CORRECTION: At about 11:20 I talk about total "power handling" of the load. I said, "with two resistors in series you don't increase your power handling". That is not strictly true. What I should have said was "you don't increase your TOTAL MAXIMUM power handling with two resistors in series". In the case of parallel resistors, they split the current between them, so the total maximum wattage that can be dissipated by the two resistors is increased, and if both are the same value, this figure is effectively DOUBLED. But in the case of series resistors, the TOTAL current through the series resistors is not split. The same current flows through both. Therefore, the total maximum power handling of this load cannot exceed the maximum wattage rating for the LEAST robust resistor in the series. I hope that clears things up somewhat. Got a lot of comments about that one, which is cool because it shows people are watching and thinking.

That’s what my dad said when me and all my buddies loaded into the back of the truck, “ well there’s a full dummy load!”

Just don't start calling yourself The Dynacologist.

I sent you this concept almost a year ago. My suggestion was a little more elaborate allowing for 2, 4, 8, & 16 ohm loads but at least you have the start of something better than what you were using. Need to add a connection for your oscilloscope. Nice upgrade to your test bench!

I'm doing the same thing at the moment , this is perfect timing for more ideas for my project.

Great tutorial Brad! I dig these style of instructional videos--great info and footage editing.

Awesome, was watching this while wearing my teal/purple Squishy wrist band and seen your black one! They sent me 2 instead of 1, so I gave one to my G/F, and donated another $20. More power to Squishy little girl. Cool project Brad, this kinda reminds me of the inside of my 80's Tom Sholz power soak... lol Great video, thank you!

Brilliant. Not only the engineering, but the reclamation of parts you had and a few purchased items.

Excellent tutorial. Thanks Brad. Can't wait to see you back in the amp world. Though I'm sure your other stuff is just as impressive. I understand that a person must evolve, especially a creator.

Thanks Brad for making video's that bring so much happiness.....

Thanks Brad. Your videos are top of the pops buddy.

You know I'll be building one. Thanks Brad!

I made two attenuators that also work as dummy loads with several 20w resistors :D

Hey Brad, great video today. I still have no idea what it’s for, but I really enjoyed watching you build like always. Keep up the great work.

That chassis could not have lended itself any better to this project! I love when stuff falls together. Nice job!

The ambient music and time lapse electronics work is hmm, very relaxing haha. More importantly, I now know what I need to do to build a dummy load! I'm going to be repairing an old Fender silver face Bassman amp, so this is perfect, thank you!

Great video Brad I’m always learning something new from your videos I love how you repurpose things we have become a throw away society and it’s sad hope some of the young viewers watch because they can learn a lot from them thanks for all your videos.

Nicely done. I've done the same thing. I like your chassis. My dummy load is in plain view on a huge heatsink. Cheers

Great video. I made one of these for high discharging RC batteries for racing (they sell for $180 USD which is the same thing you built with a fancy brand label on it). RC guys would appreciate this build, I will pass it on to them.

Power handling adds up with series resistors too, because one resistor only gets half the voltage, while in parallel circuits the current is split and the voltage is the same, but if you do the maths the power dissipation will be the same per resistor. You can also add a high impedance voltage divider in parallel to get line level out from the speaker.

Really informative , love the simplicity 👍🏻☮❤peace and love brother 😊

Now that's a real cool use of existing heatsinks and chassis, Brad. Thank you. If you use your pairs of 8's in parallel to make 4's, you can then parallel those pairs to give you 2ohm (Bassman and Super Reverb). You now have a couple of options on getting your 16ohm. I use the Weber Tru Load, which is excellent 90% of the time. It has a BNC for my scope, but sometimes the inbuilt speaker coil can cause oscillations. That's when I need to pull out my Guitologist dummy load. :-)

Damn. Destroying a nice vintage Dynaco. As a vintage audio lover, that hurts. I hope that amp was beyond repair.

thx 4 ya vids brad hope you have a great day!!!

Man, those 8RJ resistors are giving me Navy flashbacks. Actually really glad you posted this, though. Been considering how to go about this exact project.

