EKO Duke Amp restoration

[vampwizzard]

Hello again folks! Had a buddy ask about an old amp he had sitting in a basement and whether or not id take a look at it. He walked out with an EKO Duke. Pictures of the front and guts attached.

EL-84 push pull with the power section of the older AC-15 and a wild and crazy tone stack on the preamp. One 12ax7 for preamp, second 12ax7 for PI. Very little exists in the way of documentation on this amp outside of this french link http://www.projetg5.com/phpbb3/viewtopic.php?t=2231 where they have several versions of this gentleman's schematic. I was able to trace most of it out and find some inaccuracies he had.

There are some blown resistors from what looks like a previous service. Since resistors are so cheap Ill just buy a full set of replacements and get a recap job going.. already took out the death cap and will put in a new 3 prong cord. Hopefully we can get this thing playing again.

[PRR]

Interesting amp.

The other guy's tracing is clearly wrong in the phase-splitter grid-cathode area; you also expect a DC-block cap from the stage before it.

The tone-stack is plausible, and somewhat like a Fender. Obviously leave it alone for now--- the world has plenty of Fender/Marshall/clones already, find out what unique voice this beast has.

Nice sock.

[vampwizzard]

Love me some locally sourced wool socks.

Thats the part thats got me scratching my head. The power tube bias cap value was 200uf as well... which was surprising but matches early 60s ac-15s.

The .01 cap in the B+ filtering is actually the death cap attached from the switch direct to ground, but on the board. Im convinced some got a hold of this and messed with it before me.

Tone control wise the bass knob is allegedly mid boost from zero to noonish and bass boost from there on. Looking forward to this one.

[fwbik]

good afternoon.i bought one month ago one eko duke.my problem is the amp works too hot....so hot than i can touch the chassis with my hands because can burn my hand.can any help me?

[vampwizzard]

fwbik: If its new to you and hasnt been serviced regularly the amp needs to be checked for failing parts. Failed filter capacitors and blown resistors can create unsafe voltages and currents in the amplifier and can damage it. It can also cause damage to the transformers.

[vampwizzard]

Update: finished recapping. The only thing left is to figure out how this output transformer works. The two, untwisted flying wires are for the NFB circuit. The lone green wire closest to the camera is the B+. The twisted black and green pair not attached is the output. The twisted green and black pair attached to the far side of the transformer are from the power tubes.

Has anyone seen an output transformer like this? I can hunt down data sheets but I’m not familiar with these. Last piece of the puzzle.

If looking straight down from over top there are four nodes on the top and bottom and then on the far side there are another four nodes on the bottom of the coil.

My assumption is that the length of the NFB wires are correct and that they will connect to the OT on those remaining bottom nodes so that they are attached on a coil shared with the speaker.

[vampwizzard]

It turns out that EKO standardized their output transformers across all of their amps and there are detailed schematics for the OT here: http://lecaldevalvole.altervista.org/wp-content/uploads/2013/11/ViscountReverbStory.pdf

[vampwizzard]

Made a somewhat careless mistake in connecting the neutral to the amp. I found a common connection to the fuse holder where I normally attach my neutral but in a vintage amp where the death cap was removed that connection would then be grounded. Don't use that preexisting connection.

Got the power wired appropriately and went through start up testing. B+ starts a little over 400 and sits at 312.8 for both EL84s. It does make sound and breaks up super early with the NFB still disconnected. Its got a lot of 60hz hum. I had reflowed all solder on the board and grounded the power filter section to where EKO grounded it originally using a lug near the rectifier. Have to chop stick this thing out.

The owner originally had said he did not care at all about maintaining the originality of the amp and wanted something playable with potential of converting it to a head. I dont trust the wire gauge on the output section so I believe ill be rewiring that flying lead off the OT to a speaker jack and running the NFB in a way thats a little easier to diagnose for the next tech.

The adventure continues.

[shooter]

Its got a lot of 60hz hum

you probably know these but;
filaments got a CT reference?
using old/original tubes?
new wiring for filaments or old?
split ground for pre/power?
does it go away on 10 

[vampwizzard]

Its got a lot of 60hz hum

you probably know these but;
filaments got a CT reference?
using old/original tubes?
new wiring for filaments or old?
split ground for pre/power?
does it go away on 10 

lol one miracle at a time here, shooter.  Volume pot needs some deoxit and maybe replacing. Chop sticking led me to one bad solder joint on the second preamp tube which ill address. Separate pre and power grounds but the preamp grounding is a nightmare. All those gray looking wires off the pots? exposed shielding for shielded wires. Intermittently grounded to the chassis to prevent them from moving.

No tube shields. In open air. There is a ground ref for the heaters though so thats a great feeling. New tubes. original filament wiring.

More troubleshooting this weekend.

[nandrewjackson]

Looks like a mess, but a very interesting mess.


Cool project!