All of my Fender amps, i.e. Super Reverb and Tweed Bassman, require a 2 ohm load, four 8 ohm speakers in parallel. Is it safe to run those amps on the 4 ohm load available with this device or would it be better just to build one of these specifically for a 2 ohm load?
Do you recommend a good method to learn basic electronics? I've been working on my own vintage Fender amps for many years but really don't understand a lot of the basics. When I realized I need to get a more formal education is when I could not figure out all of the talk of frying my oscilloscope by mis-grounding it somehow. It really bugs me that I don't understand the neutral, hot and ground connections in all of these electronic devices and how it can bite you if you are not knowledgeable about it. Especially in these old amps with two-prong plugs which I eventually switched to grounded plugs. I think you say to lift the ground on your oscilloscope with an adapter plug but others say to get an isolation transformer to be safe. It's pretty confusing right now. It would be great if all I had to do was to be sure my oscilloscope not be grounded.
Hey, Terry! Nice simple design. I especially like how you kept all the power resistors at 4 ohms and just wired two in series to get an 8 ohm value. That means only one value to purchase and keep track of during assembly.
Yes, together you have 150W of dissipation power, but you won't let any more current than P=I2R, and a single resistor has 4 ohms and 50Watts, so 3.5A is the maximum current (in series equal for each one as for one), so in real you have 50W of dissipation. If you were to connect the resistors in parallel then you would practically have 200W.
Great video! Have a question on the resistors you have used. For dummy loads I see a lot of talk in regards to inductive vs non-inductive resistors. The ones you chose are inductive, but a lot of articles reference the use of non-inductive for dummy loads. Any input on which are best? I understand the inductive resistors will impersonate a speaker with a inductive load. Is this why you chose the RH series over the NH series? What are your thoughts of non-inductive resistors in dummy load usage?
Goodnight! First of all, thank you very much for your generosity! If it weren't for abuse I would like one Scheme for a 20w tube guitar I have. Where you can change the volume on the attenuator with a potentiometer. Would you have one of these? If so, thank you very much in advance. Big hug and success.
You don't need a double-pole switch. A single-pole can be used. Just tie the common terminal to ground and each of the yellow wires to the outer terminals.
One side is for switching the LED's that can be installed on top of the face. That's the reason why he is using that switch so in the future nothing changes.
Hello Terry! And thank you for sharing such a great idea that I'm sure 100's of people have used. I am looking to build one myself and would like to incorporate a mini fuse holder into the side of the project box so in the event of a mistake and burning out the resistor which will then require disassembly unsoldering and soldering resistors. I think unscrewing a fuse holder taking out a blown fuse, put in another and good to go again. What fuse Amp's & Volts would you recommend?
Nice video. I have a few questions: 1) the BnC connector has a silver metallic post sticking up out of the white plastic center piece in addition to the gold post coming out of the center. I have found the piece but I was wondering what is the extra piece sticking out of the internal side of the connector? 2) the Fender-style diagram doesn't seem to follow the picture. The Fender-style diagram shows the red wire coming off the center ring of the 1/4" jack and the black wire connected to the outer ring but the video shows the black wire going to the center ring and the red wire going to the outer ring. Which is correct? 3) I assume the mechanism that makes one ON position 4ohms and the other ON position 8 ohms is the way it is wired on the back-end? Thanks Terry.
The 10K resistor will attenuate the signal going to the scope. While the shape will be valid, the strength of the signal will read lower than actual. It sounded like you placed it to act as a fuse. Using a fuse would cost more but would avoid the falsely low readings. Many people want to read the voltage on the scope so they can calculate the wattage of the output.
I watched your other video on how you explained oscilloscope, tone generator, and the dummy load. You said tone generator had to be battery only or not have ground. I just bought one and it has a grounded cord. Question is it as simple as breaking ground lug off plug to not get ground loops?
Terry, is the schematic wiring on the switch correct ? On the unit, it looks like you have a jumper wire on the top two poles of the switch ? Nice Project !
I built a much uglier version of your Dummy Head 30 years ago. Practically the only thing different with mine is the ventilated metal box from Radio Shack that holds it all. Yours is so cute, with the fancy artwork, I may just make another using your design.
