How to make a simple easy 4 8 ohm tube audio amp Dummy load D-Lab
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- Опубліковано 7 жов 2024
- After many requests, here it is, the simple 4 or 8 ohm switchable resistor dummy load. Great for working on tube amplifiers. This one is a 25 watt model utilizing a Switchcraft 1/4" plug. Nice accessory for the shop. Connect your oscilloscope, troubleshoot away! Follow the easy schematic to make your own.
Speaking of Champs, D-Lab is World-class, Triple-A. Thank you, Terry!
I built one out of two 8 ohm 50 watt resistors like this. I got burned wasn't expecting it to get that hot (235 degrees F) but I was testing my amp with an oscilloscope and had the amplifier volume up high for almost 10 minutes. It melted the jacket on the test lead tip because it was touching the out side body a little along with the insulation on the wiring I had zip tied to it! Now that said, I have been using this for 2 or 3 years now for various amp testing but never had the master volume up above 2 before this. I shouldn't have used it for so long of a period. I'm sure it's ok for quick testing 3-5 minutes at the most without proper heat sink mounting. D lab is the Man! but sometimes us newbies need more instruction, I know I do !
Nice little project.
Just built this today, works great. Thanks for this, and all your videos!
I basically have built the same device and I have used it for years. With mine I use a female jack and the use different adapter cables for different amplifier outputs, tube or solid state.
If anyone is wondering he is using wirewound resistors , so calling it a "resistoroid" proved this is a dummies dummy load.
I went a step further, and bought two 8 ohm 50w resistors, where I can do 4 ohm(parallel), 8 ohm(single resistor) and 16 ohm(series)
Great idea! Can that be wired using a DPDT on/on/on switch?
I built one today, thanks for the tip D-Lab !
Thanks! Just finished mine. Works great!
Thanks Terry for another easy and super solution!!! John
Thanks Terry, always quality stuff without breaking the bank. Unless it's an Amplitrex tester of course!
Nice work Terry. Never seen this before; this will be very handy for testing.
Thanks, D-Lab, just made one, workes great. Thanks for the Pro Tip.
Very nice Terry. Thanks as always.
Built one today; works great. Parts easy to get (ordered from Amazon) but I really miss Nidisco and the "old" Radio Shack. Thanks
It's my understanding that those type resistors require a heatsink to achieve the labeled power dissipation. Learned about it the hard way. Toasted a 40W unit with much less than 40W going thru. I don't know what the derating is for no heatsink or fan. Easy enough to feel them, though, if using them with a big amp.
Yep, to keep those resistors shown at a sensible temperature that won't burn you or set your house on fire, you wouldn't want to put more than 1.3W through them.
Couldn't find any heatsinking data for the NTE product. Vishay/Dale has similar "chassis mount" product RH025 & their datasheets say to derate the wattage 50% for unmounted device at ambient air temp of 25C deg (77F deg).
Vishay's stamped ratings for RH025 assumed mounting on a 5" x 7" x 2" x 0.040" thick aluminum chassis (167 sq. in. surface area). For Milspec applications (different datasheet) they derated the unmounted 25W device to just 8W at 25C deg.
So since most audio amp power numbers are peak versus average or RMS, these ought to work for short periods of time on smallish amplifiers. If they start smelling of smoking, it's time to consider making a larger dummy. Maybe high watt resistors on a heatsinked unit at the end of a cable. Still homebrew-able, just a little more expensive.
the 25 waters will handle 6 -7 watts with out heat sink
+Jamie Phillips, yes, they'll stay within their max temperature rating dissipating 7-8W without a heatsink, but that max temperature is 200°C (392°F), you really don't want them running that hot if you're going to be handling them!
> that max temperature is 200°C (392°F)
That's crazy-hot!!! Good warning about that. Thanks!
Great tip and I'll be making one of these myself.
Thanks Terry. It was fun
Neat device, including thermal cutout (the cable-tie melts and the thing falls apart?)
Great info, thanks for posting this!
This is awesome. Can you show how to build an attenuator?
build a simple L pad design for lower watt amps for pretty cheap , but in the 40 watt range you need something greater to handle the power
thanks a lot D-man
Those resistors are only rated for a maximum of 9W when not mounted to a heatsink, and will reach 200°C (392°F) if dissipating that full 9W. For a more sensible temperature of 50°C (122°F), you'd want a maximum dissipation of just under 1.3W. To achieve the full 25W rating, with a 175°C (347°F) rise above an ambient temp of 25°C (77°F), then an aluminium heatsink of 1mm (0.04") thick, with an area of 535cm (83") square is required.
The Datasheet is www.arcolresistors.com/wp-content/uploads/2014/03/HS-Datasheet.pdf
If you need 25W without chassis mounting just buy the HS75s, they are rated for 24W unmounted.
