12bh7 plate voltage question

B

brad44

Active Member
I have an amp that uses two 12bh7 among others.

I think the max plate is 300 vdc for 12bh7. Schematics for amp suggests 285 vdc at 117vac.

I am getting 304 and 294 vdc,at the plate of both tubes, within a 10 percent tolerance if that is indeed the tolerances for this amp. The rest of the voltages on the 12bh7s are pretty much spot on.

Is this a concern?

p.s.- thanks for your patience with me asking questions here.
 
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If amp was built for 117vac.
Which was typical probably to almost 1970. Everything should be a the high side now.
You should consider a bucking transformer or at least a couple of properly sized current limiters to drop the line a bit.
I'd bet your seeing 122-123 out of your wall now .
304 for 300 isn't bad. What are you getting at the filaments?
 
I used a variac to measure those voltages at 117vac. The rest of the voltages on both tubes are almost exact to schematics.

These are old tubes, which voltages are effected by the strength of the tube?
 
I doubt it's a problem but it helps to know more about how it's being used.

Typically a plate resistor drops a lot of the supply voltage... 300v supply equals a much lower voltage at the plate-- because the tube is drawing current through that resistor.

Without the tube installed, the plate will match the supply voltage, appearing higher than it will be in operation. Was the tube installed and operating when you took those measurements?

If it is used with a plate load, there is likely a cathode resistor subtracting a handful of volts from the overall voltage across the tube... even if the plate is near 300v in typical operation you could subtract maybe 5v from that.

Now, if it's a cathode follower the plate itself will always be at a high voltage but the voltage across the tube is not high, because the cathode resistor is eating up (potentially) hundreds of volts.

Overall I wouldn't worry about putting 304v across a tube rated for 300v. But I also think if you look more closely at the circuit, you'd find it's seeing less than what you measured. It might not be dangerous but it is higher than you'd see in most applications.
 
Voltage is measured from plate to cathode, so what the tube actually sees is less than what you measure directly.
 
The 300Vmax rating for class A1 amplifier service in GE's datasheet is per "Design-Center" rather than absolute maximum, which means that a reasonable tolerance is already factored in.
 
Run it. You are within 5% of the right voltage, also note that when a data sheet specs a voltage measure that voltage in reference to cathode, not necessarily to ground. Like someone said earlier there may be 3 to 5 volts across the cathode which gets you within 1% of perfect. Pretty good considering most of this old stuff was build with 10% tolerance parts, most of which have not gotten better with age.
 
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