Friday, 18 May 2012

Jet City DIY front panel.

The Jet City JCA20 comes with a "Marmite" front, you either love it or hate it. I am in the latter camp so decided to do something about it. At the same time I changed the knobs and the guaranteed-to-fail blue lamp. The panel was MDF and the cloth was from a previous project giving an old Peavey combo a face lift.

The whole surface had contact adhesive rolled on and when firmly stuck down staples were banged in at the rear to stop edges lifting. I wiped the back panel over with some blue fountain pen ink as a colour wash as you can see this from the back.  I did try some black chicken head knobs that I already but you just couldn't see where they were pointing.

The bottom edge of the new front had some router-love in the way of a round-over bit on both the front and the rear of the panel. The one on the back of the panel, though not visible makes a good gap for air-flow.


  1. Ha-ha!!
    I also changed the front of mine!
    I removed the back panel, leaving the rear of the cab open (it seems as if all my amps end up without back panels). I now have replaced the solid front panel with the rear panel which has an expanded open wire mesh. I can now watch my output plates for meltdown (always run 'em hot and on the edge with minimum neg bias!) LoL!
    Seriously it not only looks cooler but also better air flow too!

  2. I did look at moving the rear panel to the front as it does look good, but I wanted the head to match a combo that I built and I had the speaker grill cloth left over from that project. Rounding the bottom of the panel both front and back does open up the venting without becoming visible. What a lot of people don't realise is that air won't readily move through a 2.5mm (10th inch) gap by itself so the existing gap left by leaving the front panel flat at the back doesn't do anything for air flow. Of course having no rear panel just lets it all out! The red lamp didn't last long before burning out. I live in an area where it has close to 250V mains, and the transformer looks wound for 220V. I end up with a HT close to 400V and the 6.3V heater (hence bulb) is 7.15V. This puts out about 25% more heater dissipation which can't be good.

  3. Variacs are big and expensive. Better would be a small transformer wired as a buck transformer.
    Put; Elliott Sound Products buck transformer into google, it's a good primer, note how the VA of the transformer is small to the supply required.

  4. My Superior Electric Powerstat trannys were quite cheap, are only about 20x20 cm, are 10 amps apiece, and work like a charm (charms?); on eBay they tend to be about $30.00 (US) for good ones ((without built-in ammeters). They are also very helpful in amp diagnostics.

    The lovely thing about variable-A.C. transformers is that they are not stuck at a single step-down voltage. I can run my amp at its rated voltage most places, and can drop it down for the "brown sound". Lovely.

    I will check out Elliott buck trannys. -But I tell you, variacs are the smack.

  5. Oh, yes... if you want to drop your B+ (I was JUST looking into this last night!), you can use: a resistor (with "terrible" voltage regulation as the result); Zener diodes of the proper voltage to drop the voltage in stages (I gather using a number of smaller-voltage Zeners offers superior performance to using a single large-voltage-dropping Zener); a thyristor; a dropping transformer ...and I've probably missed a lot of alternatives, too, some better than others. I'm sure you are aware of most of the good ones, anyway.

    I wonder if a device which would cut off the A.C. wave for part of its cycle would help...? I'm thinking of something like a filter set up to cut off the power for a predetermined part of the power cycle, so that you could use the polarization of the wave to your advantage...? I may be describing a common device, though. The other way to implement similar solutions is to have a fast-acting momentary switch set to turn on only when the A.C. wave reaches a certain amplitude, so that the power cycle is attenuated: the cycle is 60 Hz, but if the power is only "on" for a portion of that cycle, the effective power level has dropped, and the actual voltage should have dropped, right?

    1. Oddly I am just finalising test dropping a high B+, so very timely. The zener diodes have hit the bin and the resistor was never going to get a look in. My issue with the zener is it takes a fixed slice out. So at high mains it will bring you into a good area but with a low mains supply it takes a higher percentage cut. The overall variation is too great. I have a 15-0-15 output 250mA transformer to wire to a chassis in the loft when I can drag it down. Being wired out of phase but to the same mains supply it will scale the drop, so as the input mains reduces then so will the amount of out-of-phase voltage from the second transformer. I’ll post the results in a few weeks.