New Express build!!! Some troubleshooting required.

[Herzog]

Hey guys

First of all I’d like to say a big thanks to the forum for all the knowledge that’s been shared here that has allowed me to build an Express. Particular thanks to RJ who provided me with the kit to built it.

It’s taken a while but is finally built.

I followed the lamp limiter/start up procedure in the build guide and everything was looking good! Voltages all checked out.

Bias was a little off out the box with plate dissipation coming out at around 12.5 Watts using the 1ohm resistor method.

As I made an adjustment to the bias voltage I measured voltage drop over the 1ohm resistor and, when I went to check the plate voltage on v5 again and, as I touched pin 3 got a sparkler show that continued till I powered down (I guess a couple of seconds)

I’ve removed the flyback diodes and they test ok.

Can I ask the hive mind here what the trouble shooting procedure would be? Or if you’ve any idea what could have caused the sparking?

This is not my first amp but it is my first full size Express.

Some pics for you all.

[pompeiisneaks]

Well first off, I'm not sure how you're measuring the current, those second legs to ground you have next to the 1 ohm means that there's no need for the 1 ohm path as you have a direct short to ground already.

Normally you do one or the other, not both. If you want to use the 1 ohm resistor, you need to remove the direct short to ground to let it work. (but I can't tell 100% from the picture. Usually you just jumper pins 1 and 8 and then send that to ground either with a wire OR a 1 ohm resistor if you're going to do the measurement method.

I doubt that caused a light show.

I can't see from the pictures 'what' exactly burned, it seems almost like the socket itself did? did the 5W resistor burn up?

If not the burning may have happened because there was some kind of poor solder joint that caused arcing from B+ to anything it could jump to? Or a bad tube socket allowed the arc to happen? You touching it means maybe there was some solder blob or wire that was loose and got moved really close to where it could cause the arc to start?

If I can't see the exact source of what burned I'm just guessing.

~Phil

[Herzog]

Thanks Phil

Ok, I’ve jumpered pin 1 to ground and have removed the jumper from pin 8 to ground for the time being.

If there’s no detriment to the amp in leaving in the 1ohm resistor to ground I’ll do that.

I can’t tell myself what the cause was. The 5 watt resistor tests fine out of circuit.

Here’s a better pic. Looks like it’s just the socket that’s burned up during the arcing. I’m guessing I should replace the socket rather thank try to clean it up and rework the soldering?

[pompeiisneaks]

Thanks Phil

Ok, I’ve jumpered pin 1 to ground and have removed the jumper from pin 8 to ground for the time being.

If there’s no detriment to the amp in leaving in the 1ohm resistor to ground I’ll do that.

I can’t tell myself what the cause was. The 5 watt resistor tests fine out of circuit.

Here’s a better pic. Looks like it’s just the socket that’s burned up during the arcing. I’m guessing I should replace the socket rather thank try to clean it up and rework the soldering?

You jumpered pin 1 to ground? So you likely want the 1 ohm resistor to ground, that's your ground reference. 1 ohms won't stop anything and just creates an easy way to measure current.

You should jumper between pins 1 and 8 and then connect that only via a 1 ohm to ground. I.e. you only need one ground reference. It's possible you were getting 'bad' readings from the bias update and you pushed your tubes into redplating etc, and t hat's what caused the arc? Not sure.

I'm sure some current would still go through the 1 ohm resistor but most would go through the 0 ohm resistor (the wire). It could just be a socket failure, though? Not sure. I've never seen that much black without something actually burning though, some other components dies to create that much carbon. Weird.

~Phil

[Herzog]

Thanks Phil, I’ll use the 1ohm resistor as the sole ground reference then.

Tubes were definitely not red plating.

I’ve ordered a couple of tube sockets. Will try again Thursday.

Yeah I’ve never seen anything like it, closest I can describe it is that it was like a sparkler in the tube socket, no smoke from any components at all as near as I can tell, though I may replace the 1N4007 closest to pin 3 even though it tests ok. Very weird indeed.

[Herzog]

Having done a little reading on arcing my current (forgive the pun) is dust becoming conductive.

The arcing was between pins 3 and 2.

The build got pretty dusty in between bursts of build activity. I cleaned up the boards and chassis with a special hoover attachment but neglected to clean the sockets.

I’m hoping the OT is ok. Will test a bit later.

[sluckey]

Arcing between pins 2 and 3 is pretty common. And one common cause is to play the amp with an open speaker load. Your flyback diodes are meant to protect against this. Anyhow, the socket needs to be replaced now.

The soldering on that socket looks suspect to me, especially that purple wire on pin 3.

[Herzog]

Thanks Sluckey

Yes, doesn’t look good in the pic.

The flyback diodes seem to have done their job.

I was doing initial setup and bias with no guitar plugged in and with the amp connected to a Suhr reactive load.

That joint will be fixed once I get the new socket in.

[M Fowler]

Do you have rj's PT and OT wiring diagram you can post? The wiring of the octal sockets looks suspect to me color wise but Pacific and Edcor have their own colors.

Mark

[Herzog]

I can’t seem to dig the OP transformer wiring diagram up. The secondaries and octals were wired earlyish in the build, using the paper diagram supplied.

I mailed RJ last week to verify the primary wiring, particularly the CT.

Meantime, and prior to firing it up I found this



Which is an OEM OT wiring diagram from Pactran (found on Reverb)

Measuring the primaries from the CT I get 47ohms on one side and 49ohms on the other. My plate voltages were higher on V5 (the 49ohm measurement). Plate volts at pin 3 of v4 = 402v and v5 =405v.

Does this suggest wiring is off? Or mislabelled?

[pompeiisneaks]

No the measured resistance of the windings isn't going to match that of the impedance rating.

As an example for a 100 W output transformer, I've got the expected readings in DC ohms

this is a 4xEL34 OT, the windings on the tube side measure 18 or 19 ohms but they're for what probably 6k impedance to the tubes.

The other side was about 0.1 to 0.2 ohms per section between common and 4,8 and 16 ohms.

You can't equate resistance of the wire to impedance of the loads.

~Phil

[Herzog]

Thanks Phil, that makes sense. Secondaries are reading less than an ohm apart when measuring to common.

Can I ask if the difference in plate volts and minor difference in resistance in the windings are of concern.

I’ll fire ahead with replacing the tube socket and bias to the tube with the higher plate volts.

[M Fowler]

OK that green wire is a primary and purple or violet. That was throwing me off the green wire.

[pompeiisneaks]

Thanks Phil, that makes sense. Secondaries are reading less than an ohm apart when measuring to common.

Can I ask if the difference in plate volts and minor difference in resistance in the windings are of concern.

I’ll fire ahead with replacing the tube socket and bias to the tube with the higher plate volts.

No almost all transformers tend to have slight variances. The wire itself isn't perfect and will have slight variations in resistance per inch/mm so it will give very slight different readings per half of the coil etc.

~Phil

[Herzog]

Mark, thanks for this, caution is better, particularly given this is my first EL34 amp.

Phil that’s great info, very reassuring!