The Cobra Mark 2

[DLNB]

I recently picked up an old tube head. A Lifco Cobra Mark 2. I plan to change the tubes (I've already changed the preamp tubes and rectifier). I will also replace the bass and treble pots or maybe I can just clean them.

What I'm wondering is how I can tell what impedance this head would like? Right now I have the cab wired at 16 ohms.

The amp has two 12ax7's and two 6v6's.

Any other insights anyone would have about this amp would be much appreciated.

[Alex R]

You'd probably be ok with 8 ohms, as it's going to be somewhere between 4 and 16 ohms output and v likely 8, and a one-step mismatch like that isn't likely to endanger anything.

To find out for sure you'd need to feed in a small AC voltage one side of the transformer and measure what that puts on the other side to get the turns ratio, square it to get the impedance ratio then you can get the output impedance by putting the tube's recommended plate impedance into the ratio (it is easier than I'm making it sound). If you have an AC source (clips from a heater supply will do) and an AC voltmeter, someone here (maybe me!) will give you the method and the formulae.

But I still say 8 ohms is going to be okay! With your 16 ohm cab you run the slight risk that the amp has a 4 ohm output, a difference factor which takes us a little bit outside the broad recommended ballpark and might just conceivably cause an issue.

A confession: I have an 8 ohm 350 watt speaker by my bench, and every amp I repair gets fed into it. I never saw a problem with it yet, and we are talking about a lot of amps!

[Alex R]

btw it looks like a nice little amp to work with, point to point and all that. I marked a couple of places that might need attention:

A - is that rust or just flux? Make sure the grounding solder or screw is still doing its job here, and at the one or two other points that look a bit rusty.

B - is this insulation damaged on the neon connecting wire here? Looks like a potential mains short against the grounding shield that probably sits in the cab.

[DLNB]

I noticed the red wire but I was not thinking about the rust and a possible ground issue. Thanks for pointing that out.

[Alex R]

Good luck with it. Working out impedance ratios is quite easy and instructive, worth a go.

If the amp throws up any little problems some of those old caps might be a first place to look. Check where they feed the grids to see if any DC is leaking through them.

[DLNB]

I am wondering about one issue that could be cap related. When I start up the amp it sounds great. After a few minutes the volume level drops quite a bit and the amp sounds thin and weak. I assume it's the old RCA 6v6's but could this be a cap issue?

As for the impedance I would like to work that out.

[DLNB]

Just an update. I replaced most of the capacitors and the amp now works fine. I did not change a couple of the tiny caps and I didn't change the two metal cans. The amp is very quiet, no hum at all so I figured those filter caps are fine. Anyway I just wanted to post that the volume drop problem was related to some bad capacitors.