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Testing tubes in a mic, one hotter than the other - why?
Old 3rd March 2014
  #1
Gear Maniac
 
🎧 10 years
Testing tubes in a mic, one hotter than the other - why?

Trying some old tubes I found in my Golden Age TC1, Just tried replacing a Electro Harmonix 12aY7EH with a Phillips Miniwatt ECC81. The sound was amazing, but the mic got really hot! Is there any potential danger here? The EH tube is brand new but the Miniwatt is probably very old. Got it from a friend. Maybe used also. It really hit som sweet upper mids!
Old 3rd March 2014
  #2
Gear Maniac
 
🎧 10 years
Ive never done this before. I pushed the tube pretty far down into the socket. What should the spacing between socket and bulb be?
Old 4th March 2014
  #3
Deleted User
Guest
Push it all the way into the socket. That's where it belongs.

The ECC81 is a 12AT7. Both the 12AT7 and 12AY7 draw the same exact filament current, while the 12AT7 has slightly higher gain. I've never heard of problems substituting one for the other, other than slight noise differences. Perhaps somehow the ECC81 is biasing differently than the 12AY7 causing it to run hotter. I'd be more concerned about damaging the capsule than the tube. Hang the mic upside down to keep the heat off the capsule.
Old 4th March 2014
  #4
Lives for gear
 
🎧 10 years
You might want to watch the tube with power on. A blue glow is normal. Red glow means the tube is bad and shut it down immediately.
Old 4th March 2014
  #5
Gear Maniac
 
🎧 10 years
Is the Phillips Miniwatt ECC81 a Mullard tube? I see some on ebay sold like mullards
Old 4th March 2014 | Show parent
  #6
Deleted User
Guest
Quote:
Originally Posted by Sailor Hope ➑️
Is the Phillips Miniwatt ECC81 a Mullard tube? I see some on ebay sold like mullards
Philips Miniwatt was a brand that primarily sold Ameprex tubes made in Holland. Although some Mullard tubes were allegedly sold under the Minwatt label as well thru rebranding.

Tubes made by Amperex were usually marked "Made in Holland"
Tubes made by Mullard were usually marked "Made in Britain"

...regardless of brand label. That's one way to tell.
Old 4th March 2014 | Show parent
  #7
Lives for gear
 
LeeYoo's Avatar
 
🎧 10 years
Quote:
Originally Posted by The Real MC ➑️
You might want to watch the tube with power on. A blue glow is normal. Red glow means the tube is bad and shut it down immediately.
Red plate glow in preamp tubes with 108k anode and 47k cathode series resistors?

Imagination?
Assuming the passive parts are ok, one tube can't be seriously hotter than another tube, even when wrongly biased.
Fillament power is about 1.8Watt (both types), and plate dissipation tops at 52+120mW max.
Old 5th March 2014 | Show parent
  #8
Deleted User
Guest
Quote:
Originally Posted by LeeYoo ➑️
Assuming the passive parts are ok, one tube can't be seriously hotter than another tube, even when wrongly biased.
Fillament power is about 1.8Watt (both types), and plate dissipation tops at 52+120mW max.
Sure it can.

Plate dissipation is the max SAFE dissipation rating for the tube, not the maximum dissipation that the tube will endure under faulty circuit conditions. If the tube looses bias, then it will dissipate many times the rated max plate dissipation in it's plate, causing the tube to turn red and quickly burn out.

Also, tubes can be defective. I've had defective tubes run red under normal bias conditions, where the only solution was to replace the tube with one that was not defective.
Old 5th March 2014
  #9
Lives for gear
 
LeeYoo's Avatar
 
🎧 10 years
Hi Frank.
I was refering to the Golden Age TC1.
One half has a total anode resistor of 108k. That can't make the plate glow.
And the other half has a cathode resistor of 47k. That also can't make the plate glow.
I've seen output tubes glow red hot, once even to the point of the glass caving in, leaving a small crater.
But preamp tubes.. Can't find a proper high-current path, even with a faulty tube.
Besides, the OP said it was working ok.
Leo..

Edit: I have seen the 12AX7 in a stompbox glow red hot, and blue, and even green.
On inspection, I found an RGB LED mounted in the valve base....
Attached Thumbnails
Testing tubes in a mic, one hotter than the other - why?-tc1-schematic.jpg  
Old 5th March 2014
  #10
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emrr's Avatar
 
24 Reviews written
🎧 10 years
Yeah, I've never seen a preamp tube turn red unless in full fault condition from the surrounding parts shorting. Especially inconceivable to me if the circuit is working.
Old 5th March 2014
  #11
Gear Maniac
 
🎧 10 years
I swapped to a Telefunken tube, and back again and the heating issue went away. But thanks for all the helpful feedback!

I am looking for a vintage buttery tone with some thump in the right bottom frequencies and a clear and centered high mid that lift the vocals out.
The Oktavamod 47 capsule does a great job, and replacing the tube with the Miniwatt one was a great success! Any other good NOS tube suggestions for that colored vintage
tube mic sound? Ive hear G.E. 5 star and Mullard mentioned.. ?
Old 5th March 2014
  #12
Deleted User
Guest
In general, I like old RCA tubes first, followed by GE Five Star. Old Tung-Sols are great also. And old Western Electric tubes are the ultimate, if a compatible substitute can be found. But really, it depends on the tube. For example, I have a tube mic that uses a PF86 (i.e., not the more common EF86). For that, the best I've found are Mullards and Telefunkens. Europeans excel at some tubes, Americans at others. Try both. In the end it's what you like.
Old 5th March 2014
  #13
Lives for gear
 
LeeYoo's Avatar
 
🎧 10 years
AFAIK the only difference with E and P tubes is the heater.
E series run on a constant voltage, while P tubes run on a constant current of 300mA.
P tubes where used e.g. in TVs, where the heaters were running directly of the mains like a string of Chrismas lights.
In the seventies I would have replaced about ten tubes a day. Every working day.
Leo..
Old 5th March 2014 | Show parent
  #14
Deleted User
Guest
Quote:
Originally Posted by LeeYoo ➑️
AFAIK the only difference with E and P tubes is the heater.
E series run on a constant voltage, while P tubes run on a constant current of 300mA.
P tubes where used e.g. in TVs, where the heaters were running directly of the mains like a string of Chrismas lights.
In the seventies I would have replaced about ten tubes a day. Every working day.
Leo..
Yes, I'm already aware of the the filament difference. These days, some manufacturers are using the P version as an alternative to the E just because the Ps are still abundant in supply NOS.
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