WATCH YOUR SOLDER JOINTS !!!

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

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Igbit
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WATCH YOUR SOLDER JOINTS !!!

Post by Igbit »

Just had an "interesting" experience with my TMB head unit.

The old lady took pity on me and allowed me to drag my stuff upstairs out of the dark, smelly basement, so I could play like a human being, in the light, and with sweet smelling air!

Anyway, I got everything set up, fired it up, and.... nothing. On any channel. Pulled out the attenuator, tried again... nothing. Pulled out one thing after another, 'til I had the guitar going straight into the head, normal channel, and the output straight to the cab.... nothing.

Tried substituting another tube for V1, then V2, then the PI... nothing.

Flipped it over (not in the box, yet) and started chopsticking around; as soon as I hit the heater wiring on the PI, I got a horrific blast of speaker pop/hummmmmmm. Dragged it downstairs to the lab, got it under a big magnifying fluorescent lamp, and saw something, looked like dirt on the tube socket insulator. Poked around some more, and saw the "dirt" moving a little with the solder lug. Sharpened a chopstick into a screwdriver shape on the grinder and pushed it in between the two lugs, and the "dirt" bent away.

The "dirt" was a solder "whisker" that was touching the lug next to it, apparently grounding the cathode lead for V3A & B. Sounds GREAT, now.

Apparently, just the act of moving it around from one place to another was enough to change things for the worse.

From now on, I'm going over my work, ESPECIALLY in congested areas like tube sockets, with a halogen lamp and a magnifying glass!!!
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teletroy
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Post by teletroy »

Before I use a new build at a show I always put it in the car and drive around a bunch. If I got to a friends house, I unload it and put it inside until I leave and then load it up and go home.. If there's a problem, it helps find it.
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d1camero
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Post by d1camero »

For those anal types, here is my lengthy check list when completing and amp. After reading teletroy's post, I think I might also add to the list - "spin in the dryer for a while"

Before the first power up
# Trace entire circuit with hilighter
# Measure all resistor values
# Test all ground connections to each other
# Check all tube connections for poor solder joints
# Cont test adjacent pins on tube sockets
# Check all OT wire colours & connections
# Check all PT wire colours and connections (measure winding values)
# Check all ground connections (from the leads)

First Power Up
# tubes out
# output with dummy load
# on standby
# use variac slowly to bring filter caps up (for SS power supply)
# power on
# check voltages to transformer, heaters
# check voltages on rectifier tube - compare against schematic

Second Power Up
# rectifier tubes in
# other tubes out
# output with dummy load
# use variac slowly to bring filter caps up (for tube power supply)
# power on
# off standby
# check B+ voltages
# compare to schematic

Third Power Up
# all tubes in
# output with dummy load
# power on
# off standby
# check tube voltages - compare to norms.
# Also check line voltage.

Sound Test
# test with signal generator or guitar
# Exercise all knobs
# Exercise all jacks

Pre Burn-in Adjustment
# Adjust Bias on pre-amp tubes
# Adjust Bias on power tubes
# Calculate power tube plate dissapation
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Dynaflow_Donnie
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Post by Dynaflow_Donnie »

Gosh thats quite a list... I usually give the layout a look and make sure everything is going where its supposed to be going. I flip the power look, smell and listen for anything that looks like fire and then cringe slightly and throw the standby switch and then hope to GOD that I have some sort of sound thats like a guitar and not like a pig or a motorboat... :D On the other hand, I don't fire it up with expensive tubes in it (I'm a gigging part time musician, so I use new production stuff, NOS would be nice but I find good tones with what I use and can afford, 150.00 for tubes buys me trannies for another amp... :D ).

Regards,

Dyna

Ps: Like you say if your real cautious (anal if you will) I'm sure thats proper procedure, along with bringing it up with a variac or voltage limiter circuit. I'm just usually to excited to wait and the adrenelin of it powering up and not smoking is the main rush of building the darn thing except for playing it at least for me. Oh yeah, I'd almost bet cash that Fender didn't go through all that powering a amp up in Leo's day, but then I guess if you smoked one at the fender plant, it wasn't your money and I suspect you wouldn't smoke many before you weren't working there...
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wingnut1
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Post by wingnut1 »

Hey Igbit, I had almost the same problem with my 5F6A that I built. It was playing great for about three or four weeks after I built it. Then my wife and son went to Michigan to visit her family so I decided to turn it up and see what it could do. After a few minutes it blew a fuse and after that all I could get was a hum when I turned on the power switch even when the standby switch was open. I couldn't find anything visually wrong with the amp, no smoke, burned components, nothing. I couldn't get any bias current on the power tubes and no output except the hum.

