Gyraf G7

[mista min]

This is from Recording Hacks concerning Pearlman TM-1 and he mentions the 6AC7 tube... sounds very interesting

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 guys help me to get this together Would really love to have such a tube mic as a project

Read more: Pearlman Microphones TM 1 | RecordingHacks.com

[mista min]

I understand one must change how the voltage is regulated and the pinout for the tube... (9 pin to 8 pin).

But does anyone even wanna try?

I will ask some local techs what they think, but I think this would be cool since these tubes are a bit cheaper then ef-80's and I could build a few for myself and friends of mine.

Peace,
min

[Karloff70]

If i was you I'd go on the prodigy pro forum. That's where all the people that can help you are. And they will be able to tell you whether using that tube is foolery or not and why.

You could just do what I did and build a G7 with a PF86 instead of an EF unless you like the challenge. Mine's actually got a pot for variable voltage, so I can use either EF or PF. Thing is PF sounds great (!) and costs absolute peanuts in nice german vintage varieties........

Oh, and I wouldn't bother with the PCB. Just wire it point to point on a breadboard. I started with the PCB and after fiddling around not getting it to work ditched it and started again without PCB= now sweet as.

[Matt Syson]

Hi
Be brave and breadboard it (or adapt another).
There COULD be issues of noise or microphonics, the EF86 were intentionally 'low noise' and they attempt low microphonics by design, other valves may have had other aims in mind so you may need to start with a box of 100 to get a few that are both 'quiet' and low microphonic.
Matt S

[mista min]

Originally Posted by Karloff70

If i was you I'd go on the prodigy pro forum. That's where all the people that can help you are. And they will be able to tell you whether using that tube is foolery or not and why.

You could just do what I did and build a G7 with a PF86 instead of an EF unless you like the challenge. Mine's actually got a pot for variable voltage, so I can use either EF or PF. Thing is PF sounds great (!) and costs absolute peanuts in nice german vintage varieties........

Oh, and I wouldn't bother with the PCB. Just wire it point to point on a breadboard. I started with the PCB and after fiddling around not getting it to work ditched it and started again without PCB= now sweet as.

So bread bored... Hmm... Ok, I will see what I can do.

PF-86 might be a good option too, but I really wanna try it with the 6ac7 since I got those tubes already....
I did think of going to groupdiy, but I haven't been approved yet... kinda sucks really :/

If I breadbored it, it will be easier persay... I've never really built anything before, but I think I am ready to do this tube. Only thing is what body to use? I thought about using a t-bone mic body or something like that. Maybe, I should post that I wanna buy an old tube mic that's broken

This project is a bit abitious of me, but really I wanna do it so BAD!!! It's ridiculous. I wanna prove to myself I have the talent and knowledge to do it.

We'll see if I do... hmm... thought about a U47 as well, since Flea has the DIY kit, but Idk... a friend of mine has a U47 already. I want something different

Do you have any ideas on how to wire it (from 9-pin to 8-pin)?

Peace,
min

[Karloff70]

Doesn't take 'talent', it's painting by numbers, just patience, and reading and asking a lot on prodigy. Was my first electronics project too. Just BE CAREFUL, as the is high voltage in these things that will kill you!!!

I built mine in a Nady 1050 and so got to use the power supply too (modded). Did lengthen the body though with a bit of copper pipe (see pic).

A U47 would be a bit more ambitious I reckon. Maybe check out Max's (forum name ioaudio) new mic on prodigy. Full 47 flavour as a kit incl handmade transformer, board, basically the whole preamp as a kit....looks HOT!
There are also bodies just like U47's for sale there, but I think they're out of stock just now.....

P.S. Don't save money on the capsule, get a GOOD one.....

[mista min]

Originally Posted by Karloff70

Doesn't take 'talent', it's painting by numbers, just patience, and reading and asking a lot on prodigy. Was my first electronics project too. Just BE CAREFUL, as the is high voltage in these things that will kill you!!!

