First Build Blues: Mojo JTM45

[jazisded]

Hi Everyone,

I just finished building my first amp and power up didn't go as well as I had hoped. I could use a little help because I am fried from looking at it. It's a Mojo JTM45. Fuses don't blow and all the heaters light up, but when i throw the standby (no speaker attached), there is an audible vibration/buzz in the amp and the voltages are low on the plate and screen of the output tubes. They are down in the mid-low 200's so something is pulling down B+. This only occurs with the output tubes in the amp. If I power up without them in, no vibration/buzz and voltages are around 360 (still kind of low I think). Power up with no preamp tubes with the output tubes in and you get the same problem so i think it's in the output stage. I cannot feel the vibration in the PT, OT or Choke - only hear it.

Attached to a speaker when I throw the standby, i just get a loud monotone blast of sound.

I know I could tighten up the lead dress and will, but i don't think that is the problem just yet. Perhaps you can spot what I did wrong? I have higher resolution photos if you need them.

I'll do my best to get you whatever information you need tomorrow night.

[big_teee]

You may have already downloaded and read this.
If not it this booklet has a nice power up procedure for the JTM45.
Jtm 45 kit instructions - MetroAmp Wiki
The amp is a little different, they have KT66s, instead of the EL34s, but should be similar.
Download to your Hard drive and view it from there.
Good Luck,

terry

[carlg]

With the power tubes out the voltage on the power tube socket is 360? Or down at the preamp tubes? The preamp will be lower than the power amp. 360 sounds too low for a power amp. You shouldn't power up a tube amp without the speaker attached, bad for the tubes and transformer. If it makes a lot of noise having the speaker attached you can substitute a dummy load of the appropriate impedance, but you should always have a load connected.

[loudthud]

The twisted wires that go from the turret board near the controls to the grids (pin 5) of the output tubes are probably reversed. Before you power up the amp again, check the bias supply and adjust it for maximum negative voltage (about -50V) on pin 5 of the output tubes. If reversing the twisted wires doesn't stop the oscillation, disconnect one end of the feedback resistor, it's the 27K(?) that goes between the impedance selector switch and the Presence control. That should stop the oscillation.

[jazisded]

Thank you for all the reply's

I just got a few minutes to look at this now. I just started with the Metro power up guide (thanks for pointing that out), and apparently, the amp is failing the first test.

I have 118vac at the wall outlet, and 118vac at the end of the power cord. When i plug in and have the power off, the voltage on the hot side of the power switch starts at 114 and then rapidly drops to 99.1 vac and holds there.

Seems like i have to start there, but i'm not convinced that there isn't something else going on with oscillations as well.

Ground issue?

[jazisded]

28vac from neutral to ground on the AC input. yikes.

[jazisded]

Continuing, I tracked down the source of my bad voltages on the AC inlet to an outlet with an open ground. I rent in a house that was owned by an electrician, so I assumed.....

anyway, Now as i progress though the metro startup guide, i pass until i check the voltages on pin 3 and 7 of v1. I should be reading 1.6 volts and i am reading 106 volts. This is the same on v2 and v3. Gonna try and track this down tomorrow. Any ideas?

[big_teee]

Continuing, I tracked down the source of my bad voltages on the AC inlet to an outlet with an open ground. I rent in a house that was owned by an electrician, so I assumed.....

anyway, Now as i progress though the metro startup guide, i pass until i check the voltages on pin 3 and 7 of v1. I should be reading 1.6 volts and i am reading 106 volts. This is the same on v2 and v3. Gonna try and track this down tomorrow. Any ideas?

It should be Pins 3 & 8, 2, 7 is signal input on V1, & 1 & 6 should have the B+ from the 100k B+ Resistors.
I suspect you have a wiring error.
You are on the right track.
Here's a very good Layout that shows the wiring.
The Blue leads show the B+ leads on the PreAmp Tubes.
Compare this to the chart on the Power up Procedure.
http://site.triodestore.com/JTM45LAYOUT.pdf
B_T

[big_teee]

What Schematic does Mojo Follow in there So-Called JTM45?
They say it follows the 1987 Circuit.
So It seems to be a JTM45 with EL34s, but It has a 5AR4 Rectifier Tube.
So is this a JTM50?
Pictures of the Turret Board, this thread doesn't look like a JTM45.
B_T

[jazisded]

The Mojo JTM45 is not an exact replica -- you are correct - it is close to being a JTM50 with a tube rectifier.

Apologies for the typos on my pinouts in my previous post. Fat fingers. I was testing the right pins but had typed it wrong.

I still have 100volts dc on pin 3 and 8 of v1 and am trying to trace it back. The 250uf @25volt cap is labeled bipolar -- that means either side can be ground correct? I check the wiring to that tube and it appears correct as well.

The tube socket for v1 is very poorly made. 2 of the pins stick out much higher than the others - i am wondering if that socket is just bad. I do have a replacement but im not convinced that is the problem yet.

[jazisded]

All, Rookie Mistake. Didn't ground the can cap. Once i fixed that "little" mistake voltages came into line and the amp is up and running. I could not get the bias high enough on the output tubes so i have to try and sort that out in the next few days.

I played it though - quietly unfortunately as it is late. It works! Sounds alright too!

I am so happy - what a great feeling. Thank you all for putting up with me and for your help. I might have to ask for help again with bias but for now i am going to sleep a happy man.

[jazisded]

Alright, after taking a break for a few days, i installed some 1 ohm resistors across pin 1 and 8 to ground instead of the jumper that is called for in the schematic. At the end of the range of the bias pot, the lowest ma's i read are 44. Can't get it any lower and my plate voltage is 405. I think Mojo provided another resistor to try to get the bias in range, but i didn't have time to go any further with it last night. Changing the resistor to get the bias pot in range would be the correct course of action right?

The second issue with the amp is that there is still a ground buzz. It is tolerable on the normal channel, but the bright channel is so noisy it is distracting. Initially i thought it might just be the bright channel, but i just think that the brightness of the channel is accentuating the buzz. I did put some aluminum foil on the bottom of the cabinet to no avail. I did not try another outlet, but my other amps are quiet when plugged in to the same outlet. Any ideas on how to track this down?

The buzz is much worse with my hands off the guitar strings FWIW. I have not tried a humbucker equipped guitar yet.

Getting there!

[loudthud]

With the amp powered on and buzzing, flip the standby to the off position. Does the buzz cutoff immediately or does it fade out over a period of a second or two? If it fades out, the hum is comming from the heater supply or some other issue with shielding. If it cuts off immediately, it is usually an issue with grounding in the power supply.

[jazisded]

Im pretty sure it cuts out immediately but I'll double check when i get home. Am i looking for a bad solder joint or should i be more concerned with lead dress on the ground wires?

[jazisded]

I checked last night, and the sound dies out gently after the standby is cut. I guess the problem is with AC then. Should i just chopstick around the heater supply to see if i can increase or decrease the noise level?