Hey D-Lab, noob here. This will work for working on diagnosis for vintage receivers too right? Should I make it with lower voltage resistors or keep as is. Thanks! Love this channel, subscriber.
Thanks Terry!
first! thx 4 every video terry!! yea if you can't make 1 you don't need to be workin on an amp in the first place!! lol
Another great tip Terry. Looks almost like mine. Thanks for sharing. BTW, sent you a message. Picked up a couple of old Johnsons today and it is all your fault he he .
If I wanted to use my tube heads pre-amp only... I just pull the output tubes. Amps pre-amp works as intended... and you're not wasting your output tubes.
Could you please please please make a video on how to build an attenuator for amps 100watts and lower? That would be amazing. Im an studio and live sound engineer and I would love to build a couple for when Im working with these bands who bring these giant 100watt tube heads to a tiny bar gig and can inly turn them up to 1 lol. Also just for recording etc Id love to have one. I just dont want to pay $300-600 for a decent attenuator lol.
For what I know an attenuator works like a Variac transformer, Maybe you can build this with a variable variac transformer and some high power resistors etc. some idea to think over..
@@Retro.Studio Bad idea to try using a variac. you'll likely damage your amp
SoulSonicFX substructure plz
How about using 2 x 8 ohm resistors and having the switch take one leg of the second resistor in and out of the circuit? Gives you twice the power handling in 4 ohms instead of twice the power handling in 8 ohms.
Very compact!
THANKS!!!
Thank you so much!!
Just ordered my parts for this project. Unfortunately we lost Fry's and Micro Center doesn't carry these type of resistors, I am now dependent on online retailers and no local brick and mortar stores.
Thank You
Ned flanders lol. Thanksarino lol. Great construction.
Thank you so much....
Great to have you back Terry! It looks like a wire wound type resistor? Is there any reason to use a non-inductive type? Found a butt load of these on Amazon. Go figure! Thankz
Nope, nothing to worry about. You could also construct this with some old carbon type. For the short period of use, any type resistor is fine. TD
This would be extremely unsafe to use with any amp more powerful than 5w. The 25w rating of those power resistors is assuming they are correctly mounted to a metal surface, preferably with thermal grease. And it is also hazardous to leave them in the open air, because they will become very hot if any appreciable amount of power is run through them for more than a couple seconds. This is a serious burn hazard if anyone were to touch it. The switch and zip tie may very well melt. It looks clever, but it's not well considered. The same circuit could be built into a small ventilated metal box and be 1000% safer.
Don't touch things that will hurt you. It's hard to teach someone about a device if it's in a closed case. This device is being used on valve amplifiers that run 300-400v. The dummy load is the least of your worries.
Subject to rating, would this work ok for a SS amp that has a line out running to a recording interface? I can just turn the volume down (run the speaker silent) but I’m considering separating the classic from the speaker entirely and making a desktop recording amp with no speaker. Cheers
Gracias,por compartir!!
Thanx for sharing Mr. D-Lab, is there a schematic for your wattmeter in the aqua-coloured box? Or even a construction video? (hope hope).
I have a Fender super reissue, when I turn the reverb up past two, the amp distorts and loses volume. Can you share any ideas what may cause that? I put all new JJ tubes in it and it didn’t fix the problem. I’m afraid to use it for a job, fearing it might burn something up. It’s around a 92 or 94
Terry, have you seen this amp build? Wonder what it would sound like with one of your guitar amps driving it and Tony C. going to town.
Hi Terry, just a simple question. This dummy used 4&8 dc resistance. But speaker is in impedance. Is still ok tu just plug in this type of dummy load? Sorry.. i am new to electronic. So my question could be ridiculous. Hope i am not offending you or whatever. Thanks!
Terry, how hot does this device get.
Thanks for this Terry. I have just built this and will be finishing my 5E3 clone shortly. Any plans to do a trace on a deluxe?
Thanks
Richard
I use a reostat on a short patch cord
Aren't those wire wound? In which case they would add reactance. Yes?
Brilliant! Thanks.
hey can you tell how to do a dummy load for a 52280-A rockola jukebox amp
A bit confused. I thought the 4-8-16 ohms was impedance vs resistance. Aren't you measuring resistance vs impedance? Thx.
So the impedance is variable listed as nominal. It's key to pay attention to the wattage for choosing the right resister or resistor bank to handle the load.
You're correct as impedance is basically frequency dependent resistance, usually measured at 1000 hz. However, as far as the amp's transformer is concerned, it doesn't matter; it sees a non-inductive resistance of 8 ohms.
Thanks !
How can you do a similar Dummy for a valve amp with an output to headphones ?
