TDSL is your friend. You can even load their "TDSL Personal Edition" on your PC. It searches for the tube types you want, then gives you substitutes, as in "Identical or close" or "Different ratings or performance." You can create an infinite mirror loop of searching, too, by clicking on one of the "Identical or close" results and using that as a primary search tube type for other substitutes, find another one, then do it again, etc., etc.
TDSL stands for Tube Data Search Locator:
https://tdsl.duncanamps.com/tubesearch.php
These days, some of the references you get for the tube data sheets may be broken links, but you can do a Google search to find out what happened to that link. In some cases (Pete Millett's sites), the website simply underwent a re-organization. However, the great majority of the tube data sheet links still work. The software is still embedded with absolutely valid tube types and substitutes, however, regardless of whether you can find the actual tube data sheet on one of the links.
Bottom line, TDSL will get you what you want. When it comes to those tubes that are not "Identical or close," you have to look at the following:
- Identical Pinouts. If you don't have the same pins, you can destroy the tube or even the amplifier. Not a good idea. This is absolutely your first determination in deciding whether to use another tube type.
- Heater Voltage and Current. An amplifier is designed to use a specific heater current and voltage. This can vary somewhat, but you often need the help of the mfr to be certain. One tube may be rated at 200ma, another at 250ma and it be OK with the larger one. Go to a tube that uses 300ma and it may burn up the amplifier. Voltages are similar, but be sure to check whether the heater supply is in parallel or series (ask the mfr). A lot of tubes say 6.3VDC for the heater supply, but if the pins are tied together in series in the amp circuit, then 12VDC from the amplifier will work just fine. As with #1, if there's a big mismatch, it's potentially amplifier-destroying.
- Plate voltage and Watts dissipation. Unlike #1 and #2, this is much more of a gray area. Certainly, if the amplifiers uses a B+ voltage (tube plate voltage supply) that is much higher than the tube's plate voltage rating, then you can blow out the tube. This varies quite a bit, though, because that's exactly what guitar amps do - run voltages on the tubes higher than their plate voltage ratings, just to get distortion effects. We don't want to do that in the audiophile world, however. Amplifier voltages lower than the tubes plate rating are very, very common. The only concern is the further away from the tube's normal plate voltage parameters, the more distortion you may get. This occurs if the amp's voltage is lower than what the tube wants, but it will not damage the tube. Watts dissipation is a heat issue. You have to be careful that the amplifier's power supply is capable of handling the wattage the tube can deliver. This is much less important in headphone amplifiers, because almost in all cases, we are using signal tubes with very low power outputs.
