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7591 Amp Build (from Rowe/AMI R-4359) (Read 4380 times)
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7591 Amp Build (from Rowe/AMI R-4359)
Feb 8th, 2021, 8:36pm
 
I'm building Tube Amps, and I can't stop!
 
In addition to building a small amplifier based around Russian military tubes and a Chinese SMPS, I have been accumulating parts to build a EH 7591a based amplifier.  The Output transformers will come from a Rowe/AMI R-4359 amp chassis that came from a jukebox.  The original amp was based on 7868 tubes.  7591's and 7868's are electrically very similar, just in a different basing and envelope, so I also plan to stick with the R-4359 schematic fairly closely.
 
Output Transformers, and the basic circuit design of each channel is where the similarities to the Jukebox Amp will end.
 
The amplifier will be built in a rack-mount chassis (which is also surplus).  I am not using the Power Transformer, 5AUG Rectifier tube or any of the original power supply circuitry.  Instead, the needed voltages for the tubes will come from a combination of Switch Mode Power Supplies, as I have done for my other projects.  A 20Amp rated SMPS will provide 12VDC from a 100-240V input.  The 12VDC (at lots of Amps) will power a significantly modified Chinese Inverter module along with a rectifier, filter, and feedback system of my own design.  This will provide approximately 380VDC to the plates of the EH 7591a's.  Other modular SMPS's will provide the ~6.3V needed for the 7591's heaters, and a +/- output for the tube biasing circuit.
 
It will also contain a high performance DAC to allow USB Audio, as well as Optical and Coaxial SPDIF inputs.  Of course there will be straight analog inputs, too.  A digital, audio grade relay board will allow push-button switching between the digital inputs and 3 analog inputs.
 
It will also have a VFD Spectrum display on the front panel.
 
Yes, it's an ambitious project, and it has to wait for other projects ahead of it, but in the meantime, here is a rendering so that you can at least see some artwork:
 

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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #1 - Jun 27th, 2021, 10:14pm
 
I know it's been a while since I posted an update on this project.
 
I'm not dead, and neither is the project, although work has certainly slowed.
 
Here's the latest render:
 

 
What's Changed?

  • Using a Soviet Aerospace Toggle switch as the power switch.
  • Separate power light.  The Soviet switch has a glow light in it, but not a power light.  Amber is shown, but it will actually be blue, as on the other amps I've built.
  • More accurate measurements/layout.
  • More accurate rendering color (I've always planned for it to be black.)

 
A big change (and thus reason excuse for the delay) is that I have decided not to use the thin steel that was previously the top cover of the chassis. I have instead purchased a nice thick piece of aluminum from Skycraft in Orlando, and that will serve as the main chassis for this build.
 
If one visualizes a typical inverted 'U' shaped top cover, one may realize that if this is replaced with a flat aluminum plate, then the resulting amp will have -- shall we say -- really good ventilation on the sides -- in other words, the sides would be missing.  While I always attempt to provide good cooling for my projects, they sort of need to have sides.
 
So basically, I don't have a solution at the moment.  I am considering nicely finished wood sides, but the thickness of wood may prevent the unit from actually being put in a rack.  In reality, it is unlikely that I (or anyone else) would try to but something with tubes sticking out from the top in a rack,   but as long as sufficient overhead space and ventilation is provided, there is no reason it wouldn't work.  Even with wood sides, there should be enough tolerance to allow it to fit in a rack, but of course scratching the wood would be a risk.
 
The plan as of right now is to go with the wood sides.  While it may not really make sense, both wood sides, and the rack-mount ears and handles look nice separately, so they should also look good together.
 
Current Status:

  • The challenging cutouts for the displays, and other drilling of the the front panel is complete less painting.
  • Similarly, cutting and drilling of the rear panel is complete less cutting some of the un-used real-estate, which will be covered with perforated ventilation areas.  Similarly, painting of the entire bottom and rear of the chassis is still needed.
  • The drill plan for the chassis top is nearly complete
  • All rear panel connectors are temporarily installed (fit check).  This includes digital and analog inputs, and the speaker terminals.

