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Author Topic: Pi Filter and Mu Anealing  (Read 2597 times)

Radd 47

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Pi Filter and Mu Anealing
« on: January 06, 2005, 09:31:30 pm »

Hi Oliver! Great to see you here!

I have a question on the V76. A lot of people talk about a mod which involves removing the Pi filter. What does this do to frequency response and S/N? Will this take away part of the classic sound of the 76?

Do you run the pi filter on your products?

Also, after bending the mu cans for the 76 x-formers, do you have to anneal them?

What would be the open loop gain of the V76, that is, no feedback  at all from the gain switch. I have run my V76 like this and you can hear crickets chirping in the next county.

Why does the 76 have such a massive 30 db core? Does this core take more mic signal to excite over lets say a 15 db input?

One last question, what type of core was used in the original V76 output, torroid or just mu EI's?

Thanks!

Radd 47


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Oliver Archut

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Re: Pi Filter and Mu Anealing
« Reply #1 on: January 07, 2005, 12:20:35 am »

Hello Radd47,

you can not just remove the filter, if you do so you end up with a rise 6 to 8dB an octave over 12kHz, lots of noise and a high chance picking up RF in settings over 30 dB...

In order to properly change the frequency response, the pot inductor made from ferrite (80/120/M/fr) need to be reduced  to allow a flat response to 20 kHz. Some people replace the inductor with a resistor, in my opinion it is a half backed solution because you still have a 3dB rise... The V76S and some M versions have a NI alloy inductor.

Each of the two stages has a 60dB gain possibility, but B+ limits the max. gain (what means useable studio condition) to approx. 100dB. Open loop won't work stable, you end up with high frequency oscillation at about 35 to 40kHz, you wont hear it but it loads down your unit, if you open the 3rd/4th stage loop you end up with sub sonic oscillation at 2 or 3 Hz.

With 76 (some have 60 or 66) dB of gain and a good mic, you can listen to nearly everything....

The input core is the same size than V72/78/77/75, a specialty NI/FE/CO alloy only made in germany. The size is given by the requirement of distributed capacitance....

The historic Mu-Metal shields are RF heated in a hydrogen atmosphere to get the highest point of permeability, the new ones are RF heated in vacuum, due to the violent nature of backfiring hydrogen. A minor ding should not cause a problem.

In the new V76S and M the same EI multi taped inductor is used than in the historic V76S units.
Best regards,

Oliver


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Oliver Archut
www.tab-funkenwerk.com

We are so advanced, that we can develop technology that can determine how much damage the earth has taken from the development of that technology.

Radd 47

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Re: Pi Filter and Mu Anealing
« Reply #2 on: January 07, 2005, 01:35:30 am »

Thanks Oliver!

Can you de-mistify a few more things for me on the V76?

Stage 1: The screen voltage is higher than the plate voltage? Why?

Stage 2: The screen dropping resistor is 100K instead of 1 Meg like the preceeding stage? Seems like a lot more screen current here?

Stage 3: The 0.05 screen bypass cap is tied to the cathode instead of ground. Was this just an ease of layout issue?

Stage 4: There is a 300 K resistor from the screen to the cathode. I have never figured this out. Some type of feedback perhaps?

Thanks!

Oh, and how did they figure out where to set the 80 k feedback adjust resistor from stage 4 to 3? I know it gets adjusted for least harmonic distortion, but did they have adequate disatortion instruments back then?

OK, thats enough for one post.
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Re: Pi Filter and Mu Anealing
« Reply #3 on: January 07, 2005, 02:21:53 am »

Quote:

The historic Mu-Metal shields are RF heated in a hydrogen atmosphere to get the highest point of permeability, the new ones are RF heated in vacuum, due to the violent nature of backfiring hydrogen. A minor ding should not cause a problem



Great info!

I needed to realize this one.
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Oliver Archut

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Re: Pi Filter and Mu Anealing
« Reply #4 on: January 07, 2005, 02:32:41 pm »

Hello Radd47,

going into the V76 design is very interesting, but I consider it to far into the theoretical domain of tube design for this pin board.
You can find most of the answers in the "Radiotron Designers Handbook", shows up on e-bay all the time.
If you want to go into it deeper, Heinrich Barkhausens  "Handbook of Electron Tubes" Volume 1 to 5, the german print is readily available, the english version was published up to 1970. I bought a set (english) from Powells.com for a friend of mine at about $300...

The best audio test equipment was made 1950 to 1970, either the german Malotki, Siemens (H&B) and of course Klein&Hummel, there are some incredible RFT (east German) pieces out there. The american General Radio and HP stuff is same as good or sometimes better.
I buy them on e-bay all the time for nearly free, refurbishing is a pain, but working propperly still better than most of the new stuff.  

Hope that helps.
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Oliver Archut
www.tab-funkenwerk.com

We are so advanced, that we can develop technology that can determine how much damage the earth has taken from the development of that technology.
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