My first build from scratch...an Express

[donzoid]

Hmmm....the light bulb limiter is glowing brightly with the 25w bulb and after 2 min. of that shows no sign of dimming. I don't think it would take very long to charge up caps so I am going to re-check my PT connections. Looks like this might turn into a debug thread....hrmmmph!

[martin manning]

If it was going to go dim, it would happen in less than 2 seconds. 25W is pretty small, and it will probably glow continuously even if nothing is wrong, but you should have seen a change (bright to less-bright) when you first powered it on.

[donzoid]

Thanks Martin...I hate staring into light bulbs....it makes for a nice "sun spot". If it's dimming it's imperceptible. I just checked all my PT connects for center taps to ground and primary/secondaries and I think they are all proper. I'll do another complete sanity check before proceeding.

[martin manning]

If you want a reference for the limiter, connect it to a working amp and watch what happens when you switch on the mains and then the standby.

[Tillydog]

Here is the corrected jack: in the picture the left side opens when the plug is inserted.

That looks OK now

[donzoid]

Thanks to both of you. I am seeing something that strikes me as pretty strange. I have 120v at my IEC cable but as soon as I plug it in and measure the backside of the IEC socket (in the chassis) I have only 6.x volts on the AC setting. I thought, how can three straight-steel pins not be conducting that voltage? So I measured the pins from front to back and each is less than one ohm. Plug the cable in, and there it is again...6.7volts AC. What the heck?

[donzoid]

Hilarious! On a whim, I unplugged one leg of my (110v) jewell pilot lamp. What do you think happens? The bulb behaves exactly as it's supposed to, noticeably dimming within seconds.

[martin manning]

You are saying that your pilot lamp was shorting to ground?

[donzoid]

Possibly! I am checking that out first. Although with this type of socket I think it would have to be an internal short. The bulb was lit (though dimly) when connected. I thought I checked it but I am going to look and make sure it's not a 6.3v bulb (or defective). I would have thought it would have popped even with the 25w lamp in the limiter though. I'll pull the bulb and check the socket for shorting.

[donzoid]

Oy!...it's a 14volt bulb. No shorting in the socket (which is rated for 110) but I don't know if I have a 110v bulb to replace it with (at least not handy). But that's a side issue, I'll get that sorted. the 60watt bulb in the limiter is behaving as expected.

[donzoid]

By golly the 100watt bulb test looks good too. I think I'm ready to start measurements. Whew! That's just a reminder that the devil really IS in the details.

[ampgeek]

Re: ~6.7 V at your IEC

If you have a short somewhere in the amp, the light bulb limiter will drop the majority of the line voltage before your IEC socket. So...you would expect to see that low of a voltage there.

Probably an "after the fact" comment here...but...it might help you in understanding what was happening.

Good luck on your next set of tests/checks. We are all rooting for ya!!

Cheers,
Dave O.

[donzoid]

Thanks! No smoke coming out so I'm feeling pretty good about the TEN or so times I went over my sanity checks last night and this morning...heheh! On the serious side though the trannies are cool, heaters are glowing.

Secondary PT volts are 305 per side....610 to 617 for both and the bias volts after the diode are at -41.x so I think that's about right per the Edcor spec.

Heater volts seem a bit high at 7.1 but I only have a couple of 12ax7's in there . So I might see some drop when I throw all the tubes in. I'll start looking at the pin values and fill out the chart. So far I am pleased. I hate to tell you guys how long I took to do this because I've already seen posts by guys like Fowler that can kick 'em out in 16 hours (and they look much neater). I think I'm almost exactly at 3 times that number... with practice I will improve

[martin manning]

For sure the heater voltage will drop... a couple of EL34's will draw 3A!

[donzoid]

OK I will load up a full tube complement and finish the chart. Here is what I have so far, but not certain I measured B+1 and B+2 in the correct spots. All the B+ volts appear "in order" but a bit higher than what I have as "expected".

Thanks Martin I figured the power tubes would correct that heater voltage! I didn't know they draw 3A but that certainly ought to cause some sag.