Fun with Inexpensive Mics

[Blue Jinn]

The part I don't really grasp is the Ohm's law math part of it all. If the parallel triodes draw twice the current, and we reduce the PSU voltage, shouldn't we also  adjust the plate resistor accordingly? There are threads on this that suggest halving the plate resistor for one triode when you strap two in parallel, and posts higher up in this thread suggests that the designers here may have just raised B+ to compensate. Unless I'm missing something, it seems plausible to adjust the plate resistor for the new (lower B+) and the polarizing voltage resistors as well, so we have to look at the plate curves and do some math. Also, the tube in mine is OEM'd so it I'm not certain if it really is a 12AT7. Those are cheap enough, so I planned to get some and swap in one.

I've been reluctant to jump into this one until I get capsule mounts for the 47 style capsule, also, I need to find the time to try it out stock to see if my "improvements" are. But maybe I'll just have to jump in.

 

[tchgtr]

  Yes, what you said. I'll take some measurements after I get thru with this busy weekend and report back. Shouldn't be too hard to determine resistor values to get back to proper voltages, but I'm wondering if I shouldn't just try to turn one of these into a CCDA.
  Might be time to look at the Apex 460 schemo (since they obviously used most of that PSU), and see what makes it different from the 9000 circuit...

 

[Blue Jinn]

Just a thought, but why not do the calculations for a 12AV7  (which you said sounded better anyway?) too just to see. It has a lower gain than the 12AT7. I found the stock HST-11a to be unnecessarily hot with the 12AX7 tube. I think I'll put the 5 pin in my 9000 and give it a whirl stock at 200 volts, then do some math...

Caveat, I haven't looked at any other 12AV7 parameter to see if that is a good idea or not.

 

[Kingston]

12AV7 has rather high heater current. Make sure your PSU is able to deliver this. Most likely the usual china PSU types can't do this.

 

[tchgtr]

Thanks Kingston-Guess I'll put some AT7s back in there.

 

[Blue Jinn]

The heater current for a 12AV7 is 450 mA vs 300mA for the 12AT7. (at 6.3v) I have 1A diodes and  LM317 (1.5A?) regulator and 1/2 watt or higher resistors on my  DIY PSU, so I should be OK with either one tube, but in retrospect, I probably should increase the current capacity and the heat sink on my PSU's just for good measure.  Also, it looks like the plate current is higher on the 12AV7.

 

[tchgtr]

As always, I'm grateful to you folks for reading, commenting, and helping me understand what I'm doing. I replaced the AV7s with AT7s, and the mics sound very clean, and don't seem to get as warm over time.
I will probably put the original zener back in the one PSU, and am glad I have one in original condition for comparisons.
Just changing the capsule and replacing the output caps seems to be the best options with these mics. After I put that zener back, I will consider them to be much improved, and will make some recordings so folks can hear them. 
Thanks!

 

[Blue Jinn]

Cool. I still want to have a "generic" power supply, so I'm going to (attempt to) calculate a new set of resistor values.

 

[Blue Jinn]

I took another look at this last night, with a goal of lowering B+. So I came up with 50k on the plate and about 1250 on the cathode for a nominal B+ of around 120v.

then I noticed the heater circuit. What is the purpose of running ~9v thru R30, R31, and C30? 

 

[Blue Jinn]

I have not been able to source a complete 9000 set, (with the PSU) so I've been looking at this some more with the thought of using the HST-11a power supply. (After they are modified for a 120v) So my thought now is to lower the B+, changing R1 to 1M. I don't get what the resistors in the heater are doing though? But in keeping with powering this from the other PSU, I planned to eliminate those, and run the tube at 6.3v.

I also thought about lowering the plate resistor to about 50k and the cathode resistor to about 1k or so and see how it sounds.

So then if that works, as in the GS article, comparing this to a M49, it seems relatively simple enough to add pattern switching. Pin 3 is already free on the mic, and the two resistors and cap could be wired in to where L1/R21/C20 are, eliminating all that on the output, and wiring hte transformer direclty to pins 5 & 6 and the capsule rear to where the transformer originally met R21. I figured I'd try 100M/.01uF/150M and see.

I took another read of the GS article about the feedback network, as well. I had thought of putting a 47 style capsule, but the GS poster said he felt it was way too dark. So I thought I'd try with a C12 style with pattern.

