Got these minty RCA 6K6GT vacuum tubes in new old-stock (NOS) condition for about $10 each. Check current prices on ebay.
So what good are these tubes in the current age of digital everything? Well, they are output tubes (valves) for a variety of old radios and TVs (as well as car audio). But most people these days will be wanting them for use in guitar tube amplifiers.
The 6K6GT (tube data link) is a suitable substitute (in most cases, see below) for the more well-known 6V6GT tube. They are both octal-pin power pentodes, but the 6K6GT dissipates a maximum of ~4.5W versus the ~5W to ~8W of a 6V6GT biased moderately (you can get a lot more out of a 6V6 if you bias it hot, but tube life declines much more quickly). This might sound like you can cut output volume in half (the bedroom guitarist’s constant holy grail), but watts of dissipation does NOT track linearly with decibels of volume, unfortunately.
Is a 6K6GT Half the Volume of 6V6GT?
Generally speaking, you need to quadruple (4x) the output power to double (2x) the sound pressure level (SPL), but perceived loudness is not the same as SPL. The rule of thumb is that to to get double (2x) the perceived loudness, the output power (in watts) must increased ten-fold (10x).
By that math, a single-ended tube amp with a 6V6GT output tube would be about 20% louder than the same amp with a 6K6GT output tube. Not exactly a night-and-day difference. But still not nothing. And the other change with the 6K6GT is less clean headroom and earlier output tube breakup (if crunch or overdrive is important to you). Here’s more data on how watts vs SPL vs loudness plays out.
Can 6K6GT Be Used in Place of 6V6GT?
In a lot of cases, yes. But here’s what you need to keep in mind:
- Maximum plate voltage on 6K6GT is 315V; whereas 6V6GT max is 350V
- Heater current draw on 6K6GT is 0.4A; versus 6V6GT draw of 0.45A
Make sure you know the voltages of your amp before swapping out. But in most small guitar tube amps that are designed with a single 6V6GT, you can drop in a 6K6GT without problems. Just don’t expect a massive drop in perceived volume. So then…
Why bother with a 6K6GT?
They are cheap and readily available in both used and NOS conditions from most online tube venders, ebay, and local electronics surplus shops. Also, vintage stuff is cool!