Scoped | '75 Fender Champ Pt 3

[@stratfanstl]

Channels like this are thought provoking... They make for an interesting comparison of work skills and expectations. In the amp fixin' biz, the range of things one must understand (electric theory, tube theory, electronics theory, manufacturing, assembly) and have available for immediate recall to do the job AT ALL seems arguably wider than the average office job (like software developer) with one additional, CRUCIAL difference. The object of the work is capable of electrocuting you to death AT ALL TIMES. Unless you're writing control system software for a nuclear power plant, chemical plant or Boeing 737-MAX, most office jobs don't have those dynamics. I would like to think I HAVE many of the skills to do this kind of work but I'm not sure I have the confidence in my attention span to do it safely. And even if I had the logical understanding to find the problem, you still have to have multiple PHYSICAL skills to actually CORRECT the problem without causing other problems. Respect...

[@Dr.Sheldor]

This is incredible, Lyle. Thank you so much for taking the time out of your day to explain how you dive into these kinds of diagnostic tests. I learn something every time I watch one of your videos.

[@captainfineweather5610]

Amen!

[@edwardebel1847]

One of the most important pieces of information in troubleshooting is having the “baseline”. Once you swap the transformers, you can get that baseline for future reference. You also now have a transformer baseline for the Deluxe. Too bad there isn’t a published database of various baselines for normal voltage and amperage levels for the more popular amp models.

[@Jake-fo3rg]

Great content, Lyle. I love it when you get into diagnostics and troubleshooting.

[@Andy-ql9wh]

Lyle your videos are not boring, not to me anyway, I can only speak for myself. I guess my being able to understand what you are actually doing helps too. I don't watch Tv so I watch Your channel and others that I find interesting.

[@markN-pm2ws]

Thank you Lyle, I know how much it takes to set up a demo like that. It's gotta be a labor of love. Everything you share is a fantastic learning experience! Can't get enough, I'll keep on hitting "like" and I'll be looking forward to all that you have share with us.

[@thatampguy]

A lot. A big sacrifice for him, since time is money. He’s a great teacher, which highlights the fact that competent folks aren’t insecure about ceding ground to competitors. I recommend this channel often (as well as the Brad who doesn’t use plumbing tools to service vintage amps 😂), as it’s suitable for novice and advanced tech alike.

[@josephc3276]

Great video Lyle. I always learn from you post's. My best take away is that I can CK for loose connections and hardware and cleaning. If I have issues after that it's just best to leave it to a professional tech. I can except the fact that I just don't have the knowledge, tools and skills needed. So thanks for keeping me from making a absolute mess of my amplifier 😅😅😂😅😂.

[@bambule5268]

I really enjoy problem solving methodology thanks Lyle!

[@alexdeleon7135]

Indeed a novel way to nail down the lax low-end by the output xformer. 40.00 is a nominal expenditure to restore the Champ to it's expected performance.

[@michaelinglis567]

This was the furthest thing from boring. I really enjoyed all three of these videos from the past few days. I was actually really disappointed when the first one ended right when we were getting to the even better stuff lol. I have a small handheld Oscilloscope/DMM specifically for these types of situations. But I'm not very proficient with it since I've only really needed it for troubleshooting a handful of times so far (mainly it's cause I suspect a opamp has failed). But I intend on taking some time, hopefully soon, to look into all the ways I could use the o-scope to troubleshoot more efficiently.

[@windward2818]

Even though my repair bench is fully isolated I still like to use a Fluke Scopemeter when troubleshooting, it has fully isolated high voltage channels. I have a Fluke 289 as my general troubleshooting repair DMM, and then some specialty clamp meters for current. If I need to send a waveform to an engineer, contractor or customer, I use my Picoscope with different high voltage probes. The Picoscope app creates really nice plots that I can put into a short report if this is required. The Picoscope app can be used as a reader if necessary.