I built mine in a Nady 1050 and so got to use the power supply too (modded). Did lengthen the body though with a bit of copper pipe (see pic).

A U47 would be a bit more ambitious I reckon. Maybe check out Max's (forum name ioaudio) new mic on prodigy. Full 47 flavour as a kit incl handmade transformer, board, basically the whole preamp as a kit....looks HOT!
There are also bodies just like U47's for sale there, but I think they're out of stock just now.....

P.S. Don't save money on the capsule, get a GOOD one.....

Cool! I will use the T.bone SCT-700 and extend it too. I already know what capsule to get And yea, I live in Deutschland! So a good capsule isn't far away

I've been reading on capsules for such a long time... Gosh... I'm 100% an activist for a German handskin capsule.

I will do exactly what you said, but I will try to go with the 6ac7. If I can't find anything on it. I will go with the pf80 and email you a lot

Oh yea... what's the dimension of your breadboard? Just so that I have some clue..

THANKS ALOT!!!!!!!! You've helped me a bunch!
Peace,
min

[Matt Syson]

Hi
You don't necessarily need a 'breadboard'. Fasten any large components to something firm then wire 'point to point' and as long as you don't throw it about too much it will be happy. If you do throw it around a lot you will probably damage the valve and capsule anyway!
That would at least get you 'Mark 1' of this project. If it works as well as you hope, go for 'Mark 2'. With audio design you normally go through several 'iterations' to get a fully working unit either because it has an 'electronic' problem or the simple 'it won't quite fit in the box'!
Matt S

[Karloff70]

Originally Posted by Matt Syson

Hi
You don't necessarily need a 'breadboard'. Fasten any large components to something firm then wire 'point to point' and as long as you don't throw it about too much it will be happy. If you do throw it around a lot you will probably damage the valve and capsule anyway!
That would at least get you 'Mark 1' of this project. If it works as well as you hope, go for 'Mark 2'. With audio design you normally go through several 'iterations' to get a fully working unit either because it has an 'electronic' problem or the simple 'it won't quite fit in the box'!
Matt S

thumbsup lol......I can only say that echos my experience of building the mic. Twice. lol.

I wouldn't worry about lengthening it for the sake of it, you may be fine with how it is. Know what transformer you want yet? And how big the caps are going to be? Nice coupling caps get bloody big......lol I'd get all the parts and shuffle around for a good layout first, might be fine original size.

I'd say get comfy on the prodigy forum and read as much of the META threads on mics as you can suck up and the rest is going to happen by itself. You're welcome to email me, only I am not exactly the best person to help you. However there are plenty of them on prodigy, many building threads on the G7 and other mic, etc.

Viel Spass Alter, und rein mit der Thiersch Kapsel.........

[mista min]

Originally Posted by Karloff70

thumbsup lol......I can only say that echos my experience of building the mic. Twice. lol.

I wouldn't worry about lengthening it for the sake of it, you may be fine with how it is. Know what transformer you want yet? And how big the caps are going to be? Nice coupling caps get bloody big......lol I'd get all the parts and shuffle around for a good layout first, might be fine original size.

I'd say get comfy on the prodigy forum and read as much of the META threads on mics as you can suck up and the rest is going to happen by itself. You're welcome to email me, only I am not exactly the best person to help you. However there are plenty of them on prodigy, many building threads on the G7 and other mic, etc.

Viel Spass Alter, und rein mit der Thiersch Kapsel.........

I think I will see what I can do about all the options... I will extend the body so I can have enough room. I will go with a K47 capsule so Thiersch will be the best option unless I somehow I come upon a TLM-49 then I would have both mic and body... but that probably won't happen :P

Which transformer... well, I was gonna go Lundahl because their EU, but I haven't researched that one enough just yet. I was gonna look around GroupDIY for that info in the META bored for the mic, but really before I do that I have to know which tube I pick. The different tubes will need a different transformer.

I wish we had more DIY going on here in Geekslutz... we could definitely use it and GroupDIY can be a bit overwhelming...
I mean what's more Geeksluty then building a mic with a metal tube, K47 reskinned capsule and a very nice transformer? Can't think of anything... except building your own SSL 4000G console.... LOL!!!!!

