Blowing Fuses

[jfhudak]

Hey Fellers,

Okay, fired her up tonight, and blew the mains fuse. Fixed that (forgot the NFB wire), then fired her up again.

On Standby, no worries. When I take it off standby, it hums loudly and the output tubes glow blue. Is my bias circuit a place to check?

Thanks,

Johnny Reb

[jfhudak]

Okay, just checked it out, and I have WAY too much bias voltage. I am using the layout that Billy Batz provided, with all of the same values. What am I doing wrong?

Thanks!

Johnny Reb

[myfoot]

Check my thread here http://forum.metroamp.com/viewtopic.php?f=15&t=23934" onclick="window.open(this.href);return false;

I had to change a couple of resistors in the bias area to bring it into line . I followed the same layout as you. Also double check where you're bringing the bias voltage over from the cap board. There's lots of wires running back and forth there

[jfhudak]

Check my thread here http://forum.metroamp.com/viewtopic.php?f=15&t=23934" onclick="window.open(this.href);return false;

I had to change a couple of resistors in the bias area to bring it into line . I followed the same layout as you. Also double check where you're bringing the bias voltage over from the cap board. There's lots of wires running back and forth there

AHA!!! I think, in addition to the two errors I found, that your thread might be my issue. I have a 220, which I think I will use in place of the 150 on the diode. Right now I have a 68 on the bias pot.

I'm going to keep going on that track. BTW, is it okay to fire this up without power tubes in it? What's the protocol for that? This is my first build and I really don't want to fry my first set of KT66s.

Thanks!

[jfhudak]

Fuck me. I'm stumped. I did notice that there is a slight hum even when she's on standby. My B+ is 523 at the Standby switch.

I'll have to look at it tomorrow. It's beer-thirty.

[Flames1950]

How loud is the hum? How are you biasing, are you using the 1-ohm resistors on the output tube sockets?

My first 45/100 clone was plugged into a load resistor for testing, and when I'd hit the standby no matter what I did the bias current would skyrocket with a rig using the 1-ohm resistors. Since I wasnt plugged into a cab, I didn't recognize at first that the output tranny leads to the power tube sockets were swapped, because I couldn't hear the telltale monstrous hum with no speakers.

If you disconnect the NFB (or simply lift one end of the NFB resistor off its turret) does the hum go away? If so you may need to swap the output tranny leads to pin 3 of the output tube sockets.

[jfhudak]

How loud is the hum? How are you biasing, are you using the 1-ohm resistors on the output tube sockets?

My first 45/100 clone was plugged into a load resistor for testing, and when I'd hit the standby no matter what I did the bias current would skyrocket with a rig using the 1-ohm resistors. Since I wasnt plugged into a cab, I didn't recognize at first that the output tranny leads to the power tube sockets were swapped, because I couldn't hear the telltale monstrous hum with no speakers.

If you disconnect the NFB (or simply lift one end of the NFB resistor off its turret) does the hum go away? If so you may need to swap the output tranny leads to pin 3 of the output tube sockets.

That sounds exactly like what I am experiencing. The hum is loud as hell, and the bias current skyrockets, just as you describe.

I went through the rig again tonight and I have all of the wires hooked up right. So it's got to be something like that. In fact, I'm going to reverse those wires and try it again!

Will report back. . .

[jfhudak]

Hey Flames,

The first diagram in this post is how I have mine wired:

http://forum.metroamp.com/viewtopic.php?f=32&t=18948" onclick="window.open(this.href);return false;

I'm trying the resistor trick now with the NFB. Will report back.

[jfhudak]

Hey Flames,

I tried reversing those wires, and the humming got worse. Put them back, and it's still bad.

What happens is the tubes immediately glow blue. I have a voltage chart, what's the best way to check the voltages? Can I do that without tubes in the amp? I don't want to damage the tubes and I can't take it off standby to measure the pins.

