I'm making up an audio output meter to use during alignment and want to include switchable resistances to enable it to act as a dummy load with the speaker disconnected. Should the resistance of the dummy load be equal to the DC resistance of the speaker or the speaker impedance? I was going to make it match the impedance but then realised a resistor is going to have different characteristics to a speaker.

I've always used the equivalent number in ohms: 4 ohms, 8 ohms, etc, although with voice coils the DC ohms (resistance) is typically about 25% less than the impedance ohms.

Important to use power resistors. P= I X I X R. The current being squared can quickly increase the watts dissipation requirement.

Why do you want to disconnect the speaker? Astor had a plug at the back of the set to enable the AC volts on the plate to be measured. The Peak 200H meter that I accidentally acquired temporarily for a repair, merely uses a 0.047mfd cap. from the dedicated "output" plug

That I would assume feeds into the AC circuit. Anyone who has one of these and an AVO 7X should replace the caps. The one in the 200H had failed and I replaced it with a mains type (fail open).

All I use is one of the CRO's as I can see the shape of the wave form & It also shows if I am getting distortion.

Marc

Well I cant help you with measuring the output my brain is old and rusty
By output you mean power or voltage? (Cro's are good )

but your right on what you say, the DC resistance would be about 5R5 and the nominal impedance is 8 ohms,....this is frequency

So like you probably know its impedance is not static but depends on Frequency to what it actually is
(impedance is to AC what resistance is to DC but frequency , and leaving phase relationships out of that statement...and kinda falsifying for the sake of simplifying)

So 8 Ohms would be the correct resistance to use in place of a nominal 8 ohm impedance speaker
(at a specified frequency)

For dummy loads its all I use to do but your use is a little different as your making measurements so you need to know what the Audio standards are for that and thats were my brain is way rusty and I got to re-educate my self

The other thing as you probably well know is Valve output is a different kettle of fish to Transistors, in that the output transformer speaker combination can or is critical to the safety of the Valve ...as in keeping it with in safe operational parameters

If using wire wound resistors as dummy loads, just a point and I dont know if it matters or not really (me thinks not) but I'll say it anyway...it can be one of those little gotcha's in the world.... the design of them most have some inductance..the only ones that use to keep that reactive element down (that I know of) was Noble Brand which wound the resistor in a way to minimise the inductance... I think By using Mutual inductance to Cancel each other...not sure on that thought.... Rusty brain...

EDIT I think that should be 1 kHz below

I think for output the frequency is 400Hz sine, but stand to be corrected

Like I said rusty brain trying to recall info from over 20 years ago & thought I learned it... mostly never applied as I got into digital more often then not...so stand to be correct on any of it

Hope some of it helps anyway scraps
You got me curios now to find out the holes in info I've forgotten

Cheers

I have a bunch of 200 watt Iwaki non-inductive resistors that I will be using to make a 50 ohm dummy load for transmitter testing. Picked them up at a swap meet for just a few bucks.

Put that linear away GTC

Sounds like a bargain to me thought

I knew there had to be more making non inductive types ...just didnt know who, Thanks

Thanks everyone for your comments. A post I made during the changeover of servers seems to have disappeared into the ether. Because the impedance of the speaker is only a nominal value at a given frequency, often quoted as 400Hz and conveniently the same frequency as my signal generator, I'd decided this was the value to use. This is after all what the OP stage sees at 400Hz.

I'm not aiming for an accurate measurement, only a comparitive one during alignment.

Aligning a smaller radio with a 4 or 5 inch speaker isn't a problem but when I do a larger radio like an AWA 528 with a 9 x7 speaker the noise can become mind numbing, not just for me but everyone else in the house and surrounding neighbourhood. I do like to hear what's going on so I'm planning to include a small monitor speaker.

Another topic is whether alignment should be done at full volume or not. Some say that where the volume control acts on the AF stage it makes no difference. Personally I have found a subtle difference and always do this with the volume as high as I can bear.

Cheers,

Warren

A post I made during the changeover of servers seems to have disappeared into the ether.

Quite possible if it was made mid afternoon on Tuesday. Let me know if you'd like me to try and find it. It'll be on the other server. I'd have to add it to the database here during a quiet time though - Sunday morning perhaps.

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A valve a day keeps the transistor away...

QUOTE: always do this with the volume as high as I can bear.

I have to admire your tenacity and I can see your technique having the added benefit of clearing possums out of the roof space. Personally, I find it easier to detect changes in audible output with volume at no more than mid-way. Perhaps our ears are calibrated differently.

As you say, you are looking for a relative difference in output, not an absolute measurement, so a scope across the speaker terminals, or an analogue multimeter, should be reliable enough shouldn't it?
The main problem there would be if your circuit is one of those that feeds back to AVC from the secondary of the OP transformer, so any change in load affects AVC. I think I have seen advice to disconnect AVC while aligning. This applies to any kind of meter that affects load on the OP.

Maven

I usually just use an AC meter but it tends to fluctuate a bit. The meter I'm making up will rectify and hold the peak. It's capacitor coupled with adjustable sensitivity and would normally be used with the speaker still connected. It's only a silly project to keep me occupied and I'm including the adjustable dummy load because there's space and it seems like a good idea. I hadn't considered AVC feedback but I'll tinker a bit and see what effect it has. As you suggest, disconnecting it is an easy solution in any case..

I usually only work on single ended radios so I'm only aiming for around 5 watts using wire wound resistors. It'll be interesting to see if the inductance of these does make any difference. It may well affect the audio stage but I doubt it'll have any effect on alignment. I did just purchase an incredibly rare AWA Fisk Radiola 283 with push pull 6V6's but there is no way I'm attempting to align that beast!

Brad, that is about the time I made the post but don't go chasing it. Everything's been covered elsewhere, thanks.

Cheers,

Warren

The main reason delayed AGC and normal AGC cuts in, is due to cramming too much signal down the throat of the set.

Most generators put out volts and the signal grid only needs around 50uV

Marc

WES has a range of 10W non inductive resistors. Don't they wind half of the resistor one way and the other half the other way so the inductances cancell?
Aren't you supposed to have the volume at full, your meter at the lowest scale and wind up the Signal Generator's attenuator to the lowest output you can measure on your meter, so as not to trigger the AVC?
Doesn't todays standard say Impedance is measured at 1000Hz unless otherwise stated? Early electronics seem to use 400Hz, I guess it is like everything, things get confusing when everyone uses a different standard and spacecraft crash into Mars.

To Quote LN

"Don't they wind half of the resistor one way and the other half the other way so the inductances cancel?"

It was my understanding thats how its done...thought I've never cracked one open to see exactly how.

My memory is thinking, from info long ago ...it was folded... then helically wound but could be well wrong on that.

Sorry I am not much help Scraps but seems you got your answer, well a few too and yeah relative is good, thought I think your aim was to have a "ready to go" method to make life easier and reduce mistakes etc in hooking things up.

Anyway glad you got it sorted and I was sure you knew a lot more then me on this with radios.