High voltage on 12AX7's ?

[jamme61]

I know it's the way mesa designed it but, just curious, why on some of the 12ax7's the voltage is as high as 422 volts? On V3b on pin6, it's at 422 volts that's the way mesa made it but, that's not normal and way to high for a 12AX7 right? I guess this is part of the tone of the amp but, I'm just curious and a little worried about my 12ax7's dyeing?

 

[Old BF Shred]

If you measure with the tube pulled from the socket, the voltage will be higher; there is no "load" on the circuit.
The only real way is to measure from under the socket while the tubes are in. This can be dangerous however.... I wouldn't unless you have a real problem and you know how to fix it. 8)

 

[jamme61]

If you measure with the tube pulled from the socket, the voltage will be higher; there is no "load" on the circuit.
The only real way is to measure from under the socket while the tubes are in. This can be dangerous however.... I wouldn't unless you have a real problem and you know how to fix it. 8)

The schematic list voltages as high as 390 on V2b ( I measured 383 volts) and 422 volts on V3b ( I measured 422 volts) this worries me because, the max voltage for a 12ax7 in 350 volts? At least that's what I found on the web. Still these amps have been around for a long time now and I haven't read anything about problems? I wonder how mesa is able to do this?

 

[94Tremoverb]

Many or even most tube guitar amps run the tubes at over the maximum design spec for voltage, power dissipation or both. Check the ratings for 6L6s and 6V6s especially and it may surprise you! But usually it's not a problem since the specs are *minimum* ratings and most tubes - particularly old-production ones - comfortably exceed them.

These are a little high, but it's not quite that simple, especially for V3B. It's a cathode follower, and the cathode voltage is 220V, which makes the *plate-to-cathode* voltage (which is what the maximum rating really applies to) only about 200V, which is well within the rating. But this does bring in another problem, which is that the cathode-to-filament voltage is now very high. This is a real problem with some modern-production 12AX7s - Sovteks especially in my experience - which aren't made to the correct original spec and will often leak current (and sound bad) or fail outright in these positions.

There are even worse examples if you look at some of the amps which use 12AX7s and 12AT7s as reverb drivers - including this one, look at V5A! - 425V on the plate and 4V at the cathode. Basically this is a preamp tube being used as a miniature power tube, and it's actually surprising that they don't blow more often. At least this one is only running at about .75W (each half of a 12AX7 is rated for 1W), you should check out what the reverb driver in a Fender Silverface amp is subjected to, and yes they do sometimes melt (literally!).

Tubes are tough. Do not try this sort of thing with solid-state components.

 

[jamme61]

Many or even most tube guitar amps run the tubes at over the maximum design spec for voltage, power dissipation or both. Check the ratings for 6L6s and 6V6s especially and it may surprise you! But usually it's not a problem since the specs are *minimum* ratings and most tubes - particularly old-production ones - comfortably exceed them.

These are a little high, but it's not quite that simple, especially for V3B. It's a cathode follower, and the cathode voltage is 220V, which makes the *plate-to-cathode* voltage (which is what the maximum rating really applies to) only about 200V, which is well within the rating. But this does bring in another problem, which is that the cathode-to-filament voltage is now very high. This is a real problem with some modern-production 12AX7s - Sovteks especially in my experience - which aren't made to the correct original spec and will often leak current (and sound bad) or fail outright in these positions.

There are even worse examples if you look at some of the amps which use 12AX7s and 12AT7s as reverb drivers - including this one, look at V5A! - 425V on the plate and 4V at the cathode. Basically this is a preamp tube being used as a miniature power tube, and it's actually surprising that they don't blow more often. At least this one is only running at about .75W (each half of a 12AX7 is rated for 1W), you should check out what the reverb driver in a Fender Silverface amp is subjected to, and yes they do sometimes melt (literally!).

Tubes are tough. Do not try this sort of thing with solid-state components.

Awesome! thanks love to read about this stuff. I didn't realize the V3b is the CF what brand tube are you running there? Thanks for the info.

 

[94Tremoverb]

Currently a 70s GE. I've also tried a Mullard there and it sounds good and a little more 'Marshally' (as you would expect).

Yes, V3B is the tone stack driver cathode-follower. V4B is also a cathode-follower, driving the FX loop. You can spot them easily on the schematic because the plate is connected directly to the HT supply (with no resistor), and the cathode resistor has a high value (hundreds of K-ohms), with the signal output from there not from the plate. V2B has a fairly high cathode resistor value as well (39K) but it's not a cathode follower, the signal output is still from the plate. This high cathode resistor value gives it a lot of self-regulation for current and helps to reduce the risk from the high plate voltage. (As well as partly causing it! Since the low tube current means that the voltage drop through the plate resistor is smaller than normal.)

You can also see that the reverb driver V5A has an extra resistor (22K) in series with the reverb transformer, which limits the current and is why it isn't as hard on the tube as you would expect from the voltages.