Sylvania 6CA7 (pre ‘81) vs Philips 6CA7 (post ‘81)

[BIG Dave]

I’m looking to cop some Van Halen vibe, so I’m trying to replicate as much as Ed’s 1978 rig as possible (within reason). I recently acquired a set of NOS green font US-made Philips 6CA7 tubes, probably produced between 1981 and 1986 (Philips purchased the Pennsylvania Sylvania plant in 1981, and changed the green labelling on the tubes to blue in 1986, then closed the plant in 1988).

So my question is, was there a construction change and/or a tonal change in the Pennsylvania-made 6CA7 tubes made prior to 1981 (labeled “Sylvania”), as compared to the 6CA7 tubes made in the same plant after Philips took ownership in 1981?

Equipment for this project includes:
Amp: ‘71 Marshall JMP50, powered by a Variac set to 90 VAC
Speaker Cabinet: ‘68 Marshall 4x12 Basketweave, original 25 watt G12M speakers
Guitar: Warmoth ash body, Fender maple neck, Floyd Rose, ‘78 Duncan humbucker
Effects: MXR Flanger (original AC version), MXR Phase 90 (original Script), Catalinbread Belle Epoch (echo), and Strymon BlueSky (reverb)

[Velva]

I highly recommend blackbacks 75hz! Plus, if you are going to use a variac you have to hot bias the amp. If you do that, you must run with variac at all times (90 volts and below) or you will blow your amp. If you use a variac with the standard bias, it really just dulls the tone (IMO).

[Roe]

no need for a variac on a 50w, due to low voltages unless you have the very rare -133 power transformer