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View Full Version : Any thoughts on a custom tube amp designed for electric mandolin?



Wesley Owens
Dec-06-2012, 10:06am
My name is Wesley Owens. I own Owens Custom Guitars and have designed a great semi-hollow bodied electric mandolin. My day job is prototyping and producing custom tube guitar amplifiers. I have played the mandolin through just about every amp imagineable and I feel I have a solid grasp on how to properly voice an amp specifically for the mandolin, especially when it comes to getting the mid focus right on a distortion channel, where I find a lot of guitar amps sound boxy with it. The mandolin sits more comfortably in a higher mid range focus than the guitar, which makes sense logically. It also seems to work best using an amp with a big but tight low end and a singy but not harsh or fizzy upper range. Does anyone have any ideas on what they would like in a custom mandolin amp, or how they would change their guitar amp to be a better mandolin amp? It seems a couple 10 inch speakers with some low end girth would be optimal, and an amp in the 20 watt range. I think it would be fun to design, and there may even be a small market for it. Thoughts?

Jim Bevan
Dec-06-2012, 11:00am
I have a few amps that I use for electric mando, but my two favorites are:

1. A Jule Potter/Mercury Magnetics-modded Orange Tiny Terror, going through a Celestion Blue housed in a Bob Fusco-built small-as-possible cabinet.

2. A Dr Z Carmen Ghia head installed in a Bob Fusco-built small-as-possible combo cabinet, also housing a Celestion Blue.

I mostly use the Tiny Terror with the mando in the avatar for full-on rock, and the Dr Z with a Steve Ryder Tele-mando for a nice country twang.

If I ever get off the road and start cleaning out the basement, I'll probably sell most of my amps and just keep these two.
If I need more than the 15/18 watts, I'll run both amps together using a Lehle P-split.

The Celestion in a tight cabinet sounds better than any pair of 10s, and I've tried pretty much all of them.

Wesley Owens
Dec-06-2012, 12:03pm
Ok. I can see where a tightened up 12 would give you that bigger bottom end the mando likes. In terms of tone, what is it that you like about those amps compared to others you have tried? It seems the tiny terror and dr z amps in general are harsh in the mids for guitar, do you think that is why they sit well with your mandolins? What do the celestion blues give you that is otherwise missing on your set up? I'm interested in why this sounds good to you and why others didn't.

Pete Martin
Dec-06-2012, 12:17pm
Of course it all depends on the sound your shooting for. For me, a fat warm Jazz sound ala Wes Montgomery is it. I have found this with an '66 Ampeg Gemini II. Spruce told me to try a 15" speaker and he was right. It sure seems to deliver a better mid range than smaller speakers.

I also have a 66 Ampeg Revererocket II. Basically same sound but with a 12" speaker. I can really hear that difference the 15 makes.

Might want to try this or make it an option for a mando amp.

I also have a SRV Vibroverb clone (1 x 15) from Marsh Amps on the way. I'll let folks know how it works when I get it.

rico mando
Dec-06-2012, 12:31pm
well I would recommend having controls on the front or top for easy access during a gig . use just one speaker in a combo but have speaker outs for extra cabs ( very useful for gigs with no sound guy or monitors as you can point the extra cab at the drummer ) as well as easy transport for gigs . bring just the combo or bring combo and a few cabs to make a stack . a stand is important and I would like to have the ability to put some of my rack mount gear on top .( maybe a DI as well )

I currently use a mesa boogie 5/25 10 inch speaker with 2 extension cabs or a stand .94945

Jim Bevan
Dec-06-2012, 12:55pm
The Celestion Blue just has tons of character, more than their 10s, more than Weber's Celestion copies, more even than an old JBL D120, along with having a very clear high-end and a nice low mid fullness.

The amps, I just turn down the treble to a nice-sounding point.

I've used a Blues Junior with a Greenback and then the D120 (but not the Celestion Blue), a Valvetech Hayseed 15 through a pair of Weber Celestion-copy 10s, an old Fender-something with a Weber 10 and a JBL K110, other amps -- the Tiny Terror just sounds the best, with the most variety of crunch.

