How to measure Fender Champ tube guitar amp stage voltage by D-lab electronics
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- čas přidán 9. 07. 2024
- As requested. Here is a step by step procedure for walking thru a Fender Champ circuit. The voltages do differ from the schematic due to today's increased line AC volts. If you follow this as well as reviewing the on line schematics, you should be able to isolate faults with your classic amplifier.
- Jak na to + styl
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 !
That is a great compliment coming from you Uncle Doug. Take note D-lab, Uncle Doug is in the house!!!
@@Slugos45auto Thanks, Brett.
My 2 favorite YT channels. Uncle Doug and D-Lab!
Glad to hear it, OIE :)@@OIE82
Whether some people realize or not, you just provided some of us self-taught mooks the best theory AND practice for checking voltages. Thanks Terry!
MY THOUGHTS EXACTLY. THANKS TERRY!
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.
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 CZcams tackles this as well.
But these videos are really helpful, and keep up the education! Thanks.
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!
I can’t get enough of these signal tracing videos.
Thank you for this video. You are an excellent teacher. Again, thank you !
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.
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!
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!
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 !
thanks Terry...just got a silverface champ with a scorched 6v6 socket..all original. .so the adventure begins..
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.
Uncle Doug is a different guy, Thomas. This is 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.
These lessons are much appreciated Terry
This is a great introduction video for tube amp health check for dummies. Well done!
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.
Another masterclass Mr.Amp Terry!!! Thanks for sharing your knowledge. 👏🏻👏🏻👏🏻👏🏻
Thanks again for eductional video 👍from England 🇬🇧
Very informative. Thanks for the video.
once again another great video, thanks Terry.
Thanks Terry! You've helped me before, and this video just helped me again! Much appreciated!
great video as usual terry, thanks for sharing your knowledge
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!
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.
Very helpful video. I have a Champ just like this one and I plan on checking it out by using your video.
Really helpful for me to learn basic tube amp theory - thanks!
Very handy guide Terry, thanks
Great information there, presented in a way that's easy to understand.
You're a good teacher!
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!
Just a fantastic learning video!!!
Thank you so much.
Really enjoyed this. Thank you!
This is a really good video. When I lose my way - I always start here.
Thank you my friend, stick with me
Great demonstration!!!
Great job! Thanks for the video!
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
Thanks for explaining these details.😊
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
Great information here. Thank you!
Great video, and explanations..Informative and full of tips...Thanks for sharing..Ed..U.K..😀
Thank you! Excellent video and very helpful..
I like the Dog and what's up with the basket? Miss you guys!
Great Video! Good to know about making a dummy load. Cathode tube leakage. Thanks for talking about Meters!!!!!!
Thank you very much. I took notes.
Excellent video. Thank you
Thanks Terry trying to learn!
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 CZcams 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
Great Video! I appreciate you walking us through! I'd like to see one on a BF Deluxe Reverb and a Tweed Fender?
Thank you, sir. May I have another? Another great presentation. 👍
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.
Great video, thanks!
Soooo helpful. Thanks very much.
Very nice! Parabéns e obrigado!
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
very helpful. Thank you.
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.
Excellent.
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)
Good information, thanks
more videos like this please!
great video
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.
Gold! Thanks so much...
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.
I would like to see a video on checking and diagnostics on old Can Capacitors ( large Blue Mallory’s ) that had wires soldered....
Thanks Terry! I would have enjoyed this video even if I didn't have funny sounding Champ.
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 takeaway for me: cathode voltage as indicator of tube strength
Good job
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.
Thanks so much!
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.
Good video. What size resistor (wattage rating) do you use for the dummy load? I'm guessing 5W?
Thank you!
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.
btw, to see schematic easily - duplicate new window, full screen, scroll to 4:20 to load schematic and then alt-tab between.
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
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?
Thx, Good Video
Thank-you!
I replaced the .022 coupling cap and I still get a little voltage on the near side. The wire coming from pin 5 on 6V6 seems to have a little voltage. Any help? 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?
Correct.
Nice!
I found a leak on the .022 tone cap. Thanks.
Is there a common mod for the increased wall voltage nowadays? Like adding a resistor or cap somewhere? If not aren't all our old amps sounding like crap because of this?
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.
What wattage resistor do you recommend for the dummy load?
thanks!
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?
I'd guess its 1 watt from the looks of it.
It only needs a small resistor if the amp is putting out no signal. A 1W resistor will handle the clicks and pops while checking voltages.
Terry. Is there a benefit to using an N68X isolation transformer to reduce the AC voltage closer to 110 the designers used?
How many watts for the 4.7 ohm dummy?
My Radio Shack test leads say 1000V maximum. Do you think they're safe for amp work?
GOOD
How do you make the test load? I couldn't tell how it was wired by the video.
Connect one end of the resistor to one terminal of the plug and the other end of the resistor to the other terminal.
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...
Uncle Doug explains it should be set @ around 70% of max wattage rating, if I dont go wrong
In a vain attempt to emulate my amp heroes, I just picked up a Beckman 300A meter in the UK for less than $20 (£13.20 delivered) :)
What if you changed the the 25 microfarad cap across the 470 ohm resistor and youre still getting 25-26 volts on the cathode. Using Sovtek 5Y3 rectifier and JJ 6V6. Tks in advance
Can I go with a larger resistor than the 470 ohm? Tks again
@@rhythmshakers3878 A larger resistor will reduce the dissipation of the 6V6 and may change sound of the amp. That said, you will not damage your amp if you swap out the 470k with a resistor of 1Watt or greater and (say) 560 Ohms.