I have a 68k resistor on the bias pot and a 150k on the diode, but I don't think that is the issue, since the bias voltage jumps like it does.

[myfoot]

Did you ground the cap board? I almost missed that one myself. I grounded mine on the standoff for the cap board closest to the transformer.

You can check the voltages without the power tubes in. They will probably read somewhat high. Mine were roughly 10% high until I put the tubes in and then came right down to spec.

[jfhudak]

Did you ground the cap board? I almost missed that one myself. I grounded mine on the standoff for the cap board closest to the transformer.

You can check the voltages without the power tubes in. They will probably read somewhat high. Mine were roughly 10% high until I put the tubes in and then came right down to spec.

I did, but you know what I neglected to do? Ground the bus on the pots. I used PEC pots, so I couldn't run the bus wire across the backs of the pots. I'm thinking that is at least part of the problem.

I'm going to check that cap board again too!

For the record, I appreciate everyone's help tremendously!! Y'all are great. Will report back when I have grounded that bus wire.

Johnny Reb

[jfhudak]

I GOT IT!!! It was indeed the bus wire not being grounded!!

I'm still putting this thing through it's paces but I can already tell the following:

- I love it
- I love it
- I love it

THIS is the tone I needed! Stands to reason that a blues-rock player would like a blues amp!! It grinds when it needs to and it sings really nice. I can get AC/DC or Zep, or Hendrix, or any of that stuff. I have it on 3 with no attenuator and it is just a glorious tone.

The main thing I like about it is that the notes all have some real nice girth to them. When you nail it, it grinds but it doesn't break up like a metal panel. It has a percussive sound to it, like a blues amp should. That, to me at least, defines the guitar sound that we all grew up loving and not the overly compressed (in my opinion), distorted metal panels.

I'll report back when I am done with some more jamming but I want to thank EVERYONE on the forum for helping me out with my very first build. This thing is the shiznit!!

[myfoot]

Glad to hear you got it sorted out! Great when it's an easy fix isn't it?

[jfhudak]

Glad to hear you got it sorted out! Great when it's an easy fix isn't it?

Amen and hallelujah brother. I was hoping it was just one wire.

This thing is badass. Just going through the high treble channel, I was able to play without an attenuator and without effects. It was loud, but in actual fact I think the volume was much more gradual than my metal panel was.

I put the volume on about 5, an then worked with the guitar volume and it just gave it up! It is very organic and responds very well to picking variances, which is key for the music I like.

The funny thing is that it just doesn't "seem" as loud as the other 100 watters I've owned. It's like it gives up the goods very early on the dial, and without being overly distorted. Again, not for the gain fiends out there but for classic rock, it's a total freaking machine.

There is plenty of chime, and the frequency range is very even. You can really get some good Zep tones with it, which surprised me because I would have expected more Hendrix. It sounds very AC/DC, and though I haven't put anything in front of it yet, I'm sure it takes pedals well.

The sound is all Marshall, but I hear some Fender and Hiwatt in there as well. The other thing is that it is DEAD QUIET. I'm sure this is a result of all of the fine advice I got from the members on this forum with respect to lead dress.

I expect as the caps burn in this thing is going to lose some of the sizzle, and in that event I might add a bright cap but for now it is perfect. With the volume on 5, the treble and mids almost dimed, and the bass off there isn't anything you can't get this amp to do.

Best thing I ever did! Thanks to Mike for his clips because they started me on the journey and it was worth it!!!!

[neikeel]

I'm still putting this thing through it's paces but I can already tell the following:

- I love it
- I love it
- I love it

THIS is the tone I needed! Stands to reason that a blues-rock player would like a blues amp!! It grinds when it needs to and it sings really nice. I can get AC/DC or Zep, or Hendrix, or any of that stuff. I have it on 3 with no attenuator and it is just a glorious tone.

This thing is the shiznit!!

Now the thing is - it gets better and better over the next few months as everything beds in