Hi I know you do tube amps but I have a line6 spider IV 75w that won't power up so how would one signal trace without schematic. I can't seem to find one cheer
Clint
I'm sorry for being so critical, but I don't wan anyone to possibly break heir amplifier or other equipment if they make one of these...
Being an electronics engineer I have to cast my doubts as to how well that device is going to be able to dissipate 25W of power. In my experience it doesn't look like there's enough heat sinking to achieve that.
Even if each of the resistors would be dissipating only 12.5W, that amount of heat is enough that without additional heat sinking there could be thermal issues.
If average power dissipated is well below that 12.5W figure, OK, it could work if there's not many peaks that hit that 20W+ mark constantly.
That type of power resistors aren't rated for 25W without additional heatsinking. If you buy 5W through hole resistors that don't ahve additional heat sink mounting, even they might be rated for that full 5W only for short amount of time and if they're constantly in moving air, mounted off the board to remove that heat.
It's all in the datasheet of any proper resistor.
freakin Awesome :)
Hi doctor d .......good morning ....I have amp sansui....so wat I want is the amp op impedance is 8 ohm so iam going to connect 4 ohm + 4 ohm on per side with right polarity...so I know it'll work but when 2x4 ohm making series I is 60watt and 1 is 40 watts which will go heard means speak loud or make more drop on him self
In 8 ohm load, shouldn’t you be able to test up to 50 Watts?
What about for like 100 watt head? What resistors for that?
Im going to use 2x 15 Ohm - 50W, wirewounds and wire 2 in parallel to roughly make 8ohm load but 100w rating. You could build it into maybe a small 1590A pedal enclosure if you plan to actually play the amp under load like myself (slaving the head to another amp which has attenuator) and not risking anything over heating, the enclosure will act like a head sinc if you mount them directly to the aluminium box.
great video thanks for sharing. Could I make this with a single 100w 8ohm resistor? Just solder the lead from the resistor to the jack?
yep, i think terry is using two 4ohm resistors just for the utility of being able to use one doohickey for a range of different amps (since 4 and 8 are common output impedances, and you can get 8 ohms by putting two x 4 ohms in series)
Hello Terry, I've a couple of questions concerning amp wattage. On a few tube data sheets, plate wattage (for instance 16w) and output wattage (for instance 4.5w). Would an amp built using one of these tubes be rated at 16w or 4.5w? Another question is how wattage rating relates to volume, db? You run all of the amps you work on mostly through the same speaker and the volume sounds the same. Could be the balance on your camera but I am very curious. Thank you.
Great video. Where can I buy the 4 ohm 25 watt resistor 1%?
Try ebay for the best deal
Very nice! Question: I have a Mesa Boogie 50 Caliber Plus that has an unrepairable issue with the power amp but the preamp section is just fine. I want to use it just as a preamp. Can I run it with the four EL84 power tube out of the chassis if I use this dummy load. My gut tells me no, but I need to ask someone who actually knows. THANKS.
I don't know if anyone answered you, but it's realy simple. All you have to do is pull the power amp tubes out of the amp. That's it. When you pull them out, no current is going to the output transformer, and then you can use the preamp section of the amp. Guitar->cable->preamp in->send->to cab sim, mixer, etc.
I have been experimenting with TUBE amps for years. I even used a 200W regular bulb for dummy load. Not the smartest thing to do in life, because a bulb can brake easily, but it worked fine.
If you want to eliminate the switch, would you just solder the tip connection of the plug to one end of the resistor, and the sleeve to the other end?
how hot will one of those get during testing?
Extremely hot if you run more than a couple watts into it. Serious burn hazard with those resistors exposed and not mounted to proper heatsink.
could you make the same thing for a higher wattage amp by using resistors rated to 50w or 100w? would heat dissipation start to matter more at that power level?
You can use the same circuit with higher powered resistors, or more resistors in series/parallel to get the values you want. But it MUST be built into a sturdy metal box or you WILL have a fire.
I have my 8 ohm 100 watt loads but I would love to have a watt meter like your unit. Any ideas?
Hi. Is it possible to use a dummy load in an amp for long periods of time, for example, 2-3 hours? I'm thinking using a lunch box amp's "send" jack in the FX loop into the "return" jack of a 100 w Marshall amp. In paper, this would give me the Marshall's channels plus the lunchbox sound when FX loop is activated in the Marshall. But the lunch box must have a dummy load since I'm not using the power section... Thoughts?
Can I use the dummy load on the amp and use the amp into my mixing board to record into my computer?
if your amp has a DI out, yes.