 
Next Steps:

  • Cut the top plate from the purchased piece of aluminum. Using the basic tools I have getting the size and straightness needed is somewhat of a concern.
  • Once the top plate is cut and fits correctly, It will be drilled for the tube sockets, wiring feed throughs, and other components.
  • Remaining back panel cutouts.
  • Painting.

 
Once these steps are done, the actual building (component installation and wiring) can begin.
 
As mentioned, it is going slowly, and for now, it is not going to get much faster.  There are other projects and activities that will slow work on this project.  Check for updates on this and the other projects here on all of the Linuxslate.com "Builds and Projects" forums.
 
(Relevant and Polite) comments and suggestions are always welcome, so if you would like to join the conversation register as indicated at the top of the page.
 
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #2 - Jul 14th, 2021, 1:05pm
 
Just a minor status update.
 
I have cut the top panel (but not started drilling as of yet).
 
I have also decided to use wood side panels.  I asked my wife to bring me some thin finishing wood from Lowe's, and she brought home some plywood where the inside layers are wood paste (particle board).  It turns out that the piece of plywood that was used for shipping me the original Rowe/AMI chassis was actually better quality plywood.  It is also the same thickness.
 
So I managed to cut 2 decent pieces from the shipping plywood.  I have sanded, stained, and polyurethane'd them, and they came out pretty nice looking.  There should be no problem fitting the unit in a standard rack with the thin plywood sides.  Additional wood work may be necessary to make the rack mount ears fit properly.
 
The next step is to buy a threaded insert rivet tool and do a little structural engineering so that the chassis is structurally sound enough to be supported by the ears in the event that it is actually installed in a rack.
 
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #3 - Sep 18th, 2021, 12:55pm
 
So work is proceeding only very occasionally on this project.
 
While attempting to assemble the entire case (including the rack mount "ears") I ran into difficulty with the wooden sides.
 
I ended up cutting the sides from the original "lid" or cover, and using them as separate pieces.  The also required some additional cutting of the new top aluminum plate, but I now have the basic chassis assembled as it will be.
 
Next step is to get serious about drilling the top plate.  I have a little more finalizing of the top plate "drill plan", and then it will go under the drill press.
 
I would still much rather anodize than paint the top plate, but that is not a project that I feel like tackling myself, and I don't know of anyone locally that will do it, so it will probably either go to the local powder coater, or just get spray painted.
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #4 - Jan 18th, 2022, 10:43pm
 
It's another year, and little additional work has been done on this project;  However there's been a significant design change.
 
As mentioned in the previous post, I was attempting to work on the final layout (and drill plan) for the top panel.  During this effort, I became very concerned about both physical interference and electrical interfernce due to the proximity of the input circuitry to the Vacuum Flourescent Display (VFD) Spectrum Analyser module.  The VFD employs high voltages, and display scanning in the AF range.
 
After some addtional thought, I came up with a new layout for the amplifier.  I have moved the 2 OPT's together at the back of the chassis, and placed the Output Tubes on the sides.  This allows space to move the 12AX7's back signifcantly away from the VFD's.
 
While this has the disadvantage of moving the Power Tubes closer to both the 12V and HV SMPS's, the signals around the Power Tubes are much higher level, and therefore less subject to interference.
 
It also tends to "Hide" the power tubes behind one another, but this is not expected to visually detract from the apperance of the amplifier from any real viewing angle.
 
I will upload the latest render when I get a chance.  Latest Render added below, showing the new tube layout.
 
After a little time to review and think about this significant design change, I should be able to start drilling the top panel.
 
As of right now, the plan is to paint the entire chassis in-house to save time and money.
 
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #5 - Feb 2nd, 2022, 5:35pm
 
Finally, some real progress:
 
With the overall layout finalized, I was able to drill the top panel.
 
