JTM45 Not Loud

Structo · Post by **Structo** » Mon Aug 24, 2009 12:25 pm

Yes, so first try different tubes that you know are good.
Do you have another amp that you can verify if the tubes are good?

If none of that helps, then make a large print of the layout.
On my HP printer there is a poster printing setting that will print out on four sheets, then I tape them together to make a large layout.

Get a high lighter felt pen and trace each wire and connection on the layout.
Start at the power supply and move towards the power amp and preamp section.
Once you verify that it is correct, highlight that part on the layout.

Be sure to verify you have all the ground connections that are called for.
This seems to be the area that most guys have trouble with.
I would make sure the ground buss wire is grounded to the chassis near the input.
I don't like to rely on the mechanical connection of the pots to the chassis.

Check each solder joint on the amp starting at the power supply and then color the solder with a red sharpie so you can keep track of where you have been.

LJay942 · Post by **LJay942** » Mon Aug 24, 2009 1:15 pm

I'll give it a try tonight.

Thanks everyone!

LJay942 · Post by **LJay942** » Mon Aug 24, 2009 1:24 pm

One thing I can remember that I was unsure of.

Should the Presence pot be connected to ground? I have the capacitor between legs 2 & 3 but I do not have a lead going to ground. From alot of the effect pedals that I have built if a grounding issue occurs. Volume and tone can be lost instantly.

The only pots that should not be connected to the ground wire on the back of the pots are the Treble and Bass correct?

spaceace76 · Post by **spaceace76** » Mon Aug 24, 2009 2:03 pm

correct, but the lead on the side of the cap that goes to pin 3 is also attached to ground.

check tubes, as has been stated. a new amp is a bitch to troubleshoot if it's not the problem, so always rule out everything else first, guitar, cab, tubes, cables, etc.

does the problem happen on both channels?

LJay942 · Post by **LJay942** » Mon Aug 24, 2009 5:36 pm

spaceace76 wrote:correct, but the lead on the side of the cap that goes to pin 3 is also attached to ground.

check tubes, as has been stated. a new amp is a bitch to troubleshoot if it's not the problem, so always rule out everything else first, guitar, cab, tubes, cables, etc.

does the problem happen on both channels?

Doh. I know I don't have a ground running there. Does it matter if it is the lead from the capacitor? so I would have it attached to post 2 & 3 then have a wire coming from post 3 attached to the ground wire running along the pots.

Could this cause my low ouput?

Mars Hall · Post by **Mars Hall** » Mon Aug 24, 2009 8:38 pm

What is the plate voltage, pin 3 of either output tube to ground?

LJay942 · Post by **LJay942** » Mon Aug 24, 2009 9:32 pm

Mars Hall wrote:What is the plate voltage, pin 3 of either output tube to ground?

382v DC V4
383v DC V5

According to the instructions they should be 400v could this be the problem?

spaceace76 · Post by **spaceace76** » Tue Aug 25, 2009 4:37 am

it doesn't strike me as a huge problem, but it is slightly irregular.

does the presence control work as it is now? it could be that because it's not hooked up right, the presence is fixed at the full off position, which would make the amp sound pretty dull, and the lack of treble might reduce the perceived volume.

i don't think it's the source of your issue but it couldn't hurt to hook it up as per the instructions.

Mars Hall · Post by **Mars Hall** » Tue Aug 25, 2009 6:05 am

LJay942 wrote:
Mars Hall wrote:What is the plate voltage, pin 3 of either output tube to ground?

382v DC V4
383v DC V5

According to the instructions they should be 400v could this be the problem?

The plate voltage will affect how much headroom the amp has. 382v does seem low, reading these threads most report plate voltages in the 420's.

I recently had a problem with my plate voltage being too low on my jtm45ri. It did affect the amps volume output negatively. It turned out I had the choke wired wrong. The plate voltage was running through the choke.

Here's the thread, maybe this will help.

http://forum.metroamp.com/viewtopic.php?f=15&t=27258" onclick="window.open(this.href);return false;

It could also be the output tubes not drawing enough current. It might be worth trying a new set. Have you check your wall current?

guilds100 · Post by **guilds100** » Tue Aug 25, 2009 2:24 pm

I just completed a trainwreck clone that had the same volume no matter where you set the controls. It turned out to be a bad ground connection from the second tube to ground. The specified resister was 10k, I measured 1 meg through the resister to ground. I resoldier the connection and it took right off.

P.S. If you were ever thinking of building a trainwreck clone and you like marshalls DON'T. It is the worst sounding amp I've ever built/heard. Blaahhhh.

LJay942 · Post by **LJay942** » Tue Aug 25, 2009 5:31 pm

Everything looks alright to me. But the voltage is running at around 350-360v now which is concerning me. I don't want to keep using it to debug as I'm worried about ruining a transformer. I think I'll give Metro Amp a call tomorrow and see if it is possible to pay and have one of their techs look at it. I would rather pay someone who knows what they are doing to check it out versus me ruining something due to my lack of experience.

guilds100 · Post by **guilds100** » Tue Aug 25, 2009 5:49 pm

2 things you may want to check before sending in the amp for service. First is that jj rectifier tube. In my jtm45 build the jj i used took a dump after 6~7 hours of use, though it arced and blew fuses rather than lowering the voltage. Also you may want to build a current limiter to see if there is some sort of short. There should be directions somewhere on the site on how to build it. In a nut shell, it consists of a light bulb (75-100w) wired in series with your amps power so the ac current your amp sees is limited to what the light bulb passes. The brightness of the bulb shows how much current the amp is consuming. It should flash bright at startup, then dim after 2-5 seconds as the caps charge. You may having something in the b+ line that is going to ground through a decent sized resistor so it isn't blowing fuses but is dragging the whole system down.

LJay942 · Post by **LJay942** » Tue Aug 25, 2009 6:06 pm

guilds100 wrote:2 things you may want to check before sending in the amp for service. First is that jj rectifier tube. In my jtm45 build the jj i used took a dump after 6~7 hours of use, though it arced and blew fuses rather than lowering the voltage. Also you may want to build a current limiter to see if there is some sort of short. There should be directions somewhere on the site on how to build it. In a nut shell, it consists of a light bulb (75-100w) wired in series with your amps power so the ac current your amp sees is limited to what the light bulb passes. The brightness of the bulb shows how much current the amp is consuming. It should flash bright at startup, then dim after 2-5 seconds as the caps charge. You may having something in the b+ line that is going to ground through a decent sized resistor so it isn't blowing fuses but is dragging the whole system down.

I'll give that a go tonight before calling Metro in the morning.

I'm assuming George/Metro Amp will allow me to send the amp in for them to take a look at correct?
I planned on sending it whether or not just to have them look over it and make sure it is "road worthy" as I plan on getting alot of use out of this baby

LJay942 · Post by **LJay942** » Fri Aug 28, 2009 4:47 am

Just wanted to report back and say that it was in fact the capacitor not being grounded correctly on the Presence pot. Are correct, I should really read the manual for my multimeter

Mars Hall · Post by **Mars Hall** » Fri Aug 28, 2009 7:21 am

LJay942 wrote:Just wanted to report back and say that it was in fact the capacitor not being grounded correctly on the Presence pot. Are correct, I should really read the manual for my multimeter

What is the plate voltage after the correction?

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