largest plate voltage on a 12AX7 question

Planobilly

Member
2015-12-11 4:35 pm
Hi,

I just repaired a Mesa Boogie Single Rectifier Solo Head 50. The plate voltage on V3b which is a preamp tube in the lead channel measured 415v plate to ground. 415V was indicated on the schematic.

There is no issue with the amp but I was wondering how high the plate voltage can go on a 12AX7. This is the highest voltage I have had the occasion to observe.

Billy
 

rayma

Member
2011-04-29 8:37 pm
415V was indicated on the schematic. There is no issue with the amp but I was wondering
how high the plate voltage can go on a 12AX7. This is the highest voltage I have had the occasion to observe.

The absolute maximum is listed by RCA as 330 VDC, but perhaps some varieties can exceed this reliably.

Yes, if this tube is connected as a cathode follower, cascode, etc. then the cathode would be elevated
above ground. Then the P-K voltage would be less than the 415V power supply.
 
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Highest voltage across the tube will be at switch on before the heaters warm up an the tube begins to conduct. When this happens then voltage will be dropped across the anode load resistor thus reducing the voltage across the tube.

The 12AX7 has two(2) max voltage specs:
Va0 (V a zero) is the max when the tube is not conducting (before heaters warm up) and it is +550V max.
Va max is the the max voltage when tube is operating and it is +330V max.

I probably would'nt stress too much about up to +350 from anode to cathode.

Cheers,
Ian
 
hi there, this is from philps data sheet for ECC83 (12ax7) lists 550 volts as limit Va0 which i guess means no current as stated above.
 

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Rayma is right that's a cathode follower with 215V on the cathode, so there's only 200v across the tube. That's within the plate voltage rating of the tube but does slightly exceed another rating, the heater - cathode voltage rating.

Guitar amps have always been cruel to their tubes. Fender has been putting 430V across their 12AT7 reverb driver since the 60s. Plate voltage is 440v, cathode 8v. They regularly ran 6V6 at 420v as well.
 

Planobilly

Member
2015-12-11 4:35 pm
It is evident that tubes are pretty tough beast!!

It would be fascinating to see how things would have evolved if transistors had not been invented. I assume the data sheets would have changed considerably.

I assume 12ax7 type tubes have been pushed so hard because they are cheap and readily available.

Cheers,

Billy
 
Hi Guys

The 12A_7 family have an arc-over rating of 540V.

The 300V rating is given to accommodate transformer loading and the attendant flyback voltages. In a resistive loaded stage, the supply voltage can be anything up to 540V as the plate resistor protects the tube even if the 540 is exceeded slightly.

Tubes are still with us. Do you have a microwave oven? That uses a tube called a magnetron.

Have fun