Fun with Inexpensive Mics

[MagnetoSound]

As for the 200v B+ I just don't get it. The G7 pushes it up to 160v. But most other designs I've seen are around 100-120v. I don't think it matters that there are two triodes vs one.

Parallel triodes will draw twice the plate current, all else being equal.

That means double the voltage drop across the plate resistor. The higher B+ may be designed to compensate for this.

 

[Blue Jinn]

Parallel triodes will draw twice the plate current, all else being equal.

That means double the voltage drop across the plate resistor. The higher B+ may be designed to compensate for this.

Well, now I'm back behind the learning curve....

OK, so with the triodes paralleled the tube acts as one unit with lower plate resistance and higher current correct? But the plate resistor is pretty high in this circuit (150k) as is the cathode resistor 6.2k.  200v/150k is about 1.3mA.  If I drop the B+ to 120, I'm down to .8mA. If we're assuming it's a 12AT7 as spec'd, there doesn't seem to be a whole lot of variation there. I haven't done the rest of the math though.

 

[MagnetoSound]

OK, a few points to look at - I will be corrected if I get any of this wrong, so here goes  ;D ...

OK, so with the triodes paralleled the tube acts as one unit with lower plate resistance and higher current correct?

Yes - but, more importantly, lower impedance. The parallel triode configuration has likely been chosen to allow the use of a transformer with a lower (cheaper) impedance ratio.

200v/150k is about 1.3mA.  If I drop the B+ to 120, I'm down to .8mA.

No, it's not 200v across 150k. If it were, the plate voltage would be zero! The plate voltage is 117v. That means a drop across the plate resistor of 207-117 = 90v, which gives just under 0.6mA.

But the plate resistor is pretty high in this circuit (150k) as is the cathode resistor 6.2k.

Yes, don't forget that the cathode voltage sets the bias point, and both triodes share the same cathode resistor, so there are now two cathode currents flowing through it. The cathode resistor is increased to raise the cathode voltage and reduce the combined bias current back down to where we want it.

Similarly, we don't need more plate current, so the plate resistor can be increased to resist the signal current flow and increase the signal voltage at the plate.

 

[Matador]

Here's how I think about it:  if you start with a single triode design, then parallel the tube, plate current doubles (all other things being equal).  Thus the plate resistor must be cut in half to maintain the same output voltage swing.  Current through the cathode resistor also doubles, so the cathode resistor must also be cut in half to maintain the same bias point.  So why do it?  Output impedance is halved, because output voltage stays the same, but current doubled.

 

[MagnetoSound]

Here's how I think about it:  if you start with a single triode design, then parallel the tube, plate current doubles (all other things being equal).  Thus the plate resistor must be cut in half to maintain the same output voltage swing.  Current through the cathode resistor also doubles, so the cathode resistor must also be cut in half to maintain the same bias point.  So why do it?  Output impedance is halved, because output voltage stays the same, but current doubled.

You seem to be agreeing with me on both points, but you have expressed it the wrong way round. The resistors are not 'cut in half'. The values are higher than normal.

Curiously, they have not necessarily been doubled in value, but increased nonetheless.

 

[Matador]

Here's how I think about it:  if you start with a single triode design, then parallel the tube, plate current doubles (all other things being equal).  Thus the plate resistor must be cut in half to maintain the same output voltage swing.  Current through the cathode resistor also doubles, so the cathode resistor must also be cut in half to maintain the same bias point.  So why do it?  Output impedance is halved, because output voltage stays the same, but current doubled.

You seem to be agreeing with me on both points, but you have expressed it the wrong way round. The resistors are not 'cut in half'. The values are higher than normal.

Curiously, they have not necessarily been doubled in value, but increased nonetheless.

Normal compared to what?

Using a higher plate resistor (along with a higher B+) means you can squeeze more gain out of the circuit.  Since output impedance fell by half, they could increase the plate resistor back up and probably get another few dB out of it and still have lower output impedance (as compared to the single tube case, at least).

So yes, I wasn't meaning to disagree or correct:  just trying to reason how they came up with the circuit values they did.

 

[tchgtr]

  OK...I understand that if you are going to power both sides of the tube that you need twice as much voltage as only running one side. Then the outputs get merged again, and this lowers the impedance.
  It seems to me that the Apex 460 circuit puts the triodes in series (cathode follower or "CCDA", if you will), to accomplish the same thing, ie: to allow use of a less expensive (lower ratio) OT.
Now I'm confused...

