Blowing Main Fuse

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galenwhite62
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Blowing Main Fuse

Post by galenwhite62 » Thu Apr 09, 2015 8:44 am

Hello all,
I’m new to the forum and am just finishing up my first build on a Metro JTM45 kit. I am having issues during the testing procedures and am blowing the 3A main fuse when turning the standby on. The funny thing is I had it working initially, but was getting high readings from the PI V3 pin 6 over 400V. I started looking at the forum and retracing my work and found that the Music cap on the presence control wasn’t grounded. After I grounded and retook the initial readings from the testing procedure, the mains started blowing when turning the standby on. The music cap was the only thing that I changed. I have not installed the power tubes and only have the rectifier and preamp tubes in. Initial readings:

Line voltage 123V AC
All heaters coming in at 3.45AC
V6 Pin 2 and 8 is coming in at 5.38AC
Standby voltage at 361
V4 and V5 Pin 5 is at -53

I installed a Classictone PT and OT following the layout from triode electronics

Wondering if any one has any suggestions and I’ve included a few pics to provide a detail of my layout. FYI: I unmounted the board just to check the underneath, everything looks okay.



Thanks,
Galen
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vh junkie
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Re: Blowing Main Fuse

Post by vh junkie » Thu Apr 09, 2015 4:57 pm

1) If you have an pic of your under board wiring, we could double check that.
2) Post a voltage chart of the voltage on each of the tube socket pins (see chart in Metro instructions)
3) Cannot really see what is going on with the PT primaries, but if that is a 40-18054, the dual primaries need to be wired in parallel. See the second page of the specs.
http://www.classictone.net/40-18054.pdf
"With all due respect, sir, you're beginning to bore the hell out of me."
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galenwhite62
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Re: Blowing Main Fuse

Post by galenwhite62 » Fri Apr 10, 2015 4:58 pm

1. I checked under the board and everything is wired correctly and I also checked continuity from each turret to the pin and all checked okay. when I reassembled the JTM45 stopped blowing fuses and I was close to the suggested readings (somewhat) so installed the KT66's. The bias for each tube is not close (38 mv on one and 43 mv) on the other. Also there is a distinguishable high frequency pitch coming from the tubes. When I disconnect the cap on the presence control the pitch goes away. However there is no output to the speakers at all, nothing.

2. I'm using Classictone PT 40-18033, OT 40-18039 and a 20hy choke 40-18059. I followed the Triode amp layout to determine colors, but followed the metro instructions to complete the connections. The biggest difference is how the choke hooks up.

3. Attached are the readings that I am pulling from the amp.

Any advice is greatly appreciated and thanks VH Junkie for your help so far!!!
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JTM45 VOLTAGES 20150410.pdf
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vh junkie
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Re: Blowing Main Fuse

Post by vh junkie » Fri Apr 10, 2015 5:53 pm

1) I can't see where your speaker jacks are grounded?
2) Does the buss wire on the back of the pots have a ground wire going to the chassis anywhere? It may be that the ground there is being accomplished by the pots making contact with the chassis.
"With all due respect, sir, you're beginning to bore the hell out of me."
- Gunny Highway

danman
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Re: Blowing Main Fuse

Post by danman » Fri Apr 10, 2015 6:02 pm

The high frequency noise may be cause by the OT primary wires being reversed on the power tube sockets. It happens quite often in new builds. Just swap the wires from pin 3 of each power tube socket and try the amp again. If the noise is gone then you have them installed the correct way. As far as your bias readings, 5ma apart is not bad at all and should not cause you any issues. In a hifi amp you would like to see a perfect match but in a guitar amp, a few ma will not be noticeable. Be sure to check the items that "vh junkie" mentioned also.

danman
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Re: Blowing Main Fuse

Post by danman » Fri Apr 10, 2015 6:15 pm

I wanted to mention that instead of desoldering and swapping the pin 3 wires at the power tube sockets, you can simply unsolder the purple NFB wire at the impedance selector and then turn the amp on and give it a try. If the squealing stops then you will need to swap the primary wires at the sockets. If the amp is still squealing then you have a different issue and will not need to move the wires. This is a quick easy way to check first. I also noticed that you may be missing a ground wire from the sleeve of the output jack to ground. I see the yellow common wire from the OT but I do not see the ground connection.

