A viewer suggested that I watch this video, and I'm glad I did. What an excellent overview of safe voltage-measuring technique in a tube amp circuit. Smoothly narrated and easy to follow. Great job !
Good job. I suggest any owners of CBS era Bronco,Champ or Vibro Champs google The Gear Page, Audiokarma, or TDPRI. Basically CBS Fender performed “engineering malpractice” since as designed, especially with modern wall AC voltages of 120-125 VAC, these amps will dissipate about 18+ watts on a single 6V6 ( vs rated maximum dissipation of 14 watts) In another video, DLab partially fixes this by upping The 6V6 cathode resistor. A common fix but not the only or possibly best approach. Again I refer you to the forums to get education. I think Uncle Doug on RU-vid tackles this as well. But these videos are really helpful, and keep up the education! Thanks.
Got a 78 Silverface Champ last year that was collecting dust in a pawn shop, not knowing exactly what it was. But I had a hunch that it was a good amp, because of all the tubes in it (along with the amp being 3 years younger than myself). I've always been a Marshall, Mesa Boogie, and Vox guy but I knew absolutely nothing about Fenders. Terry's series of Fender Champ videos are very educational. I was able to recap it, identify the bad resistors and replace them, along with identifying the commonly used test points.
Thank you SO MUCH, Terry! This was not only like a "Tube Amp 101" for me, it rehashed many basic rules of electronics, not the least of which is SAFELY working around high voltage circuits! Please remember that high voltage is very unforgiving of mistakes, people. Again, you're one of the very best!
Don't go buy another meter if you already have an auto-ranging meter. All the ones I have seen, and owned, have a manual range button to override auto-ranging. You just have to remember to set it. Great video though. I am hooked on D-Lab.
One of the first things we learned in high school electronics was the One Hand rule. Any time you're doing anything with live equipment you keep one hand in your pocket and probe around using the other hand. What you're trying to avoid is having the current run from hand to hand because this is where it can kill you by stopping your heart.
Thank you so much! This is the best video I've found on the subject - and I've looked for years. I've seen videos that say you should never test a live tube amp and instead only do resistance math, and others where I can see them put both hands in the live device under test to measure the amps at the plate (shorting past the transformer). You didn't cover things like using an isolation transformer, which has been done to death, and even less highly covered things like pressing the probe too hard against a solder joint and sliding right into the case, creating a potentially exciting short (which is why I have to depower before moving probes, I'm a spaz). I've seen in other videos people popping out power tubes and stick probes into the pin slots, or say things like "never ground the cathode or you will kill the tube" (which hasn't been true when I have tested from ground to cathode pin) without clearly explaining everything behind it as you did, and plus where to really look for common issues. Bravo!
Thanks again for eductional video 👍from England 🇬🇧
7 лет назад
Nothing like a good snap of a 450 volt cap into your finger to wake you up.You will never forget Terry's advice then, for sure.,if your heart is still pumping So pay attention to the first few minutes of this video.Your life depends on it. Great video Terry !
Just the video series I've been waiting for; most I have watched have said, "hook this up and check that" without any kind of instruction what to attach where. Thanks, Terry!
Another great video from a great guy. Thanks Terry, I'll be using this info to check my Pro Reverb. Good to see your important advice on safety as well. A man becomes a burden to himself by ignoring safety.
I can see why these are considered a good "first amp" for aspiring technicians. Uncle Doug taught a great series on this. I passed electronics in school, since my circuits always worked, despite not really knowing how they worked. Thank heavens I'm much farther along now than then.
Thank you so much for shedding light on this and the oscilloscope video. I was ready to purchase a Tek 465, until you mentioned the issues they have. So I'm picking up a Leader this week for a fraction of the cost. Your videos are extremely informative. Excellent presentation, keep up the great work. Thanks again.
I’m late, but at work we have the Techtronic just like Terry has. It has the same blueish color soft case. We have a Fluke scope and it never worked as good as the old Techtronic. I use it whenever I need a scope. Much larger but a nice piece of gear that always works.
