Replacing an amp that uses 7591 tubes for Yellow Jackets with EL84 tubes

I just got my Yellow Jackets in the mail and I think they're well worth the $50 holiday special price on Amazon (the price went back to $118)! It definitely changes the sound of my amplifier. I have an old Titano amp made by Audio Guild. Audio Guild were a company formed by the guy that made Magnatone amps back in the day. He sold the amps to companies who then put their own names on them, like Panaramic, Da Vinci, Imperial, and others including Titano. Titano was (is?) an accordian company and most likely this amp was made for an accordion. It has a very nice vintage tone, the bass and midrange response is phenomenal, although it's a bit flat sounding (not much high end chime, which i like) in the high end. So I bought the Yellow Jackets and the difference was very subtle but enough to get me the chime I was looking for.

What's great about the Yellow Jackets is that you don't need to bias or anything. You just plug them into your power tube sockets. The only tricky thing is that if you have a cathode-biased amp, you will want to ground the new Yellow Jackets with the forked ground wires to your chassis somewhere. However, I talked to a tech guy at CE Distribution (Yellow Jackets are no longer owned by THD: http://www.cedist.com) and he said even if you don't know if your amp is cathode biasing, and you don't ground the tubes, it won't ruin the amp or tubes, it just might get a bit distorted. No such problem as far as I could hear, so I'm assuming my amp is not cathode biasing.

There are many different versions of Yellow Jackets. There are Yellow Jackets for 6L6, 6V6, EL34 and so on. They offer duets and quartets in standard pentode mode (reduces power to 80%, maybe 50% in some amps). They also offer triode versions in which your amp's power drops by 50-80%. This allows you to overdrive at lower volumes. I can see this being very popular with people who own very loud amps that they can never push loud enough to get that wonderful overdrive. By using the triode Yellow Jackets, you basically cut your power so that you hit overdrive sooner and at a lower volume.

Overall, I'm pretty happy with the Yellow Jackets. $50 for a bit more response from my amp is a great deal as far as I'm concerned!