Tube voltage ratings

[K O'Connor]

Hi Guys

Tube voltage ratings took a leap in the mid-1950s with a change from Design Centre to design Maximum ratings.

The old system took into account the variations of the tube manufacturer's process AND second-guessed the variations in the end user's product design, leading to very conservative ratings.

The new system only took account of the tube manufacturer's variations and suddenly the tubes sitting on your shelf or in your equipment were rated for much more voltage.

Unfortunately, you still see many data sheets for "new" tubes using old voltage values.

Have fun

[K O'Connor]

Here is an example of an application where the amplifier manufacturer "appears" to be abusing the tubes: Univox amps using 6L6GC.

The 'GC is rated at 500V for both plate and screen; Univox uses a voltage-doubler power supply to create about 700Va and 350Vs. The screenis obviously in no danger at all, but what about the plate? The plate load is about 6k6-aa, which requires 445V peak at 270mA peak for 120W peak output - all well-within the capability of a pair of 6L6GCs. The supply could sag down to 510V and the amp would make its output. That would represent 27% regulation from the supply, which is a bit excessive and does not happen in the real amp, and a bit more output than 60W is actually achieved. Were the supply to remain rigidly at 700V, the average waste heat in each tube would only be 25W, so even a 6L6GB would be okay in the real amp where there is some supply sag.

The fact the screen is operated at such a low voltage helps protect the tube. However, it means that the drive to the grid might have to be low-impedance inasmuch as the grid might have to be pulled positive into conduction to enforce high plate currents. In this specific case, there is no need for that, but even one of the 6L6 applications show this in the data sheet.

Most often we see the screen voltage applied to a tube being higher than what the data sheet suggests. For example, 6550 has a screen voltage rating of 400V and late at 600V. That is the original version - the 'C' version is 440V and 660V, respectively. Yet, we see 6550s in Marshal amps with 500V on both elements, and in the large bass amps with about 700V on both elements. In both cases, 1k-5W screen stops are used, which protects the screens, as TUTs detailed and recommended from day one.

EL-84s are frequently represented to have very low voltage ratings. However, we see them with 450V on both plate and screen, used in pairs and quads. Was Seymour-Duncan inept? Certainly not; they simply know how to read the data sheet and how to protect this particular tube with high-value screen-stops.