[@IAmChrisR]

Great video! I recently had the "Tweed Champ from hell" in the shop with a tech, which took a bunch of visits and a replaced OT before the root cause issue ended up being tracked down to an intermittently faulty volume knob. That was replaced by another intermittently faulty volume knob (not the same batch, just pure coincidence and just terrible luck). The second volume finally knob did the trick. It's crazy how complicated issues like this can be in even "simple" circuits.

[@TheStimpy60]

Love this journey into the weeds - great theory stuff

[@huddyskiba1]

Very good - as a non-scope owner this really solidifies why I need a scope to take my troubleshooting/building to another level and speed up other certain diags. Very intuitive if:then troubleshooting.

[@jonnybeck6723]

Thanx Lyle... ...and never ever boring dear teach

[@weschilton]

Wow, this is fantastic troubleshooting!

[@PsionicAudio]

Well, thanks, but this is kind of basic scope stuff I figured people needed to know about. Tracking down high frequency oscillations and such can be trickier.

[@mikewithers299]

I want a scope and don't even know how to use it! 🤣🤣 you literally pin pointed the fault with that tool.

[@jimgibson9811]

I love the scope stuff. Scopophilia - is that a word? Thanks Lyle.

[@jeffsquires6620]

There's something about old FENDER AMPS, I have 2 and consider myself fortunate. Both were abused but I had them restored to factory specs. They are amazing treasures.

[@qua7771]

A champ is something I never had. I mostly play through Marshall's which don't have the cleans that a champ has. I might have to build a clone.

[@jeffsquires6620]

@@qua7771 I have the Canadian JMP 2204, 50 watt head master volume. It rocks. My 61 Bassman has a similar tone. I use a 71 Bandmaster for clean tone. Very affordable and reliable.

[@qua7771]

@@jeffsquires6620 Thanks for sharing this information. I almost purchased a later chassis version of this amp, but it looked like it had some unknown tech work done to it, and given the age, that is to be expected. I think I will look at some schematics, and decide if I want to do a fresh build, or find a good used one. Maybe even a good replica. I would want an amp for use, not for collectability sake.

[@billmckenna7056]

Nice Video Lyle.. this is is a super cool deep dive into to how to find odd issues between amps!

[@johnkohlhauff9061]

This video was definitely not boring. I found the video quite useful. Learned some different diagnostic methods that I hadn't thought of before. Thank-yòu!

[@evanharmon7928]

This was educational and fun to watch.

[@omarmolina207]

You could tap off the plate of the 2nd triode of the Champ and run it to the input of the Deluxe's PI to demonstrate the low end capability of the Champ's preamp with a properly functioning output section.

[@spumpstein9374]

Great video, very useful exploration.

[@fiddlix]

Always great content Lyle. Keep the great content coming..!

[@christophersambuco9414]

Super interesting. I'm thinking that getting a scope is on my to-do list, and then learning to use a scope will be the next item on that to-do list.

[@retread1083]

I'd look carefully at the actual value of the cathode bypass cap for the output stage, but I'm guessing you already did that. Regardless of what's causing the LF loss in the output stage (transformer, tube, bypass cap or other) I'm not surprised you're seeing proportionally more signal at 80Hz on the grid of the Champ's output tube. You'll have less NFB at 80Hz if there is low output at that frequency, so the driver stage is trying (unsuccessfully) to compensate.

[@ditchgator1]

Always learn something new here.😉👍👍 Thanks Lyle😎👍👍

[@Slugg-O]

Not dull at all. I have a 5f2a with weak low end compared to other 5f1 and 5f2a amps. I'm using all Hammond x-formers and have thought about trying another SE transformer as a comparison. I'm looking forward to the next video.

[@philandcatherinemuzio9476]

Very interesting troubleshooting on this amp. Thanks for always keeping it interesting.

[@stevebeck3164]

Man, has the depth of oscilloscopes shrunk, or what? I still have an old 2-channel Techtronix that takes up half my bench.

[@oldguy5381]

Truly enjoyed this one, thanks for showing.

