Metro JTM 45 build Start up questions

Get support and show off your MetroAmp JTM 45 kit builds.

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kadswan
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Metro JTM 45 build Start up questions

Post by kadswan » Wed Jun 03, 2015 6:10 pm

Hello,
I'm Ken Swanson, and I am new to the Metro forum, but have been a long time lurker and admirer of these wonderful amps. I just finished my metro build, and I am getting some very high readings on V1, V2 and V3. I'm a bit leary about putting in the power tubes and firing it up before I get some advise on the high voltage readings. I've gone through the first two steps of fire up. Everything checked out normal when I put the rectifier tube in, but when I put the preamp tubes in, and took it off of standby, my voltage readings were as follows.
V1 pin 1---240v
pin 3---1.8v
pin 6---238v
pin 8---1.8v
V2 pin 1---196v
pin 3---1.3v
pin 6---354v
pin 8---197v
V3 pin 1---267v
pin 3---49v
pin 6---257v
pin 8---49.4v
I know that some of the readings are within range, but the high ones are well over 10% higher. I am using a Classic tone #40-1804 power transformer, and a classic tone choke. The output transformer is a Chris M-----. Do these look like I should proceed, or do I need to do some trouble shooting first? Any help would be greatly appreciated. And I will post pics as soon as I figure out how to. Thanks in advance

kadswan
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Re: Metro JTM 45 build Start up questions

Post by kadswan » Wed Jun 03, 2015 10:11 pm

Quick follow up. I went ahead and installed the power tubes. When I take it off of standby, I get an extremely loud buzz or hum, along with a motor boating, and then it blew the HT fuse before I could turn it off. I will get pics posted tomorrow. I'm really chasing my tail here.

danman
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Re: Metro JTM 45 build Start up questions

Post by danman » Thu Jun 04, 2015 5:12 pm

Did you notice if the power tubes were starting to redplate before the fuse blew? Did you have the bias pot adjusted for the maximum negative voltage before inserting the power tubes and powering up the amp? Some clear photos of the turret board and wiring will help us spot any problems.

vh junkie
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Re: Metro JTM 45 build Start up questions

Post by vh junkie » Thu Jun 04, 2015 9:05 pm

1) Don't put the power tubes in until you post a full voltage chart including the power tube readings
2) Be detailed about which PT you are using
3) Post pics
"With all due respect, sir, you're beginning to bore the hell out of me."
- Gunny Highway

kadswan
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Re: Metro JTM 45 build Start up questions

Post by kadswan » Fri Jun 05, 2015 8:20 am

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kadswan
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Re: Metro JTM 45 build Start up questions

Post by kadswan » Fri Jun 05, 2015 8:30 am

I have posted some pictures. Let me know if I need to post different angles. The power tubes do not appear to be redplating, and I have the bias pot turned fully clockwise. I thought I did list the transformers I was using, but here they are again
The power transformer is a ClassicTone JTM 45 upgrade transformer. Part #40-18054
The choke is a ClassicTone #40-18058
The output Transformer is a M e r r e n, Drake replica.
I cannot get voltage readings off of the power tubes because the amp blows the HT fuse, when I take it off of standby.
I did list the voltages I was getting with the preamp, and rectifier installed. I'm really at a lose!! I basically completely rebuilt this amp last night, and am still getting the same result. I know I'm missing something here, but can't for the life of me pin it down. I removed the circuit board, and checked all of my jumpers, and connections, and all looks good on that front. Thanks for the help so far, and I look forward to hearing more.

vh junkie
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Re: Metro JTM 45 build Start up questions

Post by vh junkie » Fri Jun 05, 2015 9:18 am

What are the black and the brown wires from the PT hooked to? It looks like the brown one is going to a terminal strip that is bolted/shorted to the chassis. Where is the black one going?

Those are supposed to be your hot wires to the PT.
Using the old (not safest) mains wiring scheme where the fuse is in the neutral line the connections should be:

AC black -> power switch -> other side of power switch to the black and brown PT wires
AC white -> main fuse -> other side of fuse to the black/white and brown/white PT wires

It looks like you have the paths reversed which might still work, but then I would expect to see the PT black wire going to the mains fuse holder, but I only see the brown one there?
"With all due respect, sir, you're beginning to bore the hell out of me."
- Gunny Highway

danman
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Re: Metro JTM 45 build Start up questions

