Theile M4 mic autopsy: big, beautiful and bad!

Hi!

I have just finished dissecting a Thiele M4 tube mic. These mics have been discussed here before and are not very highly thought of, but the build quality is not at all bad and they have a certain deco charm.




The design is eccentric to say the least. I don't think a full schematic has been shown here, so I've traced it out.



The tube is ECC83. R1 value is high - > 40 Meg. The unmarked resistor between the two filter caps is 40K (sorry!)

A few observations:

The power supply is built into the mic.

Safety issues: The case is not connected to mains earth! +45V AC was measured on case relative to mains earth! No fuse! I grounded the case before I started working.

There are no output caps or transformer. The output cable is not twisted or screened! (I can't say for certain if this is the original cable or not).

AC heating. Hum is cancelled using two pots in base of mic (see the schematic). AC mains and heater cabling are not not twisted.

I'm going to get this up and running as best I can in original state and see how good the hum-cancelling pots can be in a tube mic. Then I'll make a decision on what to do with it.

Full schematic and notes:
http://www.xaudia.com/omnip/Mics/MicSchLib/ThieleM4.pdf

Previous Thiele thread:
http://www.groupdiy.com/index.php?topic=9780


:shock: :shock: :shock:

Very Nice Zebra :wink: ,

Love the construction of the Mic,

Saucy :shock:

Meaty.

Nice looks, wouldn't mind to have one like that.

Indeedy!

I missed a resistor out - there is a 500R between audio + and ground. I've added it now but you may need to refresh your browser.

Also, is R1 a resistor?



From its position in the circuit I expect something a few hundreds of megs, and it's certainly beyond the range of my meter. It's marked 5/10, 63, which made me wonder if it is actually some whacky old cap. Anyone recognise these?

If it is a cap it could be acting as a pad. AC to ground via the power supply cap

Thanks Gus - I hadn't thought of it that way.

My notes say I measured -0.7V on the junction of "R1" and the capsule backplate, which makes me doubt that "R1" and the 100Meg are a simple voltage divider.

Hi,

An update on this - I've put the Thiele back together but with a heavily revised circuit. After much experimenting I settled on something very close to the C800 circuit but with a 6AK5 tube instead, basically because it was easier to fit in than the 6AU6. I experimented with the plate and cathode resistor values, but still ended up with 100K and 1K.



Repairs / mods / upgrades included...

Reskinned capsule (thanks Dale!)
Moved the tube away from behind the capsule.
Gold plated the switch contacts
Big Sowter output transformer (10:1) for balanced output
External regulated power supply (to replace the hum-inducing internal one).

The mic is sounding good now, although it does have a sound that is noticeably coloured by the grill and body. I'll try it on a few sessions and see if it is popular!

OK!
z50

Great work Zebra! :thumb:

I Also have a M4 and I was thinking in put a G7 inside, but since I already have a g7 would be better another circuit like yours.
I altready have a couple of 5ak5 tubes and also a sowter trafo (!) so could youpost the schematic for the remake m4?

Thanks!

Synthi

Fantastic, Stewart. I'll need to finish mine when I get my other projects on just now completed. I've ordered parts for the "low power" valve mic by the way.

Regarding the C800 circuit, I was wondering about the use of the suppressor and screen grid. I've seen these both tied to the plate HT voltage or with the suppressor tied to the cathode. I've also seen the screen grid taking a feed from the HT via a potential divider to lower the voltage. Are there any basic rules here?

Sorry if that's a bit off-topic.

Roddy

Roddy

Have you seen the screen grid connected to a potential divider in a tube microphone? That would be interesting.

Sometimes G3 is connected to the cathode inside the tube so for triode mode it is not an option. I have been thinking about the possable changes I don't have tube curve tracer but that might give a hint with tubes that can be wired G2 G3 and plate or G2 and plate and G3 to cathode.

IIRC I read in the 4th edition the wiring picked might be about noise

FWIW I have a GT microphone with a 5840 the plate lead is cut off and G2 is the plate G3 is connected to the cathode inside, this is the circuit out of the box. Why is the plate not grounded as a shield?

For triode plate out tube microphone I think more bias setup and tube Rp and rp and transformer and if the grid can work with a condenser capsule well. They are basically the same circuits.

[quote author="Gus"]Have you seen the screen grid connected to a potential divider in a tube microphone? That would be interesting.[/quote]

Hi Gus, Yes, I'm looking at it right just now; the screen grid is fed from a potential divider which itself is fed from after the plate load in a circuit.

Sometimes G3 is connected to the cathode inside the tube so for triode mode it is not an option. I have been thinking about the possable changes I don't have tube curve tracer but that might give a hint with tubes that can be wired G2 G3 and plate or G2 and plate and G3 to cathode.

