JTM 45 Valve choice

HHwarner

Hi all. I am thinking of putting new power valves in my Marshall JTM 45. Im a bit confused at what valves to get. Marshall say 5881 but isnt that the same as 6L6GC? 5881 seem hard to find. What would be a good valve to go for. Im looking for early breakup and a full distorted tone. Another question, when biasing would I use the amps rated output of 30 watts (15 watts per valve) or the 23 watts rating per valve (for 5881). Biasing at 60% dissipation should I use 15/423*0.6 = 0.021 or 23/423*0.6 = 0.032 for 5881 valves or should I use 30/423*0.60 = 0.042 for 6L6GC. There is quite a difference. I know the valves would handle the higher but what about Trannies etc as the amp is rated at 30 watts. 

Iv also read that plate voltage must be below 400v for some 5881 (Tung Sol). My plate voltage is 423v. Would 6L6 be better suited? many thanks. Scott 

ICBM

Use KT66s - you can get these again now, they have been reissued - and although not exactly the same as the originals, are closer than the 6L6/5881 types which were what was available to Marshall when the reissues first came out. Bias to about 15W per valve or 35mA - it's not the amp's rated power output that matters for that, it's the operating point, and that seems to be a good setting to go for. You can try slightly hotter, although that risks shortening the valve life.

The Valve Arts KT66s seem to be the preferred choice at the moment.

matt1973

I played a JTM45 with KT66s and it was a lovely tone.

HHwarner

Thanks ICBM. So KT66. I'll try some.

nickb_boy

If you're going to check out kt66s the Gold Lion offering are supposedly the closest modern valve on a scope to the original kt66s.

I've got a pair in my Satellite Barracuda and they are incredible!

HHwarner

Took delivery of a pair of Gold Lion KT66, advertised as new, matched and tested. Tried to bias them and there is minimum 12ma difference between the 2 and the lower one wont stabilise. Also crackling, sounds like arcing when i sound checked em. Got the hassle of trying to get cash back now as I bought em after the ebay listing had ended. Only saving grace is I paid via PayPal. 

knuckleberryfinn

Get a refund and then go to Karltone.  He's great.

robinbowes

+1 for @karltone.

BowynMadley

I've got a pair of gold lion kt66s in my dualist. Got them from hotrox and they are awesome. They definitely sound better cathode bias too. Not that that helps in any way
Bo

jpfamps

In the very first JTM45/ Bluesbreaker reissues the power valve sockets are too close together for KT66s, so I assume you haven't got one of these.

I've had very good results with Golden Lion KT66s and these are my KT66 or choice.

If you wanted to go with 5881s there are now a few more options than there where, but I'd go with KT66s.

ICBM

Until reissue KT66s became available, I always thought they sounded best with those Chinese 'coke bottle' 6L6s, so those could be a good cheap alternative if you can still find them, or if it's an early one with the close-spaced sockets as jpfamps said - or an early Bluesbreaker reissue, which have too shallow a cab to take KTs.

It's an ideal use for them since they aren't really sturdy enough to hold up well in normal 6L6 amps (eg Fender Twins).

ESchap

I'm with @ICBM on the Valve Arts KT66's they are the closest I've found to genuine GEC's, which it has to be said, are in a class of their own.

John_A

gold lions are great, but either 6l6s or el34s sound good in a jtm45