Thank you for the info Brad. Love your vids!! 💚🎸

Sweet Brad, been needing a dummy load thx.

I've actually been wanting to build one of these from an old paint can filled with cooling oil for a ham radio dummy load for the radio, has to be able to handle high wattage. I like this for that idea much better, never thought of it for some reason. I would also like to see about using this as a laod for the tube amps as well. Great video's as always!

Extremely interesting. I never thought of using a dummy load for an amplifier other than on transmitters in our radio business.

Reminds me of the Scholz Power Soak I had back in the 80’s. Worked just fine for its purpose especially back then.

Have you considered adding terminals to the dummy load so that you can connect your scope easily?

Great job brother. How about designing a pedal using 6 volt tubes like a tube driver

Excellent work KOOL

Fine work,brad👍🎸🔊🎶

I mounted four 300W / 4R on a huge heatsink for testing PA power amplifiers more quietly. I made two pcs for a total of 2x1200W. If i need more power, i just connect them in series. Works great!

Thanks for the clarification. I have been looking into this idea mostly because I want to go Cab-less, but want to keep my amp head.

Very interesting video never knew what it was or used for thanksfor your time

Brad I used to use my ex as a dummy load but she kept complaining about it, I couldn't convince her it was pre-menopause thing. Good stuff here as always!! Dynaco! How kewl is that!! Them Ohmites were built for mucho watts and I still have them in use today!! Ohmites were never pussyfied - also I still have and use my Ohmite 194X variac just not very fancy as todays stuff. Take care!!

I used to have one of these Dynaco ST120 amps---HATED IT! The tube amps I had were MUCH BETTER! Stayed with them and sold the ‘120. I built it from a kit-just one afternoon to build. When I attended college at the University of Springfield,South Dakota in 1971-1972-the Dale resistor factory was in Yankton SD just a few min from Springfield Sd. The Dale folks donated several of those chassis-heat sink mount resistor to the school. Some rated to 500w! Later in life as I worked on AM broadcast transmitters had to make a dummy load for a 10Kw AM transmitter modulator was 5kw of audio into 6kohm. Used a bank of 1 Kohm 200W ‘dog bone “ resistors in series parallel.worked great. The modulator had a pair of 12BY7 tubes in the font end-they drove a pair of 6CA7/EL34 as drivers for a pair of 4CX3000 ceramic tetrodes via a mod transformer-that ou-led to the RF PA stage.Most of the time could test modulators in the transmitter as is with the Tx feeding the antenna or dummy load-often the stage had to be isolated to troubleshoot noise issues or crossover distortion that could be caused by a shorted turn on the mod transformer primary. Like the output transformer in a usual tubed PP amp the mod transformer has a high resistance single end secondary wing to couple to the single end RF PA stage. Fortunately the mod stage was ok and it’s mod transformer. Glad of that mod transformers are expensive and heavy at 600 lbs¡ your dummy load made me think of the one for a modulator stage.

It's good to show simple ways to make a home brew speaker load.. Many years back I used heater element. Now use fan cooling just to push the load resistors a littler harder.

had a line out that didnt have a dummy load but worked on standby and was not affected by master volume, wanted to know if i could connect a speaker cable to a 100 watt 8 ohm resistor and have that plugged in just incase i accidentally switched it from standby to on with volume at 0, and this video was super helpful. I learned i can run the two 8 ohms i have in the 4 or 16 configuration since the head has both options and it will probably be fine ayyy time to go make my first thing and see if I understood this at all.

whoua man thanx I will build it for sure !!

Wicked, I was just about to whip one of these together. CPU coolers should work great for the application. Super- cheap (or even free) for 80 to 100W dissipation each. Biggest down side is the 12V requirement for the fans. I used a PC PSU in my last dummy load. Built it in an old Dell chassis.

