Tube Amps

 

This Engl Fireball head suffered a tube failure.  This sort of thing is usually not a big deal, but after the owner replaced the output tubes, a red “FAULT” light came on.  The amp played, but sounded a bit odd.  The Unbrokenstring Crew To The Rescue!

At first look, the rear panel is pretty simplistic.  However, let’s indulge in some Tech Porn and take a closer look.

OK, just a quick tour of the unit…  Here’s your typical IEC cord socket with integral mains fuse.

Footswitch jack functionality is straight-forward.

Interesting to note here is that the mix between the original signal (dry) and the stuff in the loop (effect) is set back here.  Since this control is on the back of the unit, it’s kinda ‘set and forget.’

So when this head reaches ‘end of life’ they want you to recycle it rather than just toss it in the waste bin.  Okay…

All you need to know is found here!

The output circuitry is very versatile; four, eight, sixteen ohms cabs, series and parallel, are no problem for this amp.

As the front panel is chrome, it’s a little tricky to make out the control nomenclature.  But very cool!

Not just gain, but Ultra Gain!  Is it a laundry soap?  Or more like A Clockwork Orange‘s ultra violence?

This amp has a ‘Power Tube Monitoring” circuit that indicates that a defective power tube has been isolated from the electrical power circuit.  This monitoring circuit protects the expensive components (rectifiers and power transformers) from further damage in the case of tube failure.  Oddly enough, something in the monitoring circuit went bad in this head, and now, known-good tubes still trigger the indicators.

Let’s take a look inside.  The quickest way inside is to enter through the back door.

With the back cover off, let’s unfasten the chassis from the amp cabinet.  This is a view of the bottom of the head.

These red LEDs add a little cheap bling to the appearance of the amp when it’s powered on.

Here’s another view of the bling LEDs.

The capacitors sticking through the chassis is kinda cool, but are a testimonial in favor of a good 3D CAD package.

These preamp tube shields are unique.  They clip through slots in the chassis itself.  A good squeeze liberates them.

Something new to me is the ENGL-branded preamp tubes.

Peeking around the backside of the front panel, we see the input jack and the end of the circuit board that carries the volume control and push buttons.  The braided sleeving over the power cable is a nice touch!

Here are the tone controls.  Note the flat ribbon cable sporting some hot glue as a strain relief.

 

Note the split ground planes in the circuit boards.

See the long metal straps that make electrical connection to the front panel LEDs, hanging under the circuit board?

The power transformer, sporting a sharp brass shield over the outer winding.

The output transformer lives under this steel shell.  It is attached to the circuit board, so all this comes off in order to remove the circuit board.  The weight of the output transformer makes the subsequent dis-assembly a little awkward.

The components we want to look at are on the other side of this large circuit board.

Many of the wires attached to the circuit board are ‘lap’ joints.  A little strip of RTV relieves the strain on the joint.

The low level circuitry is shielded by this steel sheet.  I’m removing it to get it out of the way.

Some of the lead dress will be loosened to allow the main circuit board to be inverted for service.

Rather than disturb a lot of solder joints, I just unfastened some of the smaller assemblies to free the main PCB.

This little resistor became a fuse, protecting the rest of the amp during tube failure.

This shows a little smoke, frozen in time.  When the smoke leaves a component, the component fails to work.  Thus, in the electronics world, we recognize “Magic Smoke” as the ‘stuff’ that makes electronics work.

The body of the burned resistor has been de-soldered.  Note the identical circuit, for the other output tube, to the right.

I’m using isopropyl alcohol for the cleanup on the circuit board.

While I’m at it, I’ll collect and save the Magic Smoke found on the chassis.  This can be used to resurrect other components.  If you don’t believe me, send me an email via the contact page so I know you are still reading this.

The affected portion of the protection circuit was repaired, and the other channel was repopulated with matching components so that they will behave exactly the same way if they are ever called into action in the future.

New tie wraps complete the restoration of the lead dress.

Now we’re pretty much back together.  This is a nice view of the chassis.

Remember the LED bling?  Well, here they are at work.  They are BRIGHT!

Both tubes are push/pulling and the bias is perfect.  The clean section is VERY clean on this amp!