Anyway, seems a good OT upgrade for the 9000 might be the Neutrik NTE4 that some folks have used in mic circuits around here. They're quite reasonably priced.

BTW, thanks Matador and Magneto for chiming in. It seems we less-enlightened are often left to reinvent the wheel around here these days, and I for one appreciate your input. It does me some good to see that even you guys are sometimes confused by all this, tho mainly by syntax, and not your knowledge.
I'm trying to figure this out myself, but things sure move along faster with such helpful comments.
A Guiness to you both.

 

[MagnetoSound]

Normal compared to what?

Fair comment. Missing point of reference. 

I should have said typical as in 'a typical single-triode arrangement'. One would typically expect to see 100k on the plate in most cases.

 

[MagnetoSound]

@tchgtr: One thing is for sure, we never know it all.

As my Irish friend Liam used to say, "Chairs!"  ;D

 

[Blue Jinn]

Thanks again, I'm still trying to do the math.... 

 

[Matador]

  OK...I understand that if you are going to power both sides of the tube that you need twice as much voltage as only running one side. Then the outputs get merged again, and this lowers the impedance.
  It seems to me that the Apex 460 circuit puts the triodes in series (cathode follower or "CCDA", if you will), to accomplish the same thing, ie: to allow use of a less expensive (lower ratio) OT.
Now I'm confused...

Anyway, seems a good OT upgrade for the 9000 might be the Neutrik NTE4 that some folks have used in mic circuits around here. They're quite reasonably priced.

BTW, thanks Matador and Magneto for chiming in. It seems we less-enlightened are often left to reinvent the wheel around here these days, and I for one appreciate your input. It does me some good to see that even you guys are sometimes confused by all this, tho mainly by syntax, and not your knowledge.
I'm trying to figure this out myself, but things sure move along faster with such helpful comments.
A Guiness to you both.

A small correction:

"if you are going to power both sides of the tube that you need twice as much voltage current as only running one side."

For the Apex 460, the first triode is a standard "common cathode" triode gain stage.  This is fed directly into a cathode follower stage.  This combination forms a "constant current draw amplifier", where each stage's plate current always adds up to a constant.  This means the draw from the supply is always constant (e.g. if the current entering the first stages plate drops, then the current through the follower increases by exactly the same amount).  Why is this important?  Much easier to design the power supply.

There's an excellent description on TubeCAD here:

http://www.tubecad.com/2009/03/blog0161.htm

The Apex circuit is one of my favorite sounding tube circuits....somewhere behind the AB763 circuit of course.

 

[Blue Jinn]

There's a brief discussion of paralleling in this thread too:

http://www.groupdiy.com/index.php?topic=47764.0

 

[Blue Jinn]

There are a few other threads on this microphone, but I'm not sure some of the commentators knew that the triodes were paralleled, and there were recommendations to change the plate and cathode resistors to *normal* (as in "typical"  ) values.

From what I've read thus far, if the goal here was to be able to use a lower ratio transformer because of the lower output impedance and push the gain up, the 12AT7, as opposed to the 6072/12AY7 (and it is possible a 12AX7 is actually in there) is already a fairly high gain tube. So increased gain, with lower turns ratio transformer, it still seems like a lot, and other posters reported this being pretty hot, but not being hot enough with e.g. a 10:1 transformer. I'm wondering if it would make sense to change the plate/cathode resistors, change the voltage divider (to lower the B+) and either decouple the second triode stage like the C12, or change to a cathode follower (like the HST-11a) that would require finding an appropriate cathode resistor and finding a place to solder it, (which shouldn't be too hard) and keep the stock transformer for now. (I'd like to measure its ratio at some point)

 

[tchgtr]

It makes sense to try it with a CCDA circuit, as the current (thanks Matador!) is already there in the stock PSU, and the stock OT ratio is probably similar to what would be needed.
Presumably, that is one reason the CCDA was used in the Apex 460, to lower OT costs. Perhaps that schemo could be used as a guide.
I need to do a lot more reading to understand why the parallel triodes were used in the first place...