Open to suggestions and criticisms of course. As is I can't use the mic, as I don't have a PSU for it, although I probably build one, with a B+ of around 200v, which would probably be close enough.

 

[tchgtr]

    I've been looking at the Aurycle site, and they have a cardioid tube mic kit for $140. Someone like you might be able to use the PSU from that, and find a project for the rest of the components in that kit. Seems a bit expensive to get a PSU, but might be an alternative for you.
Otherwise, I'm following your idea and think it's a good one. Need to read that GS thread again, but have been busy with so much other stuff...
    They also have a multi-pattern kit for $170, and I'm pondering that for a C!2-style project with the 2nd capsule from my group buy a while back. It's near impossible to bid for any used HST-style mics in evil-bay these days.
 

 

[Blue Jinn]

    I've been looking at the Aurycle site, and they have a cardioid tube mic kit for $140. Someone like you might be able to use the PSU from that, and find a project for the rest of the components in that kit. Seems a bit expensive to get a PSU, but might be an alternative for you.
Otherwise, I'm following your idea and think it's a good one. Need to read that GS thread again, but have been busy with so much other stuff...
    They also have a multi-pattern kit for $170, and I'm pondering that for a C!2-style project with the 2nd capsule from my group buy a while back. It's near impossible to bid for any used HST-style mics in evil-bay these days.

HST-11a's are available in the white market, (where I got mine) and it looks like "chunger" is getting some as well? (there is another thread on that)

The silly thing is I don't have a 7 pin female XLR. I have a spare PSU that could probably put out about 200v although the heater section is probably pressing the limit (LM317 with a tiny heat sink)

But I have already two HST-11a's to mod (just waiting for the tubes to arrive later this week) and I think it would be handy to be able to use those PSU's so I'm on a sort of "standard."  I ordered a 12AT7 as well, so I'm going to plug in the design tube to be certain.

Another thought I had was, again given the feedback network, maybe try a different 67'sh capsule at some point. Which reminds me, I'll need to get teflon posts from Mouser.

 

[abbey road d enfer]

It's an odd circuit, with the typically odd choices by whoever designed it regarding the output caps - a 33uf electro in parallel with a .1uf film cap - so I replace those with a single 1.85 uf film cap,

There is a good reason for having large value capacitors there: the higher the value, the lower the frequency of the inevitable LF hump. The output transformers in these cheap mics have a lower inductance so they tend to resonate in the audible LF spectrum when they are mated with 1-2 uF caps.

  and remove all the RF filtering junk just prior to the output transformer.

What do you mean? The only RFI/EMI "junk" I see is after the xfmr. Do you mean L1/L2/C20/C21 ? You may regret having taken them away one day...
 

  It also calls for a 12AT7 in the schemo, but often comes with a 12AX7 installed.

Far-East manufacturers tend to put anything they have on hand. After all many of them haven't got a clue what the products they make is intended for... 

Enter the M-Audio Nova. ... Whoever designed these put a 10uf output cap on them, probably to tame the brightness of this capsule

No, increasing the value of the output cap doesn't change the HF response. Same explanation as above.

 

[abbey road d enfer]

For output impedance, plate should be loaded with no less than 10x the output impedance of the plate stage.

This would be for bridging. For matching, you want Zload = Z out_tubes. This reflects to Z sec = 312 ohms, which is only 50% off the rated impedance. Considering the minimum input impedance of mic preamps (barring a few exceptions, such as GT's The Brick) is 1.5k, this reflects at the xfmr primary as 24k, which is adequate, translating in less than 2dB loading.

 

[abbey road d enfer]

  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.

There are two reasons for lowering  (or increasing) voltages in a condenser mic.
But there are three voltages involved:
Reducing the plate voltage reduces noise (look at the old Neumanns with 50-60 V plate voltage), but also reduces headroom, increases output impedance, decreases gain.
Heater voltage also has the same effects so there is an optimum for noise performance, generally about 10% below nominal (ca. 5.5V instead of 6.3)
Capsule polarization (bias) directly governs the sensitivity of the capsule, but has notable side-effects by increasing or reducing the diaphragm tension, which in turns increases or decreases the resonant frequency. Increasing bias may shift the resonant frequency in the "nasal" region of the spectrum. Increasing bias also attracts the diaphragm closer to the backplate, reducing the high-SPL handling capability.

