Gyraf G7 with 6AC7 tube

Hi!  New guy here!  I have a question.  How can I get an 6AC7 tube to work in the G7 circuit.

Ok... so I read lots of the META.  I already know which body I wanna use and I even created a thread on Gearslutz asking this question.  Even asked a collage of mine to look at the thread and see if he could answer my question.

I've gotta some good tips so far, but I am stuck because I honestly do not know enough about circuits to comprehend what I must do.

Here is all the info I have acquired.  That I've also posted at Gearslutz.
Mechanical Data

Bulb .......................................... MT-8
Outline ....................................... 8-1
EIA Base ...................................... 8N
Cathode ....................................... Coated Unipotential
Mounting Position ............................. Any

Electrical Data

Heater Voltage ................................ 6.3 V
Heater Current ................................ 0.45 A
Maximum Heater-Cathode Voltage
Heater Positive with Respect to Cathode
DC Component .............................. 90 V
Heater Negative with Respect to Cathode
DC Component .............................. 90 V

Direct Interelectrode Capacitances (approx)

Pentode
Input ......................................... 11 pf
Output ........................................ 5 pf
Grid to Plate ................................. 0.015 pf

Maximum Ratings (Design Center Values)

Pentode
Plate Voltage ................................. 300 V
Grid No. 3 Voltage ............................ 0 V
Grid No. 2 Supply Voltage ..................... 300 V
Grid No. 2 Voltage ............................ 60K V
Plate Input ................................... 160 W
Plate Dissipation ............................. 3.0 W
Grid No. 2 Dissipation ........................ 0.38 W
Grid No. 1 Circuit Resistance
Self Bias ................................... 250K Ω

Characteristics and Typical Operation

Class A Amplifier
Plate Voltage ................................. 300 V
Grid No. 3 Voltage ............................ 0 V
Grid No. 2 Voltage ............................ 150 V
Grid No. 1 Voltage Derived from
Cathode Bias Resistor ....................... 160 Ω
Plate Resistance (approx) ..................... 1M Ω
Transconductance .............................. 9000 µ
Plate Current ................................. 10 mA
Grid No. 2 Current ............................ 2.5 mA

Characteristics and Typical Operation

Class A Amplifier
Plate Voltage ................................. 300 V
Grid No. 3 Voltage ............................ 0 V
Grid No. 2 Voltage ............................ 300 V
Grid No. 2 Dropping Resistor .................. 60K Ω
Grid No. 1 Voltage Derived from
Cathode Bias Resistor ....................... 160 Ω
Plate Resistance (approx) ..................... 1M Ω
Transconductance .............................. 9000 µ
Plate Current ................................. 10 mA
Grid No. 2 Current ............................ 2.5 mA

Also

IO
1 - m
2 - h
3 - g3
4 - g1
5 - k
6 - g2
7 - 7
8 - a




From Jakob E.
"This is probably the main question: How does this tube act with a much-larger grid-bias resistor - in condenser microphones these are from 100M Ohms to several GigaOhms. But tube data alone does not tell the full story - you need to set up the circuit and try the tube in real-life.

A hint could be trying to under-heat the tube (turn down heater voltage) - often this has good influence on the required grid leak resistor.

Jakob E."

So my questions are basicly how do I have to wire the tube as the different pin-configuration confuses me?

And also what values need to change on the capacitors so that I can run a heater voltage of 4.5volts? Also, what value should I insert in the power supply before it hits pin6 to do this? Should it be right after LM317T or after the two joints?
Also, is 4.5 volts too low? Should I send more or less 5 volts? Also, how do I adjust the amperage to go up to .45 A from the original .2 A?

Any help would be greatly appreciated! Also, perhaps somebody could draw out the differences. I know that's a lot to ask for, but I think I would really need this to understand it properly.

Thanks,
min

HI,



    I think you really ought to build it as per the original circuit if you cant answer these questions! There is lots of info and troubleshooting available, - very important for a new starter. When you have built something successfully, THEN think about using a non-standard, non-documented tube.


  What is wrong with original anyway . . . .


        ANdyP

  . . . . and what makes you think this tube will be any good for a microphone . . . .


    I dunno much at all, but most tubes I know of in mics are not high gain . . .  (Someone chip in . . . .)

Thanks for the advice.  But I just had the idea.  And thought it would be cool.  I will figure it out one way or another.  ;D

About whether it works in a mic or not well David Perlman said this...

Dave Pearlman
The European tube that I’m using in the TM-1 is the EF80 (6BX6 or 6BW7). It is a pentode wired in the mic as a triode just like the EF14. It is a replacement for the metal EF14 and sounds so similar that sometimes I can’t tell the difference. It has more of a high-mid-forward sound that made the EF14 so desirable. A great sounding mic.
The American tube replacement for the EF14 is the 6AC7. I have a few thousand of these and again, it sounds so close to the original that it is hard to tell the difference. It might be in some circumstances a bit softer sounding than the EF80. One can also use as a replacement without any change in this socket the 6SJ7 and 5693, both of which are readily available and in massive supply. There are Russian military spec versions of all three of these tubes that are amazing as well. These are great sounding mics, too.
Just to add to the confusion I have also made the mic with the EF86 (used in the U67) and EF804 (V72 tube). Both these models are a bit more “boutique” but really have very similar sounds to the other versions. These tubes are not easily found and can tend to be expensive at times ( not over $50 usually).

So yea made go... hey kinda cool 

Thanks,
min

Maybe this should have been posted in the drawing board??? If so, then maybe an admin can move it....

Any ideas are welcomed!

Thanks,
Min

You already got the best piece of advice: As a novice, build it *exactly* as described by Gyraf. If you get it right (and as a novice, chances are you'll run into problems), you can always experiment later on. But don't try to make the second step before the first or you'll fail, and you won't even know where to begin troubleshooting.

Also keep in mind that even if a certain tube type is usable in a microphone, it does not mean that any tube of that particular type is microphone grade. Usually you have to select from a number of tubes. If you only have one tube of the type you want to use, chances are it is not microphone grade. For example, only 1 or 2 out of 10 EC92 tubes are low enough noise to be used in a Gefell UM57 or UM92S. The rest of the tubes will work fine in a radio or guitar preamp, but not in a microphone. Basically, mic circuits are out of spec for just about any tube. Therefore you can only hope they'll work, nobody will guarantee it. Usual measurments on a tube tester mean nothing, you have to test the tube in the actual mic circuit.

Well,

I did some research, spoke to a friend of mine and came to the conclusion that I won't gain anything out of modding the circuit, except I get to use tubes I already own.  It's not because I couldn't do it   I got to the point where I figure out what to change on the powersupply, but you guys are right, there isn't a point to it  ;D

Still, I think everyone is right, but I may go with an EF806 instead ;D

Thanks for the help everyone!!!!
min

P.s.  Anybody know a good use for 6AC7 tubes outside of making a radio