Hi heater voltage in tube mic?

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Freddy G

Well-known member
Joined
Jun 4, 2004
Messages
463
Location
Canada
Hi,
I modified an apex 460 tube mic about 2 months ago and now it's making alot of noise. I cleaned everything inside, especially the hi impedence parts and although it's a little bit better there is still a constant "white" noise.
The mod is the typical remove cathode follower thing (used a cinemag 2480 transformer and 6072a tube)
When measuring heater voltage it reads 6.45 V dc.
Could this be the problem? Is it killing the tube? Should be 6.3V no? or maybe a little less for the underheating thing....
Any help appreciated.
Freddy
 
Could be the tube some just get noisy in tube microphone because they were not designed to work at the grid to ground resistance used. 1meg vs 1 gig. You could try a 200meg grid to ground

The china power supplies I have seen have a 6V reg 7806 and a diode 1n400x in the ground leg. The diode is an old trick to 'lift the reg up" a diode drop. so you get about 6.7VDC at the reg output light load.
I guess the idea was to run the tube at 6.7VDC - the connector and IR voltage drops in the wire. So you end up at about 6.3VDC

If you take out the CF and disconnect that side of the fil the current drain goes down and the IR drop is less. about 300ma vs 150ma

Three simple things you can do.
jumper the diode for 6VDC - the cable and connectors IR drop

replace the 7806 with an easy to find 7805 for about 5.7VDC

install power a resistor in series after the stock regulator find one that gets you at about 5.7VDC measured at the tube socket

Now some/all? 12a-7 type tubes don't work at under about 5.7VDC without getting noisy after time. As best I can tell it has to do with the cathode metal alloy and oxide mix.

So the simple safe thing with 6072a 12ay7s 12a-7swith the china supplies I have seen like used with the 460 is to jumper the diode using the stock 7806 and run at under 6VDC at the tube due to the IR drop.

NOTE I have seen a china supply that the diode was installed but the way the reg was installed caused the diode to be shorted. The 7806 heatsink was electricly connected to the ground tab and the heatsink was soldered to PCB ground. The tab and ground lead are connected in the reg.

Some tubes just don't work well in tube microphones. I just had a microphone go noisy that was fine for maybe 40 hours. It was a tube from the same JAN batch and company 4 others from the same batch with more hours are fine.
 
white noise = tube noise, if it stops and picks up again, after a few snap, crackles and pops.

constant white noise at the same volume level, that may be something else.
 
Thanks guys,
Gus, I'll try your diode suggestion....you da man! :thumb:
Interestingly, I threw in a 12ax7 and no white noise....pretty quiet.
I did notice a faint "motorboating" sound though.
I'm going to order some NOS 6072 tubes. The original tube was an Electro Harmonix....I've read that they are more than likely not "true" 6072...but some Russian leftover that's "close".
I've ordered NOS 6072 before without these problems, but who knows?
I'll report back with my findings
Freddy
 
http://recforums.prosoundweb.com/index.php/m/238372/0/?srch=806#msg_238372

http://recforums.prosoundweb.com/index.php/m/241324/0/?srch=6072a++EH#msg_241324
 
update:
I tried Gus' suggestion about bypassing a diode in the psu. Worked great...now I'm down to about 6V.
Replaced the Electro-harmonix with a NOS GE 12AY7 and not only is the noise gone but the tone is soooo much nicer!
This is not the first mic I've modded with an electro-harmonix....I guess they just don't work well in this circuit.
Anyway, big thanks to Gus for the insight :thumb: :sam:
Freddy
 
[quote author="Gus"]

If you take out the CF and disconnect that side of the fil the current drain goes down and the IR drop is less. about 300ma vs 150ma
.[/quote]

Gus, I still have the 2 heaters attached to the PSU after removing the CF. Could that One heater cause "plops" ?
 

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