plz post more im a college student studying ce trying to make me own guitar amps and pedals as projects. i really enjoy your content and your one of the few people on youtube who can actually teach me these things!
Excellent video Stuart.. Much like a doctor, it's all down to diagnosis to find the fix for the patient .. Very informative and great explanation of how different systems of biasing valves work.
Good fix. Enjoyed that. Good job it wasn't one of the KT88s. They cost a kidney. I have four in a 200 watt Carlsbro PA, which I plan to turn in to a beastly guitar amp. Thanks for another great video!
I love gear with "magic eyes". I have several Grundig and Telefunken radios with them, and much of my bench test equipment has some type of magic eye. I think they are excellent for repairing vintage electronics and I use them to verify the results of my digital test equipment. If nothing else, they look really cool and give that mad scientist vibe ;)
Stuart, you have provided another entertaining/educational video. I had to laugh at the placement of the screws on the cover (another way for engineers to mess with techs). Thanks again for the talk through the troubleshooting and the explanation with diagrams. Yes, I can follow as you explain (you do well with this ). Thank you again for your time.
A brilliant explanation regarding “auto biasing” but sometimes, what you call the “wasted voltage”, developed across the cathode resistor is intentional and used to introduce negative feedback for that stage. Although this reduces the gain, as in this case, it produces gain stability and increases the bandwidth. I frequently used to find exploded electrolytic bypass capacitors in amplifiers where the cathode resistors went open circuit. The voltage across them used to rise to way above their rated value, used to make such a mess. Keep up the good work, really enjoy your channel.
The defective cathode bypass capacitor appears to my eyes to be bulging slightly. It's also located closer to the hot-running cathode resistor then the capacitor for the other channel is, so its not surprising for that particular cap to fail first. I would have installed it with the leads left a bit long so that the capacitor could be bent or tilted away from the heat generated by that resistor. PS, the bypass capacitor also increases gain, improves frequency response, lowers distortion and improves linearity.
An old serviceman's trick for replacing screws such as the hidden one down at the bottom of the tube cage: put a dab of soft, sticky beeswax inside the screwhead and push the screw onto the screwdriver tip. We would collect the soft wax that comes out of hot-running transformers, and every tech had a little vial with a ball of it on their bench. In a case like this, the problem with using a magnetic screwdriver is that the screw itself will tend to tip sideways off of the screwdriver tip, going off in search of the nearest ferro-magnetic device, like the transformer housing or the amp chassis. Anyway, if you don't have any soft beeswax from a transformer, a soft resinous putty of almost any kind will work, and you only need the tiniest dab of it inside the head of the screw.
@@stuartukguitarampguy5830 , You can temporarily magnetize the tip of any steel screwdriver simply by rubbing it on a speaker magnet or the other large magnet, several times. You can also buy a hollow, torpedo-shaped magnetic adapter made by Makita (it's blue, like their tools are) that will slip right over the shaft of a screwdriver to magnetize it; you can slide the magnet away from the tip for less magnetic force or towards the tip for a stronger magnetic force.
@@user-rf9me7xm1w , auto-body ribbon putty, like a softer version of Mortite ribbon caulking, also works. The trick of course is to use just a tiny bit of it, just enough to hold the screw but sufficiently little that it will easily be displaced if you need to remove that screw again.
Hi Stuart. Another top video, so thanks a lot for that. Do you use an isolation transformer with your oscilloscope? I have read multiple articles on whether you should or shouldn’t use one when using a scope with a guitar amp. Also read a few things saying not to use use the group cable on the scope probe as well. Would love to know if you have any strong opinions on the subject. Many thanks again. Paul
Thanks Paul. No I've never heard of using an isolation transformer for the scope. I can;t think of a good reason? The scope ground and amplifier chassis ground are the same ground. Wonder why they want to use a transformer?
The volume control appears to be a Chinese clone of a high quality Noble potentiometer. The wafers are sandwiched together with a pair of bolts, and so if there's no opening where you can easily inject control cleaner ---- there might or might not be slots underneath the control where the terminals exit, or it might be sealed ---- you can remove one of those bolts and loosen the other and squirt a little cleaner into the gap or space that develops between the wafers, or into the hole that the bolt passes through, and the control cleaner will work itself down inside the control by capillary action. Of course, this means you have to remove the potentiometer from the amp chassis first!. It does appear that there's a section of the main circuit board right behind that control which doesn't have a trace running through it, and so you probably could take a Dremel tool to the edge of the PCB and grind away a little bit of the main board, an 1/8" or so, and perhaps create enough space to angle the control out. Be advised that I have seen eBay sellers hawking what appear to be or are said to be high quality Alps "blue velvet" pots but are actually fakes and have poor resistance tracking between the 2 halves of the stereo control.
