Apex 460

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yikelu

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OK, the mod is done and I can report that it did help tame the highs, reduce the harshness, and reduced the THD (well as far as I could tell by ear). Unfortunately haven't built anything to measure it directly. I assume to take the capsule out of the circuit you just short it out with a jumper and clip in test leads where the cap attaches to the PCB?

New capsule comes in the mail in the next few days (always intended to put a K47 style cap in it for variety).
 

yikelu

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Nice catch, however i believe 12v is wrong, these psu have usually 6v, but definitely worth checking.
In either, case, the heater connection is unchanged from the original except that it is disconnected from the second half of the tube, which only affects the current not voltage.
 

blue_luke

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Dec 13, 2008
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Montréal, Québec
I have a pair of those (Apex 460) I tried them for a few minutes, just to make sure there was no issues, there were none.
Then I did a few basic modifications, throw away the 12AX7, which is a bad choice there and replace with a low mu low noise valve, 6072 is excellent, I think I installed old RCA 12AY7 I had on hand, I replaced C4 for polypropylene type and also C8 for nonpolarized polyester. This cap does not have to be 450V! there is only a few volts at this point!

Remove C9,C10, they are supposed to filter RF interferences and are actually too big of value... Just take them out!
Just these basic mode will make the mic pretty darn good, if you like over bright and fizzy highs, but at least, it becomes usable with some EQ.
Low-pass around 18K 12db/oct or 14K 6db/oct and cut a few db, wide Q at around 10K, not perfect but decent on female voices.
The bass is also very good.
The biggest difference will come when you change the capsule, I used Microphone Parts matched set of RK-12, and now we are talking! :)
Gone the 'chrome polish' on the highs, the bass is very flat down to 20hz, and the whole thing sounds tight, crisp but solid...
I like these mic very much and I use them often to record choirs and classical music ensemble.
I use the stock transformer and kept the circuit original, except for the mods I mentioned.
A friend of mine owns a real C12 and honestly, mines are sounding very close to the original.

I forgot to mention that in the PSU unit I replaced the zeners for polarisation voltage to 120Vdc, 160 is excessive and think I beefed up some electrolytics value.... not sure, but I remember that all these mods, on two mics took me just one afternoon to do...
 
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RuudNL

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Most of these Chinese power supplies have an output of about 6.7 Volts for the filament.
This will shorten the life of the tube.
They use a 7806 with a diode connected to the center pin of the 7806, to 'lift' the voltage 0.7 V.
Bridging the diode will bring the output voltage down to 6 Volts.
And of course you will lose a little bit of voltage in the microphone cable, but this will generally be not more than 0.1 Volt. (Unless you use a really bad cable...)
 

yikelu

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Chicago
Put in a 47 capsule, somewhat disappointed in the results. I A/B'd against 2 other mics with 47s (one with the exact same kind of 47) and the others had more richness in the low mids.

I'm thinking the problem is the tube (still has the stock 12AX7 ATM) and possibly the transformer. Going to order a 6072 to do the swap. Wondering if changing the transformer is worthwhile though?
 

kingkorg

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I have measrured the 460 transformers and they are excellent!
No need to change them. Frequency response from 10 Hz-40 KHz within 1 dB.
+1. I measured both in "typical" plate out circuit, 460 cathode out circuit and just with audio interface output (50ohm). I can also add THD is very low even in LF, much lower than T14 type transformers.

I noticed of course HF response might vary with different tubes and topologies, but in most cases 10-20K its still flat, which is true for any transformer i guess.

I heard recently David Royer was not satisfied with the early ones found in MXL mics, not sure why, but i guess they upped the game in the meanwhile.
 

RuudNL

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According to Brian Fox:

"The stock circuit (CCDA) with the stock transformer in the 460 is flat within .1 dB from 30Hz to 30Khz and it's only down .5dB at 40Khz!"
 

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yikelu

Member
Joined
Jul 3, 2022
Messages
18
Location
Chicago
I have measrured the 460 transformers and they are excellent!
No need to change them. Frequency response from 10 Hz-40 KHz within 1 dB.
+1. I measured both in "typical" plate out circuit, 460 cathode out circuit and just with audio interface output (50ohm). I can also add THD is very low even in LF, much lower than T14 type transformers.

I noticed of course HF response might vary with different tubes and topologies, but in most cases 10-20K its still flat, which is true for any transformer i guess.

I heard recently David Royer was not satisfied with the early ones found in MXL mics, not sure why, but i guess they upped the game in the meanwhile.
Thanks for the feedback! Much appreciated. I was also specifically curious about plate out as I had just changed my mic to that.

Your responses and my research jointly indicate that the likely culprit is the 12AX7, so I've got an EHX 12AY7 coming to swap out. Here's hoping that gets me the desired result. Otherwise I gotta start hunting for rogue LF rolloff.
 

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