Blue Jinn-since you are starting without the stock PSU, the sky is the limit. I think your original plan to run it off of a Royer PSU is a good one. With a nice OT, it will likely sound good.
At some point I will disconnect the two OT wires and measure...

@Magnetosound--I once was taught to speak in an Irish accent by repeating the phrase "Whale oil beef hooked." over and over again....

 

[Enchilada]

Hey mate, I believe you helped me out with my MXL 9000 last year! Turned it into a pretty decent, really tubey sounding mic in the end.

I painted mine though because I destroyed original paint trying to clean the logo off with acetone. Looks much better now anyway  ;D

 

[Blue Jinn]

e end.

I painted mine though because I destroyed original paint trying to clean the logo off with acetone. Looks much better now anyway  ;D

It does look nice, what did you use?

 

[Enchilada]

It does look nice, what did you use?

The same paint as I used for my G7. White Knight "Squirts" quick dry gloss enamel. The colour is Metallic Charcoal.

 

[tchgtr]

  I lowered the B+ on my Nady 1050 recently, based on all the talk about C12-style capsules and polarization voltages that we've had here. It definitely seemed to reduce the high end bump on that mic, so I thought I'd give it a try with one of my 9000s to compare.
  Upon opening up the PSU (see photo) I found that it looks like it was made from the same schemo as the Nady, minus the pattern voltage circuit, and possibly came from the same factory. Even the numbers and printing for the zener diodes were the same.
    Quick listening tests switching between adjusted and stock PSUs shows a definite difference between the two B+ voltages, but after re-reading the discussion above, I'm wondering if I'm off-base here, and have lowered the plate voltages too much from a lack of a deep understanding of the circuit.
  B+ is adjusted in these circuits with two 100V zeners. I replaced one with a 47V zener, as I did in my Nady. Just realized they are .5 watt, as opposed to the 1 watt diodes that were stock, so I guess I'd best update those as soon as possible.
  More reading/listening to do, but I do prefer the lowered B+ with these mics and their swapped out "U47-style" capsules. As with the Nady, the highs are obviously not as hyped, and at first it seems the mic is flat-sounding, but closer listening implies that it's just a flatter response.

 

[Blue Jinn]

If you lower B+ w/o adjusting R1-3, aren't you also lowering the polarization voltage? (Note I did that inadvertently on my first Royer mod, the PSU transformers are underrated somehow, and I only get about 70v under load, so my polarization voltage is down around 40v, but it works ok.) I haven't tried mine out yet, I wanted to try it out at a full 200v, but I don't have any 7 pin female XLR's so I was going to swap out the microphone end with an A5M to eventually mod it to be compatible with the rest of my 5 pin "Royer" supplies.

 

[tchgtr]

If you lower B+ w/o adjusting R1-3, aren't you also lowering the polarization voltage? (Note I did that inadvertently on my first Royer mod, the PSU transformers are underrated somehow, and I only get about 70v under load, so my polarization voltage is down around 40v, but it works ok.) I haven't tried mine out yet, I wanted to try it out at a full 200v, but I don't have any 7 pin female XLR's so I was going to swap out the microphone end with an A5M to eventually mod it to be compatible with the rest of my 5 pin "Royer" supplies.

  Yep, that's where I'm at---better get out the DMM and take some readings. I just figured that replacing one diode was easier than trying to turn it into a CCDA or going with a plate-out circuit. I'm sure you're absolutely right and now I'm not getting full polarization voltage. Might be the same case with the Nady.
  My last dual-capsule Royer build came out great with some unusual transformers from the local electronics store. B+ is almost exactly 120V under load. I just cheaped out on the capsule and transformer, so it's a bit bright, but clean.
  I found some nice 7-pin connectors on evil-bay a while back at truly cheapskate prices. So far they have worked well, and the seller was very good at communicating. If you're in the LA area, you might check out All Electronics. Seems like they sell them online too:
http://www.allelectronics.com/make-a-store/category/194/Connectors-XLR/Mike/2.html
  Unfortunately, no 7-pin panel mounts...
...and here are the xfrmrs I used for the Royer/G7 build:
http://www.allelectronics.com/make-a-store/category/790600/Transformers/P.C.-Mount/1.html
  ...the lowest one on the page. I used two arranged in the Gyraf scheme.
  I also got my xfrmr for my first cardioid Royer build there, which fit the specs that Royer outlined. Works great.