With the lights down low, the bling is just a little understated.  I like it, and it sounds good!

Thanks for reading all the way to the end!

CONTACT INFO – David Latchaw EE

281-636-8626 voice or text

 

 

This well-worn road warrior was back home and ready for a fresh set of new output tubes and an annual checkup.

With the front and rear grilles removed, we can get to the tubes.  Perhaps we would remove the chassis and clean off a year’s layer of foreign object debris from dusty roads and smoky honky-tonks, while we’re at it.

We pulled the output tubes and cleaned up the chassis.  This unit is still like new underneath the surface grime.

Click on the picture to get a hi-res close-up of the underside of this unit.  By The Way, if this picture is distorted, then you need to upgrade to a browser from the 21st century.  Just sayin’…

Here is a close-up of the top side of the circuit board where the output power tubes live.  Nothing to see here.

On the other side of the circuit board, we see that the sockets were hand-soldered in place at the factory.  The smokey areas are just the fumes from the solder rosin.  However, we will need to clean up the carbonized flux before it becomes conductive.  Solder flux does not conduct electricity, but the dust and moisture it attracts does.  And there’s hundreds of volts to be found here, so this contamination has to go before it becomes an issue in the future.

Here is a close-up of another hand-soldered socket.  This one is in a little better shape but will still be cleaned.  The solder bridge between the two pins in the foreground is supposed to be there, but I checked the schematic to make sure, just in case.  This amp was working before the re-tube, so there was very little chance that this solder bridge was ever an issue.

I am using an acid brush with the bristles cut almost all away as a stiff scrubber.  Straight rubbing alcohol is a safe solvent for this sort of clean-up.  An inexpensive source of lint-free, absorptive paper is a coffee filter, as you see here.  Using an absorptive wipe gives the ‘junk’ some place to go besides somewhere else on your circuit board.

The tube socket site to the right is good-to-go.  Three more and we’re done here.

A fresh 12AX7 will perform phase splitter duties.

These old soldiers were all barely functional, suffering from a build-up of gas.  The interior vacuum inside an operational tube is not perfect.  Most of the oxygen and water are captured by the silver ‘getter’ seen at the top of these tubes.  Over time, heat will drive the moisture and some oxygen out of the getter region and back into the interior of the tube, where it interferes with the electron beam between plate and cathode.  Time for a well-deserved retirement!

More porn for all you tube nerds,  Here is a matched quartet of 6L6 tubes.  Ruby brand was the OEM choice for Peavey amplifiers from this era.

Oops.  I did it again.  Guitar Tech Porn!

Almost there!

We’re warming up, using the purely resistive loads which you see on top of the cabinet.  An oscilloscope samples the output from the amplifier, and a sinewave generator is connected to the input jack.  A total harmonic distortion meter (THD meter) is paralleled across the resistive load and monitored separately.

All crossover distortion is gone.  With four output tubes, the bias check is a little tricky.  There was a little bit more tweaking done after this picture was taken, in order to get the middle a little fatter and the total THD on the clean channel as low as I could get it.

Here’s the back side view of a cool-running amp after it’s million mile checkup.  We’re ready for another million miles!

Thanks for reading all the way to the bottom!

Contact information : David Latchaw EE

281-636-8626

The music died when the AC line fuse blew in his Mesa 20/20 stereo amp.  What are we gonna do?  Call The Unbrokenstring Crew!

JC’s rig was built around the amp, so of course it was installed first at a place of honor in the bottom of the road case.

Exploratory surgery is fairly easy in this unit.  Everything is accessible once the top black panel is removed.

Here is a little Tech Porn.  A lot of versatility is designed into this amp.

Every tube in this unit is original.  These were all checked and packed safely to the side.

Here’s what a blown AG3 fuse looks like, just in case you hadn’t seen one before…

With the tubes removed and the power applied gradually through a Variac, the problem persisted.  Here, one of the red wires that sends high voltage to one of the output transformers is temporarily isolated from the circuit.

And this is the output transformer at the other end of that red wire.  As power was applied, it began to get hot.  Do you see the little round dents in the insulation?