Here it is, drilled and anodized:
 

 
Thanks to Chem-TeK Metal Finishing for doing a very nice job on the anodization, for doing it in less than 2 days, and for a very reasonable price.  
 
In addition, cutting and installation of the vent mesh on the back panel is complete.  I also found hole plugs at Lowes that perfectly fit the RCA holes next to the SPDIF connections, so those holes will not be cut or covered, they will probably just have the hole plugs inserted.
 
The next step is painting the rest of the chassis, which as mentioned, I have decided to do in-house with just spray paint.  This will also include disassembly, cleaning and painting of the output transformer shells.
 
2 things are delaying painting -- First, it has actually been cold enough here (<50°F) as to be below the optimum temperatures for panting, and Second, Lowes was out of black metal flake paint due to the "Supply Chain Issues" we are facing here in the US right now.
 
Once painting of the chassis is complete, actual building/wiring can begin.  I will wire the AC mains connections to the low voltage power supply, and then wire all of the input connections to the input selector board, as well as the buttons, encoder and display for the DAC.
 
Before any actual building of the Tube Amplifier circuitry begins, the bottom chassis will be separately tested as a premium DAC, digital input selector, and spectrum display.
 
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #6 - Feb 3rd, 2022, 1:48pm
 
As we get closer to the actual assembly/wiring part of this build, I realized that I had not shared the following picture here:
 

  • Not Shown: Output transformers. They are on-hand, but were still attached to the Rowe R-4359a chassis when this picture was taken.
  • Not Shown: (Most) fasteners, discrete components, and quantity components such as wire, heat shrink etc.
  • Not Shown: 6VDC Buck converter. I do have one of sufficient current rating on hand, but I'm not sure if I'll use it. The alternative is to just series wire the 7591a heaters for each channel.
  • Yes, there really are tubes in the boxes. The 12AX7's are also branded Electro-Harmonix. All are new and new build.
  • As you can see, most of the metal work on the chassis is complete.
  • Inside the Chassis left to right: 90-250VAC to 12VDC SMPS (position approximate) -- Brackets for the DAC OLED display, and the VFD -- Input switch board -- DAC. Closer to the opening is the VFD Spectrum Analyzer board (partially viable) and the Lexan piece (with protective blue tape). The Speaker binding posts are installed.
  • On the right is the High Voltage SMPS with temporary heat sinks. The actual heatsink will be cut from the part shown at the right edge.
  • Front center are the Bias supply inverter boards, and the bias fail protection relay.
  • The 2 switches just to the left of the Bias inverters are for the Digital and Analog input select buttons.
  • Blue LED (appears clear) = Power, and the Red LED = Bias Fail warning light. I was going to use an Amber LED for power, but the family insisted that power LED's must be blue these days.  Command override.  I have decided to go back to using Amber for the power light.
  • Note that the OLED DAC control board on the very left will have to be disassembled, and wired separately. IR (remote) will not be supported. The volume encoder will be on the back panel.
  • Note Little Rubber Feet "LRF Technology" is a very important factor. Amps are like puppy dogs. If they have big feet, you know they will grow up big and strong.
  • The area around unused holes in the back panel will be cut-out, and covered with perforated metal for ventilation. The top will also be vented to allow convection cooling airflow around both SMPS's. (Perforated metal for both ventilation, and protection around the HV SMPS is not shown, and has not been ordered yet.) Note that as shown below, the HV SMPS will actually be on the left side.
  • The Chassis top (extreme top of the picture) has been cut apart.  Only the sides are retained.  The top chassis is the aluminum part shown in the previous post.

 
I want to give that large power switch on the left a separate paragraph. That is a Soviet Military Surplus NOS toggle switch as was actually used in Soviet Aircraft such as the MIG-21, Transports, and Helicopters. The spot on the end of the toggle glows in the dark. It gives a very satisfying clunk when switched.  It is rated at 20A/27B (Note that the "B" is the cyrillic letter "V" = Volts). The eBay vendor claims that it is OK for use at up to at least 250VAC.
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #7 - Jun 7th, 2022, 1:44pm
 
Glow Test !!
 