 

[abbey road d enfer]

The part I don't really grasp is the Ohm's law math part of it all. If the parallel triodes draw twice the current, and we reduce the PSU voltage, shouldn't we also  adjust the plate resistor accordingly?

Depends... If you don't change the plate res, the plate voltage will drop with B+, which may be just what the doctor ordered. Yes, if you want to maintain the same Vp, you need to reduce the plate res. But my understanding is that the goal is to have a different operating point to get a "browner" sound, less noise and less pronounced diaphragm resonance.

There are threads on this that suggest halving the plate resistor for one triode when you strap two in parallel,

If the intent is to make both halves operating like the single one before, you want to halve the cath res too. Halving only the plate res would make both halves operate at lower current, which would result in increased plate voltage (higher current in same cath res increases bias current, thus increases cath voltage, thus reduces current in tubes -that's the paradigm of cathode bias).

...posts higher up in this thread suggests that the designers here may have just raised B+ to compensate.

Raising B+ has one advantage: it allows the use of larger plate res, which in turn slightly increases gain and headroom. But the ulimate motivation is that manufacturers can build one type of PSU and adjust the operating point of the particular tube by adjusting the plate and cath resistors. Makes inventory management much simpler.

 

[Blue Jinn]

The part I don't really grasp is the Ohm's law math part of it all. If the parallel triodes draw twice the current, and we reduce the PSU voltage, shouldn't we also  adjust the plate resistor accordingly?

Depends... If you don't change the plate res, the plate voltage will drop with B+, which may be just what the doctor ordered. Yes, if you want to maintain the same Vp, you need to reduce the plate res. But my understanding is that the goal is to have a different operating point to get a "browner" sound, less noise and less pronounced diaphragm resonance.

There are threads on this that suggest halving the plate resistor for one triode when you strap two in parallel,

If the intent is to make both halves operating like the single one before, you want to halve the cath res too. Halving only the plate res would make both halves operate at lower current, which would result in increased plate voltage (higher current in same cath res increases bias current, thus increases cath voltage, thus reduces current in tubes -that's the paradigm of cathode bias).

@abbey road d enfer Thank you for your insight.  Note that the triode stages are already in parallel in this mic, It isn't clear from the schematic that was posted. I also neglected to mention halving Rk but understand that is the "rule of thumb" calculate values of plate resistor and cathode resistor for one triode section, halve those values for parallel triode.  I attempted to do that with a nominal 120v B+ and the 12AT7 plate curves, which is where I came up with the values I posted above.  I would think that with a 12AT7, gain would not be a problem, compared to e.g. a 12AY7. One can only speculate, but perhaps the lower output impedance (and therefore lower ratio transformer) is the motivation.

...posts higher up in this thread suggests that the designers here may have just raised B+ to compensate.

Raising B+ has one advantage: it allows the use of larger plate res, which in turn slightly increases gain and headroom. But the ulimate motivation is that manufacturers can build one type of PSU and adjust the operating point of the particular tube by adjusting the plate and cath resistors. Makes inventory management much simpler.

Yeah, I'm wondering if the V69 PSU isn't the same "under the hood." I've never seen a schematic for that mic, and I do not have one to take any measurements. 

  and remove all the RF filtering junk just prior to the output transformer.
What do you mean? The only RFI/EMI "junk" I see is after the xfmr. Do you mean L1/L2/C20/C21 ? You may regret having taken them away one day

I planned to remove that to use the solder points to add pattern switching. It is perfect for a resistor/capacitor/resistor network, adapted from the M49 and connecting the transformer leads directly to the XLR connector. Will let you know if I regret it. I may have another one of these coming now with PSU, so will be able to do some comparisons.

 

[david-p]

Does anyone know whether Aurycle Microphones are still in business?  Their website aurycle.com has not been updated since October2012.

David

PS:  I was going to post this as a new topic, but cannot find out how to start one... Anyone know?

 

[abbey road d enfer]

I was going to post this as a new topic, but cannot find out how to start one... Anyone know?

Go to the section of interest, e.g. "Microphones" and click New topic at the right top of the page.