Plus, my two favorites are really light, the best tone-per-pound ratio, much-appreciated. :)

Polecat
Dec-06-2012, 3:26pm
Wesley, this is an interesting thread, thank you for starting it. I actually went down that road about 15 years ago, having had an emando built for me and not being able to find an amp that sounded how I imagined it should sound, I built myself one. I posted a link to a soundcloud file of how it sounds here (http://www.mandolincafe.com/forum/showthread.php?69977-Tube-Amp&p=1079642&viewfull=1#post1079642) (I hope the link still works, my browser no longer allows me to access the page). Unfortunately, no one saw fit to comment on the sound in the thread, perhaps you, as an expert, would care to give me your opinion. I forgot to mention in the original post that the speaker is a 10" celestion G10S50 in a relatively small open backed enclosure. These days I mainly play accoustic, but in my louder days, I was always happy with the amp.

mandroid
Dec-06-2012, 3:55pm
Bi Amp It ..
with Optimized High frequency clarity, by having the crossover split,
and run the mid to high frequency signal thru its own Amp and speakers.
20w + 10w?

mandroid
Dec-06-2012, 4:09pm
I got a Selenium concentric 2 way speaker ,
Clever bit: high frequency Horn fires through a Mesh Dome in the Center.

have yet to use it , so still in a Box.. No Cabinet..

Pete Martin
Dec-07-2012, 12:38pm
Polecat, the link works and you and the mando sound great. Do you have any pics of the amp?

michaell
Dec-07-2012, 2:28pm
Wesley:

I've used a Polytone Mega Brute with a ten inch speaker (no tubes). I've also used a Musicman hybrid with two ten inch Celestion replacement speakers. Keep in mind that Wes Montgomery often used a solid state Standel amplifier, and he sounded great (obviously)!

Polecat
Dec-08-2012, 4:10am
michaell: Was that an external speaker with the Mega Brute? I thought they came with an 8". The Mini Brute II was the only amp I tried that came close to sounding how I wanted, and I considered buying one to try to mod., but pcbs and solid state electronics do not lend themselves easily to monkeying about, which is why I chose to go down the valve route - I'm sure it would be possible to custom build/modify a transistor amp that sounds great with an emando - it's just much easier with tubes.

pete: thanks for that, over the weekend I'll dig out the amplifier and take a couple of pics. I'm interested to read that you use a 15" speaker; that was a possibility I considered, but let myself be talked out of, perhaps I'll try it after all.

Astro
Dec-08-2012, 10:11am
I have tube amps but I wont gig out with them. Too delicate and expensive to chance. My mando sounds great through my Train Wreck Rocket clone. You could look up the design opinions (they vary) on the off label Trainwreck forums or through the Gear Page forum. The design can be fairly simple. Ken Fisher only made a few Rockets before he died. There are more Express and Liverpools around. Mine is an awesome amp that is too good to waste on me but I'm over the guilt.

As crazy as it sounds, I play my mando through my SWR California Blonde. Its not a tube amp, but its great for so many things that I doubt I'll ever sell it. Originally designed as a pro level acoustic bass amp , its a 12 inch speaker and has a separate horn (which I turn off for mando). For mando I roll the treble all the way off and that de-quacks my piezo pretty well. For guitar my settings on it are very different and though I dont play bass, its of course great for it. I also use it for vocals too when I scale down to solo or duo. Sounds way better than my buds Fishman Loudbox. It is a tank which I guess is good to be rugged for gigging out, but its very heavy at 50 lbs.

Polecat
Dec-08-2012, 11:44am
Here, as promised, a couple of images of my amp.