Click to expand...

I don't fully understand the effects of the two extra grids, but I have read a little about them. See here:

http://www.freewebs.com/valvewizard/pentode.html

[quote author="valve wizard"]Screen-Grid (g2): The screen-grid exists to accelerate electrons toward the anode, and "screen" the control-grid against Miller capacitances. It is this extra acceleration that gives pentodes their high amplification factors. The screen voltage (Vg2) is usually made only slightly lower than the anode voltage, either by a voltage divider from the HT, or simply by connecting it to the HT through a fairly large resistor (Rg2). Since some electrons are collected by the screen-grid, a small current flows through it, causing a voltage drop in the screen-grid resistor, placing the screen-grid at a lower voltage than the HT: Vdropped = Rg2 * Ig2. The screen voltage has much more control over how the pentode amplifies than the anode voltage. Lowering the screen voltage reduces headroom and compresses the grid curves. Do not exceed the rated maximum screen voltage or average power dissipation![/quote]

FWIW I have a GT microphone with a 5840 the plate lead is cut off and G2 is the plate G3 is connected to the cathode inside, this is the circuit out of the box. Why is the plate not grounded as a shield?

Click to expand...

I've read of people doing this. Unfortunately I can't rememeber why! I think you might need to know about the construction of valves and the impact on their characteristics to know this. Perhaps PRR might know. I'll look and see if I can find that original text where it discussed this technique though.

For triode plate out tube microphone I think more bias setup and tube Rp and rp and transformer and if the grid can work with a condenser capsule well. They are basically the same circuits.

Click to expand...

My understanding is still quite simple - set bias according to curves, then select a suitable transformer. I couldn't tell you what grids work well as I've not looked into that. Since I'm planning on making a pentode mic circuit, I thought I'd take a look at the two extra grids to see where I'm going with those.

Hi!
Synthi - It's basically just the C800 circuit but with a 'silicon' power supply regulated to 240V DC / 6V heater. I used a 7806 for the heater supply.

I picked 100K for the plate and tried cathode resistors 1.5K, 1.2K, 1K, 750R, but settled on 1K based on sound and output level.

I tried 4 6AK5 tubes and picked the quietest. One was very noisy, and the best benefitted from a burn-in period.

I also tried several Russian 6Z1P tubes - it is NOT an exact equivalent to the 6AK5 and gave very different operating voltages,



Roddy, Gus,
This has come up before and I still haven't got my head round it. It's certainly interesting. Do you think it is worth building a test mic with easily switchable grid connections? Maybe we agree a test program between us?

http://www.groupdiy.com/index.php?topic=27138
http://www.groupdiy.com/index.php?topic=1773

OK!
Stewart

Thanks Stewart!

I think I`ll try your aproach Dling the 800 schem now...



:guinness: :guinness: :guinness: :sam: :sam: :sam:

Synthi

[quote author="zebra50"]
Roddy, Gus,
This has come up before and I still haven't got my head round it. It's certainly interesting. Do you think it is worth building a test mic with easily switchable grid connections? Maybe we agree a test program between us?

http://www.groupdiy.com/index.php?topic=27138
http://www.groupdiy.com/index.php?topic=1773
[/quote]

Thanks, Stewart. I hadn't seen these threads before and they raised some of the same questions I've been asking. I'm a little more confused as to what makes pentode operation now as it seems a bit of a grey area.

Anyway, parts arrive today for a new mic to be built, so I'll hopefully start experiementing.

That mic looks sharp!!!

Stewart

The supercaps in the fil circuit C105 and C106 do seem to drop the noise from the fil supply. I used two .1F and balance resistors that I had in the one I built I installed them in the power supply.

The Schoeps CV60 had G3 to cathode and a 6au6. I found that schematic on the web in the past but can't seem to find it again. It looked interesting with fixed bias and a neg fil supply.

[quote author="dale116dot7"]That mic looks sharp!!![/quote]
Good - because the singer is usually flat. :roll:

Gus, I don't think you can see them in the photo but the supercaps & balancing Rs are there, quite close to the tube. They should help but I didn't try without them. I can't detect obvious noise from the heater side.

Changing the tube (NOS Raytheon) changes the noise level a lot!

I just saw an ebay auction linking to this thread and using my schematic in the listing!

If you've found this thread via ebay, be prepared to do quite a bit of work on these mics to get them working safely again - mine had live voltage on the casing! Also, please note that some of the pictures here are of my rebuilt Thiele with the Sony C800 circuit.

Hi!
Someone has taken advantage of your figures, having exposed the microphone on sale?