Output transformers, working out output impedance?

ESchap

Done some further investigation and the Woden transformer is the interloper i.e. a replacement.   WEM used the larger transformer in all the photos of these amps (10 examples) that I could find.  Pretty certain that it's a Dagnal Transformer and they seem to still make that transformer albeit with a 4, 8 16 ohm taps now.  They list it as a 10watt transformer but the dimension are identical in every way to the old one in my WEM.  Not much work to change the transformer so may go for it, cost is £30, so not too bad.

Final question, is the Zobel network in the signal path? 

jpfamps

ESchap said:

Done some further investigation and the Woden transformer is the interloper i.e. a replacement.   WEM used the larger transformer in all the photos of these amps (10 examples) that I could find.  Pretty certain that it's a Dagnal Transformer and they seem to still make that transformer albeit with a 4, 8 16 ohm taps now.  They list it as a 10watt transformer but the dimension are identical in every way to the old one in my WEM.  Not much work to change the transformer so may go for it, cost is £30, so not too bad.

Final question, is the Zobel network in the signal path? 

Indeed it is.

ESchap

jpfamps said:

ESchap said:

Done some further investigation and the Woden transformer is the interloper i.e. a replacement.   WEM used the larger transformer in all the photos of these amps (10 examples) that I could find.  Pretty certain that it's a Dagnal Transformer and they seem to still make that transformer albeit with a 4, 8 16 ohm taps now.  They list it as a 10watt transformer but the dimension are identical in every way to the old one in my WEM.  Not much work to change the transformer so may go for it, cost is £30, so not too bad.

Final question, is the Zobel network in the signal path? 

Indeed it is.

Thanks, probably don't want wire wounds in there then?  Also the one with the correct transformer definitely has the wrong values for the network, 33k + 0.02uF  should be 4.7k + 0.002uF  Appreciate that both are x10 higher than they should be, but will this affect the tone? 

jpfamps

ESchap said: jpfamps said: ESchap said: Done some further investigation and the Woden transformer is the interloper i.e. a replacement.   WEM used the larger transformer in all the photos of these amps (10 examples) that I could find.  Pretty certain that it's a Dagnal Transformer and they seem to still make that transformer albeit with a 4, 8 16 ohm taps now.  They list it as a 10watt transformer but the dimension are identical in every way to the old one in my WEM.  Not much work to change the transformer so may go for it, cost is £30, so not too bad.Final question, is the Zobel network in the signal path?  Indeed it is. Thanks, probably don't want wire wounds in there then?  Also the one with the correct transformer definitely has the wrong values for the network, 33k + 0.02uF  should be 4.7k + 0.002uF  Appreciate that both are x10 higher than they should be, but will this affect the tone? 


Well the resistor is 10x lower.

Anyhow I think the idea was to try to keep the product of the resistance and capacitance the same, which for a 1st order filter would have the same effect. The filter with the higher resistance will have a larger minimum impedance, and as you are trying to compensate for the rising impedance of the speaker I'm not sure that this is a good idea.

It would employ the original values.

My guess is that the person doing the repair didn't have the correct value parts, so bodged it. 

You need a power resistor for the Zobel filter, and going higher powered that is "needed" is always better. I've used 3W metal oxide resistors in this circuit with no problems, but a higher powered wirewound would be fine as long as the voltage compliance is good enough (some wire wounds are rated for quite low DC voltages), so check the datasheet.

ESchap

Many thanks indeed @jpfamps.

ESchap

Just been sourcing a replacement transformer for the old WEM and realised that the Woden that's currently in it has a 14k primary.  Should be 8k for a pair of EL84's.  It's a single out at 4 ohms and I've been running it into 16ohms! 

So is that 3 steps away from where it should have been?  i.e. to get 8k approx. on the primary I should have been running it into 2 ohms?   Can't have been doing the valves any good, fortunately they are old Mullard's and I didn't play the amp too often. 

Compared to the other amp with the original transformer, this one did sound very dull and lower output,  I'm assuming the totally wrong output transformer values accounts for this?

jpfamps

ESchap said:

Just been sourcing a replacement transformer for the old WEM and realised that the Woden that's currently in it has a 14k primary.  Should be 8k for a pair of EL84's.  It's a single out at 4 ohms and I've been running it into 16ohms! 

So is that 3 steps away from where it should have been?  i.e. to get 8k approx. on the primary I should have been running it into 2 ohms?   Can't have been doing the valves any good, fortunately they are old Mullard's and I didn't play the amp too often. 

Compared to the other amp with the original transformer, this one did sound very dull and lower output,  I'm assuming the totally wrong output transformer values accounts for this?

Yes that's correct, by presenting way to large a load to the power valves you will be getting much less than optimal output.

You also would be getting very large voltage spikes that could cause arcing, either internally in the transformer or externally in the valves (or possibly around the valve base although this is less likely with EL84s due to the pinout).

jpfamps

Incidentally, in my experience, guitar sound their best when the output loading is optimized for maximum power.

When prototyping a new design, I always do a plot of power vs loading to find what is the optimum loading for that design, my logic being that changing the loading is cost neutral, so if you can improve the sound of the amp at no additional cost it would be daft no to.

ESchap

Many thanks @jpfamps, that might explain why it occasionally, briefly, lost volume altogether, I just put it down to the knackered Hunts caps!