[@rustymohican8280]

Thank you Mr. Wizard!

[@Joshua_Stephens]

Good Info - Thank You

[@robertbull6895]

I have built and experienced similar loss of low end in the single ended Champ output using the “correct” replacement Hammond output transformer. I sourced other single ended 5 Watt transformers (physically larger core) and experienced better overall output sound quality (less distortion) and improved low end performance. I suspect this is due to a combination of how well the SE transformers deal with core saturation and hysteresis as well as optimizing the bias current/ output valve (tube😊) dissipation. As you suggest, it is possible that physical wear and tear and heat load on the output transformer over time has changed the physical characteristics of the transformer iron and air gap which in turn has affected the low end transfer characteristics. I will be interested in your findings.

[@wesleymorris1]

Its the output transformer i had the same thing happen. I bought a se ended transformer for a champ, i was upgrading to a 6l6 and it was weak and i checked everything, it was the trans.

[@Happy_Broom]

Thanks for yet another great informative and insightful vid!

[@merlesson1164]

Excellent!

[@markbenson9379]

Boring? Stop it....I appreciate the education. Thanks Lyle!

[@bigchrisgulley66]

Thank you for the information that's cool as all get!

[@ThomasACarlos]

Makes me think about having some type of standard test to look at the health of an Output Transformer, other than simple DC measurements. I guess it couple be done having a simple amp circuit (either a salvage amp or something built) and comparing an OT to a known good (or new) OT, looking at output over a range of frequencies.

[@PsionicAudio]

Well, I was doing simple AC tests. But those tests exist for transformers.

[@kevinfarrellUK]

The best things in life have been fixed, found, made and invented in man caves and sheds.

[@unclemeat8422]

Needs some 3n1 oil n gauze pads! Its all ball bearings these days!!

[@tom-iv6lc]

"Proper Spock" but im a gen X too and i havent herd that one before.

[@ThomasD66]

Awesome video. Is the minimal change in the third probe reading between the different inputs on the Deluxe attributable to the small value of the bass resistor in that amp's tone stack?

[@PsionicAudio]

Well, it’s the mids resistor, but yeah, you’re seeing the result of the Deluxe’s 6.8K vs the Champ’s 15K there, and it affects the bass, not just the mids. It’s all interactive. Note that I was measuring 1K and 80Hz with probe 2 on the output of the 100nf cap in the tone stack. If had measured 1K at the output of the 220pF then the voltages would be a good bit higher but the 80Hz would be dramatically lower.

[@soapboxearth2]

Love this kind.of content

[@Mark70609]

So the output transformer is working but only lacking bottom end. I have the very same issue with a Mesa Boogie SOB. There is bottom end but not as much as I was expecting. What fault within the output transformer causes this kind of fault.

[@michaelmoore7975]

Does the 5k or 8k (brown or black wire) match the anode impedence of the valve according to the datasheet? A mismatch can cause harmonic distortion, changing the tone. Too high impedence equals low output before clipping, too low impedence mean current drive issues. A 6v6 pairs well with a 5k single-ended and a 7.7k in push-pull, but an EL84 goes good with 4.5k single-ended and 8k in push-pull. I realize they are both so similar that using brown or black makes little difference.....that is until you need phase matching for stereo or NFB loops. And when scoped the best one to use would be the more symmetric waveform. But what do I know? I'm just spitting possibilites. I'm not telling you anything you don't already know. Really digging the content.

[@PsionicAudio]

The Heyboer has a fixed 7K primary for this era of Champ. The Hammond has a choice of 5K and 8K. As did Classic Tone. As do Mercury. I may go Hammond with their paper coil option. Then see which primary sounds better.