Peace,
min

p.s. Wo hast du dein Deutsch gelernt? Und ja, ich bin kein Deutscher :P

[Karloff70]

You could always get a Neumann K47...or even a K67/870, as I have just done to put into my G7. Comes recommended by Jakob as working great in there, I'll soon hear for myself, just waiting for a mount and some other bits for a general upgrade of the beast..... and isn't the capsule in the tlm49 only one sided? I would just start reading and asking questions on prodigy....things will come to light, as the dude might say....

And it sounds like you're describing Max's creation there!

P.S. Tja, in Deutschland, ich bin nämlich Deutscher. Nur schon ewig nicht mehr da am wohnen.

[mista min]

Originally Posted by Karloff70

You could always get a Neumann K47...or even a K67/870, as I have just done to put into my G7. Comes recommended by Jakob as working great in there, I'll soon hear for myself, just waiting for a mount and some other bits for a general upgrade of the beast..... and isn't the capsule in the tlm49 only one sided? I would just start reading and asking questions on prodigy....things will come to light, as the dude might say....

And it sounds like you're describing Max's creation there!

P.S. Tja, in Deutschland, ich bin nämlich Deutscher. Nur schon ewig nicht mehr da am wohnen.

Yea, your right! I forgot the it was single sided! It was really a joke But yea :D
Here is some info on the tube hoping someone might wanna help me


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

I also posted a few specifics of the tubes via photos and the original schematic.

THANK YOU GUYS!!!!!

[Karloff70]

300V plate voltage sounds a bit hefty for a mic to me, but I am far from knowledgeable, so....

I would really post this on prodigy, where you'd get instant knowledgeable advice.

[gyraf]

Pentode
(...)
Grid No. 1 Circuit Resistance
Self Bias ................................... 250K Ω

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.
Pentode
(...)
Grid No. 1 Circuit Resistance
Self Bias ................................... 250K Ω

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.

[mista min]

Ok, so under heating the tube plate voltage may help with this? My main problem is not only the specifics of how much voltage to send, but the wiring itself.

I'm not knowledgeable enough to design this circuit. I was perhaps wondering if some one could give me some more tips.

Concerning the DC plate voltage. The EF-86 runs at 250 volts in a pentode and triode.

But Jakob is right about the the Self bias being a problem and yea I'd have to try it to know for sure... but like I said I can't even start to figure out how to mod the circuit because I don't really understand the differences between the two tubes designs... except the ef86 has a sheilding section (pins 2 and 7) while the 6ac7 doesn't.

Hope that helps understand... what I don't understand

[Jonkan]

I think what Jakob meant by underheating was to lower the heater voltage, not the plate voltage.

Oh, and you dont really need to run the tube at max plate voltage. Look at the g7 schematics (160v).

The capsule polarisation is then derived from this 160v, giving +-80v compared to the backplate.

I had to install extra voltage dividers to get my capsule polarisation to 60v, otherwise the membrane got stuck to the backplate, but it is different for different capsules. Sounds fab now anyway!

The g7 circuit is self bias, so just match the pinouts if not already the same and pop your tube in there and listen how it sounds.

good luck!
/J

[mista min]

Yea, I got that he meant the heater voltage. I was just replying to Karloff's comment about the 300 volts, but yea.

I don't even know how to do the pinouts for the 8 pin tube. I guess I will just take it to a buddy of mine and see if he can tell me what I exactly need to do.
Which resistors to increase and decrease in value and so forth.

The interesting thing about the heater voltage is that I was actually thinking that lowering it would help somehow... but once again... I have not a damned clue about any of this

THANKS EVERYBODY!!!!

[mista min]

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

[mista min]

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. I got to the point where I figure out what to change on the powersupply, but there just ins't a point to it.

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

Thanks for the help everyone!!!!
min

p.s. If and when I start this mic, I will go ahead and make a blog about my progress here