Hi I've just been rummaging through some cupboards (the stuff that never been unpacked since 2001 ) looking for some stuff for the wife.

I've found a speaker that I rescued from a skip some years ago, that I'd forgotten about, with the following stamps on the chassis T1281 GG15





It looks a bit rough around the edges but I've been thinking about replacing the 10" in my Epiphone valve special, so im wondering what the safest way to test it.

Is it safe to measure the impedance with a multimeter ?
Should I test it in my dodgy tranny Kay 30W sound fashion first?

Any advice or info appreaciated, cheers

Certainly measure the DC impedance before you connect it to anything. If you don't know the ohmage then anything between 4 and 16 should mean its fine. Use the nearest standard speaker value - 4 8 or 16 ohms - to check its compatability with the Epiphone. An open or short circuit reading means its knackered. Quite often the ohmage is stamped on the chassis somewhere.

Gently push the cone in and out to make sure it moves smoothly and hasn't jammed or is catching.

And then yes, certainly test it on a tranny amp first. If you start to see smoke or smell burning, turn the amp off quickly!

T1281 should be an G12H30 16-ohm. And GG15 dates it to mid '74. Probably worth more than the EVJ if it's working fine. Good find!

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Don't let your mind wander - one day it won't come back.

Thanks guys

I guess if the speaker is valuable then it would probably have been skipped because somethings broken - I'll give it a try anyway

Quote Wonkey Wabbit :

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Test it on a tranny amp first. If you start to see smoke or smell burning, turn the amp off quickly

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In this scenario are you talking about the amp or the speaker getting to hot ?

Quote loverocker:

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T1281 should be an G12H30 16-ohm. And GG15 dates it to mid '74. Probably worth more than the EVJ if it's working fine. Good find!

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I think the EVJ wants an 8 ohm speaker, how important is it to match the impedance, I don't mind a quieter amp, but I don't want to blow anything up.

Edited by Jumpeyspyder (18/04/08 09:44 AM)

I think the first thing to do is gently push the cone back and forth as Wonkey suggests. If the coil is ok, it should feel smooth. If it feels gritty, the coil has probably burnt out.

When connecting it to a transistor amp, I would have though that it would be pretty obvious within a few seconds as to whether it is knackered or not. It'll either work, not work or sound very distorted. I'd also imagine that it is wise to keep the volume low at first and if all appears ok, turn it up a little.

I've only blown a speaker once - it sounded very distorted as it happened and afterwards on inspection, the cone movement felt gritty when pushed.

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my music

Quote Jumpeyspyder:

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Quote Wonkey Wabbit :

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Test it on a tranny amp first. If you start to see smoke or smell burning, turn the amp off quickly

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In this scenario are you talking about the amp or the speaker getting to hot ?

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I guess it could potentially be both!

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my music

More likely to be the speaker. Most tranny amps will have short-circuit protection and will sit for ever on an open circuit (unlike valve amps), but if the speaker coil insulation starts burning off then you'll know/smell very quickly.

God its been a long day, all I've been thinking about is trying this damn speaker.

Finally I got home from work and did the Wonkmeisters super speaker cone touch test....

...seems smooth ,but I don't want to push it too hard in case I break it.
...Got the multimeter out and got about 15ohms so the voice coil is probably OK

....have some dinner and put the kids to bed ..

wait till they're asleep [PLEASE GO TO SLEEP]

Got the tranny out ,
find a screwdriver ...
got to solder the terminals
(wheres the soldering iron !! )

Turn all amp controls to zero, find an RCD and plug it in

[fingers in ears] switch on...

KBooomMMMM !!



!! Hang on where's the smoke and shrapnel ??

Gets guitar, gently turns up gain and volume YAY ITS ALIVE

Its working in the tranny and does seem add a creamy kind of sound but seems about half the volume of the stock(8 ohm) speaker (possibly a good thing)

Special thanks to Wonkey, Loverocker and Mike

I really want to try this thing in the EVS but I'm not sure if I should.
Both the internal speaker and extension are rated at 8 ohms and I can't afford to fry EVS

Never got any kind of manual with th EVS so I'll try google. I'd be very happy if anyone has any further advice.

Edited by Jumpeyspyder (18/04/08 09:17 PM)

Glad the speaker is in good working order!

From what I gather, transistor amps are more forgiving to impedance mismatch. Valve amps arnt - I'd guess that worse case scenario would be output transformer death, in all honesty though, I don't really know.

I'm sure that the Wonkmiester, Loverocker or someone else more in the know will chip in with the facts.

Mike

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my music

Ok found the manual - useless

On closer inspection the external speaker jack actually says "8 ohm min" but I'm unsure if the internal speaker should still be connected or unplugged.

The output transformer seems to be rated at 8 ohms which doesn't fill me with hope.

There are some schematics here if anyone knows how to understand them.

Edited by Jumpeyspyder (19/04/08 01:11 AM)

If the EVJ/EVS were designed properly from the scratch, the designer deliberately chose the load he wanted to get the EL84 output valve working the way he wanted (for tone, response, etc). If so, using a speaker with double the intended impedance (16 instead of 8) changes the load on the EL84 valve and this may:
1) sound fine anyway and work great
2) sound bad but work great
3) sound fine but not work so great
4) sound bad and not work so great

My suspicion is that Epi's "8 ohm min" means that the designer reckoned it would still be fine with a 16 ohm speaker, but that a 4 ohm load is not suitable (most likely because of the higher current in the secondary winding of the output transformer). The schematic doesn't throw any more light except to indicate that the output transformer only has a single output tap (and there aren't any unused taps internally).

Now, some amps are quite happy with 2:1 mismatches, others are far more fussy. However, the fussy amps are generally far powerful 50-200W amplifiers, where there are many more amps and volts running around in the output transformer.

So... if you wanted to be 100% sure, you could upgrade the amp's output transformer (many people do to one that's bigger than the standard EVJ/EVS one, which'll improve the bottom end response) to one that happens to have all three of the common impedance taps: 4, 8 and 16.

But... personally, I don't think it's worth it. I'd use it with the amp as-is.

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Don't let your mind wander - one day it won't come back.

Yes, the extra output is in parallel with the internal speaker, so anything less that an 8 ohm speaker plugged in as well would give a load of less than 4 ohms.

Plugging in the 16 ohm speaker would give an equivalent 5.3 ohm load, so within the amps capability, though most of the sound would come out of the standard speaker so you may get a little benefit from your re-discovery, but not a lot.

I just don't know if the 16 ohm speaker on it's own would be OK. You could try and find another 12" 16 ohm speaker and make up an 8 ohm 2x12 cab.

Might be worth thinking about the Valve Jr head, which has 4, 8 and 16 ohm outputs. That's come down a lot in price recently.