[@samuel_towle]

If I recall, you mentioned that the output transformer was changed out. Did someone replace the original with one that has less primary impedance? I wonder if a section of the primary shorted across layers and the winding in between opened up? A significant change in the number of turns on the primary should lead to a lowered inductance and lowered Impedance. Would it be possible for you compare the inductance and DC resistance between the original and replacement

[@PsionicAudio]

Original was replaced with this Heyboer which seems defective. To be honest I’m not really interested in spending time on an autopsy. I know the rest of the circuit is healthy now, so once the new iron is in I’ll be moving on.

[@PsionicAudio]

But I’m thinking along the same lines you are.

[@sundogaudio851]

good stuff man!

[@jimilee459]

Oh wow, that is most interesting.

[@rob-french-data]

Awesome!

[@captainfineweather5610]

Posted this already on "Poor Thing": ??? (Like you ask to mark questions in your live chats, I know this isn't one, but nevertheless) Dear Lyle, very nice! Confident command as usual. There is a question burning in my mind that I hope you might be prepared to clarify, if it's not a trade secret. What are the two larger than stock C7 and C14 electrolytics on the AC15C1 mainboard shown in your video "Transmogrified"? They are the cathode bypass capacitors for V1 resp. V2(a). Did you just replace them by ones with higher voltage (hence of larger design) or raise the capacitance to 33uF too? I'm considering to try just that and I wonder if the maestro would concur ... ???

[@PsionicAudio]

Stock are both 22uF. I change to 10uF and 4.7uF respectively. Keeps the low frequencies of the guitar but doesn’t waste power amplifying subsonics you don’t want.

[@PsionicAudio]

They’re slightly larger because they’re 63v Nichicons rated for 10K hours at 105C.

[@captainfineweather5610]

@@PsionicAudio THANK YOU SIR, most appreciated! I already worked my way through the schematic and knew they were 22uF 25V stock. Didn't realize that lowering the values here would be the right thing to do. So 10uF for C7 and 4.7uF for C14 it is. Thanks again, man! Would it be of any use to replace R34, the plate resistor of V2(a) which is 100k, by a 220k resistor like you recommend for R5 and R6, plate resistors of V1(a+b)? Now to something completely different: "Aw'right, so let's do some proper Spock", did I understand that right? If so, let me tell you that as always, you spocked it properly indeed, like the famous character I think you are referring to would have done himself!

[@PsionicAudio]

Thanks man. Keep the 100K on V2A.

[@captainfineweather5610]

@@PsionicAudio Thank YOU! (A positive variation on the dialogue in the chapter of Jim Jarmush's "Night On Earth" in which Armin Mueller-Stahl plays an East German taxi driver in New York who picks up YoYo and later his sister-in-law, only to see them fight nonstop including the inflationary use of the f word)

[@dortoh1]

Lyle, earlier on this amp you talked about the negative feedback resistor and the fact that this output transformer was for both 4 and 8 ohms. Does the resistor value change depending on which tap you use? I'm finishing up a 5f2a build, and I'm using the 8 ohm tap instead of the normal 4.

[@PsionicAudio]

If you connect NFB to an 8 ohm tap, roughly double the resistor. 2.7K becomes 4.7K typically, though 5.4K would be more exact.

[@dortoh1]

@@PsionicAudio Thanks!

[@dannbrauckmann1545]

I have a question. I've got a 1968 drip-edge Vibro Champ, and I built a 1x12 cab that I sometimes use with it. Inside the 1x12 is an 8 ohm EVM 12L (kinda overkill, I know, but I found the speaker for $60). I also have a spare 8 ohm Jensen P8R (the 8" Alnico). I've already swapped the RCA speaker jack on the VC to a 1/4". I know that it can be okay to run the 3.2ohm (I think that's correct) OT into an 8 ohm speaker, but what if I wanted to run the 8ohm 8" inside the VC cab, and the 8ohm EVM 12L in its separate 1x12 at the same time in parallel, without having phasing issues? I bought a 1/4" Y cable, but I'm pretty certain that when I tried it, the speakers were out of phase. It's been awhile since I tried this. Is this a dumb idea?