Thanks for sharing the schematics of this wonderful and very USEFUL project for geek amature like me :) More power to your channel! Watching all your videos here from Oman!
I built one of these a while back that's I think 25 or 50 amp in one of those Hammond aluminum guitar pedal sized enclosures. It's only four and eight ohm but since it's nice thick cast aluminum I thermal pasted the resistors when I mounted them to the enclosure. I used an RCA instead of a BNC (what I had on hand), but I have an adaptor. Very handy tool to have in the shop. thanks for putting up the tutorial for folks, I know that it will be appreciated.
After watching one of your videos long ago, I built a dummy load. This is what I use it for. I have an old BadCat unleash attenuator, a large 100 watts tube amp and live in a small apartment. The full amp into the BadCat was too much power for it (even when I removed two of the four Power Tubes from my amp).The BadCat is supposed to be used with the speakers! Ridiculous! By using the dummy load in conjunction with the attenuator... voilà! I could crank up the power amp, crank up the attenuator (to get a decent sound), no speakers... it sounds great out of the attenuator direct line out. Thank you sir for sharing your info!
Your BadCat uses the speakers as the amp doesn't stay at a fixed output load through its range of frequency it is amplifying. A speaker isn't a fixed load but varies over a frequency range. Although you can use a fixed dummy load, you'd be better off creating a variable load, and there are various tutorials on employing a speaker's voice coil structure to create the variable dummy load......you don't need a full speaker with the cone......you just need the voice coil motor (magnet and VC with spider) from a speaker of the appropriate wattage for the amp or a bit higher. Think an EV with a torn speaker cone or a dry rotted cone as opposed to a dead speaker. AND you'd mount it a appropriate sized enclosure. Your best bet would be using a speaker motor that would match the full rated load of the output transformer.....hence a 16 ohm if the has a variable 4/8/16 output transformer.....to take advantage of the full transformer. Generally the pre-amp of an amplifier is what shapes your guitar signal and the power amp via the phase inverter makes that signal huge. Generally a power amp section with 6550's or 6L6's for big power tubes tend to be on a clean linear scale up for the output power......the EL-34's tend to color things way more as well as EL-84's for a small power tube type. It also can have a influence as to how HOT you bias the power tubes along with what the power tubes are rated for......the Groove Tube scale of 1 to 10 or other manufacturer's ratings that serve the same purpose......as ultimately if there is power tube colorations.....its more when the amp is full on and wide open.....thinking everything on "10".....or the Tufnel ELEVEN...one more. You mention using the Line Out......I'm assuming you are patching in to a computer and then using headphones. But using a speaker motor will give more dynamics than a fixed load. One of the original designs for an attenuator was by Tom Scholtz back in the 1980's as he was not just the Guitar player & leader of BOSTON, he was also an MIT engineer...and he designed all his stuff and launched a line back then. His attenuator was a ventilated box full of cement block power resistors in an array......I have one here in my collection as an example and I never counted the power resistors but there are at least maybe 20 ! BUT his attenuator being an early design and based on his MIT background was thought to be great but met with bad reviews when other attenuators started to appear on the market when the market opened up for the power reductions for the big amps......THE FLAWS. SO the flaws were addressed by using a speaker motor to keep things dynamic as the actual load was a Voice Coil in a magnetic field
@@mikecamps7226 Thank you for that response! Hmm. Now, I'm not sure if I'm wrong or not. My setup has the power amp speaker output to BadCat Unleash (ver 1). Isn't that a variable attenuator like a speaker without cone? The problem is that BadCat has an output for speaker that when it's left unplugged gives a lousy sound. The dummy load in the badcat speaker output seems to make all the difference and I can crank up the main volume which gives great control. Control over preamp settings and power amp volume. They are quite different yes, and I love that power amp amplification,
@@jfjoubertquebec In getting back to you.... I just did a brief look up of the unleash. One appears as a general attenuator which is kind of a take off on a home stereo speaker situation. Generally ina 2 way or a 3 way stereo speaker system, there are sometimes control knobs to turn up or turn down the mid-range driver or the tweeter.....and generally they employ a big wire wound rheostat as the mechanism to do so....think of it as a giant control pot but being wire wound. THIS was adapted over to guitar amps and certain attenuator systems......and this is what appears to be in one version of the unleash. Another version of the unleash apparently has a solid state class D amplifier built in where you patch in the guitar amp via its speaker jack which then is applied to the solid state power amp in the unleash box and then you still plug in a speaker. SO NO, there is no speaker motor involved as far as I know in any version of the Unleash.