Hi Uncle Doug at D-Lab, I really appreciate the time and effort you put into your tube amp videos. I just want to thank you for all the ways you've helped me to understand this wonderful world of Fender vintage tube amp circuitry. I just want you to know , you are not wasting your time. I look forward to future videos. Thanks, Tom Cowan, Digby N.S. Canada.
I want to thank you for dong this video. I'm troubleshooting a kit-built tweed vibrolux, and not just how/where to take readings, but what they might mean, are super clear with this video. Thanks!
Great Terry! So, if I may, since you have opened this can of worms, there are several topics you could cover in the future. I am aware of your other videos, which cover a wide variety of amp brands and servicing, maybe viewer request will show more of what your viewers want to see. Hell, I like em' all! Your showing what test equipment and safety issues are great. Safety tips and procedures are also very helpful! Thank you continuing these great videos! Thankz
Excellent as usual and very timely. On my channel I started a project yesterday where I am converting a Philco 84 to a Champ 5F1 circuit. Very very good stuff Terry, thank you.
Thanks for a great video, D-L E. I am not an expert yet, but would recommend that you NOT rest your other fingers on the chassis while probing with the positive lead. Cheers!
Once again Terry a great video and I really appreciate how you stress the safety factors when working on an amplifier as you know that is first rule of thumb and there are some idiots on RU-vid trying to teach people how to work on tube amps like Roy Blankenship that are going to get people killed the guy is a complete moron grabs a chassis with one hand while probing with the other uses no current limiting and it's a breath of fresh air to see somebody like you that knows what they're doing or talks about safety God bless you brother and thank you so much for your help
Oh boy you are just loaded with Fender champ tips, great stuff and i have those exact tunes too. So this little test should be perfect, the earlier one was no power shorts and opens testing. This one with power for component health is great i must see them all again and get my list of testing compiled. Thank You very much. Dennis
I got a few Beckman meters because I saw you using them in your work. They are very solidly built and work well if you have one with a good LCD screen. The only problem I have is some have bad or leaky LCD screens and bad plastic LCD screen holders. A common problem of LCD screens of this vintage. I think it's possible to use a common LCD screen and re-wire it, it's something I'm looking into but I still need to find technical data on the original LCD screen, that would be a big help.
Hi Terry, First and foremost I would like to thank you for sharing your knowledge. Thanks to you and others like you, your knowledge allows for people like me to learn and safely diagnose problems. I would like to ask if you could explain the current shunt purpose with the 1 ohm resister, why you apply it, how to use it, for example off a 6L6, 6V6, or other tubes. How is the current flowing through it, why does at this point, millivolts can be read as milliamps, and where are the test leads attached when testing. I guess I'm just a little unclear on this procedure but if used and done right, I can see how this would be very useful. They always say the dumb questions are the ones not asked, I have a good idea of how to do this, but until I know without a doubt, I won't attempt it. Thank you for your time and instruction, much appreciated, Lee
Awesome video I watched this one 3 times and then I used the knowledge to repair my friends Fender Champ Then I used snother video and adjusted the bias using all the tech tips you gave in that one Another 500 vids and maybe I will be smart like you LOL, sort of Thank you somuch for all the vids VA6RDR, got the call right this time Radar
Great safety video bud.👍If you do not now what your doing get someone like Terry to do it for you.It's not worth your life to poke around on a electrical devices.
Hi, just been forwarded your videos from a forum, and can’t believe I didn’t find them before. Really informative! Do you have any advice on how best to lower the voltage to suit more modern higher wall voltages? Thanks.
You touched on the point of a screaming hot 6V6. How hot does a properly biased 6V6 tube in these amps get with no signal? I have heard that a poorly biased tubes will glow red hot inside. Now that is hot. That is an obvious issue. Thanx.
Excellent video Terry! Very educational! One thing remains a mystery to me. I would like to know how the cathode of the 6V6 is fed the 21 volts (where does that voltage come from) and whether that voltage is negative or positive...I am guessing it is negative? Thanks Terry. Brian
At some point, Fender went to a grounded, three conductor power cord. My '73 Vibro Champ has the three conductor cord. Like the V-Champ in the video, the tube heater is a single wire off the pilot light. Would I notice a difference if I re-wired the tube heater with two twisted wires? After removing the existing grounds, of course.