Due to [Insert pretty much every excuse possible], progress has been slow, but last night was the night for a tube glow test and photo opportunity.
 

 
The pedantic always need to provide notes to a picture:
  • This is about as close to what it will actually look like when finished as we are going to see for a while.
  • The High Voltage Power Supply is not present/not connected, so this does not constitute a full load test.  In all honesty, at this point doing a full load test is somewhat NVA (No Value Added).  See below for more discussion of further testing.
  • Tubes are running at 6.1VDC filament voltage.  Slow start is implemented, and it takes about 30 sec to ramp up to that steady state voltage.  It may actually be a little too soft.
  • The 12AX7's were not installed for this picture, but driver tube glow was verified before this picture was taken.  Filament voltage was tested at the driver tube heater pins while the power tubes were installed.
  • Note masking tape under the Output Transformers.  They are only installed for this photo (which also serves as a fit check), so I wanted to protect the chassis from being scratched.  The tape will be removed when the OPT's are installed "For Flight".  Nothing is connected to the OPT's at this point.
  • Obviously the display bezel is not installed.  Again, no point in risking scratches at this point.

Lights and Displays  (Left to Right):
  • Amber LED -- Power -- Specifically indicates presence of 12VDC Low Voltage Power
  • Red LED -- Bias Fail.  Obviously, this light would not be on for normal operation, but in this case it is accurately showing Failure of the Bias Power Supplies because they are not installed yet.  A failure of either or both (Logic OR) Bias Supplies will interrupt the 12VDC source to the HV Power Supply.
  • The DAC OLED Display can be seen at the upper left of the display cut-out area.  The dashes show no digital input (No bit rate to display), and the line just under that indicates that the DAC volume is all of the way up, or almost all of the way up.
  • Below that is a Blue LED indicating Input #1 (Analog Input #1) is selected.
  • The VFD is showing time since power on since the clock is not set, and there is no signal present to make it go to Spectrum Display mode.

All-up and Thermal Testing
The largest load/largest heat producer is the High Voltage Power Supply, so this test does not represent any significant load to the Main (12VDC) supply.  
 
The HV power supply was not installed because (1) This test is much safer without it, especially since the HV PS cage has not been fabricated yet. (2) I have not drilled and tapped the "real" HV PS heat sink yet.  More testing with the temporary heat sinks would not have provided much additional meaningful data.
 
As expected, just running the Power Tube heaters did not produce any significant temperature rise in either the 12VDC supply, or the 6.1VDC Filament supply after running for about 5 minutes.
 
The unit was audibly quiet (No PS wine) during this test.
 
No more playing Mr. Machinist -- No more renders or doodling in Libre Office Draw -- No More cutting the displays off of DAC's and then wiring them back again. No more putting it off --  It's time to actually build a Tube Amplifier.
 
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #8 - Sep 16th, 2022, 1:17pm
 
Project Update:
 
Note:  This entry is also posted to the Build Thread for this project on AudioKarma.org
 
This thread is very nearly 2 years old, but the project is not dead. In fact I've gotten back into it and made some progress.
 
The High Voltage Switch Mode Power Supply (Inverter) heatsink has been cut/drilled/installed, and the PS board and heatsink have been mounted to the chassis. The Heatsink is isolated from the Inverter Driver MOSFETs, and is bolted to the chassis for noise reduction and heat dissapation.
 
I think I have reached an engineering milestone -- I have made something so overly complicated that design changes can no longer add complexity, only simplifiy it -- or perhaps that's just wishfull thinking.
 
I have decided to eliminate the two 12VDC Negative boost converters that were going to generate a single (-) bias voltage for each channel.
 
