I was reading an interview with Peavey's project manager in charge of the 5150 amp development.
He said Peavey had a lot of new old stock Syvania 6L6 tubes in storage and those tubes were used in the early prototypes and production 5150 amps now known as "Block Letter" amps.
He said as those original Sylvania tubes dried up, Peavey ordered Chinese made Sino tubes becasue they could get mass quantities of them which were needed because tube amps were back in high demand and the industry was looking for big purchase orders of 6L6 tubes to meet demand.
The Sino tubes were hit and miss and resulted in some customer complaints.
Peavey being a large purchaser of tubes approached New Sensor who was a large importer of tubes and known for their Sovtek line of Russian made tubes. New Sensor had their Russian manufacturing contacts develop a new 6L6 tube especially for the 5150 amp line.
Some people have converted their 5150 amps to EL-34 tubes with dissapointing results. The amps sound muddy with those tubes. The high gain preamp of the 5150 likes 6L6 tubes.
I have found the Electro-Harmonix tubes do the job well. 5150 amps are very tube picky and if you are running Elextro-Harmonix tubes in both the pre-amp and in the power amp, the amp warms up; especially with a propper bias.
I have found you can dial in a pretty good sounding classic VH tone with the 5150 combo or the 5150 II head. That's because the crunch channel circuits on both are simular. You have to keep the pre gain around 3-4 and get into the power amp tubes. You need to dial in the ressonance, pressence, and EQ. There is some classic VH in these amps. You will not find it in the Lead channel but it is in the crunch channel.
I've tried sevral tube brands with hit and miss results and Electro-Harmonix work great and since the 6L6EH tubes were designed for the 5150 line of amps, then it's logical that's what to use.
I still think a used 5150 combo is a bargain if you want balls to the wall and classic rock tones. At least for humbucker equipped guitars. With the right tubes they do sound good and I have never heard of one breaking down on someone. A lot of them look beat to hell but still work.