Hi Terry, you have some great video,s,,what resistors can i use for a 150 watt amp ..8 ohms ??
regards
i can see that this is an old comment and idk if this info is still useful to you… but maybe is, or will be useful to someone else
the resistance value depends on what sort of impedance the amp expects to see at the speaker out; 4 or 8 ohms are quite common, but some will want to see 16 (check the back, it should be labelled)
this is not the same as wattage/power dissipation: terry is using 25W components here, which means that they can comfortably dissipate that much power (current per unit time). for more powerful amplifiers, you'll probably want to use components rated for higher wattage/dissipation… you could maybe use lower-rated components, but if you turn the amp up you'll probably just end up blowing/melting them
soupalex so say they want to make a 150 they need two 150 watt resistors??
@@vemonguy686 Thank you.best regards.
Does anyone know if I could use this to plug into the extended speaker jack on a deluxe reverb to get the overall volume down on the amp? If I remember science, shouldn't this work to do that? Maybe would approximate 40% volume reduction? Thanks!
Hmmm
As a quick fix for my 100 watt amp 8 ohm needs... I guess I could just solder one 8 ohm, 100 watt resistor between the + and - ? Does orientation count?
Orientation of resistors only counts when they're color coded and you want to show how meticulous you can be :D If there's an instance a resistor does need oriented a certain way, there's typically either poles or an arrow showing the signal direction.
Can this circuit be expanded to hook 2 heads( one solid state and one tube head) that are both 8 ohm to a single 1x12 cabinet(8 ohm) with only one 1/4 inch input?
👍
But we should never switch it from one setting to the other while the amp is on, correct?
The amp would never know you flipped the switch, no worries
No you should not. that is not good, the amp and you would "know", if you use a break before make switch, which many unlabelled switches are, and you can hear it make a loud thump, from the brief unloaded transient spike, which is not good for the tube or transformer. If you used a make before break switch then that would be fine, the amp would still "know" from the parallel drop in load for the moment, but that is safe for that moment as it's still loaded, and you would not hear anything or stress the tube or transformer with a big spike
Why doesn't any company sell a product that does exactly this?
They do. Search “load box”
Does the wattage need to match the wattage output of the amp? For instance, if I made this with two 50 watt resistors to use with a Bassman 100 head, could I still use it with a little 10 watt champ?
Will this dummy load work with a field coil speaker or does something have to be added to load the field coil?
Would it harm the amp or hinder troubleshooting using an 8ohm dummy load on a 4ohm output?
If your simply troubleshooting, not driving the output, that load would be fine. Always better to have a load than none. No load can cause issues fast.
I have a bit of a strange question. I’m looking to get an open chassis amp(Milkman hi-fi 5w) to use as just a cool tube glow novelty kind of thing, , but I know it needs a load as to not ruin the power section. Can I build one of those and leave it in there indefinitely? If so, is it better to build one higher than 5w? I know this probably sounds like a stupid thing to do.
is the 8 ohm load 50 watts?
Attenuators please:)
can use this to make an 4 ohm amp head to power and 8 ohm speaker cab? I want to do this with an input and output jack but only 8 ohm load on or bypassed with switch can I use the same components? besides extra jack what else do I need?
Hello!
A have a question.
I've assembled an 8 Ohm (100W) dummy load lately and it gets warm with my 50W power amp. I guess it is OK for resistors as the OT stays cool, but anyway I would like to clarify if that is OK just to be sure.
Are there any restrictions while working with/troubleshooting an amp loaded on dummy load like that instead of an ordinary loudspeaker?
Best regards,
TT
It's OK, but I would probably limit the time you have it on, or bolt it to a piece of aluminum with conductive paste. Warm is fine, hot to the touch may limit the life of the dummy load (check the resistance when it's hot; is it still 8 ohms?)
I would at least put a fan on it and make sure it doesn't get any hotter than you can touch for a few seconds. Heat will eventually cause the resistance to fluctuate or fail and possibly kill your power supply as well.
So, if I use 200W resistors, will I have a dummy load that can safely handle a 100W Marshall?
Yes
Vic Hardy what about 120?
Well, professional dummy loads, almost all those that cost a bit, are NOT a couple of resistors soldered together, if you want a REAL dummy load, then you must provide inductance and mechanical resonance, a couple of inductors and capacitors in the right place can do that. Test with a purely resistive load won't tell you how the amplifier reacts in real-life situations.
I have a Rivera Fandango 55w (2 12inch speakers). what resistor size should i use? The amp setting is 4 ohm.
Measure the impedance of each speaker. You should have two 8 ohm speakers wired in parallel thus making a 4 ohm load. So in reality you could make a 4 ohm ONLY version using just one of those fixed resistors. His use of two resistors just makes a nice 4 or 8 ohm dummy load.
where is the easy to follow schematic?
Schematic is at 3:07 mins
Nice (dummy)!...LOL
funny intro :)
Get past the nonsense at 1:10