95014

95015

95016

AS you can see, it's hardly in mint condition, but it's seen a lot of hard use over the years. I decided not to pull out the chassis to show the wiring, but as in most point to point wired amps, its pretty much a birds nest. I would feel sorry for any tech who had to service it. For me, it's OK.
I have never been able to understand the point of owning an amp which sounds great and not using it for what it was designed for, i.e. making your instrument louder onstage. Even 5W is too loud to use at home, let alone the 20-30 that I presume the Rocket produces (I followed Ken Fishers example and run my power amp tubes at a relatively low plate voltage, they sound better, last longer and there's less danger of overheating, even so, my nominal 15W is hellishly loud - the gismo next to the speaker in the second photo is a power attenuater constructed of old refrigerator parts). If you're over your guilt for owning the amp, Astro, I would encourage you to try to conquer your fear and use the damn thing - that is what it was designed for.

Astro
Dec-08-2012, 12:52pm
Polecat-

You are right the Rocket is 30+ amps and must be used with an attenuator (I have a custom prototype "Alex's Attenuator").

Way too loud for anything I do. Its very, very heavy and enclosed in a beautiful exotic wood cab. Its a luxury item I splurged on for personal play and I never intended it to travel. I use it mostly for electric blues but its clean works on most anything. I'm not good enough to deserve it and I dont play out anywhere that needs it. Sort of like owning a claasic car and not wanting to drive it to work everyday. Its for Sunday cruises.

Actually this thread got me testing and I'm going to have to change my tune anyway. The Rocket is a little bright for mando. I have to roll the treble all the way off, bass turned up, and shaping knob all the way over to make it sound good.

I have a little boutique Princeton based circuit 5 watter that actually sounds better on Mando. The SWR CAlifornia Blonde (ss) also sounds good but again on all these I have to roll the treble all the way off and use some shaping.

I'm a little embarrassed because I just now went to the top and read the op first post and I now get what he meant for this thread.

Yeah an amp designed from ground up for the highs of the mando is a great idea. Something where the sweet spot sits in the middle position of the treble and bass knob and where a tone eq wave knob does something useful.

I'll post a pic if I can.

Astro
Dec-08-2012, 3:20pm
95038

On the right over the cane front 1x12 Boogie cab sits the Rocket. Its a monster in every way but mostly quite clean until the end. (souped up AC30 chime). The attenuator is on top of it.

On the left is a little boutique amp on an old Newland chassis using roughly the Princeton circuit. Its in an antique wood radio cab and it sits on a simple 1x10 pine cab.

I don't play electric much anymore but when I do, I like my old Parker Fly.

I'm 80% basic acoustic guitar and 20% beginner Mando now.

Charlieshafer
Dec-09-2012, 2:14pm
I'm running a Swart Space Tone with an 8" speaker, and it sounds fantastic, still not sure why the need for a large speaker cone, and the hassles of carrying it. In a large space, you're miking the amp cabinet anyway. In a small space, you don't need the large cone. If you want mid and upper range tightness, which ought to be the preferred tone, as it is a mandolin, and not a guitar, a good sharp attack, which is what a smaller cone delivers, would be perfectly fine. Obviously, everyone has their own tone preference, but I want clean and bright myself.

Charlieshafer
Dec-09-2012, 2:48pm
Forgot to add: I WOULD like an amp I can get that slightly hot, overdriven sound to without a lot of volume. In a small venue, you simply can't turn a normal-sized amp up enough to not ruin everyone's hearing. In a small rehearsal space, I want to be able to get a little crunch, still preserve the highs, and NOT have to use a pedal for any distortion boost. Clean tops with a natural crunch. It's easy to boost volume with a p.a. system, not so easy to keep the sound under control at lower volumes.

Tom Wright
Dec-09-2012, 7:36pm
I'm running a Swart Space Tone with an 8" speaker, and it sounds fantastic, still not sure why the need for a large speaker cone, and the hassles of carrying it. In a large space, you're miking the amp cabinet anyway. In a small space, you don't need the large cone. If you want mid and upper range tightness, which ought to be the preferred tone, as it is a mandolin, and not a guitar, a good sharp attack, which is what a smaller cone delivers, would be perfectly fine. Obviously, everyone has their own tone preference, but I want clean and bright myself.

