Metropoulos Forum

Okay I'm in need of some help. I've completed assembly on the JTM 45 kit, all tubes installed, all parts soldered on. So I go to turn on the amp for the first time to test the bias and voltage (I have a weber bias rite), and after I flipped the stand-by switch to on...um...a horrible loud roar erupted through the speaker cabinet and all knobs on the front panel of the knob were turned down to 0. So, obviously I screwed up somewhere, and I guess I need some pointers on where I should start troubleshooting this. I was pretty certain I had soldered everything correctly, but apparently...um, I didn't.

Thanks in advanced!

Double check which output tranny primary wire went to which socket -- the brown one to the power tube socket closest to the preamp tubes, and the red to the other socket (pin 3 on both.) Often getting those switched around will result in a howl.
The same symptoms can happen if you reverse the wires at the speaker jack end, but that would have been harder to do I'd think with all the impedance selector wiring. Still, it bears double checking as well.

Flames, the blue wire goes to pin 3 of the output tube socket closest to the power trans. Red goes to the filter cap with a choke wire.

George

New steps for the instructions are posted in the sticky thread.

George

Excellent job with the PDF installtion manual George. I just downloaded the remaining files and combined them all as one PDF file and printed them out like a service manual.

VelvetGeorge wrote:Flames, the blue wire goes to pin 3 of the output tube socket closest to the power trans. Red goes to the filter cap with a choke wire.

George

Sorry, I spaced off that he was dealing with the MM trannies instead of the Heyboers. I just flipped my chassis over to verify wire colors. And looked at the wrong pin to boot on one tube, one more reason not to play with amps when groggy.........brown and blue to pin 3's on mine, durrr......
So I'm showing a black to Pin 3 of the socket next to the preamp tubes and a red to the socket next to the filter cap to make sure that the OT primaries are correct, for the MM tranny?

Thanks for the pointers guys, I'll check my OT connections tonight after I get home from work.

I did the same thing when I upgraded my Bluesbreaker - scared the piss out of me (happened in my apartment)! That sound is like a full volume low frequency howl - I thought I would blow my speakers if it happened again. Man was I relieved when I got that straightened out.

Eamon

I did the same thing when I upgraded my Bluesbreaker - scared the piss out of me (happened in my apartment)! That sound is like a full volume low frequency howl - I thought I would blow my speakers if it happened again. Man was I relieved when I got that straightened out.

Yeah it scared the you-know-what out of me too! I don't think I've ever hit the power switch so fast in my life. Anyway here's hoping I can find my screw up!

Okay, apparently I must have misunderstood something in the instructions. Anyway I soldered the red wire to the tube socket closest to the pre-amp sockets, and the black wire to the tube socket closest to the filter cap. Voila! The amp when turned on came up just fine. The bias is a little hot, one tube is right at 42mv while the other is about 48, and that's with 82k resistor, so I'll probably need a trimmer pot. Also the plate voltage came in around 437.

So, all in all, I'm quite happy, though I do have a couple of questions for the experts.

1) When I play the amp at max volume I can see a fair amount of arcing (blue flashes) in the first power tube, is this normal, or will this go away when I get the bias down to the correct values.

2) When I strike a note and let it die out, there's some funny flubbiness near the end of the note's decay that can, at times, almost sound like a phaser pedal. This gets better if I turn the presense knob down. Is this the dreaded parasitic oscillation, or is this "ghosting". I thought I had the wire running to the presense knob routed as far as possible away from the tone controls, but I could be wrong.

Thanks again!

Sometimes tubes have a blue glow inside them caused by gasses being ionized, and this type of glow can flicker with the intensity of what you're playing. But this "glow" is very generalized throughout the tube's glass envelope if present. Shut off the lights and play to really distinguish what you're seeing, if it's anything but a faint blue glow spread throughout the tube (which usually is present when the amp is idling as well) then I'd get the bias in line first, see if you've still got flashing, and if you do contact George for some tips on anything else you may need to try. Any sharp "lightning bolt" flashes need to be addressed promptly, you may have a soon-to-be-bad tube. The 6mA bias spread is a little much for matched tubes as well, borderline there.......
Turning down your presence knob increases the feedback on the highs, and since it improves your "flubbiness" oscillation is possible. If your purple wire was too close to the other wires usually you'd hear a high-pitched whine as you increse the presence control (mine does that a little bit yet.) If you can get pics up it may help, as wire routing still is often the cause, although it can be difficult to isolate. (My 100-watt Metro Super Bass did the presence whine as well, and for some reason it improved substantially when I spaced the grounds out on the control buss wire.)
Hope this is helpful.....keep us posted!!

Yeah, the arcing is pretty noticable when I turn the lights off. I'll check that once I get a trim pot and can get the bias more inline. Or maybe I'll need a different set of tubes.

I'll also try and get some pics up too.

You might try the higher filtering by jumping the 16uf can. That's the way I have mine and there is NO ghosting or that sag your probably experiencing. I twisted all relative wires in my build with a drill and it is quiet as a mouse, no hum hiss, just plain dead quiet. I do that with all my builds and I can't stress enougfh how quiet they are by doing it. Also, with that spread on the tubes, you might try switching the tubes around to opposite sockets. I have done that before and got lucky narrowing that spread. Is the plate voltage 437 with the tubes installed? I ended up with 403 with the valves installed and I think Flames did as well. You must have the MM trannys, my OT wires are brown, blue and red, which the brown goes to v4 pin 3, the blue to v5 pin 3 and the red to the filter beneath the fuse holder.

You must have the MM trannys, my OT wires are brown, blue and red, which the brown goes to v4 pin 3, the blue to v5 pin 3 and the red to the filter beneath the fuse holder.

His does have the MM's so we can't vouch for exactly where his plate voltage will settle in.......sheesh, I'm surprised our two with Heyboers came up as consistent as they did.

You might try the higher filtering by jumping the 16uf can.

I do have the dual 16uf can jumpered, so I am using 32uf because I do like a tighter bass. The 437v is without the tubes installed, with the tubes its about 395v. Also I hand twisted all the wires, as I didn't think about the drill method until after I read this forum a little more, so that could be the cause of some noise too.

Flames, could I get a little more info on how much you spaced out the grounds on the buss wire for your amp?