JTM-45 build help

miter53 · Post by **miter53** » Sun Oct 04, 2015 12:07 am

I got this completed today, and it worked great for 10 minutes! I started off at lower volumes, and then I turned it up to about 7, turned up the guitar vol, and it suddenly went dead with a blown HV fuse. I went back, removed all the tubes, re-checked my wiring, which seems to be fine. I went back through putting in the rectifier, checking voltages, putting in the pre tubes, and checking voltages again. Everything looked good at this point. I put in the power tubes, and when I hit stand-by, I got a very loud hum. I quickly shut it down, but the mains fuse had blown. More or less I am following the Metro instructions, with Heyboer iron. I'm getting 90-100 ohms between the OT primary center tap and the legs and about 500K between the primary leads and ground. I've just got the one pair of KT66s and I'm thinking of putting some less expensive 6L6s in their place to test tomorrow when I get some more fuses. Any advice would be appreciated.

axeman · Post by **axeman** » Mon Oct 05, 2015 4:50 pm

First of all, I think your amp is a master piece. Clean work like mine haha. Remove the tubes then turn it on, then take it off of stand by. If the fuse blows then the PT is bad.

danman · Post by **danman** » Mon Oct 05, 2015 5:28 pm

Beautiful amp! It sounds like you may possibly have a bad tube. Do what axeman suggested and power up without power tubes first. If the fuse holds when you switch it out of standby, get a quick reading of the negative bias voltage at pin 5 on each power tube socket. If the bias voltage is within normal range and all other voltages read good, you should be safe to try a new set of power tubes.

miter53 · Post by **miter53** » Mon Oct 05, 2015 6:27 pm

It looks like one of the KT66s is shorted between pins 2 & 3. I don't have any more of those, so I tried 6L6s and EL34s after confirming that all the voltages checked out without the power tubes in place. With the impedance switch on 16 ohm to better match these tubes to the 8k primary I've wired (on the advice of others), I'm not blowing fuses, but the output is extremely attenuated (barely audible) and distorted (completely fuzzed out). Switching back to 8 ohm, the output seems normal (loud!!) and clear, although with more hum than I would like. It's my understanding that an anode/heater short is pretty unusual. I'm wondering if the short may have damaged the OT. Here's the wiring diagram I'm working with:

danman · Post by **danman** » Mon Oct 05, 2015 7:18 pm

Are all of the voltages, including filaments, reading normal at the sockets? Check the socket thoroughly for any traces of carbon left from the arcing. It can become a conductive path and cause the flashover to occur again.

miter53 · Post by **miter53** » Mon Oct 05, 2015 8:40 pm

My last readings with EL34s in place:

Sockets look good...

vh junkie · Post by **vh junkie** » Tue Oct 06, 2015 7:56 am

Some better pics of the wiring at the impedance selector would be good. Do reality check on the OT wiring... be sure that none of the Primary wires show a connection to any of the Secondary wires and vice versa. The secondaries should show small impedances to each other... same for the primaries. We want to be sure that the OT was not mis-manufactured.

What voltages are you reading across the bias sense resistors? Also, might want to change the bias feed resistor if need to get a more negative bias voltage might want to get to move more towards -50v.

miter53 · Post by **miter53** » Tue Oct 06, 2015 11:22 am

Here it is before I disconnected all the secondary wires!

It's not clear in the photo, but the center terminal is completely clear of the terminal behind it.

The primary has several unused taps, so I trimmed some length off them and heat shrank the individual wire ends. They are neatly stashed under the board. I pretty confidant they are not shorting, but I'll pull the board if it's necessary.

neikeel · Post by **neikeel** » Tue Oct 06, 2015 12:54 pm

I did not ry that switching with my RS OT - just fixed for 16ohm with 6.6k primary, there is another switching diagram somewhere, can't find it at the moment, I thought Brian Wallace did one with a large wafer switch?

miter53 · Post by **miter53** » Fri Oct 09, 2015 3:32 pm

Up and running now! Got a new set of KT66s, and emailed Heyboer about the OT. They informed me the wiring diagram I was given had the brown and green leads reversed, so the 8R winding was bucking the other two windings when in the 16 ohm position. I got it all straightened out last night, and everything checked out. I sounds awesome. Thanks for the help all...

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