I just replaced only V1 with a TT 12AX7 / ECC83 Classic (https://www.tube-town.net/ttstore/tt-12ax7-ecc83-classic-v1) but it made a big improvement: more defined and fatter sound.
A pair of nice matched Tesla E34L's in the power section, two matched Tung-Sol's in V1 and V2, and a JJ in the PI. ---- For some reason a couple of months ago, my SV20H just didn't seem to sound right. I think one tube was bad. But at the time I didn't have a lot of time, and I did have a fair amount of inventory. So I just swapped them all. The amp came back to life and still sounds fantastic. I use it every day and still with those tubes. It has a fantastic 70's classic rock plexi crunch when plugged straight in, and shapes well with a pedal or two. I bought the tubes from both Antique Electronic Supply (TubesAndMore.com), and Tubes Unlimited.
I have two sets of the original power tubes/valves from Marshall. I believe they are Shuguang (not sure on spelling) but are likely Chinese, which have great tone, but are short lived if you play hard or for hours at a time. All four have separation issues where the intense heat and vibration (combo VM) made the glass separate from the base. I even tried to super glue them back together but the heat is just too much. The leads from the base (pins) to the actual electrode eventually fail. They might light up, but no one his home or so I've learned the hard way. The factory claw holder only hold the base and not the glass so I suggest the older style spring retainers for the combo. Actually, the full size spring retainers are even recommended in any head version too. Vibration is a different topic and not easy to solve but there are different kinds of rings you can buy that go over the glass bottle. A simple "high temp" silicone o-ring will do in a pinch. Sorry, I got off on a side tangent, but it's all realated. Either way, the VM is worth finding solutions for and keeping in the amp stable. Rock On !
This is really good info since I'm looking real hard at the SV20H. EL84's are can take way more punishment than KT66's, even though the "big bottles" do deliver a very special tone when heated up and cranked !
ANYTHING will be better than the stock tubes. Right now I have Chinese tubes, Ruby EL34 and PM 12AX7 in it.
You have luck with Ruby tubes? I only had them once, 6L6's in my attenuator (Fryette PS2) and they failed in about 2 months.
I replaced all my tubes with diodes and transistors! CP Mullard here. Sounds smoother with more growl, if that makes any sense, or more Marshall like IMHO.
I still have the stock tubes in it. Now that we're talking about it, I probably should think about replacing them, it's been played quite a bit up until the pandemic hit.
Tried some GT mullard reissues which I really like - both in the DSL and my 50 watt plexi. Didn't like them in the SV so I put the stockers back in. That said I took the GTs out of the DSL because one of them had gone a little bit microphonic. I've since bought some new ones so I suppose I ought to throw them into the SV just to see. I don't hate the stock power tubes. They sound very 'plexi' to me. Haven't tried any other preamp tubes.
The VM Combo KT66's get very hot after a good hour of playing hard. I don't have (but want) the SV20h so I can't comment on that just yet. My Marshall Artist 30 Watt Head (not the Artiste) has EL84's and they don't really get that hot and I push 'em real hard and sometimes for over 4 hours.
Stock JJ el34s for now i did use different brands didnt seem that much different. Just dropped a Tad 12ax7 in the 800 combo it smoothed the highs out better and a little tone over the ruby Chinese. A Ruby Chinese v3
