Rode K2 noise troubleshooting

[Kona Steve]

I was given a Rode K2 by a friend who was dismantling his studio. I brought it home and got a really nice take on a project track. (Recording through a Mackie Onyx into Logic.) Then, the mic made a dreadful screeching noise and went quiet except for a low hum, which persisted even with the power supply turned off but still plugged in. Before continuing, let me make clear  that I am definitely not an electric tech guy. So, please bear that in mind.

The hum sounded like ground loop, but on flipping the lift switch (with the unit back on) it just got louder, so in reading about it I figured it  was the more than 10 year old tube dying.
I replaced it with generous and thorough advice from Christian Whitmore. The tube is a Amperex/Phillips PCC88/7DJ8

I followed directions carefully on tube replacement and then the Rode manual for sequence of hookup. I had headphones on and as soon as I plugged the power supply box into the outlet I heard a significant hum, without even turning on the unit. I flipped the ground/earth switch again, but that made it worse. On turning the unit on, I got a pretty loud, deeper hum that diminished in maybe 30 seconds, which I guess was the tube settling in? The initial higher pitched hiss/hum remained, and that was all that came through to Logic.

I tried switching XLR cables, power outlet, cleaning contacts, checking that the tube was seated, but the sound remained and nothing was being picked up by the mic.

I read that the mic power should be plugged into the same house circuit as other parts of the chain. Tried that, no change. I switched power cords, tubes, nothing. I checked tightness of connections and found that the seven pin plug into the power supply had a fair amount of wiggle. With it switched on, I had that steady low hum/hiss, and when I wiggled the plug, I got loud cracking like when you plug in a guitar to an amp. So, I bought new 7 pin cable which definitely was a more snug fit in the power supply and eliminated those noises on moving the cable. However, there was still a hum as well as some electric cracking, along with the initial short-lived, loud, deep hum that happens every time I turn it on. I tried swapping tubes back to the new one and that periodic cracking stopped. However, still no analog signal from the mic. I tried a different house circuit and a different power cord. Nothing. Still just plugging in the power supply with it off, I could hear hum in my headphones, worse when turned on.  So, that leaves something within the mic or something within the power supply?
I live on the Big Island of Hawaii, so finding a tech to test the components is tricky and mailing them expensive.
What did I miss? Recommendations?

Many thanks for wading through this long description and any ideas you might have to help resolve this issue.
Steve

 

[FarisElek]

Did you try changing the 7 pin tube xlr? I used to have a Rode NTK and I had almost the same problem as you. it's XLR had a broken ground connection and when I opened it up it had terrible connections. Like someone soldered with their non dominant hand for fun. So I contacted Rode and they replaced it no questions asked. I cut the connectors off the old cape and installed Neutrik ones and used it for a U67 build.

You can test if it's that by jiggling both ends of the cable from the PSU to the Mic itself and see if the noise comes and goes.

If that seems to give you moments of reprieve: I recommend notifying Rode and having them send you a new cable. If they wont because you're outside warranty, then buy a replacement or the much cheaper and better option would be to buy nice 7 pin neutrik connectors and some heatshrink and fix it yourself.

If that doesn't seem to help, there is plenty of things you can check, but if you're not comfortable doing soldering work and working with high voltage, I'm not sure what I can recommend.

Last thing, you have the PSU set to the right voltage, right?

 

[Kona Steve]

Thank you for the quick reply. As the mic is over 10 years old, I figured it was well out of warranty, so I bought a new 7 pin cable (Iconic Cables) with gold pins. Yes, the PSU is set to 115 (Hawaii). Just tried the wiggle test, no change.
If you have any other brainstorms in the middle of the night, let me know!! High voltage is a scary mystery to me!
Thanks again for the time.

 

[FarisElek]

I would check the psu for any obviously burned up resistors or swollen caps. Then the capsule for any obvious damage. Then the hi-z section of the mic pcb for damaged resistors or caps. Probably in that order.

Also make sure the 7 pin connector on the mic doesn’t have any bent pins. 

 

[Ricardus]

So from your first post it sounds like you DID change the tube? Yes? What did you change it with? New or used?

 

[Kona Steve]

I would check the psu for any obviously burned up resistors or swollen caps. Then the capsule for any obvious damage. Then the hi-z section of the mic pcb for damaged resistors or caps. Probably in that order.

Also make sure the 7 pin connector on the mic doesn’t have any bent pins.

