Hi jmac,

We'll not jump on you as you're a newbie.

I think the answer is "Yes, you can use 274B rectifiers in the Mk IV." Though the tubes may be a bit stressed and as a result not last as long as normally they would in a circuit more designed for them.

There is a 274B made by Valve Audio (and I believe Shuguang) which is electrically more like a 5U4G rectifier and works quite well in the Mk III and most likey the Mk IV. They don't seem to be stressed by the circuit; at one time Steve was sending these with CSP2+ preamps.

I recall it similarly as Lon. If you search Steve Deckert's posts for 274B, you will hopefully find more details, but the traditional 274Bs, being expensive and vulnerable to the circuit electronics, they seem a little sketchy to me. Some folks have had problems with the Valve Art 274B flashing in the MKIII. I have had Valve Arts in the MKIV, but with EL34 or Psvane 5881s. I think the fit is too tight with the Tungsol KT66s. They have an interesting "big" open feeling. Also, as with the MKIII the 40s and 50s RCA 5U4G-STs tend to work very well and fit with the bigger power tubes.

I have not tried the Sylvania 5931s in the MKIV yet, but in the MKIII they sound very accurate, open, and refined, but almost to the point of "too good" for me. Alternately, I find most RCA-STs (and the Valvo 274) less rigid with more textures, harmonic and ambient info.

Rectifiers are great to play with though! I have a ridiculous number of them!