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Old 20th July 2013, 11:55 AM   #11
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Quote:
Originally Posted by Ukulelegordy View Post
Thanks for the replies folks I guess the mystery for me now is why I can only get an output when I'm connected to the 100v output? And I have safety concerns. I was thinking of gigging this amp tomorrow but I think I will hang fire for a bit!
As I mentioned above, it's a speech/background music only amplifier, not intended for gigging - it's not an instrument amp. Quality would be low, and it's probably not going to stand hard use.

If you're only getting an output connecting across the 100V and 8 ohm taps, then it sounds like the output transformer is O/C at the bottom end?. It would make more sense connecting between the 8 and 16 ohm taps though.

I don't suppose you have a schematic?, I've just downloaded the user manual (too big to post here), and it seems to show the normal transformer connections at the output.

BTW, whereabouts in Derbyshire are you? - PM if you want to keep it private.
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Old 20th July 2013, 03:28 PM   #12
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Originally Posted by Ukulelegordy View Post
Well thanks for the replies. I've re-wired the speaker as suggested negative to ground and positive to 8 ohm. The result is no output at all not even a buzz. So I then connected the negative to the 100v output whilst leaving the positive in the 8 ohm output. This gives a sound more like what I'd expected from a pa type amp clean and not overheating?
There may be a common tap on the output transformer not connected to earth ground, look for that.
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Old 21st July 2013, 07:46 AM   #13
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A further observation on this amp is that the combination of decent guitar speaker and cab with a lower quality amp produces a great "Blues Harp" amp! This could be a solution to a problem I've had for years!
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Old 28th July 2013, 06:08 AM   #14
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Solve the wiring puzzle first or your years long quest will continue.
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Old 28th July 2013, 03:58 PM   #15
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Well the wiring puzzle is now resolved as much as it can be with the speaker wired between the 8&16ohm pots thus removing the need to connect to the 100v pot. This seems the only option. Still for a celestion gt100 speaker in cab costing 8, and an amp that cost 6:50, I am not going to complain!
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Old 29th July 2013, 06:07 AM   #16
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For info -
70V line is normally considered a 50 Ohm output.
100V line is a 100 Ohm output.

Are you sure there is not a terminal labelled COM, COMMON, GND, EARTH or similar. That is where the speaker -ve should go with the speaker +ve to the 8 Ohm terminal.

You may like to do a continuity test between that terminal labelled "PRIORITY" to see if that is ground. If so that should be where the speaker -ve goes.

Connecting an 8 Ohm speaker between 8 and 16 Ohms is not correct either. You have to remember that impedance ratio is the square of the turns ratio (and hence also the voltage ratio).

If you a are looking for the same voltage across your speaker as you would get from the common to the 8 Ohm output it means that you want to double the voltage, that means double the turns ratio and that means 4 times the impedance. That means that you would want to connect an 8 Ohm speaker between the 8 Ohm and a 32 Ohm output - if there was one. If there is a 70V line terminal (50 Ohms) you could connect the speaker between that and the 8 Ohm as being "close enough" but FAR better to identify the 0V/GND Connection and use that with the 8 Ohm output - check that terminal labelled "PRIORITY".

Many early HiFi Output trannies came with 2 off 4 Ohm secondary windings - you connect them in parallel for 4 Ohms output (voltage stays the same) or you connect them in series (X2 the voltage)
for a 16 Ohm (X2 squared = X4 the impedance) output.
Cheers,
Ian

Last edited by gingertube; 29th July 2013 at 06:28 AM. Reason: Fixed factual error.
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Old 29th July 2013, 08:07 AM   #17
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Originally Posted by gingertube View Post
For info -
70V line is normally considered a 50 Ohm output.
100V line is a 100 Ohm output.
Actual impedance is entirely dependent on the output power of the amp, so it's not considered to be anything, and there's no reason to ever think of it that way, just total the watts of all the speakers connected to it (which is one of the points of 100/70V line).

Quote:

Are you sure there is not a terminal labelled COM, COMMON, GND, EARTH or similar. That is where the speaker -ve should go with the speaker +ve to the 8 Ohm terminal.
There is, and presumably the bottom end is O/C?.

Quote:

Connecting an 8 Ohm speaker between 8 and 16 Ohms is not correct either. You have to remember that impedance ratio is the square of the turns ratio (and hence also the voltage ratio).

If you a are looking for the same voltage across your speaker as you would get from the common to the 8 Ohm output it means that you want to double the voltage, that means double the turns ratio and that means 4 times the impedance. That means that you would want to connect an 8 Ohm speaker between the 8 Ohm and a 32 Ohm output - if there was one. If there is a 70V line terminal (50 Ohms) you could connect the speaker between that and the 8 Ohm as being "close enough" but FAR better to identify the 0V/GND Connection and use that with the 8 Ohm output - check that terminal labelled "PRIORITY".

Many early HiFi Output trannies came with 2 off 4 Ohm secondary windings - you connect them in parallel for 4 Ohms output (voltage stays the same) or you connect them in series (X2 the voltage)
for a 16 Ohm (X2 squared = X4 the impedance) output.
Sorry, but I would completely disagree - the early transformer you mention would have had two 8 ohm windings, in parallel to give 4 ohms, or in series to give 16 ohms.

Check winding details of valve output transformers, same number of turns from 0-8 ohms as 8-16 ohms.

It's not really any different to speaker impedances.
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Old 29th July 2013, 08:14 AM   #18
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Hi Ian, there is a terminal marked "COM" so I have now connected the -ive to this. That seems to work so if this is the safest option I will stick with that connection. As yet I have not used this amp apart from these tests but I may gig it soon to see how it works. I only play in small fairly quiet settings (my 30w Fender is rarely above vol. 2) so I do not intend to punish the amp. Thank you for the advice.
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Old 29th July 2013, 08:21 AM   #19
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Hi Nigel, thanks for the link to the manual it was much appreciated. You can see this experiment is something of a learning curve for me!
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Old 29th July 2013, 05:03 PM   #20
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Quote:
Originally Posted by Nigel Goodwin View Post
Check winding details of valve output transformers, same number of turns from 0-8 ohms as 8-16 ohms.

It's not really any different to speaker impedances.
That's plain WRONG! Do the math. On the typical 4-8-16 ohm winding, the 4 ohm tap is the center tap.

Pick any power level. Calculate the voltage using the formula V=SquareRoot(P*R). 16 ohms gives twice the voltage that 4 ohms does.
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