Cobra Amplifiers

Canadian Amplifiers – Cobra

Here is another one of those ‘oddball’ brands that were more than likely manufactured out in Eastern Canada during the late 1960’s. Cobra amplifiers have a look very much ‘borrowed’ from the Blackface Fender heads during the same time period. An example can be seen below; it is the Cobra Model 800 MkII. These photographs were provided by reader known only as Chris Dayhoff of Boise, Idaho. Thank you Chris.

CobraCobra amplifier head borrows heavily from the Fender Blackface exterior cosmetics.

The first thing that catches everyone’s eye is the ‘Blackface’ styling of the exterior. Unfortunately, that’s where the similarity ends. Apparently, it was hoped that the looks alone would entice you over to this amplifier. It is purported to sound quite good, as I am sure it would, seeing that a simple tube circuit is used. The interior is nothing like a Fender, as evidenced in the photo below.

Cobra_interior2
A sparse circuit + point-to-point wiring = a very basic amplifier.

While the photographs did not turn out as well as I’d hoped (sorry about that), you can still see that the very sparse interior is really wired point-to-point, and no Fender-style fiber board is seen. It is also evident that the chassis is used as often as possible for a convenient ground connection. This was a common (although not recommended) practice by a ‘stencil-line’ amplifier maker from Montreal know as Pine Electronics. The concept of a ‘stencil-line’ is relatively simple; the amplifier maker produces amplifiers for ‘wholesale’ to virtually anyone who asks, and puts the buyers name on the panel. Usually, it is the same amplifier produced over and over again; only the front panel name is changed. Going back to the first picture, I have seen a few heads that look identical, with names on the front panel such as Riveria and Pepco. These may well be the very same amplifier. If any reader should have further information, it would be greatly appreciated. Finally, you may notice that the coupling capacitors inside the chassis are about the size of a Volkswagen. This is not unusual in older equipment, but is something you don’t expect to see in an amplifier this ‘young’. Perhaps the manufacturer bought surplus capacitors, or simply had them on hand from his days as a television/radio repair technician. So many mysteries, so little time.

The amplifier itself is pretty straightforward. It ‘features’ two 6V6’s, two 12AX7’s, and a 5U4 rectifier. The rectifier choice is unusual because a 5U4 is very ‘over rated’ for an amplifier of this power output. A 5Y3 would be more or less ‘expected’ to be used here. It is a single channel circuit, with a two-knob EQ. The lone Volume control oversees the three input jacks, labeled Guitar/Guitar/Bass. The ‘middle’ jack (labeled ‘Guitar’) appears to be the ‘high-gain’ input. When you draw out the circuit, it is very much like a Fender, albeit with a low voltage power supply, and an output of only about 15 watts. Other ‘odd’ points-of-interest I have noticed, although this is after only examining less than a half-dozen of these amplifiers is as follows.

  • The split-load phase inverter is not fed from a higher B+ than the preamplifier stages. This is very similar to a certain model of Fender Princeton amplifiers. A popular ‘mod’ seen on websites is to move the B+ feed over to an unused power supply node. The idea works well; it provides a few more watts and a little more headroom. Both are welcome ‘improvements’ over the stock circuit.
  • The amplifier is apparently operating in Class A. I say apparently because it is done so poorly, that the possible ‘improvements’ are dramatic. The common cathode resistor seen on the 6V6’s is usually 350-ohms. This is way too high; I lower it to 180 or 200-ohms, watching the plate current. Adding insult-to-injury is the absence of a cathode bypass capacitor. I add a 150uF/100VDC unit, and this completed mod can bring the amplifier to life!
  • In the few heads I have examined, none had negative feedback. There is also one gain stage without a cathode bypass capacitor. Bypassing this cathode reduces the headroom, so make sure the B+ mod is done before bypassing that lonely cathode resistor.

There you have it. I usually see these heads for sale at very modest prices, and with just a little tweaking, they sound fabulous. You can think of it as a Fender Princeton in a head, but at a fraction of the price.

2 thoughts on “Cobra Amplifiers

  • 25 October 2015 at 00:00
    Permalink

    any chance you can show me how to do these mods to my cobra 800?on a layout diagram thnks

    Reply
    • 25 October 2015 at 04:46
      Permalink

      I’m very busy at the moment, but I will try to find the time to get this done for you.

      Reply

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