For very initial testing I'll be working with power supplied running though a light bulb to current limit. I've also thought it may be easier to use a high wattage 5K resistor in place of the OPTs. This will make it easy place to measure waveforms and current. Obviously this doesn't completely mimic the OPT/speaker combination but is there any issue with using this approach?
A 5K high wattage resistor won't mimic an output transformer. What is the DC resistance of the output transformer? If this is a single ended amp, you can use a resistor that's approximately equal to the DCR of the output transformer. For a push-pull amp, you'll need to use two resistors.
You cannot substitute an output transformer with a resistor and make waveform measurements without making a ton of other changes.
You cannot substitute an output transformer with a resistor and make waveform measurements without making a ton of other changes.
That doesn't make sense to me.
Assume 3W output into 8ohm. Therefore voltage swing at the output is circa 5V. The OPT is 5K/8ohm so the turns ratio is SQRT(5000/8)=25:1. Therefore to get my
5V swing I need an anode swing of 125V. My OPT has a DC resistance of 72ohms.
So If I used a 72ohm resistor as suggested I would get a instantaneous current of nearly 2A
If I use a 5K resistor I get a current of 25mA which exactly matches the 25:1 current ratio of the 25:1 transformer
You need to use the 72 ohm resistor so your DC operating point is correct.
If you put a 5K resistor in place of a 72 ohm one (the DC resistance of your OPT), then the DC current your amp draws will decrease to almost nothing.
If you want to use a 5K resistor and your amp draws 25mA, then you need to increase your B+ by 123V.
If you put a 5K resistor in place of a 72 ohm one (the DC resistance of your OPT), then the DC current your amp draws will decrease to almost nothing.
If you want to use a 5K resistor and your amp draws 25mA, then you need to increase your B+ by 123V.
If you have a choke, you can use it to simulate the output transformer.
Put a 5k power resistor across the choke.
The DCR of the choke simulates the DCR of the transformer.
The 5K resistor simulates a 5k transformer that has a proper load on the secondary (like an 8 Ohm power resistor on the 8 Ohm tap.
Now test the amp at 1kHz.
Do not expect to get good response at 20 Hz (chokes are generally made for 2X line frequencies (100Hz and 120 Hz from full wave rectifiers).
And you may not get a good response at 20kHz (or you might).
How about that?
Put a 5k power resistor across the choke.
The DCR of the choke simulates the DCR of the transformer.
The 5K resistor simulates a 5k transformer that has a proper load on the secondary (like an 8 Ohm power resistor on the 8 Ohm tap.
Now test the amp at 1kHz.
Do not expect to get good response at 20 Hz (chokes are generally made for 2X line frequencies (100Hz and 120 Hz from full wave rectifiers).
And you may not get a good response at 20kHz (or you might).
How about that?
If I am quarantined in my home, and have a choke, and it is 8 weeks until I get my output transformers, I am going to pre-test the amp with a choke and 5k power resistor.
Then, when the transformers come, mount, solder, final test.
Ready to listen.
I am not saying that is your situation.
I kept a major production line up and running when we were running out of switcher transistors for a switcher supply.
I built an adapter with switchers, and pre-tested the circuit boards to 95% of the tests.
Stored labor kept the workers working, and when the switcher transistors came in, we were able to meet customer commitments by shipping on time.
I am Just Sayin'
Then, when the transformers come, mount, solder, final test.
Ready to listen.
I am not saying that is your situation.
I kept a major production line up and running when we were running out of switcher transistors for a switcher supply.
I built an adapter with switchers, and pre-tested the circuit boards to 95% of the tests.
Stored labor kept the workers working, and when the switcher transistors came in, we were able to meet customer commitments by shipping on time.
I am Just Sayin'
Well, technically you can use resistor instead of OPT. It'll just require different PSU.
Say, if the output stage is running at 300V and 50mA, and OPT's primary impedance is 5k, you can use 5k resistor, raising the B+ to 300+5000*0.05=300+250=550V.
And this resistor should be able to dissipate 250*0.05=12.5W, so you'll need something like 50W resistor.
Other than that everything should be fine 😀
Say, if the output stage is running at 300V and 50mA, and OPT's primary impedance is 5k, you can use 5k resistor, raising the B+ to 300+5000*0.05=300+250=550V.
And this resistor should be able to dissipate 250*0.05=12.5W, so you'll need something like 50W resistor.
Other than that everything should be fine 😀