OK. I need a definitive answer on maximum 12A_7 voltages.

[viceroyal]

I built a power supply I can get ether 180V or 375V B+ out of it. Fearing that 375 was too high for the 12A_7 tubes that I'm experimenting with I chose to use 180V. I have spent that last two days looking at schematics for tonestack ideas and I have noticed that some amps run their preamp tubes above 300V. I have even seen one go up to 400V. Were tubes listed with higher max voltages back then or were these engineers playing with fire? Is there a safe way to use 375V on a 12A_7 tube?

I tried various ways of lowering the B+ and after a bunch of reading the simplest idea involved using a higher value series resistors in the power supply which is supposed to lower the voltage but no matter what resistor I use the B+ stays the same. Why is that?

[paggerman]

CAUTION: amateur answer. If you're testing without valves in place, there is no current. No current = 0 amps. Ohm's law : V=IR. If I=0 the voltage drop will also be zero. Your resistor can be any value, if there is no current, you will get the voltage of the HT. But I'm a potcher, not a player and I could have this wrong.

[ThermionicScott]

Are you talking about the supply or idle plate voltage?

[MWJB]

375v at the B+ rail will be fine for any of the 12A#7 tubes, as voltage will be dropped accross the plate resistor and voltageat the tube's plate will typically be <75% of the B+ supply.

[viceroyal]

375v at the B+ rail will be fine for any of the 12A#7 tubes, as voltage will be dropped accross the plate resistor and voltageat the tube's plate will typically be <75% of the B+ supply.

This is what I was looking for. I had heard that before the tube draws any current that the plate resistor has no effect and the tube sees all of the B+ for a short period. This is what had me worried.

[viceroyal]

CAUTION: amateur answer. If you're testing without valves in place, there is no current. No current = 0 amps. Ohm's law : V=IR. If I=0 the voltage drop will also be zero. Your resistor can be any value, if there is no current, you will get the voltage of the HT. But I'm a potcher, not a player and I could have this wrong.

I had heard this before about the plate resistor and that before the tube pulls any current it will see all of the B+.

[viceroyal]

Are you talking about the supply or idle plate voltage?

This is with the tube in place. It seems to work but I don't want to melt anything.

[mooreamps]

This is with the tube in place. It seems to work but I don't want to melt anything.

What's the matter ? Don't you got any "kahunas"?


-g

[MWJB]

When the tube is not drawing ANY current, it's because no current is being drawn through it, or the plate resistor & it's not doing any work. The tube only takes a few moments to get up to full current (for 12A#7 types, more of an issue with big power tubes like KT90 which take several minutes to draw full current).

[Chuck H]

Is this a preamp or a full amp? If it's a preamp you could always drop some volts with a totem power supply arrangement. Not enough current to cause substantial sag. If it's a full amp you could use zeners to regulate down for the preamp only.

[ThermionicScott]

This is what I was looking for. I had heard that before the tube draws any current that the plate resistor has no effect and the tube sees all of the B+ for a short period. This is what had me worried.

Remember that the plate isn't dissipating much power at this point, so it all works out. If it puts your mind at ease, Mullard gave a zero-current voltage rating of 550V for the 12AX7: http://www.tubezone.net/pdf/12ax7ecc83.pdf

[viceroyal]

Is this a preamp or a full amp? If it's a preamp you could always drop some volts with a totem power supply arrangement. Not enough current to cause substantial sag. If it's a full amp you could use zeners to regulate down for the preamp only.

This for a preamp. I've never heard of of a "totem power supply". I'll have to look it up. Thanks for the tip!

[Chuck H]

That's just a term "I" used to describe the stacked filter cap arrangement (each with a parallel resistor across it) used in some power supplies. In a two cap stack, if you tap in at the point where the filters meet +/- you'll get a 50% voltage division. You could stack three caps for a 66% division giving you a B+ around 250Vp. I don't think the current drawn by the preamp would cause too much "sag" since it isn't a power amplifier, it's a voltage amplifier.

[viceroyal]

What about cathode followers that don't have a plate resistor to protect them from the high B+?

[MWJB]

"Plate voltage" is really just the difference between the voltage on the plate and that on the cathode. A Cathode follower has a high value resistor between the cathode & ground, so you might have 400vdc on the cathode follower plate, but typically there may be 200vdc on the cathode, so plate voltage is only 200vdc.