First Marshall build - JTM45 content
- Thread starter RiverRatt
- Start date
You guys are killing me. Those would cost half as much as I already have in the amp.
Got some new glass courtesy of George at tubeaudiosupply.com! I’m scared to put them in the amp since 2 of those GT KT66C tubes died a spectacular death at totally different times when the amp was dimed. It didn’t follow the socket either. I’m about 99% sure it was a problem with the tubes, confirmed by my amp tech and several people online who I trust.
Anyway, here they are. Not smoked glass but still better than any CP tubes.
Anyway, here they are. Not smoked glass but still better than any CP tubes.
This is one of the tubes that failed. Both tubes failed outside the envelope. The copper leads started arcing inside the glass where they go to the base pins. It looked like a little sun formed and got hot enough that the vacuum pulled air in through the base. You can see the glass melted and formed a little pipe where the air came in. I was watching the amp and it started arcing while my friend was playing. He had both channels dimed and it would arc when he really dug in hard playing lead. I have a video. But the dealer I bought the tubes from was condescending and blamed it on my “homebrew” amp. I got so pissed I told him just to keep his damn tube. I didn’t even mail it back for an exchange. I’d rather have the dead tube as proof.
Im247frogs
Well-Known Member
That wiring is very decent, dude. Good job.
Im247frogs
Well-Known Member
Yeah, it's not supposed to do that...
Thanks, I appreciate that. I’m gonna see if I can kill my old 5881’s in it before I put the GEC tubes in.
Chris-in-LA
Well-Known Member
What do you think caused the arcing? You might want to check all your b+ voltages before you put the tubes in. I’d also check the tube sockets for signs of arcing. Let us know what you find.
Way ahead of you. The plate voltage is a solid 447VDC and the screens are a few volts under that. Tubes are biased around 60%. I honestly believe it’s just a batch of bad Chinese tubes. After looking at them a while, I’m pretty sure that the leads from the plate and screen are where the arcing occurred, because they are way too close to each other. There’s no sign at all of arcing on the socket. I’m using old Bakelite sockets which would boil if they got that hot.
Check the left side of that tube in the first pic. See how close the leads are where they go through the glass? That’s exactly where the burn through happened on the other GT KT66. The bottom photo is the GEC for reference.
Chris-in-LA
Well-Known Member
Well, good luck with your new tubes.
Clayton Custom Cabs
Active Member
I'm curious which of your builds do you use the most?
Chris-in-LA
Well-Known Member
Probably the 2204 clones. I have 3 of them, all have good OT’s in them and sound great at living room volume. I rotate them. Since I do most of my playing at home, they get the most use.
Thanks! I was playing through it earlier in stereo with a weird amp I made that sounds decent. The first tube on the weird amp is an EF184 pentode and the socket is not making a good connection. The sound cuts out completely and I have to wiggle the tube to get it back.Where there's a will, there's a way!!!!!!!!!
Fantastic work!!!!!
I realized that I’m thinking of the JTM45 as if I just happen to own it, not that I built it. It behaves itself and does what it’s supposed to do. I don’t have to wiggle tubes or whack the chassis to get it going.
I’ve thought about doing a PPIMV mod on it, but I’m kinda afraid to touch it. The Greenbacks are broken in and it sounds so good. I have a bunch of pedals, from a clean boost to full on Brown Sound. I never thought it would be as versatile as it is.
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