In addition to the 380V winding, the HV PS has an 18V winding on the secondary of the transformer. Since it is a separate winding, it should be possible to use this winding to generate a negative DC supply in the same manner as is typically done in a classic grid biased or fixed bias amp.
 
Then I came across these on a "certain popular auction site":
 

 
These boards contain a bridge rectifier wired for a negative voltage, filter capacitors, and (4) voltage dividers to provide a bias for each of 4 tubes.
 
Note that because of the higher frequency, I should be able to use much smaller capacitors. I intend to leave the ones it comes with, and just bypass them with some smaller value non-electrolytic caps to help with the higher frequencies.
 
This will give me individual bias adjustment pots for each tube.
 
It is also much safer, as it contains only passive components. Failure of the HV PS will be fail-safe.
 
This means that I can also get rid of the "Bias Fail" relay arrangement. This will give me back much needed space inside the chassis, and greatly simplify wiring. It also eliminates the load of the 2 automotive relays
 
It also gives me more negative voltage if needed. The previous boost supplies maxed out (minimumed out?) at ~-15VDC when driven from a +12VDC source. I'd be in trouble if I needed more (absolute value) than that. Running from ~18VAC, the pictured board should be capable of well over (again absolute value) -20VDC if needed.
 
Ideally, the 4 bias pots should be accessible through holes or an opening in the chassis. Due to how far along I am, I don't think that will be possible in this design. Adjusting the bias will require opening the chassis.
 
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #9 - Oct 2nd, 2022, 10:39pm
 
First Sound
 
The Amplifier described in this thread has been powered up, and played music for the first time!
 
The first song played was Clubbed to Death - Kurayamino Variation by Rob Dougan -- More commonly known as the theme from the movie The Matrix.
 
A lot of work remains.  Here are a few initial notes and observations.
 
  • Only the first (Left?) Channel is currently built up.
  • I am having problems with the HV SMPS stability.
  • The power supply seems to run erratically at some voltages/loads.
  • Since the Bias voltage is now taken from the 18V Tap on the HV transformer, when the HV is erratic, the supply to the bias board is affected.  I am running the tubes within current limits, but I cannot adjust the bias as I should be able to.
  • I have tack soldered in just a resistor for global negative feedback.  I was very conservative in the choice of this resistor to insure that the amp was stable during initial testing.
  • Due to the above, the Amp lacks power and bass.  At this time, I do not have any indication that anything else is wrong that is causing the lack of power.
  • Also partially due to above, the amp is almost dead quiet in idle.  We will have to see if this remains true as I tune the feedback.
  • What I do hear is very crisp and clear, but of course I have not tried any actual testing yet.
  • I am using a known weak 12AX7 for this initial testing.
  • Again, I am only running one channel, and I only ran it for about 15 minutes continuously, but at this time, I have no indications of any thermal issues.

 
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #10 - Oct 15th, 2022, 2:46pm
 
Here's a Minor Update, Cross Posted from AudioKarma.org, with edits.
----
 
 
Engineering is always a compromise.
 
I don't like compromise.
 
...So I compromised.
 
Many articles debate the value for the Grid Bias Resistors for 7591 based amplifiers -- Particularly Fishers, but this applies to other grid biased 7591 amps, too.  Many of these articles suggest installing 220K, or even 200K, with the lower values helping to prevent bias run away. So I had previously installed 200K Grid resistors.
 
However, I felt that the Amp (only a single channel at this time), was not putting out the power that P-P 7591's should produce -- probably because the Phase Inverter was being loaded down by the 200K Grid Bias Resistors.
 
I didn't think that the change from 200K to 220K would make a lot of difference on the load on the PI; But going to 270K worried me, especially with my experience with (used) 7591 family tubes.
 
240K is also a value for Metal Film Resistors, and an assortment pack I bought from MPJA contained this value. <-- Yes, that's an unsolicited plug. 2W is overkill for a grid resistor, but 2W resistors in the values included in that assortment have lots of applications in Tube Amps.
 