Those little dents are from these rubber bumpers that help support the wire bobbin of the output transformer.  Current leaked from high voltage winding to the steel core laminations of the transformer.  The head generated by the electrical current melted the nylon bobbin, further damaging the transformer windings and leading to a cascading insulation breakdown in the unit.

Mesa Engineering had a few of these output transformers left in stock.  Here is a fresh one, via FedEx.

The new part is the output transformer on the right.

Now we are going to dress the wiring back through the chassis of the amplifier in a manner similar to the way the old part was installed.  Since the chassis is about one and a quarter inch thick, everything is super-easy to work on!

Except when the cabling goes underneath the circuit boards.   A pair of round-nosed pliers are used to snake the wires where they need to go.

And we come out the other side.  Recognize the red wires from the earlier picture?

These three wires are the four ohm / eight ohm output leads.  These were trimmed to length and tinned.

If you look closely, the colors of the wire insulation is painted on the circuit board, where each output lead belongs.  However, this color information did not match the original configuration of the amplifier.  I’m glad I took all those pictures before I started removing wires!

Here is another view, showing the final lead dress.  The amp is back together for final test.

This is Channel B at 20 watts into an eight ohm resistive load, driven by a 400 Hz sinewave.

OK, we’re back together again.  Here we have another gratuitous picture of the front of the rig.

The back of this unit is not nearly as pretty.  JC said, “Fix this mess!”

So we stripped everything out and went back with one channel of the audio chain.

This cable was intermittent and was replaced.  These ends are crimped onto the cable and are not serviceable.  Several layers of black duct tape attests to the fact that I am not the first person to come to the conclusion that this cable had a problem.  Hint: Why not try fixing the problem right when you find it, next time?

This is all the AC wiring that did NOT go back into this rig.

The AC cables were shortened and these rectangular plugs were fitted.  These plugs were chosen because they could fit on the AC outlet strip without blocking any outlets.  The black tie wrap augments the strain relief.

The AC power strip was disassembled so that a longer AC cord could be permanently affixed.

Here is the AC wiring and one channel of the audio cabling.  The other channel was similarly wired and dressed.

One Happy Customer!

Thanks for reading all the way to the bottom!

David Latchaw EE

281-636-8626

 

Ian of Jealous Creatures found a loose tube rolling around inside his amp. The Unbrokenstring Crew to the rescue!

We need to take a closer look (and find some Guitar Porn along the way!)  Off comes the partial rear cover.

The smaller tubes, all 12AX7s are held in with spring-loaded bayonet tube shields.  There is no hardware whatsoever to keep the tubes larger tubes in their sockets.  Houston, We Have A Problem!

While we’re at it, Ian wanted us to take a look at the RCA sockets that handle the pedal duties.

Now that I’m getting older, I tend to forget where I found things, like cables.  So I labelled them “I” and “O” to correspond to “REVERB INPUT” and “REVERB OUTPUT.”

I need the tubes out so that I can add retainer clamps to the sockets.

Let’s remove the chassis and take a tour of this fine amp.  This is the chassis under the rectifier and output tubes.

Everything is wired point-to-point.  Here’s the phase splitter and the end of the preamp tube string.

Here is the beginning of the preamp tube string.

Here is the front-panel pilot light.

A brass grounding strip is affixed behind the front panel controls.

These are the controls for the vibrato.  Note the black ground wire wound around the cable bundle.

We see the VIBRATO input jacks in the center of this picture.

Volume and tone controls are here.  This brass grounding strip is in the best shape I’ve ever seen compared to other units of this same approximate age.

Here we see the volume control and the NORMAL input jacks.

The power cord, accessory power socket, and ground lift switch.  This is probably a replacement power cord because it uses the international color code scheme.  American color codes are black/white/green.

From left to right, we see the fuse holder, power switches and the output/speaker jacks.

RCA jacks for pedals and reverb tank connections.

And this, ladies and gentlemen, is the end of our chassis tour.  Back to work!

Ian said that the pedals were intermittent.  The connectors were not broken, but I removed them anyway to scrub them down with contact cleaner.  Here, a cotton Q-tip has been resized to scrub the bore of the RCA jack.