I am really happy with the the Eminence Alphalite 6-A 6.5" speaker. Nominally a woofer, its range goes up to 5K easily, and its midrange is sweet, while it has enough low end to handle the bottom of my 5- and 10-string mandolins. It is not as efficient as a larger cone, and even with 100W pushing it (cone is rated 100W RMS) you won't drown out any drums. My Crate PowerBlock will overdrive nicely, and the crunch is mellow and musical, although I don't depend on amp overload for tone--that comes from a superb pedal from AnalogMan, their King of Tone.

I built a super-lightweight box for it, and since the speaker has a neodynium magnet and I used 3/16" ply with bracing, the total weight is 5 lbs. Cabinet dimensions are roughly shoe-box size:

8 liters volume

W----8 1/2 in.
H-----13 1/2 in.
D-----5 in.

port diameter----1 3/8 in (no duct).

Resonance frequency +1.7db---108 Hz

Cutoff frequency, -3db---99 Hz

Wesley Owens
Dec-09-2012, 8:08pm
Astro-
I think you get where I'm coming from. You can make a guitar amp work for mandolin, but wouldn't it be better to design an amp with a useful eq section, proper voicing, and an interesting gain structure tailored specifically for the mandolin. It would give you more tonal options. Also, I think the e-mando rig should have it's own unique voice and not sound like it's trying to be a little guitar rig. I have an idea in my head of what that should sound like, I just hope to draw a little insight from the experience of others. So thanks to everyone's comments and keep it coming.
To Polecat-
I wasn't able to pickup on the low end of your soundclip because of the little speakers on my tablet, but as far as I could tell it had a nice woody mandolin tone and a tame top end, which is all good to me. I'll try to listen through some better speakers, but I will say it sounded like a mandolin to me, which means the amp didn't appear to be fighting the natural tone of the instrument, which some guitar amps can do.

Polecat
Dec-10-2012, 1:28pm
Astro-
...I think the e-mando rig should have it's own unique voice and not sound like it's trying to be a little guitar rig. I have an idea in my head of what that should sound like, I just hope to draw a little insight from the experience of others. So thanks to everyone's comments and keep it coming.
To Polecat-
I wasn't able to pickup on the low end of your soundclip because of the little speakers on my tablet, but as far as I could tell it had a nice woody mandolin tone and a tame top end, which is all good to me. I'll try to listen through some better speakers, but I will say it sounded like a mandolin to me, which means the amp didn't appear to be fighting the natural tone of the instrument, which some guitar amps can do.

Thank you for your comments, Wesley, that was just what I wanted to hear, that my amp sounds like a mandolin, because that is exactly what I was aiming at. If you would like the schematic (minus the mods), I'll be happy to send it to you, it is from a german publication about audio and guitar amps. It is the only amp I have ever built (I've refurbished a couple of others, but I'm far from being what you might call experienced), and it is a long time ago, but here's what I remember from my experimenting:

Reducing the b+ Voltage to the end tubes definitely helps in the problem of "too many highs" in an overdriven sound - when you wind up the volume on your instrument (provided you've got that helpful cap on the volume knob) the end tubes distort without giving you a lobotomy.
Altering the values of the coupling caps in the preamp didn't appear to have any great effect on the overall sound, whilst increasing the value of the caps parallel to the anode resistor to theoretically improve the bass response caused the amp to behave unpredictably if I overdrove the preamp tubes.
The standard (fender) values of the caps in the the tone section are, seen top to bottom, 250p (treble), 0,1u (bass), and 47n (mid) - radically changing the value of the 47m cap causes all the tone controls to interact in a way that is very interesting but not practical, so I left that alone - I increased the value of the "bass cap" and decreased the value of the "treble cap", effectively pushing the "mid damping" (if that is what it is called, all the literature I read on the subject was in german) to a higher frequency range and increasing the bass response - that was what had the most effect on the overall sound of the amp.
I experimented with a master level control both between the control grids of the end tubes and before the phase splitter, and was unhappy with both - too much treble damping. I also tried connecting the end tubes as triodes to reduce the output but the effect was not noticable. A power attenuator proved to be the best solution, but it is still necessary to turn up the treble knob to keep the sound the same.
Hope these comments are helpful.