So based on totally empirical "guess the number", I installed the 240K resistors.
 
The amp is definitely making more horsepower with theses resistors installed.  For all you Fisher owners, and other Owners of 7591 based amps that may be worried about run-away tubes, but don't want to hurt your sound, 240K may be the value for you.
 
I also changed the diodes on the diodes on the Bias Board to High Speed Glass Diodes that are more appropriate for the High Speed Switching PS that is providing the 18VAC for the (-) bias.
 
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #11 - Oct 16th, 2022, 2:10pm
 
Just another quick update:
 
The other Channel is complete, and playing music.  I have installed the brand new 7591a's in the left channel, and set the bias.  The amplifier is playing in stereo.
 
Supply voltages all look good (391VDC ob the plate, and 349VDC for the screen/pre-amp/phase inverter supply).
 
During build up of the 2nd Channel, I noticed that I used a 3.9K resistor on the cathode of the Channel 1 pre-amp.  I had 1.8K on the schematic, with 2.7K written in as "as-built".  I don't have any other notes or memory of where I came up with the 3.9K value, so I built Channel 2 with 2.7K.  I like the voltages around the Preamp/PI better on Channel 2, so I will change Channel 1 to 2.7K.
 
With the built-in Spectrum Analyzer disconnected, the amp is dead quiet in idle.
 
With both channels working, I now have a full load on all power supplies.  Nothing but he power tubes are getting hot.  
 
I am still suffering from Dummy load anxiety, and I don't have a pair of dummy loads appropriate for continued feedback tuning.
 
My intent is to complete final assembly (Chassis sides, handles, etc.), and in the mean time maybe-- order some dummy loads.
 
Pictures soon.
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #12 - Oct 16th, 2022, 9:23pm
 
As Promised, a Couple of Pictures:
 

 
 

 
 
Notes:
  • No High Voltage Cage.  Home Depot was out of the perforated metal sheeting I was going to use.
  • There is still protective masking tape on the top.
  • Front Lens/Bezel not installed -- Again, for protection.
  • Only the 2 Tubes on the left are the new ones purchased for this project.
  • Not Visible, but the little metal "L" brackets that allow the top to remain stable when the unit is opened are still installed.  There is still a non-trivial amount of work to be done inside.
  • Little Rubber Feet (LRF) installed.  That's kinda like when an airplane under construction has "weight on wheels" for the first time.

 
 
UPDATE:
 

 
Added picture of back panel.
 
  • Note USB/SPDIF Optical/SPDIF Coax inputs on the left.
  • DAC Volume Knob installed.  Note that this is a digital encoder, there are no wipers, and no audio flows through this device.
  • Only the 8 ohm terminals are currently connected internally.
  • Hole plugs installed.  At this point, it is hard to tell that this chassis was re-used from another device.
  • Hinge brackets to allow easy servicing are visible in this view.  These will be removed.
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Re: 7591 Amp Build (from Rowe/AMI R-4359)
Reply #13 - Oct 21st, 2022, 8:45pm
 
This project is (kind of) on hold again.
 
There's nothing wrong with the project, it's just that I don't have 2 decent audio dummy loads.
 
I had a whole separate project in mind to make an "Audio Tester" that would include dummy loads, but I have scrapped that idea for reasons that I will not get into here.
 
So why is it "kind of" on hold?  While the Amp itself has been off the bench for a few days, I have been working on the cover for the High Voltage SMPS (The board seen on the top of the chassis in other pictures.)
 
The original plan was to bend it from a sheet of perforated metal; However, I decided to make it out of materials I already had on hand.  Namely Lexan (Clear Polycarbonate) and steel mesh.
 
 

 
It may get one more coat of Polyurethane, but it is basically complete.  I was thinking of providing some lime green trim (Opposite scheme as the OPT's), but I may just use some green LED's mounted inside to achieve a similar, but more intense effect.
 
Maybe a "Warning High Voltage" sticker?
 
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