Restoring Electro-Voice Sentry IV-A

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I am in the process of restoring a pair of EV Sentry IV-A's, The Gefco woofers needed new foam surrounds and the boxes are more than a bit rough. One bass baffle was missing. The good news was that all of the speakers and drivers measure within spec for DCR.

The Sentry IV's were EV's answer to the Altec Lansing Voice of the Theater line. For a long time I have lusted after a pair of the EV's. I heard the EV's in a "shoot out" with Klipshorns maybe 15 years ago. I liked the EV's better. They had smoother mid range and high frequencies, but not quite the low-end extension of the K-horns. The K-horns were an early vintage with mostly EV components. The K-horns, obviously took the corners and the EV's were placed next to them, so placement advantage played a role in the perceived bass extensions. (It wouldn't have been fair to NOT give the K-horns the corners. It was not like we were switching this brutes about constantly!) I still have a strong aural memory of the dynamics of 102dB plus horns!! It was pretty amazing.

I am running them with my BottleHead Paramour 2a3's and I am pretty impressed, even in these early stages of restoration. Still need much work. All of the Gefco 12" woofers now have new foam surrounds. The horns didn't sound all that good on initial fire up...screechy! Replaced the old zip cords connecting the horns and the wire for the woofers with solid core silver plated copper/Teflon...more focused imaging and detail. I took the tweeter protection circuit out...better still. Then I replaced the x-over caps with Russian PIO; KBG 2uf,(tweeter), and MBM 30uf, (mid). The old caps were mylar and one of the 30uf was an electrolytic. Everything smoothed out, timbre was much better, imaging much more focused and a the sound lost a coarse, nasal quality. I am spec'ing out replacing all of the coils.

So, the bass? One would think, based on the size, and 2 12" woofers per side, that bass would be subterranean...Nope, doesn't get to the sub woofer realms. But placed in the corners of my room, (approximately 15'X 40'), the bass has much more impact than my Gallo Solo's, (Rogue Audio M150 Magnums or monoblock modded Golden Tube Audio SE40's) w/ SS powered subs, or my open baffle project, (GTA's or bi-amped Bottlehead Paramours). (OB =>Audio Nirvana cast 8 and Eminence Alpha 15). I am running 1 BH Paramour per side. Will try bi-amping with the Paramours, when I figure that out from the x-over schematic. (The 1/4" phono jacks on the bass bin's were replaced with a 2 bladed connector, the x-over switch to bi-amp is actuated via a 1/4" jack.)

What I hear is marvelous. Approximately 102dB 1w/1m is really amazing in terms of dynamics. I really like the horns. I could probably live with the bass, but the bass bins are huge. As one can see in the pic, there is nowhere for additional subs! So, I am going to indulge on my wife's tolerance and let the new parts in Sentry's burn in for awhile. Still need to work on mounts for the horns. Clean up seems to be never ending as every time I touch the speakers I find new nooks and crannies full of decades of vintage crud!!

I am going to re do the x-overs.



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Simplify Crossovers

The original crossover can be switched in by inserting a dummy 1/4"phono jack into the J1 bi-amp switch. This takes the woofers, out of the x-over circuit. To bi-amp one would need to provide an external crossover and amp for the woofers? The mid/high's would be driven from the stock crossover input? I plan to reduce the x-over to eliminate the tweeter level switch,bi-amp jumper and the STR. Without the level switch the frequency versus power handling for the tweeter is 10w@15K. (See Curve 1 on chart)

This is my revision. This layout can be strapped for single amp or bi-amped. I dropped the .6mH,(Not sure why it was there!) coil in the switch circuit and added a 3uF cap in the mids to maintain 2nd order roll off. These values are basically the stock values. When I model, the crossover points are more like 500Hz & 5K Hz, 2nd order Bessel. The spec's call out 400/3500 Hz.

I am definitely not real experienced playing around with x-overs. The plan is to replace the current x-over box in it's entirety with the new crossovers, rather than hack up the stock units. Does it look reasonable? How can I improve the design,without getting too complicated? I would appreciate thoughts, suggestions and comments.



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These are grand old girls! Any progress to report?

