Dummy Load Question

[Champ_49]

I am building a dummy load so I can plug the amp into it while using an oscilloscope to trace the signal of the amp. I'm new to all this.

I am going to use aiken amps passive dummy load schematic using 8ohm 1% 50W resistors with switchable 4,8 and 16ohms.

I noticed the 8ohm resistor changed to about 9.6ohms when going through the switchcraft jack and the three way selector switch.
The resistor measured directly was 8.1ohms so very close.

My question is does it really matter if the dummy load impedance is slightly different? Or slighly higher? How much difference is too much difference?

The jack, wire and selector switch introduced some additional impedance. Would it be much if an effort to get lower resistor values to compensate for the additional impedance added by the jack, wire and switch?

[shooter]

9.6ohms

does it really matter

yes n no
yes, IF your setup is dodgy n you blow up some OT's because of it
No, as long as you know the numbers, the math answers will be correct.

[Dave]

A speaker is a reactive load. A resistor is not. So a big ole resistor can never present an equivalent load to the output transformer that a speaker could. But, an approximate resistive load can provide a good enough load to not blow up the output transformer. That's not to say that it can't happen, but I think we all pretty much use them sometimes. I've never heard of anyone blowing up their amp because of it.
Most figures that I have seen show that a resistive load, in order to most closely approximate a speaker load, would be somewhat higher than the speaker's nominal impedance. So, the 9.6 should not be an issue. However, an additional ohm and a half across the switch is more than I would expect. Maybe you should clean those connections.


Dave

[DummyLoad]

You Rang?   


--Pete

[PRR]

Find that extra ohm. Something is dirty? Jiggle the switch and plug.

Not that the Ohm is fatal, but dirt/tarnish only gets worse, and if it goes to infinity while testing MAX power the output voltage will spike and may blow stuff up.

I always just clip-leaded a 7.5r 44W ceramic to the output. (Even on 150W+ amps-- boiled some water.)

[shooter]

You Rang?

does it really matter

[Champ_49]

Find that extra ohm. Something is dirty? Jiggle the switch and plug.

Not that the Ohm is fatal, but dirt/tarnish only gets worse, and if it goes to infinity while testing MAX power the output voltage will spike and may blow stuff up.

I always just clip-leaded a 7.5r 44W ceramic to the output. (Even on 150W+ amps-- boiled some water.)

There is a new switchcraft jack,  wires and a 3 way switch in line. All components are I guess "clean" and new. Not sure why it would cause this extra ohm. I'm going to try and clean it. Also I added a shoulder washer so the jack doesn't contact the chassis. Maybe that has something to do with it?

[sluckey]

Show us a hi-rez pic.

[Champ_49]

Show us a hi-rez pic.

I had it wired up with alligator leads so its not wired yet. But these are the two components that I am using. I just wanted to show that they are new and clean.

[sluckey]

I had it wired up with alligator leads so its not wired yet.

That's the problem. Use some 18AWG or 16AWG to wire it up properly.

[Champ_49]

I had it wired up with alligator leads so its not wired yet.

That's the problem. Use some 18AWG or 16AWG to wire it up properly.

I wanted to test it out to see if it works. But wouldn't thick wire also add to the resistance? For speaker jacks I would definitely user heavier guage wire. And when I solder it down..I'm also trying to use a three way switch for 4/8/16 ohms. It's an on-off-on switch. I don't think I can wire it using that if i were to use the aiken amp schematic so I ordered a 3 position rotary switch.

[sluckey]

But wouldn't thick wire also add to the resistance?

Just the opposite. Big wire has less resistance than flimsy gator clip leads.

[Champ_49]

Thanks. It makes sense to use heavier guage wires.

Would I have to ground the input jack of the dummy load to the chassis? I am planning on isolating the switchcraft jack with shoulder washers. Is this safe?

[sluckey]

Thanks. It makes sense to use heavier guage wires.

Would I have to ground the input jack of the dummy load to the chassis? I am planning on isolating the switchcraft jack with shoulder washers. Is this safe?

No. I would not bother isolating the jack from chassis.

[Champ_49]

Thanks everyone. Got the dummy load at least to read correctly..

For position 1 - 4.2 Ohm
       Position 2 - 8.2 Ohm
       Position 2 - 16.1 Ohm

Thanks Sluckey. I used 12, 14 and 16 Gauge wire and it reads very close now.

I tried plugging into the amp and put a signal into it with my guitar. At higher volume I can hear sound or the signal coming from either the amp itself or the dummy load. Most likely the amp. The sound is very very faint however. It's like the sound of the amp but at very very low volume. Is this normal?

[sluckey]

That's normal when pushing the amp hard. Most likely the OT laminations vibrating.

