Omni level much quieter than cardioid/figure 8 TCM1150

I've got a TCM1150 I did a bunch of the standard mods too, and finally got working. But now the level is only sufficient in any position other than true omni. Even then it's quite low. I close miced a 12 string in figure 8, and the gain on the Octane was cranked. I never got over -12 db on the pro tools track. I put it in omni, and the level gradually decreases significantly over about 5 seconds. There's only one capsule being used in omni, correct? But I thought if one of the capsules wasn't wired, there wouldn't be that gradual drop, and there would be no signal at all. Can someone help me out on this? Where should I look? Thanks!

standard mods? :? what exactly did you do?

Read the G7 page on gyraf's site, it explains the basic capsule polarization scheme:
http://gyraf.dk/gy_pd/g7/gic.htm

What are the "standard" mods you did. From what site(s)

You know...
Upgraded the capsule to a peluso CK-89 dual sided
TRX to a Cinemag 2480
tube to a NOS Mullard MilSpec 12AT7 CV2024
C8 to an Ansar
C6 to a black gate
removing the LPF caps on the transformer output
removing the Cathode follower
changing two diodes to zener 56v

and I also upgraded a couple of the PSU filter caps to elna cerafines for the heck of it, and got a SP litz cable.

I've been reading up on this here and at homerecording.com for the most part. Some gearslutz and studioforums.com too. Here's my thread at Home Recording
I'm AMZavareei. The schematic is the same, except I fixed that stupid grounding problem between the xformer outs. Should 5 be tied to ground too?
Any leads would be much apreciated. Thanks!

Was there a 12AT7 in there to begin with? I thought these used 12AY7/6072. You'd have to change the value of the cathode resistor, and possible other circuit values, if you changed tube type.

Sounds like the stuff that Dave Thomas guy does.


Try swapping the wires to the capsule and see if the problem follows a particular side of the capsule first.

Not what I do to mod one. The stock 1150 capsules I heard are not that bad. Where did the hack dropping of the B+ voltage come from?

Your problem is most likely flux somewhere in the high Z capsule section causing a leakage path. Does the noise level change with patterns?

If cleaning does not help put the stock capsule back in and test it again.

When you are new to changing parts it is often a good idea to only change one thing at a time.

Here you can find the schematic of your original mike:

http://www.apexelectronics.com/pdfs/smapex460.pdf

The omni position uses both sides of the capsule.

I would use a 6072a or 12AY7 rather than a 12AT7.

[quote author="Gus"]Where did the hack dropping of the B+ voltage come from?[/quote]

Don't these chinese power supplies use a single rail for both B+ and capsule polarization. So if you want to lower the latter, you must lower the former as well?

The version of this mic that I have sounds much better with 120V instead of the stock 200V. Might be a different capsule.

There was supposed to be a 12AT7, according to the lit, but most people say they come with a 12AX7. Before I did any mod, or even got the mic, I looked for what a good 12AT7 was, and the CV2024 popped out. Yeah, it seems like a lot of people are using the 6072, and I might change to that at some point, but right now I don't think that could be the source of my problem. And I haven't read of anyone changing anything else to accommodate that tube in this circuit. Is there a formula for that resistor?

The diode change is discussed here

There isn't really noise. It sounds clean when I boost it in pro tools. Just the level drops off in omni a few seconds after switching to that mode. The other modes have much higher level, but still slightly low. Might just be the tube gain factor though.

What do mean by cleaning it? I'll definitely try swapping the sides of the capsule.


I know, I've learned about doing things one step at a time at this point.

Thanks again!

electrorocket

that thread has some info I question

my 1050 capsules are different than the 1150s I have seen and heard

The v69 tube does not matter that much because of the way it is used. I have two and started to trace the circuit Not worth modding IMO if I don't sell mine I will change the insides.

2480?

Info you question, such as?

Yeah, the Cinemag 2480 mic output transformer. What about it?

I would guess most of the nady tube microphone mods at other sites were inspired from here

Funny thing is they did not get some of the stuff right IMO.

Transformers I bought >$1,000 worth of transformers for microphones. I won't give away what I like but it makes me wonder when I read peoples posts at other sites about transformers. I wonder what their sample base is (one transformer?)

I would/have used the stock nady before a 2480 with the 1150.

I helped someone from here with two mods to a 1150 you can get an OK microphone with the stock capsule and transformer.

I also chuckle with the caps some of the old stuff had electros tube to transformer.

Cleaning is cleaning the flux and dirt.

People change the caps and miss all the other things.

I can't post what I am posting about because I won't give away free information to the microphone "moders"

Sorry, I have to disagree with Gus about the transformer. I have modded a couple of 1150s with 2480s and done extensive testing with both with instruments (frequency response and distortion measurements) and listening tests and there is a significant difference between the stock transformers that were supplied by Alctron and the 2480. The most noticable difference in listening was improved transparency, particularly in the mid highs. The most noticable difference in the frequency response was that the stock transformer really poops out below 100Hz and is smoother (lessened odd peaks) in the 4-6KHz range. The core material in the stock transformer appears to be transformer grade iron. The 2480 is shiny high-nickle stuff. The THD was half a decade better on the 2480 than the stock. Again, I have to say, that you might, in a Trion, have a better transformer than the stock Alctron, but we won't know unless we can see it.

burdij

Actually you are not disagreeing with me if I understand your post correctly. I think we are hearing the same things with the transformers but finding different uses.

Some old microphone transformers were designed to roll off the lows If what I have read is true.

I find the 2480 to be a very boring transformer for a vocal microphone. (very clean maybe good for classical etc recording) If I want that sound I will use a boring transformerless circuit.

Now think about the MIX how many times do you roll off some of the lows so the mix does not turn to mud.

How does the voice- microphone- pre amp- A/D etc fit in the mix.

Can you use that peak in the mids, will that little iron blur help someones voice etc.........

Some people like their G9 with the OEI transformers. I have heard a well buit G9 one channel Lundalls and the other OEI.

And there are things one can do to help out the stock transformer.