Gumtown's compact and powerful Bass amp-combo build

Started by gumtown, March 30, 2015, 10:01:34 PM

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gumtown

Here is my current DIY 'thing'..
After years of carting large gear around for gigs, and lifting an awkward shaped powered monitor upstairs to the practice space
I decided it was time to make something....compact, light weight and powerful.

I had always thought small drivers might be the answer, so I chose these 6.5" drivers from a local supplier,
they seem to be copies of KEF or RCF drivers with yellow Kevlar cones, and rated at 125W RMS, 250W Music power.

a 6.5" quad box it is, and a HF horn too (why not).

I started fiddling with some box parameters using WinISD, and came up with the required recipie.


and this was the plot from the give specs and sizes


So I set about with some 18mm 'form ply' (lightweight and dense resin/ply) and made a box to the calc design

meanwhile I had been shopping on eBay for modules and kitsets
requiring a 2 x 350 watt amplifier (run 2 x 8 ohm driver pairs at 4 ohms), a 700watt SMPS (switch mode power supply),
a stereo 6J1 tube buffer preamp (one for the power amp and one for the D.I. out) and parts to build a tube preamp/tone circuit,
which is an Alembic design robbed from Fender, the TMB preamp with bright switch.


this is pretty much what the 6J1 buffer preamp is

and here are the power amp modules being assembled


Then a dry fit into the top space

and at last the knobs arrive



more to come .....
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CodeSmart

Looks great  !!!
Our bass player wants one, his gear too bulky  ;D
But I got more gear than I need...and I like it!

gumtown

next...

the items are placed onto a gear plate and a 2 U face plate front panel painted black, with the pots and switches drilled in


top view of the layout


and a side view


the top of the box is cut down to a 2 U space, covered in a 'faux leather' vynayl covering from the local fabic store.
a test drive in the garage indicated the two vent fans (one blows in the other sucking out) were not enough to dissipate the amplifier heat,
so another fan was fitted over the heatsink.
The test drive had astounding results, far exceeding my expectations, the blue leds are fitted behind the tubes, and the front grill with black curtain netting and a fabric 'jolly rodger' fitted with white leds behind the eyes, which are connected across the speaker output, light up as you play.


Then the big test, we had to skip the last practice before last Friday's gig, so I tested on the night...
Second song after thrashing it for the first song, and .... it died....
The aluminium strap holding the power devices to the heatsink bend out on the ends and the outer devices cooked after lifting off the heatsink..

never mind, I am going to ditch the linear poweramps and am now waiting on delivery of a class D power amp.

the front panel from left to right are fuse - power switch -standby switch - ECC83/12AX7tube view hole - input - Gain - Bass - Mid - Treble - Bright Switch - HF driver switch - Master volume - 6j1 tubes view hole - Send/Return loop - D.I. Out and DI ground lift switch

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gumtown

#3
Here are the parts purchased off eBay

500 watt class D amp module
http://www.ebay.com/itm/121501215933?_trksid=p2059210.m2749.l2648&ssPageName=STRK%3AMEBIDX%3AIT

Switch mode Power supply 50-050 volts + 20-0-20 volts
http://www.ebay.com/itm/121141952907?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

speaker protection relay module
http://www.ebay.com/itm/251786498992?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

6J1 stereo tube preamp/buffer
http://www.ebay.com/itm/400771396952?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

skull knobs
http://www.ebay.com/itm/261727540478?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

350w + 350w amplifier kit which I have now abandoned
http://www.ebay.com/itm/201212178246?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

and from my local suppliers

4 x 6.5" Kevlar cone speakers
http://www.surplustronics.co.nz/products/576-wide-range-kevlar-cone-woofers-65-inch-

horn flare
http://www.surplustronics.co.nz/products/571-horn-flare-for-hf-drivers-110mm-x-265mm-x-170mm
and piezo driver
http://www.surplustronics.co.nz/products/583-piezo-horn-driver-standard-version-1-38-thread-65mm-

As for the tube preamps power supplies, the 6J1 module has a built in voltage multiplier, so all it requires is a 12 volt AC supply.
The 12AX7 preamp (which I built from the schematic on vero/strip board) runs the plate voltage at around 160 volt DC,
I used a multi tap transformer (24VA), 12 V AC for the heater, and another smaller 12 volt AC transformer (12VA) fed 'backwards' from the 7.5 volt tapping into the 12 volt secondary and back up to 140 volts AC. That saved having to get a special wound transformer .
(mains voltage is 240 Volts here).
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gumtown

#4
And here are the glamor shots







The bass response out of this is phenomenal, it really shakes the walls, and with a really clean crisp top end (as good as a peizo horn gets), I may yet replace the 1" HF driver with a real titanium compression driver.
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whippinpost91850


HAMERMAN409


gvidelock

Wow! As we say in Boston ("Bah-stun", that is): wicked cool!

Once you have your tone dialed in, you should pull the skull knobs off and re-orient them to face the same, smiling direction. Then you can see at a glance if one of them got turned by accident during moving.
-------------------------

Godin Spectrum SA
Godin Freeway SA
Fender Telecaster with Ghost and Hexpander
Fender Stratocaster with internal GK3 with Synth-Linx
Warmoth custom with internal GK3
Boss SY-1000
Roland GR-55
Roland GP-10
Katana 100W Head

Elantric


gumtown

#9
An update on my current project:
The first gig and the amplifier section only made it to the second song, something in the amp module was unstable and went into thermonuclear self destruct.

So I got the amp module I was originally going to use, a 500 watt Class D amp, which goes with the original concept of pairing old school preamp tubes with a modern class D (some call them digital even though it is basically PWM) amplifier.
It runs stone cold at full power and the combo has now been used for 3 practice sessions and 3 gigs, and absolutely no complaints at all.
Everyone is surprised at how much noise and solid bottom end such a small combo can make (it does as well as either of my 400 watt Marshall bass stacks)

http://www.ebay.com/itm/121501215933?_trksid=p2057872.m2749.l2648&ssPageName=STRK%3AMEBIDX%3AIT
Free "GR-55 FloorBoard" editor software from https://sourceforge.net/projects/grfloorboard/

Frankster

I would very muck like to see and hear a video of the finished project.

I built a version of that Alembic preamp myself many years ago, my brother still uses it for bass. It's one of those designs that has been copied so many times it's public domain now.

whippinpost91850

Congratulations on the new project. I'm all about small , powerful and light weight. But then again I'm old ;D