Finally Building JTM45: Stupid Questions As They Arise

[The New Steve H]

You're right, Jelle. I put a KT66 in the wrong socket. I'm worried that the KT66 tubes will not be far enough from the cabinet. I have never built a JTM45 before, so I don't know anything about the way the cabinet fits. I told my friend to hurry up and bring it to me. I don't want to wire the whole amp up and then find out we need a new chassis.

Thanks for the positive comments. This modification was a lot of fun.

If anyone else gets stuck in this situation, I would recommend using two straight pieces of aluminum instead of a spacer that goes around all four sides. You don't need a milling machine if you use separate pieces.

Believe it or not, I decided to use the drill press and a flush-cut carbide router bit to cut the hole in the chassis. It was somewhat quicker than the mill. I attached the spacer to the chassis and rested the bearing on the inside of it, and it guided the bit. I can't recommend this as a sound practice; it's a little rough on the bit, and the chassis will jump around while you're doing it. But it worked for me.

[NickC]

.................. I'm worried that the KT66 tubes will not be far enough from the cabinet ................

I used a standard big-box Marshall clone cabinet for mine. Plenty of space for everything, including those big KT66 bottles. Also good ventilation. If you do the same, or something similar, you should be fine.

[The New Steve H]

Here's my latest question.

I'm doing the wiring on the amp. Right now I'm working on the PT. I can turn it two ways. If I turn it one way, the heater supply wires to the power tubes will be short. If I turn it the other way, the heater supply wires to the rectifier will be short. Which is best?

I'm thinking the power tube wires should be shortest, since they are closer to the stuff that makes sound. It seems like if there have to be hum-inducing fields, they should be down at the rectifier end.

[ChrisM]

Here's my latest question.

I'm doing the wiring on the amp. Right now I'm working on the PT. I can turn it two ways. If I turn it one way, the heater supply wires to the power tubes will be short. If I turn it the other way, the heater supply wires to the rectifier will be short. Which is best?

I'm thinking the power tube wires should be shortest, since they are closer to the stuff that makes sound. It seems like if there have to be hum-inducing fields, they should be down at the rectifier end.

Doesn't really matter.

I usually orient it so the primary wires are closest to the back (where the mains voltage comes in).

I'd evaluate all the wires though and pick the orientation that gives you the shortest lead length overall.

[The New Steve H]

Here's another one. Merlin Blencowe says you should put resistors on the anodes of the rectifier. I ran into this while trying to come up with a plan for diodes to protect the tube. Won't hanging all this crap--diodes and resistors--off the rectifier lead to increased hum, because you can't have a nice tightly wound set of wires this way?

[The New Steve H]

I found the 5AR4 datasheet and Googled some forums. I decided to put 10W 100-ohm resistors in line with the anodes, and I also stuck some 1N4005 diodes in there. The supply wires are twisted tightly until they get to the resistors. Best I could do. If it sucks I'll take them out.

[The New Steve H]

Here is what I did. I left a lot of wire because I don't know whether this is going to work. If it works, I'll shorten it up.

[Reeltarded]

Nice twisting!

[The New Steve H]

Thanks. I've done a lot of fishing, so I've had to twist a whole lot of wires together by hand. These came out pretty uniform.

[Cliff Schecht]

100 Ohms seems excessive.. Where did you read to use such large resistors? What is the resistance of your PT seconddary HV windings?


I've always just left these resistors off TBH and am yet to hurt any rectifiers. With that said, I tend to use the right rectifier for the job (i.e. choose my rectifier depending on the transformer I'm using).


IME the Chinese 5AR4's are pretty good (I think mine is a Sino). I've heard mostly good things about them from others too. The hi-fi guys even like them. They're cheap, reliable and I'm yet to have one fail (given you change them when they are going weak). Any rectifier will eventually fail, you just gotta keep an eye on them. The SS diodes are always a good idea, cheap protection.

[Roe]

wire the rectifier and standby like ceriatone does

[M Fowler]

Merlin certainly knows tube amp circuits but I can't think of one thing I do from Merlin Blencowe's book in my amps.

Sometimes you just need to consider some of the amp books out there as entertaining reading.

Mark

[The New Steve H]

I got the resistor value from the data sheet.

I'm not using a bypass. The guy I'm building the amp for says I have the go-ahead to get rid of it.

The main thing I'm worried about here is noise from all the untwisted, current-carrying stuff.

[Zippy]

Here's another one. Merlin Blencowe says you should put resistors on the anodes of the rectifier. I ran into this while trying to come up with a plan for diodes to protect the tube. Won't hanging all this crap--diodes and resistors--off the rectifier lead to increased hum, because you can't have a nice tightly wound set of wires this way?

I'd be concerned as well regarding having that much mass hanging off the socket - better to secure at least one end of each resistor to a short local terminal strip. Of course, if it breaks off the pin and flops around with a voltage on it, you could get some really cool fireworks!

[The New Steve H]

If I keep that stuff, I will probably anchor it with a screw and cable tie.

I kind of wonder what the next guy who works on it will think.