Crucial TUBE question

Wow. Props to Upscale for being so calm in their response here lol. Those tubes are not pin-compatible; they’re not even function compatible. The 5AR4 is a full-wave rectifier and you maybe sort of got this KT150 scheme working as a half-wave rectifier? - I don’t even know.

Anyways, what this tells you is that you like the sound of LOTS of distortion (the euphonic kind, not the harsh kind). I know how that feels - I once inadvertently subbed 5814 (12AU7) into the 12AX7 phase inverter slots of my Rogue Apollo amps, and thought it sounded awesome! Then discovered the mistake a couple days later, and felt silly.

But yeah, this is much worse than that 12AU7 / 12AX7 swap - please respect the gear and don’t keep doing it. At the least, it’s truly a testament to how resilient tube circuits are to most mistakes, versus Solid State.

I thought OP replaced both rectifiers with a matched pair of KT50 - maybe he can confirm.

Well, pin 2 on both 5AR4 and KT150 (same as KT88 pinout) hits the heater, but the other side of filament is pin 8 (also ties to cathode) on the rectifier and pin 7 on the KT. I don’t know enough to say how that works out, but if it does fire up - the ratings of the 2 filaments are close enough for the KT tube to be ok.

On the other side, the KT tube has its screen grid hooked up to what should be 1 of 2 plates in the rectifier (each plate handling a "half wave"). There appears to be nothing hooked up on the KT to serve as the other plate. So at best it’s acting as a (sort of) half wave instead of full wave rectifier, and the PSU isn't outputting as flat DC as it was designed to. KT150 is generally extremely robust on many parameters, but (total guess here) the danger is vastly exceeding the screen grid’s power capabilities and melting it at some point. You might get away with it...for a bit.

Hopefully someone with actual tube circuit knowledge can clarify.

Just to reiterate, the problem with wiring up a the KT screen grid like a "plate" here is going to be power dissipation - the fine wire obviously can’t handle power like a solid plate. We know that plates glow orange or red when they’re over-driven - imagine what happens to grid wire!

It’s amazing the tube can take it like this, even for a short session - but I admit I’ve made some serious mistakes and quite often the tubes (somehow) weather the storm.

Another anecdote: My VAC Master preamp has a switch to allow running 12DJ8 tubes in place of 6DJ8. The 12DJ8 uses fully twice the heater voltage. Once I had the switch on 12V for those tubes, and rolled in some nice NOS 6DJ8, but forgot to change the switch. Ran that for like 20 hours - it sounded great! Next time I looked in there, the little 6DJ8 were GLOWING LIKE THE SUN. Haha. Corrected my mistake, and tested the tubes. They’d lost ~ 20% of their transconductance since before the incident, but still tested "good", and still matched! They were NOS Siemens E88CC (nice tubes). Yes, I think they probably sounded a bit sweeter driven to molten hot temp lol. The preamp was perfectly fine too (this was years ago).