Wow Brad. What a way to treat a Dynaco 120. That was my first ever amp project kit circa 1974. Ran my floor wedges until I blew up one side during a gig. Ooops! That aside, 2 x 100W - 8ohm resistors in series or parallel will handle 200W. Even though when in series the current is equal in each R, the voltage divides across both. Similar to when in parallel the current divides, but the voltage drop is equal across both. You can do the math as I x V , or I^2 x R, both = same W. As long as the resistors are equal this will hold true. It's when you have unequal resistors that things get sporting; you have to calculate voltage and current through each leg and determine power handling requirements for each R (that's when I^2 x R is best formula to use). Have fun with your load, but don't do electrics loaded!;-)

Hey Brad! Love the video. Could you do a series on Guitar Tube Amplifier Design where you tackle the basics (somewhat like Uncle Doug) but go much further, explaining how each design type, component values affect the end result, the tone. All the while you would be showing Schematics to illustrate how the topics you're covering are implemented, with some chassis pictures and real demos as an added bonus when you can get your hands on the appropriate amps! I have learned a lot following your channel, and I know you're already kind of doing what I've talked about, but I would LOVE (and I think that a series like this would be VERY POPULAR) it if you could structure your teachings in order to help people like me in their quest to build tube amps, effects pedals, etc. You have a very dynamic and clear, concise way of explaining things with a lot of insight. Thanks for considering this proposed project!

Nice....!!!....background music is relaxing too. A Made in USA label, some probe jacks for a meter/scope and a fuse could be handy additions.

Thanks Brad. I fried 2 pots the other day trying to make an effects loop attenuator. I need to up my soldering game. Ill give this a go.

Very cool to watch and learn thank you.. Question what was wrong if anything with the transformers on that are they salvageable ? Im wondering who would rewrap those type transformers these days ?

Nice job. I am looking at building an attenuator for my Mk IV. Boogie. I have come up with a T topology resistive pad that should give about 20dB and an impedance of 8 ohms. I’d be interested in hearing your thoughts.

I need to build a load for a 5 watt blackheart, how much resistance do l need to bring a low power 5 watt TUBE amp down to line level?

Excellent!

You're a gifted musician and electronic engineer

Brad I loved this one. I assume you are making the music in these videos, I want to hear about it!

Very cool. Can you do a video on how to test a speakers impedance and it’s power handling capability? I have speakers with no labels and don’t know where I can use them.

Another dummy load I used that type of resistor in-just thought of it. This load was used to test cook a broadcast console mixer power supply-+ - 15VDC thThe console power supply would fail intermittently-cooked it under load until it died. Then was able to fix the supply.-funky regulator chips.re-laced them-cooked the supply again and it was OK . Had to make a “HOT DO NOT TOUCH’ sign so others in that shop would not touch the load. Mounted it on a rack panel to work as a heat sink and chassis. The load served it’s purpose-probably still in that shop-the VOA maintenance shop in Wash DC. Sometimes folks like to touch resistors being used for test. Your load has the cover to prevent. That.

What are the chances the holes line up you might want to play the lottery

you should expand on this and show us how to make a home made attenuator

Fella I grew up with who was sort of an electronics savant had an idea that used an ordinary lite bulb base and regular lite bulbs of varying wattages. I don't know the other components, but the idea of drawing off various wattages with different lite bulbs seems interesting. Just a thought.

Hi this is very informative. What do you with the Ground connection that is tied to the TS connector sleeve ? Thanks.

11:17 Power handling also doubles in series, as they get full current, but drop only half the voltage. However keep in mind that those power resistors also have a maximum temperature rating. With purely passive cooling, you can calculate the approximate temperature rise per watt for your heat sinks. I think you need surface area, cubic content and thermal conductivity but don't ask me for the formula. Only if they stay below the max operating temperature they'll dissipate full power with no damage. Just by looking the heat sink is probably too small, but you're not gonna need that for guitar amps anyways, so you're fine.

You could slightly enhance this as noted and then market this! A handy tool for amp work.

Is it possible to make a dummy load with a line out jack?

Made one of these years ago with 4 16 ohm 100 watt resistors, gives me 100 watts of 16 ohm, 200 watts of 8 ohm, and 400 watts of 4 ohm. I use a much more massive heat sink than you but still I have contemplated water cooling because it get's scarry hot when soaking an amp. For testing and intermittent use it's fine.