The same technique was used to apply a thin coating of Blue Shower protectant and lubricant inside and out.

The sockets were unscrewed and new tube base clips were installed.  These are from the same OEM as supplies Fender and Peavey.  Dunno why they weren’t installed in the first place… maybe CBS was cutting costs?

The RCA and its washer stack goes back home where it belongs.

After tightening the nuts, we’re soldering everything back the way it was.

This should wrap up everything electrical that has to do with the rear panel.

I hit the RCA jacks assigned to the reverb tank using the same process as I used on the pedal RCA jacks.

All the quarter-inch jacks were cleaned and lubricated.

The front panel jacks received the same attention, using the cleaner and lubricant process.

More input jack love.

This is a little better view of what’s actually happening during the cleaning cycle.  Note all the gunk on the Q-tip.

This beautiful Fender-branded 5U4G rectifier is the first tube to slip into the tube base clips.

Your turn.  This is a Ruby-branded tube.

This is a JJ/Tesla tube.  We’re ready for final test.

Wow, this thing is loud!

Thanks for reading all the way to the bottom.  With all the Guitar Porn, this is the longest post yet.  I hope you enjoyed reading today’s post as much as I enjoyed working on this unit and documenting this little trip down Amplifier Avenue!

David Latchaw EE

281-636-8626

A local musician lost this amp head during a gig.  The bassist also blew some fuses in his rig.  Everything failed at once.  Besides making a great testimony for carrying your own power conditioning equipment, a lot of stuff had to be fixed.  The Unbrokenstring Crew goes to work!

There were no signs of life other than the tube amp section appeared to be fine, and, surprisingly, the LEDs in the foot switch assembly lit up.  Some Google-ing revealed that Line6 built this amp, which included Line6 modeling technology to implement the reverb and other internal effects married to a Reinhold Bogner power amplifier.  Now that I could locate a manual, we could narrow down where the failure occurred.

There is a fuse holder built into the AC power receptacle.  All is OK here.

The Line6 design project was called “AV/Cali” and the “HD” on the sticker indicates that this is a ‘head’ version.

Moving out of the left side of the picture is another fuse.  OK here.  Let’s keep going.

I don’t know why the ETL sticker appears a little faded.  But here’s where the foot switch plugs in.

This effects loop specifies parallel connection.  Glad I’m not doing a pedal board for this amp!

With the grilles off, we see a little ‘bling’ including a custom-milled tube cover.  Wowsers!

The aluminum foil is a shield for the open bottom of the chassis.  This unit is well built.

This is a high-resolution pic of the bottom chassis wiring.  Click on the picture to fill your screen with tech porn!

There are a couple more fuses under the chassis that we need to check.  Here’s one…  No problems here.

Another fuse is reserved for the big transformer.  No problems here either.  It’s time to check the various supply voltages against the service manual.

What’s this?  This wire was pinched between the chassis and the amplifier enclosure.  The insulation is a little deformed but otherwise OK.  This was poked back where it belonged as this was NOT the problem.

We didn’t have the 3.5v and -3.5v power.  The schematic showed that this was derived from a single winding of the power transformer, sent through a full wave bridge rectifier, and filtered with a big capacitor.  The issue is somewhere around the rectifier.  Here, I’ve flipped the Power Supply Unit (PSU) board upside down so I can remove the rectifier to more easily troubleshoot the circuit.

These solder joints were cooked.  All the solder was removed and the holes cleared of solder.  Now I can see that the circuit board was open-circuit on the trace at the bridge rectifier.  The holes in the pads shown in this picture is where the bridge rectifier was.  I’ll fix the open circuit board trace with a short piece of wire when I put this part of the circuit back together.

Here’s the view of the board without the rectifier.  It’s a little hard to see here, but the bottom oval pad was cracked.  That will be repaired.  The rectifier is OK, so it all goes back together.  That is what you call ‘troubleshooting beyond the component level.’

All lights are on, all knobs work.  This beast is ready to go now!  I delivered this to the owner’s house as he lives on my way home after work, and he said “It sounds great!”

Thanks for reading all the way to the bottom!

David Latchaw  EE

281-636-8626