@@jfjoubertquebec on a side note, Fender did an addition on a newer amp they recently put out on the market. I only browsed it once online as for the schematic, and there might be something on Rob Robinette's website for it. Essentially its an Eric Clapton edition tweed tremolux or something like that, so its not a big power amp. It might be 30 watts tops if that....maybe as best as I can recall. So what they designed in was a sort of attenuator/ power scaler situation to dial the power back in steps. They have the speaker load the output transformer was designed for, matched to the speaker in the cabinet obviously. BUT to lower the output, they use a Power Resistor network to scale back the power going to the speaker......so the power resistors are in series and also can be in parallel with the speaker load.....and there might even be a situation of series/parallel with the power resistors too. I only looked at it once as a reference, so this is all off the top of my head......without going back to review and be accurate. BUT its a very interesting situation for a low wattage amp and cutting it back......so essentially the speaker is always hooked up and this network is added in between the output transformer and the speaker connection at the speaker. And being a small wattage amp, its all conventional common power resistors.....probably the cement block types. So if you are studying the Dummy Load and how he has them set up in the array with the switch......it would be kind of similar as its a series & parallel situation . SO example, two 8 ohm power resistors in series would yield a 16 ohm load, and two 8 ohm power resistors in parallel would yield a 4 ohm load.
Just made this thing. Haven't built or soldered anything in 20 years and it just works. Was quite fun! Now... time to fix my Fender Bassman! Thank you Terry!
Hi and thanks for your amazing knowledge. What a great treasure. So I’m building the dummy load unit but have four 100w 4ohm resistors. 1) can I keep the 10ohm ¼ watt resistor shown between the Jack and the bnc? Or do I need to modify? How? 2) what gauge wire do you recommend? I’ve almost got my diagnostic tools together, this is the last piece (unless I really need an isolation transformer). Thanks again for all the wisdom.I’ll try and get you a pic of my version if you’d like.
I’m making one of these, my enclosure is a good bit larger than this one. Does anyone know- would it be possible for me to add a small (2-3”) internal speaker with an on/off switch somehow? If someone could give me a quick description of how a speaker could be wired into this circuit, I’d seriously appreciate it. Thanks for the hours of incredible videos, Terry! You’re a living legend and a national treasure.
Great Idea for a dummy load box. Is there a kit available? The hard thing for me is where to find a good box to put everything in. Any other information that you could share would be greatly appreciated.
Terry, I've wanted to be able to do such a project for a long time. Thank you very much for making it available. One question: can I test with amplifiers of up to how many Watts? Thank you, hugs from Brazil.
Nicely done and a great example of how investing a little time in some test gear gives you something that will save you time for years. I will either build the dummy head as spec’d, or I may build a switchbox to utilize the 100 watt dummy load resistors that I’ve had for years. I always seem to have a mismatch between the wire/plug/terminals that I have attached to the resistors and the unit under test. Reconfiguring wastes time :-) Thanks for the inspiration! Dave
@@Stu66orn A resistor is a lot cheaper than a fuse- and an 8k 1/2 watt resistor would be quite all right. Anyway, the resistor is NOT actually a "fuse"- it's a current limiter to protect the amplifier output in the unlikely event that the BNC jack terminals are shorted together. Unless the BNC jack terminals were actually shorted together- the dissipation of 8k or 10k resistor is negligible- and even then, the dissipation is only slight.
Great video, looks like a fun and useful project. My question concerns the HA-520. I was given one, it had been dropped and the chassis and transformer had been damaged. I managed to resurrect it and I was planning on building the 6AQ5 AB project but that 6K6 with trem is the Bee's Knees for a practice amp. Please run a vid on the build, I need a winter project. If not how can I get a schematic and a few tips. Love your channel.