Great truly useful project. Could you make a cheap power brake?? Please.

If you can find the time I'd love to see a video showing how a reactive loadbox circuit works.

Dang! You completely dismembered a vintage Dynaco stereo amp just to make something to dissipate some extra heat? You big dummy!

I have a speaker 1000w in to 4ohms can 2 resistor of 4ohms 100w in parallel to make 6ohms to my speaker can the resistors handle thank you

Straight forward but effective. Wonder if you have any thoughts on the differences between a purley DC resistive versus an inductive dummy load. It would seem a resistive load offers negligible inductive reactance and so serves mainly as a load for the DC component but an inductive load has both impedance (inductive reactance for the AC component) and DC resistance. Maybe not an issue with most tube amps as far as basic troubleshooting and maintenance goes but it takes me to one of those ..'Hmmmmm'...moments. Cool video once again. Cheers!

A regular space heater on "Hi" ( 1650 watts) is quite near 8 ohms and can dissipate.... well 1650 watts. Just change the plug. Best to use one without a fan. If it has a fan you can disconnect it if you don't drive it near the max rating. The fan is inductive instead of the resistive load of the heater coils. It wont hurt anything but the impedance will vary a little with frequency and will have a reactive component. A lower setting on the heater will give higher impedance ( a 900 watt setting would be 16 ohms). If you have a 1000 watt setting it will be a little lower than 16 ohms. You can calculate the impedance of the heater from the listed watt draw at a particular heat switch setting by the formula 14400/watts=impedance.... assuming 120 volt ac line voltage. You can hear the guitar sound from the heater a little too. A bonus...

Could have probably had better thermal properties if you had put like 1/4 as much thermal paste, it is just meant to fill in the imperfections of the metal, very thing layers work excellent - after that you start to insulate it as the metal (copper and alum) will disperse it better than the paste will.

Thanks for the video. I have 2 dummy loads of 4.2 and 8.4 ohms, can I use for load testing of 4 and 8 Ohms? What's the maximum tolerance? Thanks.

With a dummy load for troubleshooting, measuring etc on workbench, if the dummy load is not used when playing, does less than full power handling do the job. Say being enough for a bass amp, since you don't introduce power spikes with transients when you're not playing it?

OHHH!!! Hell yeah Brad. Good on ya, man! Now where talking.

can i use speaker cables for the internal wiring?

Can you make a video showing how to make a Line Out Box because some Line out Boxes use an isolation transformer and the line out box needs to convert speaker level signal to instrumental level or line level signal. The amplifiers output is going to have an impedance of 4 ohm, 8 ohm or 16 ohms which the signal is not an audio signal level because its a speaker signal level which needs to get converted back to instrument level or line level signal. It would be nice to learn how to do this correctly.

Question... 😊 if I build one of these... I could hook up my marshall head without a speaker and play to my DAW without the worry of killing my head even if I used it full blast for an hour... I have a marshall jcm 900 50w it has a couple of line outs (direct out and recording compensated)

Love the background music! Zen as fuck! Cool Project Sir!!!

Hi Brad I was wondering if you could help me. I have a 16 ohm Hotplate and my amp is 8 ohms output into the 16 ohm hotplate which I know is fine with that one step mismatch BUT what about the output coming from the 16 ohm attenuator does it have to be a 16 ohm cab or can I run an 8ohm cab? I remember calling THD years ago and asking them about this and they said no problem going from an 8ohm amp into 16 ohm Hotplate but going out of the 16 ohm Hotplate you want to run a 16 ohm cab. I can't remember if they said it was not a good idea or it would ruin the Hotplate but I don't always have a 16 ohm cab and more commonly have 8ohm 2x12 cabs so I have been going into a 8ohm 2x12 out of the 16 ohm hotplate but my amp is always 8ohms into the 16 ohm Hotplate. You think it's hurting the 16ohm Hotplate by running out into an 8ohm cab?

Hi! Awesome video! I just ordered 4 of the resistors. I am just now realising that my Engl head is only 50w. If I build it the way you have, will there be any negative effects or is it just more headroom? I guess what I'm asking is, are the resistors 100w "max". Thanks to anyone who helps me out!