Thanks, I am going to do another run through on my own now. I am having a bit of a different problem with excessively high vdc out of the rectifier. 469 vdc on a pair of 6v6's, and the schematic is calling for 440vdc. I gave a solid state rectifier a test drive. Hoping that going back to the 5AR4 tube will resolve it, but looks like I got on the back order list for that. Anything else I might look at? My VAC appears to be within specs coming in. I will have to look over that cathode voltage now as well. (I am just a novice here and this is on a Princeton Reverb 65 RI)
Before I did my trade as a tech I worked in a boat factory. There was a father & son working in this place who both had burn scars from metal watch bands shorting +ive to earth on batteries in aluminium boats!!! Yeah yeah we spell aluminium different down under.
Hi Terry, I have a question regarding a output transformer that I took out of my Fender blackface Bandmaster. I watched your video on checking your transformer with a multimeter . If they're pretty closely matched on the primaries you're good to go but how much of a difference in readings would indicate a bad OUTPUT transformer? And how do you check if a choke is bad?
I’m about to start working on a 1966 Champ so this was super helpful! I do have a resistor and cap that look heat damaged. Could I send you a pic and have you tell me which values I need to replace?
Thanks for the informative video, one question I have for you if I was going to set up my own amp repair/building shop what would be the basic tools you would recommend? Multimeter, scope, drill, hand tools, soldering irons, valve/tube tester.
D-Lab, This is ALMOST an excellent video...However I would like to give some feedback in the interest of continuing education and better videos! When I first read the title of this I thought this might be a great teaching too for people. and you gave some great advice and was a great concept. But you missed the mark in my opinion not switching back to the schematic. For those out there just learning. being able to SEE both where you are at on the schematic and IN the amp would have been VERY beneficial! for someone to see where you are probing both on the schematic and in the amp I think would go a very long way for those students out there that are just getting into this. I would like to Encourage you consider a video in the future as such! I hope you will take my comments as Positive feedback and not necessarily criticism. Thank you Dave Meyers Overkill audio inc.
Hi Terry - *BEWARE* CRAZY SOUNDING QUESTION, MAYBE BETTER ANSWERED BY SOME OTHER CHANNEL 😃 --- Say I wanted only a preamp of a given amplifier (i.e. a 5F1, 5F6-A, etc.) with the intended function of plugging it directly into a DAW or a power amp for tube flavor. Would it work to build, e.g. a 5F1 circuit up to the coupling capacitor right before the 6V6 and replace the grid leak resistor of the 6V6 with a master volume pot? Essentially removing everything included in and after the power tube stage, removing the B+ to the non-existent output transformer and 6V6 screen, and removing the feedback loop connection to the secondary of the non-existent output transformer? How would one ensure the output impedance is correct to feed into a DAW or power amp with a circuit like this? Would mismatched voltage be an issue? What could be done to correct impedance or voltage so it would be suitable for the inputs of a DAW or poweramp? Thanks, Chris
Can I measure the RMS output on a combo amp to determine what speaker I can connect to the amp? Trying the determine the RMS power out in Watts so I can be sure I'm purchasing a speaker that is safe to swap with the current speaker. Thanks!
Thank you. You know your stuff. I have a question: the tone caps and plate caps that show no voltage on the ground side, they should not pass any DC, correct?
Hi Terry, thanks so much for your videos... about the best thing on yt regarding tube amps. I'm working on a Champ clone just like this. 21.6V / 470 = 46mA @ 380V --> 17.4W Aren't 6V6GT rated for 14W? Isn't this bias too high? The amp I was working on was biased at 19W. Of course is worked alright and was very loud, but sounded compressed. That was at least my conclusion after getting the bias down to 13W -- then is sounded more full, musical etc...
Great video Terry! What is the wattage of the dummy load resistor? Is a small resistor like that safe considering the rated 5 watt output of a champ? How about higher rated amps?