vh junkie
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Re: Blowing Main Fuse

Post by vh junkie » Fri Apr 10, 2015 6:33 pm

Checking for the black and red wires being reversed should come after we get some sound at all out of the amp. I have not seen that the ClassicTone trannies suffer from the reversal as much as the Marstran/Heyboer. Check the speaker ground first.
"With all due respect, sir, you're beginning to bore the hell out of me."
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galenwhite62
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Re: Blowing Main Fuse

Post by galenwhite62 » Fri Apr 10, 2015 9:04 pm

You guys were right in that I forgot to ground the speaker wire. I connected a ground and powered up. Now quiet as church mouse but still no output to the speakers. Is there any way to test the caps on the board? I did install a a lower recommended 10v 330uf and wonder if that may be an issue.

vh junkie
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Re: Blowing Main Fuse

Post by vh junkie » Fri Apr 10, 2015 10:03 pm

Nah... that 10v is not too low for that 330uf cathode cap... we need to keep looking...
"With all due respect, sir, you're beginning to bore the hell out of me."
- Gunny Highway

galenwhite62
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Re: Blowing Main Fuse

Post by galenwhite62 » Sat Apr 11, 2015 8:38 pm

It’s alive!!!!! The biggest issue that I’ve found is the grey matter between my ears. After much thinking and looking. I was perplexed as I thought the circuit was sound. I retraced every step, checked every solder joint, and tested continuity and resistance on everything. Thanks for the catch, EV Junkie on grounding the output jacks as that would’ve had me chasing my tail for a while too. But my issue was the impedance selector. I didn’t think to realize it was a 6 position switch and had the selector positioned on one of the unused terminals , 180 degrees from what I thought (look at the red line dummy). After finding that out I suddenly realized how good someone could feel about being so stupid. Lol!!! However, on my journey, I also discovered that one of the JJ gold pins I purchased for the project was bad and was part of my fuse issue.

I’ve installed Mullard 12AX7 reissues in V1 and 2 and Tung Sol reissues in V3-5 and a JJ for the rectifier.

I can’t get my bias above 42 and am wondering if I can swap the resistor out to increase bias??? My standby voltage is 361 and based on that I should be biasing at 52 mV, right?

Initial thoughts are Wow!!!, this is a harmonically rich amp. I’m playing through a tele and it had some noise but not bad. I’m sure it will clean up once I get it in a shielded cabinet. I love fender and Marshall amps and have a Fargen modded DRRI and ’78 JMP and feel this jtm45 clone falls somewhere between those with much more headroom.

I want to thank everyone for their help and am awed by how quickly you responded to my concerns.

If anyone can help me with my bias question it would be greatly appreciated.

Thanks!
Galen

vh junkie
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Re: Blowing Main Fuse

Post by vh junkie » Sun Apr 12, 2015 1:39 am

Glad you figured it out! B+ in the 360s sounds kinda low. Do you get that reading with the amp running (off standby). Be sure your meter has fresh batteries (that can through off readings, but usually to the high side). It is kind of weird because I have been seeing posts by so many that used the 40-18033 that their B+ was too high (470+). Different recto tubes might yield different results .... which JJ are you using? Anyway, to run the amp hotter you could increase the 220k resistor that feeds the bias supply... try a 270k and see if that moves it in to the range you are looking for.
"With all due respect, sir, you're beginning to bore the hell out of me."
- Gunny Highway

galenwhite62
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Re: Blowing Main Fuse

Post by galenwhite62 » Sun Apr 12, 2015 8:02 am

You were right on the B+ voltage and after taking a reading from Pin 3 of V5 I yielded a result of 471V. This equated to 40 mV and I biased at 35.5. The rectifier tube is a standard JJ GZ34. Thanks!

galenwhite62
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Re: Blowing Main Fuse

Post by galenwhite62 » Fri Apr 17, 2015 9:11 am

Thanks VH Junkie for your help!!! The amp is biased and sounds awesome. I will get some pics and clips posted soon.

vh junkie
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Re: Blowing Main Fuse

Post by vh junkie » Sat Apr 18, 2015 3:28 pm

Very cool! Just keep playing it and let those Sozos settle in... most folks start changing things too soon let those caps break in for quite a while... then think about tweaking...
"With all due respect, sir, you're beginning to bore the hell out of me."
- Gunny Highway

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