This makes me wonder if you can use an old punctured speaker that still has an intact voice coil. Basically a speaker that doesn't make much noise. Maybe put it inside a sealed box. I can't remember if this is already a thing or not. Hmm... maybe I'm confusing a dummy load with an attenuator.

Does reactance come in to play at all?

Can you build a attenuator dor a 100 watt tube amp?

Is this also “reactive”? What’s the difference and should I care?

Am I crazy or did you not do anything with the ground (tip?) of the speaker plug?

Well Brad, I think I know someone who also read and used the Forest Mims Engineer's Mini-Workbooks. ;p)

Hey Brad, my amp guy says that nothing will truly substitute a speaker cabinet for a tube amplifier and that any dummy loads otherwise will end up damaging it. I would normally look at the fact you're making one, as well as the fact they are sold everywhere and seem to be widely used as evidence to the contrary. However, this guy is definitely has some weight behind his opinion as he's worked on stuff for big names for a long time (more on the vintage synth end of things, but he's done many amps). Is there any truth to it, or do the resistors truly serve the same function that the speakers do in a cabinet?

The resistors you have used are metal clad wirewound resistors. These will be an inductive load to the amp, you realy need to use high wattage carbon film resistors, this is what Hughes & Kettner use in their valve amps as dummy loads.

Hey brad, thanks for the post. Question... what makes a dummy load a ‘reactive’ load? Also... what does it take to add a line out?

Tip: I use a Badcat amp attenuator... but it's used to lower the volume of your speaker not eliminate the speaker. So... long story short: Dummy load it...instead if using speakers. Now the output goes straight into mixer. Magic! Will not work if this isn't done, the attenuator does not sound the same unless you use "loud enough" which is way too loud for my bedroom.

Cool!

You do know that some people love those amps, right? My jaw is on the floor, scrapping a decent Dynaco for the case! That said, thanks for the tip on the Dummy Loads! I'm going to build one, but try not to destroy a classic amp in the process.

Liking before watching. I have faith in you, Brad!

Watt a cute little low tech load bank and a great use for a dynaco 120 horrible amp my load band is rated conservatively at 1,000 watt a side it’s forced air conked and water cooled the resistors used are Dale250 wt nonindutive with switching between 2, 4 or 8 rack mount with tempature switchs to control the fan speed and pump for cooling

Hi! great project! I´m testing an 8 Ohm prototype made of 2 pairs of 8ohm 100W resistors, and I´m making initial tests with 3 amps, 24 volts and making augments of 1 volt. I have noticed two things, I will appreciate if we can share some info about it. Here is what I found: 1.- Do you have any specific test to confirm it´s working? I made this calculation: To get the maximum input values: P= 124 [W] @ 1k [Hz] (info from manufacturer, hand made amplifier) Z= 8 [Ohms] => I = sqrt(P/Z) which is a bit less than 4 [Amp] => Vout max from amp = 4*8 = 32 [V] So with this I decided to make the system stable for a DC signal of 32*sqrt(2) = 45.3 [V], and an AC signal, 1k [Hz], V peak = 45.3[V] 2. - The resistors gets VERY hot : I ´m making measurements, and just got a thermometer to confirm but I could estimate about 80°Celsius by touching it. Do you know if there is a wat of estimating or calculating that? I´m concerned with loosgin resistance stability due to heat in the long run. I´m building an enclosure with a 1590 C Hammond box, a fan underneath the resistors, and CPU dissipation and vents on the outside of the box. Have a great day!

Can this be run with a speaker to lower the overall vol of a Mesa Boogie 5:50?

Don’t forget a 2 ohm load for the occasional Super Reverb/Concert/Bassman full bore power test!

can u power a fan off the power amp current?

These power resistors in aluminum case have heat sinking built in, that's why they are finned, extra sinking can't hurt but it seems like overkill to me.

@Brad is that transformer blown in the Dynaco ? if not what wattage is it again and how much do you want for it ? you have my email...

Hey Brad, could you make a power brake circuit in a similar way?