Post by danman » Fri Jun 05, 2015 1:55 pm

You mentioned that you can take it off standby with only the preamp tubes installed and it will not blow a fuse....is this correct? If it is, do you have another set of known good power tubes to try? You could possibly have a bad tube which is causing the fuse blowing.

vh junkie
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Re: Metro JTM 45 build Start up questions

Post by vh junkie » Fri Jun 05, 2015 2:34 pm

We need to get some voltage measurements from the power tube sockets before you put power tubes in the amp.
"With all due respect, sir, you're beginning to bore the hell out of me."
- Gunny Highway

kadswan
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Re: Metro JTM 45 build Start up questions

Post by kadswan » Fri Jun 05, 2015 5:08 pm

I do have the opposite. I contacted Magnetics corp. who makes the classic tone transformer, and they told me to connect the black and brown wire together for the common to the main fuse, and to connect the black/white and brown/white for the 120 power. Do you think I have it backwards? What I did with the brown and black wires was to run them to the terminal strip, along with the one wire from the lamp, and then I ran a single wire from that to the main fuse. I did this because I didn't have enough wire from the lamp, and the two transformer wires to reach the fuse, so I thought this was the cleanest way to do it. If you think the terminal strip is shorting it out I can just solder some wire to the three existing wires and heat shrink the solder joints. One other question. The red/yellow wire is the high voltage center tap, which I have running to the HT fuse and then to ground. According to Magnetic corp, the red/blue wire on the transformer is the bias lead. I just tied that off with the rest of the extra wires. Do I need to connect it to something, or run it to ground? I didn't because there isn't this lead on the Metro transformer, so I figured it just needed to be cut and covered. I want to thank you so far for trying to help me with this mess. I really do appreciate it. I think it is very cool that you are taking your time to help me sort this out. Take care

kadswan
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Re: Metro JTM 45 build Start up questions

Post by kadswan » Fri Jun 05, 2015 5:42 pm

Danman. I do not, but I will get some and try that.
VH Junkie. I will get the measurements right now and post them.

kadswan
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Re: Metro JTM 45 build Start up questions

Post by kadswan » Fri Jun 05, 2015 6:19 pm

Here are the power tube socket voltage readings without the tubes installed.
V4-pin 2 3.5ac
pin3 499v
pin4 497v
pin5 -54.5v
pin6 497v
pin7 3.5ac
V5 are the same identical readings
rectifier pin2 494
pin4 368
pin6 368
pin8 494

If the power tubes are bad, can I try a pair of 6L6s instead of the KT66s just to test the amp until I can get some new KT66s?

vh junkie
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Re: Metro JTM 45 build Start up questions

Post by vh junkie » Fri Jun 05, 2015 6:37 pm

Given what you are saying, things should be OK as long as that stand-off terminal is not contacting the chassis. You don't need to connect the red/blue bias wire to anything. Make sure all of your un-used wires are individually insulated and not touching anything (including each other). Your voltages are too high for JTM45. You should disconnect the red wires going from the PT to the recto socket and use the orange wires instead. This should bring the voltage in to the proper range. Make sure you have a speaker cab connected before turning on the HI-V, and keep you bias pot at its most negative setting. Once you have all that done try the power tubes again.
"With all due respect, sir, you're beginning to bore the hell out of me."
- Gunny Highway

kadswan
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Re: Metro JTM 45 build Start up questions

Post by kadswan » Sat Jun 06, 2015 8:36 am

Okay, I switched the red HT wires for the orange ones going to the recto, and cleaned up some of the wiring. I ran the wires for the common directly to the fuse holder. I checked the voltages before installing the power tubes, and it did bring all of the voltages down, but they are still slightly high. I'll post them this afternoon, can't do it know as I am at work.I attached a load, and made sure the bias was set for maximum negative setting, but I'm still getting the very load hum and motorsboating coming out of the speaker. I immediately turned it off before I blew the fuse. Do you think it's the tubes, or could it be the output transformer? I have literally rebuilt this thing, and I can't seem to find what the heck I'm doing wrong. If the tubes are bad, I can't get ahold of any KT66s until next week. Can I use a pair of 6L6s just to test, until I get the Kt66s? Thanks again.
I will post clear pictures of the power transformer since I rewired it this afternoon, and get all the voltage readings up as well.

danman
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Re: Metro JTM 45 build Start up questions

Post by danman » Sat Jun 06, 2015 10:03 am

The voltages will read high until you get the power tubes installed. Give the 6l6's a quick try long enough to see if the fuse will blow or not. The tubes have slightly different dissipation ratings and biasing requirements but you will be fine to try them out.

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