Hi All,
Nearly finished the Kriesler 11-110, just waiting on some new belts for the reel to reel.
I got everything back together today and fired it up and one of the speakers has an intermittent crackle.
I did try swapping the speaker wires from L to R and it's definitely the speaker.
Put a multimeter across the bad speaker, measured 12Ω. When I push on the speaker cone, the speaker goes open circuit. Which I think is where the problem is. At low volumes (low speaker movement) everything is fine. But as the volume goes up, the speaker moves more and then goes open circuit.
So now I'm looking for a replacement...

The specs of the Rola C8MX are:
10w Peak
45Hz to 12,000Hz
15 Ohms at 400Hz

Now, the closest speaker I seem to be able to find is this:

http://www.altronics.com.au/p/c2000a-redback-200mm-8-inch-8-ohm-10w-dual-cone-pa-speaker/.

The only problem is that it's 8Ω. Now is that a problem? I seem to remember reading that you can't just chop and change impedance on a valve amp.

Alternatively, could I run 2 of these? And just modify the cabinet?

Any input is really appreciated. Want to get this one finished to move onto an old STC A4100!

If you halve the load impedance, then you effectively halve the reflected load seen by the output valve.

Whether or not that's a problem depends on the amplifier design and the impedance range that the output valve can tolerate within that design.

Without knowing the particulars of your amp, I would suggest that you aim to retain a 16Ω load impedance.

15 or 16Ω speakers are not particularly easy to find these days. The usual source is a parts donor set.

Is there a manufacturer's model number on the output transformers?

Now, the closest speaker I seem to be able to find is this...

There are plenty on ebay and you can use a magnovox used in the same model. Seems they used different speakers depending where they sourced them from. I may be able to help you out if you can't find any locally.

Cheers
Vic

You might try repairing the bad speaker. There's flexible wiring (looks like braid, the kind used for removing solder off circuit boards) between the speaker terminals and the voice coil, there may be a break you could resolder together. Near the center of the speaker cone you usually see some black shiny paint like goop that covers wire leads to the voice coil. There's usually a connection between the voice coil leads and the flexible wire under the blob furthest from the center (voice coil area). I'd try resoldering the connection of the flexible wire at the speaker terminals first, that might fix it, before digging around the above blob. You were going to toss the speaker anyway, nothing to lose.

I have had a couple with wire breaks in the VC external to the winding. One at the joint with the braid, but both due to damage. One break in an Airzone, (see Silicon Chip); I saved it as it was branded Airzone.

The biggest pain was a Jensen as the wire was on the outside of the cone. I managed to scrape the wire & tin it, then splice a bit on. Micropore (band aid solution?) non woven tape is then used to hold it all & patch cracks.

To give more strength, I will paint it with an acrylic binder. Water based is ok, but drowning it is not. Water & paper = mush & distortion.

Marc

I recently purchased a brand new 15 ohm speaker on Ebay. You might find one there.