New to combos

[buzzy]

I'm used to playing through half stacks (usually Marshall) but recently bought myself a 30yr old Peavey Renown combo with 12" Scorpion speakers. After rewiring the reverb, replacing a pot and re-gluing one cone I now have a good solid amp with endless power and a nice tone... except the enclosure adds a 'boxy' quality. I reckon it would sound better with some damping in it and a back on the cabinet - but that got me wondering why combos are usually open backed and cabs closed? Was it a tube ventilating thing or is there some other reason?

My renown is solid state with an enormous heatsink on the back which hardly gets warm so ventilation wouldn't be an issue..

Any thoughts?

Buzz.

[long813]

Sonically the difference in open/closed back cabs is in the bass.

I suspect most combos are open back to allow you to swap speakers. A closed back cab demands a tight seal, so getting a tight seal on a combo will increase it's size, and be a pain to undo if you want to change your speakers.

... that's my guess.

[sumgai]

long,

Sorry, but buzzy's guess takes the cake.

It is a matter of heat, being as tubes were used in amplifiers for nearly 40 years before transistors came along. (Let alone 19 years more before transistors were first used commercially in a line of guitar amps.)


There actually were several designs that did enclose the speaker, leaving the tubes and such out in a ventilated area, but that was very early in the dawn of instrument amplifiers. Back then, we didn't have TS parameters and the like to help us design cabinets correctly, so it was all a craps-shoot - some designs worked, some didn't.

Also, you have to remember, in those halcyon days speakers didn't yet use Permanent Magnets, they were of the "field coil" design. This is where the high-voltage of a power supply (usually a battery) was passed through a coil for filtering purposes. This coil of course generated a magnetic field. In turn, it was thought to use this magnetic field as the "energizing field" of a speaker, thus allowing a voice coil to have something to work against. Field coils needed both ventilation, and access for troubleshooting and repairs, as they and their wires weren't the most sturdy components in the box.
</end Today's History Lesson>

---

buzzy,

I'd be careful of doing anything to that cab. Although Peavey didn't exercise any great care as to how to dampen the speaker with just the right amount of 'back pressure' from the back panels, they did design/select a speaker that was meant to operate in more-or-less free air. Enclosing such speakers usually results in unrealized expectations in terms of perceived tonal quality.

I suggest that if you are going to do something like this, you should: One: do lots of research; Two: talk with speaker cab builders of all kinds about what they use; and Three: use an exension cabinet first for experimentation purposes - no use risking your Peavey's cabinet in the early stages of your Mad Scientist-in-training escapades.   Like others here, you may find that an extension cab is all you really need in the first place, in order to get "that Tone".
</end Dutch Uncle>


HTH






sumgai

[JohnH]

My main amp is a 1x12 combo, by Marshall, and it needs all the ventilation it can get so cool the valves, which like most such combos, hang upside down at the top of the speaker space. The sound of this open-backed unit is clear and bright, but cant do the thump of a big closed back cab, unless it is connected to one. But there is some sonic pay-back in the more dispersed, room filling qualities of an open backed design.

I ofen have a closed back extention cab beneath, but I also am getting very nice results in rehearsal studios using just the amp, with an amp stand to tilt the combo back, raise it slightly, then put all of that near a wall - it really magnifies the sound, particularly the lower range.

Marshall have run several series of amps with solid state output sections, and closed back designs when in combo form, such as the Mg and AVT series. They try to capture some of their classic stack vibe in a small amp. They work OK, but it is hard to get that deep bass punch in a small cab.

Ive been getting good results in the extention cab, with some carefully considered ports - to add a bit more bass to to a small cab. Some manufacturers such as Mesa also make small cabs that work this way. It is critical to calculate these correctly, but not too hard to do.

Id say - experiment provided you take care of heating issues, but dont do anything you cant replace. If you try a closed back, keep the old back so you can reverse the mod.

Try a tilt-back arrangement near a wall.

If you want to check numbers, or think of ports, WinISD is a really helpful free program.

I/ll post more on any of that if you want....

John

[buzzy]

Thanks guys - I take all your points and what I propose to do first is try and EQ the boxiness out with an EQ stomp box in the FX loop which, on these amps, puts it between the pre-amp and the power amp just in case it's not the speakers but the pre-amp. I'd like to sort something as I prefer the general tone to Marshall (I'd like a 212R Frontman really but I can't afford one!) but for this subtle boxy undertone. I'll report back!

[long813]

Jan 14, 2013 12:25:31 GMT -5 sumgai said:

long,

Sorry, but buzzy's guess takes the cake.

Was it ice cream cake?

After I posted that, I looked at my amp and thought "There's no way to do this right".

Thanks for the history lesson.