I finally found an amp tech that had his own shop and would let me sit and help while he troubleshot the amp. He found that there was 3.3 volts AC in the bias circuit and when he took the red lead from the OT out of the circuit it went away, but cam back when reattached. He then measured the blue and brown wires to the OT and found the 3.3 volts there. He disconnected them from the tube socket pins and the 3.3 volts went away. He then re-twisted the wires tinned them and reconnected them to the tube socket and the problem was solved.

It had to be the same thing as your issue, a stray strand of wire from the OT was touching the tube pin lug for the heater and drawing the 3.3 volts AC. I've learned that I need to do a better job of twisting and tinning my stranded wire from now on.
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PCollen
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Re: WATCH YOUR SOLDER JOINTS !!!

Post by PCollen »

[quote="Igbit"]

The "dirt" was a solder "whisker" that was touching the lug next to it, apparently grounding the cathode lead for V3A & B. Sounds GREAT, now.

[quote]

Better known as a 'tin whisker'...they can GROW over time. We can not use tin in our manufacturing materials in the 'business' I am in because of this phenomenon.

http://www.calce.umd.edu/lead-free/tin-whiskers/

http://nepp.nasa.gov/whisker/background/index.htm

http://www.empf.org/empfasis/sept03/tinwisk1.htm

http://en.wikipedia.org/wiki/Tin_whiskers
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johnny5
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Post by johnny5 »

I remeber reading somewhere about powering your amp up for the first time with a lightbulb in-line with a power cord to limit current. aka a poor man variac.

any of you heard of this? tried it? how did you do it?
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Toots
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Post by Toots »

One more important thing to put on your checklists, before the "Before the first power up" section. Call it "Preparation":

Check your measuring equipment:
batteries of DMM OK
measurement leads OK

Not so long ago I measured some resistances with a DMM that had a run-out battery, but for some reason it didn't warn me about that. It happened to be about 30% off when measuring resistance and voltages. When I finally found out I had incidentally rolled over the tips of my measurement leads with my chair (yes it has wheels), damaged them and I didn't know that.
Took me some time to find out what was wrong with the amp.
Nothing actually, just my measurements.
---------------------------------
Toots
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LarryLarry
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Post by LarryLarry »

D1 - that is one fine list you got there. I'm pretty anal myself (programmer by day) so I can appreciate your thoroughness! I will cut and paste that bad boy for a future reference. Thanks!
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NOSMullard
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Post by NOSMullard »

johnny5 wrote:I remeber reading somewhere about powering your amp up for the first time with a lightbulb in-line with a power cord to limit current. aka a poor man variac.

any of you heard of this? tried it? how did you do it?
Its called a current limiter and I learned how to make one from Gerald W**** , I just built one but havent tried it with a freshly wired amp yet, hopefully soon tho. To make one you get 2 single outlets (not a double) and wire them in series. You can add a switch to the outlet with the light if u want to switch from limiting to full current.

Also thanks 4 that list. I will use it!!
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rjgtr
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Post by rjgtr »

johnny5 wrote:I remeber reading somewhere about powering your amp up for the first time with a lightbulb in-line with a power cord to limit current. aka a poor man variac.

any of you heard of this? tried it? how did you do it?
I do this with every new amp I make and used amp I service. The main reason is that if you've made a wiring or other mistake the bulb will limit the current and you won't lose a transformer or something important. More than worth the $20 in parts to make.

There are diagrams for this all over. Both the Tube Amp Book and the Webber books have a diagram to make one. It is dead easy to do. It is essentially putting a light socket in series with the HOT line of an extension chords and you plug the amp in one end and then the other end into the wall. If the light glows bright you don't have something right.
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chainsawmillerman
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Post by chainsawmillerman »

The light bulb set up is better than a variac as the variac dos'nt limit power it just changes the voltage to current ratio which can still blow tubes before the fuse blows.
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servant
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Post by servant »

How about using one of those "dining room" dimmer switches? Do theylimit current, or just reduce the voltage. (We have none at home so I can't test one...)

And yes, d1camero, great list!

Thanks,
Dean
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Jakel
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Post by Jakel »

servant wrote:How about using one of those "dining room" dimmer switches? Do theylimit current, or just reduce the voltage. (We have none at home so I can't test one...)
I wouldn't try it. I think they use some kine of pulse width modulation to vary the voltage/power. I doubt that would be very good for an amp. It wouldn't limit the current anyway.
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NOSMullard
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Post by NOSMullard »

Here is a pic of mine, I added a spst switch (2lugs) to switch between full power and current limiting. To do so, wire the switch up to the outlet that your bulb goes to.
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