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7591 Amp Build (from Rowe/AMI R-4359) (Read 3094 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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