Making some progress....mid horn's chips and broken tweeter mounts have been repaired and threaded inserts installed to hold the tweeter horn mounts and keep the mid horn mouths at proper opening height. Should solve the "Damn the tweeters are loose again, get me an even BIGGER wood screw!" problem. Other than the expected wear and tear, this is the only cosmetic issue that required structural repair.

Crossover components arrived today, so I will start them over the weekend.

Next will be to sand and refinish the mid horns. Then lot'sa cover up of the "Grand Old Girls"* age and travel related blemishes on the bass bins.

Still dealing with the "Can I keep them?...WAF issues!" Gotta make them look like furniture...well maybe appliances...wooden refrigerators? ;) But I got the extremely rare admission that they sound "Really nice!"


*Like that!!!
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Joined 2004
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Those look great! Congrats on getting them up and running. (I'm running Altec A5 VOTT, myself).
Your crossover looks "reasonable" at first glance, tho perhaps a bit too ideal or simple. The crossover is where the speaker will live or die. And it's not easy with a big horn system like this.

To build a good crossover for these giants, you really need to measure, measure, listen and measure. These sorts of speakers need a lot of "tricks" in the crossover to get the best results. It can be a long process, especially if you are new to the game.
The bass

I have an RCA "directional baffle" going in my largish shop and the same thing is happening; no subterranean lows but above 40 Hz or so it makes no mistakes. At the moment it has a pair of 60's Fisher 15s that have a fs around 10. I'm lucky to have a pro sound pal with surplus drivers since he's gone to LAB 12s for the low end in his rig. So driver rolling will ensue. It's good to have room for these big boxes to play in.
Were the Gefcos original?
You will not get low bass with a w horn. 40's are about it. The horn path just doesn't have the length. As mentioned corner loading does help. Keep in mind pro sound stuff back then didn't do much below the 40's back 20 years ago. If you put a horn extension on them. Look up the levan horn for an example. You might gain a db Or so to in efficiency.

Listen to Pano though, he's an expert in this subject.

After reading this thread I come across a pair of EV Sentry IVs advertised in the Los Angeles Craigslist. The guy only wanted $100 for the pair so I jumped on them. I've just brought them home. All of the parts are there but the cabinets need some work. They spent a year exposed to the southern California sun and definitely show the damage. Yours are clearly in much better shape.

My horn is done at 60 both in path length and circumference. The corner, and a bit of eq, gets it as low as it needs to go. Of course another octave down would be fun but that's where my friends surplus 2245s might fit in...the "ring of fire"
won't be going on in my low power world.
After reading this thread I come across a pair of EV Sentry IVs advertised in the Los Angeles Craigslist. The guy only wanted $100 for the pair so I jumped on them. I've just brought them home. All of the parts are there but the cabinets need some work. They spent a year exposed to the southern California sun and definitely show the damage. Yours are clearly in much better shape.

That's a steal! Good job! How are the drivers?

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The previous owner had bought them new back in 1978. He still had the sales receipt for $1300. He used them regularly right up until just a few years ago. When he took them out of service he removed the horn and crossover assemblies and stored them inside. The cabinets were put under a tarp in a storage shed and more recently outside up on cement blocks. At some point squirrels got inside and chewed holes in the the drivers. As result the EVM 12L drivers will all need recone kits. Those are $60 apiece. I will do the recone myself. The crossover, tweets, mid horns & CDs are all in good condition.

After getting them home I more closely examined the cabinets. They are not in as bad of a condition as I had first thought. Only the outer veneer of the plywood is damaged and only on a few panels. All corners and joints are still tight. EV used a quality plywood when they built them! I think a couple of busy weekends spent using a belt sander, poly resin & hardener along with a good industrial primer should make them look like new again.
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Here are the mid horn tweeter rails repair in process:

They just kept using bigger wood screws and more glue! :eek: I replaced with 8-32 threaded inserts and SS cap screws.

Roughed in repairs with 2 part epoxy putty. I used small brass wood screws to form foundation posts in the broken out structure.

Cleaned up wood screw holes in the "undamaged" horn, screwed in threaded inserts with Gorilla Glue.



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