 6.3v to the heaters. This should also boost the current a little but it is difficult to measure in this circuit. I have tried both the default 5.05v and the 6.3v values and I much prefer the higher values.

-C2 im told is very important for noise suppression. I chose a VCAP TFTP for this as I have heard amazing things about teflon caps. I would recommend this to anyone interested. This mic is amazingly quiet in spite of my soldering .

-You may be tempted as I was to use a aftermarket Transformer (Cinemag). It is very difficult to secure a different transformer in the bottom bell assembly and still have room for the resistor Point-to-Point wafers. This may work for some of you but I ran out of room for my Mundorf PP caps in the bottom bell as well. For this reason I used Tab's BV08 classic which gave me just enough room for both Mundorf Evos. The BV08 will be just fine with the C3g due to the low Rp from this tube.

-Make sure all leads on the wafers are cut very short and that solder blobs can not touch the mic body when assembled.

-The Neumann K49 capsule is very delicate WEAR GLOVES! For cardioid only connect the side with the writing to your grid assembly. Leave the back side disconnected. Remember to connect the charge power from C2 to the ring mount.

-Use a dish to store your screws while working. The body alignment screw in particular is easily lost.

After rigorous testing in my studio I can confirm that this build resulted in a mic that is 3-6db louder than any other condenser I own. The transient response is unbelievable, better than I would have imagined. The frequency response follows the original U47 chart closely (as best as I can test due to the lack of Anechoic chamber for testing). I chose not to do professional Anechoic evaluation due to cost (around $1k USD) as I do not intend to manufacture this for sale.
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