My first build from scratch...an Express

[martin manning]

The preamp node voltages will likely fall into line when you put the preamp tubes in. A few mA through the dropping string will do it.

[donzoid]

OK the tubes are in, and of course I just HAD to mess with that pilot lamp...something fishy is going on with that thing! Found a 110v bulb put it in and immediately blew the fuse at power on. I dunno...maybe there IS a short in the darn thing but the meter didn't indicate it. So, disconnected one lead of that, replaced fuse and we're going to remeasure and get my chart filled in.

[donzoid]

Looks like B+ values dropped a bit with the tubes in, as did the heater volts but looks pretty good so far. I'm letting the tubes cool so I can put in my bias probe (dang...I knew I should have put in those measurement sockets on the back panel. And I may yet.)

[donzoid]

Bias is now set to 44mA. At idle the amp is making some funny scritchy noises. So I took my chopstick and of course the crap startup tubes I threw in for power up has an extremely microphonic one in V1. I'm anticipating guitar noises within the hour.

[donzoid]

WOW. Just wow. This thing is a BEAST. I haven't even started rolling tubes and its sounding great. I tried to make a cheesy phone recording but it turned out awful fizzy...I'll attach it but be forewarned, it would drive Bill Fartbloom off the edge. Hey waitaminnit . None of that fizz is really present its just that crappy mic. No, I'm not playing that great tonight but you get the idea. Dig in, and she really talks back.

Question: I am going to assume since I fired it up, and it didn't go straight into oscillation that I have the two OT primaries in the correct phase right?

I have to get those buttoned down. AND figure out what's up with that pesky pilot light. Oh, and my choice of red chicken head knobs was a bad engineering move...they run into one another. Rats! I have 5 Marshall knobs around here I will have to hunt them down. It just won't have the look I was after though...

This amp is amazing and considering the aggressive picking attack I use, its just gonna be marvelous. I'll start rolling tubes tomorrow afternoon.

[geetarpicker]

Congrats. It's ALIVE!

GK

[donzoid]

Thanks Glen....I could never even have dreamed of doing this if t'werent for TAG. I think now I got the bug!

[martin manning]

Woo-hoo!

[donzoid]

OK I know its a small thing....but this pilot lamp deal is really bugging me. I just tried a whole different socket, completely isolated from the chassis and again, my fuse blew the second I turned the power switch on. Its running straight off the power switch hot and neutral, with a 110v bulb in the T3 socket, and I know the bulb is good because it flashes (when the fuse pops).

On the plus side, I am glad this is the only issue I have to debug. But this is so simple, I don't see what could be wrong here. Its just a 2" lead from the switch hot/neutral sides to the socket. I'm now out of 2A fuses so I will have to go get some more!

Any ideas? Do I need to diode the socket? Or cap it?

[donzoid]

You guys are fed up with my silly questions aren't ya? That's OK....even I understand.

[M Fowler]

Is that a 6.3v bulb or 120V?

A wire must be bare conducting across the power at the switch.

[donzoid]

Mark, its a 110 volt. I've checked....unless I am having an arc inside the switch when I flip it, there aren't any shorting wire fragments (at least on the outside). But if its internal (to the switch)....why did I just spend an hour on the amp turning it off and on to roll tubes without blowing the fuse (with the damn pilot light disconnected). That's what I don't get!

[donzoid]

I think tomorrow I'm going to try an experiment...I have a 6.3 volt T3 bulb and I could wire the heater circuit to it like an old fender. If it doesn't blow the fuse, then I guess I can live with that.

[donzoid]

Ok Mark I'll have to backpeddle a little (sorry). Is it possible for that to happen through the MOV cap? For example, could I have wired it in reverse polarity? I didn't think MOV's had a specific polarity but I'm just trying to understand what's going on, and I agree it's acting like a direct short. I just looked for the 4th (plus) time, and there aren't any shorting wires across terminals.

[JazzGuitarGimp]

The bulb will draw big current until it warms up. Do you have the datasheet for the bulb? If so, what is the inrush current? Are you using a slow blow fuse? What is the current rating of the fuse? What is the output power of the amp?

If you have a variac, try bringing the voltage up slowly to avoid the inrush current of both the bulb as well as the PT. If you can get it up to full line voltage without popping the fuse, it's possible that you just need to increase the fuse ampacity a bit.