Will need a tech for this level of stuff. Will look into it. Thanks.

 

[Kona Steve]

The New tube is, I believe, new. Got it from Christian Whitmore, a PCC88/7DJ8 Amperex/Phillips, Holland 2nd Gen. He hand selected it, burned it in and listened to music for a while with it.

 

[RuudNL]

Was there a special reason for selecting a PCC88? (The original K2 used an ECC88, both tubes have a different filament voltage.)

 

[Khron]

"Mojo", obviously  :

Different filament voltage, and slightly different pinout as well (pin 9 is some shield, as opposed to the common / "center tap" of the two filaments).

Was there a special reason for selecting a PCC88? (The original K2 used an ECC88, both tubes have a different filament voltage.)

 

[kingkorg]

Before any tinkering with Røde stuff i suggest taking a look at this video. The tube circuit he is talking about is basically K2.

Randomly swapping tubes makes less sense after watching this video. One improvement might be optimising circuit for each individual tube to really make it shine. Getting noise levels even further down.

In short, the tube is operating in extremely linear fashion, random tube rolling  might just get you in trouble. Best case scenario it will work just like stock one.

https://youtu.be/niZizzHBanA

 

[Kona Steve]

Thanks for all the input. I will check out the video. As I mentioned earlier, I am not educated in electronics (in other fields, yes, but not this one). I had a number of detailed exchanges with Christian Whitmore of Pro Audio Tubes. He talked about a number of tube possibilities and this one was one of his top two or three. Based on its described sonic properties, this seemed a really good option. He hand selected it and tested it. He also alerted me to the not infrequent failure of cables. But again, I am reliant on people like him and you folks who can give guidance.
I do wish there was someone here on the island who I could just hand it to and and ask for it to be repaired (like taking in a car to a tech). I will explore some more. And maybe sending it to Rode is an option...just expensive to mail. It's not simply that the mic is noisy; it doesn't work at all, though it did briefly when I was given it.
Thanks again, all.

 

[Ricardus]

Was there a special reason for selecting a PCC88? (The original K2 used an ECC88, both tubes have a different filament voltage.)

The K2 and the NTK use the same PCBs. I have an NTK that was modded into a K2 with a new capsule and a pattern pot, and I used a 6922 (E88CC) which I guess is the mil spec version of the ECC88?

Anyway, put a 6922 in there and it will probably be happy. The Rode web site calls for a 6922 also:

http://www.rode.com/microphones/k2

 

[Kona Steve]

I found an audio tech not too far away and it is now in his hands. I will post his results just in case anyone is interested in how the story comes out. Thanks again!

 

[Bowie]

The K2 and the NTK use the same PCBs. I have an NTK that was modded into a K2 with a new capsule and a pattern pot, and I used a 6922 (E88CC) which I guess is the mil spec version of the ECC88?

Anyway, put a 6922 in there and it will probably be happy. The Rode web site calls for a 6922 also:

http://www.rode.com/microphones/k2

Just so there's no misinformation lingering here, PCC88s tend to work out very well in the K2. I prefer them over equivalent NOS 6922s because they usually lower in microphonics, have a better noise floor, and the added headroom and overall tone is well-received.

I found an audio tech not too far away and it is now in his hands. I will post his results just in case anyone is interested in how the story comes out. Thanks again!

Hi Steve! I checked here to see if you had posted. Let us know how it turns out!

 

[kingkorg]

K2 has -5.5vdc for filament. PCC88 requires 7.6vdc, or -7.6vdc in this case. Am i missing something? What does that do to tube life?

Increased headroom? According to Røde, K2 has 1% thd at 162db!
155db according to Ford.

This is probably best valve mic headroom spec ever, unless you are recording jet engines. 

 

[Kona Steve]

Hey Christian! Thanks for checking. Will let you know!

 

[Gus]

If you do a search here and the web you should find a schematic.

I tried a number of different 6DJ8 type tube numbers I even changed the heater wiring for a 12ay7.
I don't believe a tube exchange changes anything except noise and microphonics in that circuit.

Interesting circuit.

Have you check the body of the microphone and grill to  the microphone XLR ground with a meter set to ohms?

Do the leds light up inside the microphone?

does anything smell like something burned  in the microphone or power supply?

Any visible damage to parts in the microphone or power supply?

Do the electrolytic caps look OK?

I would not assume the noise is only from the tube.