[@PsionicAudio]

I wouldn’t use either 8 ohm speaker by itself with the stock output transformer as it’s too wimpy to handle a mismatch reliably. But you could run both in parallel giving a 4 ohm load, which is not really a mismatch for the stock OT. If the speakers are out of phase, one is miswired. Easy to correct.

[@jutukka]

Very nice ANALysis. In small SE OTs the low end response is very often a problem. A proper class A bias current causes the SE OT to go to hysteresis, primary inductance falling etc. that kind of s**t. Sounds thin and dirty in a wrong way. In many cases very small and cheap push and pull OT can work well and sound good but not a cheap SE OT. Too small/cheap SE OT nearly always sounds a bit thin and weak. In some good quality 50's radios the SE OT is twice the size of Champs OTs and they sound veey good.

[@KB_Joys]

Or just maybe a better, or bigger OT for more bass response

[@Tibbon]

Are you using an isolation transformer, or you're just careful to not connect the low side of the probe to anything with a significant voltage to the safety/earth ground on your scope? I've considered getting some cheap differential probes, but it seems like if you're careful, then they are only useful maybe a few times a year at most.

[@PsionicAudio]

I was measuring AC points in the circuit here, so all low voltage things. Not an issue.

[@jonnybeck6723]

Any idea just what went bad in the OT? ...like is it repairable? (theoretically) Ventured guess is heat damage (?) (Thanx in advance)

[@PsionicAudio]

Could be oscillation if run connected incorrectly, could be operating without a load, could be operating with the wrong load. The stock output tube socket had some charring at the cathode/heater area and was very loose, but that was changed before this test and this test was done with a fresh 6V6.

[@PsionicAudio]

And I doubt it’s reparable other than a rewind. But it’s a $40 part, so thank you, next.

[@victorbeebe8372]

Mahalo Lyle!

[@Expedition18]

👍

[@rockson8965]

So how did you make out in the end with the new transformer?

[@PsionicAudio]

This video was yesterday afternoon. I haven’t ordered the new transformer yet.

[@mrbuttons1243]

Sherlock Ohms strikes again...

[@thatampguy]

Haha how many scope requests did it take to break you 😂

[@sempercompellis]

ive never heard a champ sound like a "fender"-- they are very boxy very nosey and pretty underwelming (this is my opinion here) The jump from champ to princeton is so night and day; princetons sound every bit as sparkly every bit as full as any fender amp.. Finding a good mod for a champ would be a godsend..If anyon could do it , i suspect it would be Mr Psi.

[@dominicdiclemente8877]

Call me lazy but this is why the new scope shopping I am currently doing is only for scopes that will draw a full transfer function.

[@PsionicAudio]

This Rigol will do just about anything you want. I chose a fairly simple test to show viewers this one point.

[@dominicdiclemente8877]

@@PsionicAudio The cheapest Rigol with the ability to do a transfer function (integrated tone generator) is what I have narrowed it down to. I have been spoiled by the $$$ test gear from work where I can remote log into them to use them.

[@PsionicAudio]

It’s a good choice. I didn’t get the sig gen version because I like having it separate from long habit. And I love this app - does sweeps, pink, white, and brown noise, etc. Though basic sines and squares usually do what I need.

[@tomk1tl39]

LMAO . . . Champ looks like a patient in an OR . . .

[@PsionicAudio]

That’s exactly the case.

[@joerectifier]

Not boring at all…..I’m not a fan of guitarists that don’t understand their input to output system and all the interactions and transfer function - and at least some basic troubleshooting. Tools of the trade and to me at least extremely interesting.

[@kmetal83]

Uh...maze...ing

[@edwardebel1847]

Yeah, NOT boring! Just sayin’…

[@chipsterb4946]

Boring? Heck no! Thanks

[@ThomasBrock74]

We totally trust your judgment.Dude, just change the output transformer already.You don't need to explain yourself.... I'm pretty sure if you say that's what's wrong with it.I'm ninety nine point nine percent Is sure that that's probably what's fucking wrong with it.