[sumgai]

long,

Well, depending on the build of the cab itself (meaning, how well it was designed), I'd hazard a guess that 90% of all combo amps can have their speaker changed out/replaced within a half-hour. Usually less, sometimes more. I've seen boxes where the amp chassis actually covered, by design, one or two speaker mounting screws/nuts, hence the additional time required to take the chassis out.

But most of the time, it's not so hard to replace the speaker in a combo.


.....


Come to think of it.... Wait until you have to open up a Showman 2x15" bottom - 27 screws later, you'll come to a high understanding of why they make a 1/4" hex driver with a #2 Philips head, for use with a drill motor!







sumgai

[gumbo]

"I've seen boxes where the amp chassis actually covered, by design, one or two speaker mounting screws/nuts, hence the additional time required to take the chassis out."


...like every 4-10 Bassman I've ever seen (including mine... )

[buzzy]

Nah Peavey's speakers are front mounted - grille pulls off then unscrew the speakers! That's typical of Peavey and the build quality is excellent too. I don't know if today's rate but back in the eighties they certainly new how to build amps.

[long813]

Jan 15, 2013 9:34:29 GMT -5 buzzy said:

Nah Peavey's speakers are front mounted - grille pulls off then unscrew the speakers! That's typical of Peavey and the build quality is excellent too. I don't know if today's rate but back in the eighties they certainly new how to build amps.

I don't think my VK is front mount. And I've read that it's quite difficult to put some speakers in if you keep the tube in tact.

Their amps still seem fine, but are more directed to the metal crowd now.

[buzzy]

Yeah they hooked up with EVH a while back didn't they - I suppose I do play some metalish tunes occasionally but it's more fast-rock songs and the aggressive thick sound of the Peavey appeals ...cue the 'aggressive & thick' comments lol!

[Deleted]

I think my crate combo has an open back as well. Talking about swapping speakers, when/why should someone replace a speaker? What is lets say the overdue period to replace it? 20yrs? 30yrs? Later than that? Would that make a significant change in sound? Does a speaker swap make any effect for an otherwise working amp? Would some brands/types of speakers be better for specific kinds of music?

[long813]

Jan 16, 2013 0:06:20 GMT -5 @pyrros said:

I think my crate combo has an open back as well. Talking about swapping speakers, when/why should someone replace a speaker? What is lets say the overdue period to replace it? 20yrs? 30yrs? Later than that? Would that make a significant change in sound? Does a speaker swap make any effect for an otherwise working amp? Would some brands/types of speakers be better for specific kinds of music?

To change the sound (or broke) and I guess that only can come from experience or getting info from other people.

Speakers can make a HUGE difference, it depends on what was in there before. Some stock speakers can be quite a doozy. Maybe for cost reasons? I don't know. But, then on the other spectrum, boutique amp makes will spend a lot of R&D to get the right speaker for their amp.

Company say that some speakers are good for style X or style Y, but I feel that is really just a generic lumping of sounds. Like, "Oh this speaker has scooped mids, so it's better for metal" ... type of thing.

[sumgai]

long hits it pretty good - changing speakers is a matter of taste, unless the speaker is visibly and audibly kaput.

newey did this, what, 18 months ago, give or take.... His Fender Sideman went from being a so-so practice amp to something he'd gig with, if he ever left his bomb shelter. (!) He did it just for the potential of improved tonality, and for him, it paid off.

Many players will "upgrade" their speakers, some for valid reasons (the amp was/is known to come from the factory with cheezy crap), others in search of "that Mojo Tone". I myself changed out the speaker on my own Fender amp only once, a 1x15" Bassman as it happens, and pretty much deteremined that it was a waste of time. I put the original back in, and sold the replacement for what I paid for it. (Lucky me, craigslist was my friend that day!)

My FTR still has its original speakers, but if I were playing, I'd be searching out a pair (or two) of Fender Gauss speakers. Heavier than sin, but Gawd what a sound to die for!! Clean until the cows come home, they simply can't break up. Not even JBL D-120F speakers sound this good, trust me.

But short of that, I'll just leave the amp stock, speaker-wise. Ditto for my remaining bass amps and PA speaker cabs. To these tired old ears, they're close enough to the real thing that I'm happy. ;D

HTH





sumgai

[buzzy]

Oh to be able to swap speakers to tailor the sound.. I took mine out the Renown to glue the cones back in!!

[buzzy]

Update: I couldn't make the amp sound as I wanted (it must just be the voice of the pre-amp) so I sold it on and picked up a Laney GC120 (for £30!) which I really really like. Plenty of power and a nice neutral voicing - I just need to make a footswitch for channel changing and I'm sorted! Cheers, Buzz.