Power Up; Time to push some electrons!

[gearhead]

Heh all,

Despite the number of posts, I've only recently been able to get going on my Express (finger/joint health problems- no guitar either, ugh). Had the parts for over a year!

Got it all wired up except for the transformer primaries/secondaries. Done a decent number of cross-checks with the layout as well as checking for in-jack/power-in connectivity and grounding/shorts.

Trying to decide between Pauls safe-start and the light-bulb limiter methods. What you all use? One thing with Paul's has me concerned; the indicator light takes all the incoming power, not the heater voltage. Won't it get fried if left by itself (vs hooking up the primary PT also)?

I might just do both.

In any case, is it just me, or do you all feel both excited and fearful at this stage? LOL.

Dave

[Tubetwang]

Big moment!

Always fun to fire it up especially if it has been a long build...

I re-check on everything and hit the switch...

There is usually something wrong but... there has been a few times when it did work...

[paulster]

The indicator light isn't going to be 'fed' more power because it's the only thing connected; it'll simply draw as much as it is expecting to based on the voltage supplied and its resistance.

The idea is you can make sure you don't have a heater secondary shorted by using the low-value mains fuse and then check that you actually get 6.3V out (i.e. you are using the correct primary taps, if applicable, on the PT).

I like the lightbulb method with no tubes in as a start, as it provides a means of protecting the PT in the event of a short and also gives you an opportunity to get some baseline voltages to see if things are broadly in the ballpark. They won't be all that close, but you want to make sure you haven't got sky high voltages on the HT or no voltage where you'd expect it.

Not a problem in a wreck but you do need to ensure that your downstream filter caps can handle the full B+ voltage though, as there won't be any load on the dropping resistors to bring the voltage down along the chain and they'll all sit at the highest B+ voltage.

[Firestorm]

I always follow the Firestorm Amps power-on checklist:
1) Carefully insert plug in wall
2) Using proprietary-design digital manipulation probes (fingers) flip switch to "On."

Seriously, though, the light bulb thingy works pretty well at catching shorts and lets new caps have a moment of peace before the onslaught. No disrespect to Paul, but I don't have the patience ...

[Bob-I]

In any case, is it just me, or do you all feel both excited and fearful at this stage? LOL.

Dave

Me too. I've built maybe 15-18 amps and I never get past the anticipation of firing up an amp for the first time.

Use the lightbulb limiter and leave the tubes out. It's saved my skin more than once. Take your time and go carefully, checking all voltages at every stage.

[UR12]

Same here..... I leave the tubes out, fire it up, check voltages and install an old crummy set of known good tubes if everything looks fine. I usually just hook it up on a bench and have the thing hooked to a sig gen , scope and dummy load. I just power the thing on and if it blows then so be it. Sooner or later you just got to hit that switch and see what happens.

Congrats Gearhead!! Can't wait to hear how it goes. Keep us posted!!

[gearhead]

Thanks all for the kind words!

Was doing some last minute checks before any power; such as shorts to ground, and came across something that's got me baffled. It might be right, but don't know why.

The OT secondaries (4, 8, 16 oh taps) all have near continuity to each other, including the ground wire. The greatest seperation is 1.3 ohms. I've checked them as installed on the switch; and each with the chassis. Just happen to have another one of Mooses for a Liverpool, and those secondaries check out the same.

I confess that transformers are one of the least things I understand with amps. Can't believe that I have two bad, unpowered OT though. Hopefully

Is there some electromagnetic field/inductance/reactance affect that kicks in when you feed the primaries?

[UR12]

Thanks all for the kind words!

The OT secondaries (4, 8, 16 oh taps) all have near continuity to each other, including the ground wire. The greatest seperation is 1.3 ohms. I've checked them as installed on the switch; and each with the chassis. Just happen to have another one of Mooses for a Liverpool, and those secondaries check out the same.

That's normal as you are measuring the DC resistance and not the AC impedance. Just make sure you have a speaker or dummy load plugged in when you fire it up with tubes in it for the first time.

[gearhead]

Thanks UR12, I was thinking there was something like that going on.

Fired up the amp with no tubes installed, no load, light bulb limiter in circuit. Standby seemed ok: B+1 was up there at 400VDC. I thought it would be lower due to the limiter, but nothing smoked.

Flicked on the rest of the circuit. B+2 is close to 399V, and B+3, 4, and 5 graduated steps downward. However, the lowest is still in the 385 range or so. Normal ops is 100V below that number! The heaters were running low (4V or so), which is what I did expect with a bulb limiter.

Without the tubes and a limiter, is this all normal? Either I got something wired up wrong, or not enough current is being drawn to pull down the B+ voltages?

Thanks all,
Dave

[Bob-I]

Flicked on the rest of the circuit. B+2 is close to 399V, and B+3, 4, and 5 graduated steps downward. However, the lowest is still in the 385 range or so. Normal ops is 100V below that number! The heaters were running low (4V or so), which is what I did expect with a bulb limiter.

Without the tubes and a limiter, is this all normal? Either I got something wired up wrong, or not enough current is being drawn to pull down the B+ voltages?

Thanks all,
Dave

That all seems right to me. Time for tubes

[Firestorm]

Check the negative bias voltage, too. That should read just about accurately even with no tubes in and a limiter on the AC line. Set it nice and low to start -35 or more).

[jjman]

With no tubes in there DC should be all about the same on all PS nodes, as you are seeing. This is because there's no current flowing thru tubes to drop the voltage down on each rail node.

You could check the primary ac voltage coming in to the amp to get an idea of how low the heaters should be. Where to probe depends on whether you fused the hot side or the neutral. If the main primary is X% lower than the wall (due to the limiter bulb) the heaters should also be about X% lower than ~6.3vac.

But as mentioned, all sounds good so far.

[gearhead]

Made progress up to the point where I had tubes (no limiter) and flicked from standby to full power.

It blew up. It actually took about a minute, but made this huge screeching sound (and the volume was all the way down).

Did get a chance to take some voltages, and B+2 was WAY down, by about 80 VDC. Checked across thg big 1K cement resistor, it was across that. Don't know if something in the power tube area was sucking down the DC or not.

Kind of in a funk now. Standy works fine, get all the right voltages, but can't go to full power without it screeching, so I can't fault isolate, EEEk!!

Where do you go from here? Did I blow the OT?

[Firestorm]

Try swapping the primary leads on the OT to the opposite tubes. Sounds like your negative feedback loop has become a positive feedback loop.
[Edit for slow reading ]
Are you sure you're seeing 80 volts dropped across the big 1K? No way the screens are pulling 80ma (unless there's a short on the tube socket).
Check for shorts downstream of the B+2 node.

[gearhead]

Further recollections; the B+3, 4, 5 were somewhat in line albeit not quite in the zones. There was NOT a 80+ VDC drop between 2 and 3, which is what I was expecting to see. I don't recall exactly what it was, but B+3 was probably as close to B+4 as it was to B+2.

Could there be a short downstream of B+2? I would think that if it was at B+3, the current across the two 9.1k resistors would have really dropped the voltage down at B+3?

Have done some further checks. With the tubes out, everything is like it was before.

Another thing; are the heaters supposed to be near ground? I'm getting a continuity between either tap and ground.