Reverb 12AT7 Tube Heat ?

Amp Kat · 11-13-2009, 02:18 PM

Do any of you back off the voltage on the 12AT7 in the Fender reverbs ? If you do a touch test those tubes they just run so freaking hot that it bothers me as it seems it would burn out way faster than the others. I realize the 12AT pulls more current than a 12AX by Siemens but does it kill the tube quicker I guess is what I'm getting at ?

MWJB · 11-13-2009, 02:35 PM

"Touch test" isn't really a test at all.

AB763 schem shows the AT dissipating less than 1.5W (.75W per triode), well below max dissipation, even allowing for rise in AC wal voltage. Some late SF's had a 820ohm cathode resistor, these often get changed back to 2200ohms/25uf bypass cap.

Amp Kat · 11-13-2009, 02:49 PM

Think I'm gonna do a current test because this tube is hot to the point of not being able to touch it and at that dissipation wattage it should be way cooler. Maybe there is a problem also. I don't know what a 1.5 watt dissipated tube feels like but this one is really,really hot.

MWJB · 11-13-2009, 02:51 PM

Do the current test by all means, but Fender reverb 12AT7 tubes are usually too hot to touch comfortably.

Think about it, you would worry about a hot rectifier or a hot power tube (which is exactly what the 12AT7 is in this case).

Amp Kat · 11-13-2009, 03:17 PM

Good enough

pdf64 · 11-13-2009, 06:47 PM

SF fenders ran the 12AT7 reverb driver with a 470 to 1k cathode resistor, which runs the tube way over dissipation spec. As MWJB suggests, 2k2 is the value needed.

tubeswell · 11-13-2009, 07:21 PM

I've found out from experience that those BF (12AT7) reverb driver tubes wear out faster anyhow, even though each triode is designed for 5mA plate current.

11-13-2009, 02:18 PM	#1
Amp Kat Senior Member Join Date: Jul 2006 Location: Baton Rouge,LA Posts: 1,062	Reverb 12AT7 Tube Heat ? Do any of you back off the voltage on the 12AT7 in the Fender reverbs ? If you do a touch test those tubes they just run so freaking hot that it bothers me as it seems it would burn out way faster than the others. I realize the 12AT pulls more current than a 12AX by Siemens but does it kill the tube quicker I guess is what I'm getting at ? __________________ KB

11-13-2009, 02:49 PM	#3
Amp Kat Senior Member Join Date: Jul 2006 Location: Baton Rouge,LA Posts: 1,062	Think I'm gonna do a current test because this tube is hot to the point of not being able to touch it and at that dissipation wattage it should be way cooler. Maybe there is a problem also. I don't know what a 1.5 watt dissipated tube feels like but this one is really,really hot. __________________ KB

11-13-2009, 03:17 PM	#5
Amp Kat Senior Member Join Date: Jul 2006 Location: Baton Rouge,LA Posts: 1,062	Good enough __________________ KB

11-13-2009, 07:21 PM	#7
tubeswell Supporting Member Join Date: Feb 2007 Location: Wellington NZ Posts: 2,654	I've found out from experience that those BF (12AT7) reverb driver tubes wear out faster anyhow, even though each triode is designed for 5mA plate current. __________________ Building a better world (one tube amp at a time)

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11-13-2009, 02:35 PM	#2
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	"Touch test" isn't really a test at all. AB763 schem shows the AT dissipating less than 1.5W (.75W per triode), well below max dissipation, even allowing for rise in AC wal voltage. Some late SF's had a 820ohm cathode resistor, these often get changed back to 2200ohms/25uf bypass cap.

11-13-2009, 02:51 PM	#4
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	Do the current test by all means, but Fender reverb 12AT7 tubes are usually too hot to touch comfortably. Think about it, you would worry about a hot rectifier or a hot power tube (which is exactly what the 12AT7 is in this case).

11-13-2009, 06:47 PM	#6
pdf64 Senior Member Join Date: May 2006 Posts: 609	SF fenders ran the 12AT7 reverb driver with a 470 to 1k cathode resistor, which runs the tube way over dissipation spec. As MWJB suggests, 2k2 is the value needed.

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