[Champ_49]

That's normal when pushing the amp hard. Most likely the OT laminations vibrating.

That's good to know everything is OK. Thanks for the confirmation.

This reminded me of the time I had an amp  and forgot to plug the speaker cable in. And while playing I heard the sound coming from the amp. Luckily at the time the amp didn't blow anything.

[Champ_49]

I'm also looking at including a bnc connector for the dummy load. There are a variety of them to choose from but they have an Ohm spec on it. 50ohms, 75ohms. Would any suffice?

[sluckey]

I'm also looking at including a bnc connector for the dummy load. There are a variety of them to choose from but they have an Ohm spec on it. 50ohms, 75ohms. Would any suffice?

Why? So you can connect a scope to the dummy load. A chassis mount female BNC connector has no ohm spec. Male BNC connectors such as you would put on a cable are made for specific type cables, ie, RG-58, RG-59, RG-6, etc. There is no universal one size fits all cables. Now the cable types do have an ohm spec, ie, RG58 is 50Ω, RG59 is 75Ω, RG6 is 75Ω, etc.

This is what you need to put on your chassis box...

     https://www.amazon.com/QMseller-Chassis-Socket-Connectors-Adapter/dp/B07FB39S1L/

[shooter]

^^^ what he said


50 ohm cable is "typical" in RF, 75 ohm is "typical" in audio/video

[Champ_49]

I'm also looking at including a bnc connector for the dummy load. There are a variety of them to choose from but they have an Ohm spec on it. 50ohms, 75ohms. Would any suffice?

Why? So you can connect a scope to the dummy load. A chassis mount female BNC connector has no ohm spec. Male BNC connectors such as you would put on a cable are made for specific type cables, ie, RG-58, RG-59, RG-6, etc. There is no universal one size fits all cables. Now the cable types do have an ohm spec, ie, RG58 is 50Ω, RG59 is 75Ω, RG6 is 75Ω, etc.

This is what you need to put on your chassis box...

     https://www.amazon.com/QMseller-Chassis-Socket-Connectors-Adapter/dp/B07FB39S1L/

Thanks Sluckey.  Yes. It's to hook it up to a scope through the dummy load.

This is the one I saw on digikey.
https://www.digikey.ca/en/products/detail/amphenol-rf/31-10-RFX/100642

It's says 50ohm impedance. 

[sluckey]

You don't want that! That's an isolated jack. No provision for the shield common/ground connection. They can call it a 50Ω jack if they want since it's being advertised as an RF jack. But until you connect a specific type cable to it, it has no impedance.

[Champ_49]

You don't want that! That's an isolated jack. No provision for the shield common/ground connection. They can call it a 50Ω jack if they want since it's being advertised as an RF jack. But until you connect a specific type cable to it, it has no impedance.

This guy uses it.

You can see the part number for bnC jack at about 6:32.

You can see on the layout the part number.

The link you have unfortunately I'd need to pay import fees so would prefer to get it on digikey.


For the last link couldn't I just wire a lead from the ground sleeve to the chassis?

This looks non insulated.
https://www.digikey.ca/en/products/detail/amphenol-rf/112424/1011723  this should work then right?

[sluckey]

My bad. I looked at the 360° pic of the DigiKey jack and saw that it indeed has two connections on the jack, one for center conductor and one for the shield. That will work just fine.

[sluckey]

There's really no reason to connect any part of that circuit to chassis. But it's fine if you want to use non isolated jacks for the BNC and phone jack.
 

[Champ_49]

There's really no reason to connect any part of that circuit to chassis. But it's fine if you want to use non isolated jacks for the BNC and phone jack.

Would you recommend insulating the circuit from chassis? I know maybe it's not that big a deal to do or not. But which would you recommend over the other?

[Dave]

As long as the load is good, it won't matter.


Dave

[sluckey]

You're thinking way too much about this. It doesn't matter if it's connected to chassis. Toss a coin. Or connect one jack to chassis and float the other. Dummy Load is just that. It doesn't care!   

Here's my no frills Dummy...

[shooter]

here's a search for "speaker dummy load"
https://el34world.com/Forum/index.php?action=search2


**DL get's a few cameo's in the search 


fwiw; my dummyload can be seen behind the amp, 3 dale 100w screwed to a 1 by.  bolt up series, series parallel, or parallel to get close.

[Champ_49]

Here's mine.  It was my very first time just a few minutes ago using an oscilloscope to put signal into the amp via built in signal generator. Was getting the jitters not knowing what would happen. Hence i wanted to make sure . I measured the high voltage plates and broke the ice.  Went smoothly and was able to see the signals and see how it changes with volume. Exciting!

Only thing I need now are some heat sinks.

Thanks everyone for the help!