How to check capacitors on old valve amplifiers

[@IronandWire]

I really enjoyed your video. When I rebuild an amp, I gut out all the old caps and carbon resistors. I want the amp to be dependable and sound as good as it did new. A vintage amp once had new circuitry and somehow sounded great. No "mojo-pixie dust" excuse needed, just a well built circuit that's been maintained. I understand if you're a collector wanting the history, but If you're an actual player you want dependable gear and no drag on the circuit.

[@stuartukguitarampguy5830]

Hi David I couldn'r agree more. Load of rubbish talked about 'tone' on old capacitors!

[@lowheadroom]

As someone who recently started their own tube amp shop, I’m very pleased to have found this channel! Thank you!

[@stuartukguitarampguy5830]

Great! Physical shop or online?

[@lowheadroom]

I run it out of my home in South Carolina USA. It appears I'm the only one in this city that really does it. haha.@@stuartukguitarampguy5830

[@MrsGULCH]

Thank you for this video. I found myself taking capacitors completely out to measure capacitance and had forgotten you need only de-solder one end to get the same reading. A time saver method thank you again.

[@stuartukguitarampguy5830]

Great Have fun with those caps!

[@vihuelamig]

3 decades ago I went through my crazy Hi Fi phase. Having built an EL84 PP amp it wasn't long before I was replacing the resistors with 'better sounding' replacement types. Of course the capacitors were the next on the list for replacement. Then I heard that a 6V6 sounded better than an EL84 so I converted it to 6V6 PP. Most of my original build was cobbled together from parts that I already had including the output transformers. It even had a choke power supply. It's all too easy to convince yourself that any change you make must be an improvement. The changes that I made to my amplifier was extra experience but that's about it. I don't think any of the changes improved the sound one bit. It got converted back to EL84's because I could see the heater glows much better!

[@stuartukguitarampguy5830]

Hi, you've summed it up perfectly. It's mostly smoke and mirrors. I'd bet serious money nobody could tell the differece between 'audio grade' caps/resistors and the cheapest ones out there!

[@smartquark]

Thanks Stuart another knowledgeable video. I tend to agree with you regarding a lack of difference in sound quality when changing the signal capacitor/s is necessary. Personally whilst I haven't noticed any difference it is nice to appreciate the vintage quality by keeping the original components. Very thorough and methodological trouble shooting. It's nice to see a expert with a passion for vintage AMPS.

[@Steve-wz5pz]

"...because people 'believe' that they affect the tone...". Well DONE, Sir! : )

[@stuartukguitarampguy5830]

Yep it's all smoke, mirrors and wishful thinking!

[@TonyLing]

All you need to do is play it daily, whilst very gradually increasing the gain over a period of three decades in order 'break in' the new capacitors. If you get it right, preferably limiting it no more than three chords, your golden ears will be pleasantly caressed by the amp's fine tonality.

[@stuartukguitarampguy5830]

@@TonyLing FINALLY! Someone who really knows how to get maximum tone out of these caps :)

[@the0v0man]

Thanks for the great video. I’ve just acquired an amp and it would seem a couple of the smaller caps are damaged, so I’m going to do those tests on them to see if I need to replace them. Cheers

[@stuartukguitarampguy5830]

Great, good luck.

[@jonathanhorne6503]

Fantastic lesson. I would have loved to see a merger showing some leakage. Not a complete fail but one that showed a volt or two. Enough to effect bias of the subsequent stage.

[@stuartukguitarampguy5830]

Thanks Jonathan I'm about to do a video on an old Fender with leaky caps so keep an eye out for that one.

[@jameshenz1780]

Ty for sharing!! How can I test for leaky Orange drops in circuit? The way that I've been doing it is with a standard digital voltmeter. I put the black probe on chassis ground. Then use red probe to check across other side of OD. For example, in tone stack, there's high DC present from plates. On the other side of OD it reads .4 or .5 mv. Is that cap fine or am I doing it wrong. Ty for your time! , Jim

[@fkitch]

Very good video thank you! Do you know if there's 25uf/25v caps that are not electrolytic (polyester)? Need to change quite a few on a 70s Italian amp and I'd rather not put ELs back in, thanks and Merry Christmas!

[@stuartukguitarampguy5830]

Thanks for watching. No I believe that's way too high a value for a polyester. If there was such a thing it would be very large.

[@daydreamer102]

Thank you, Stuart. Very informative and clear. Very useful.

[@stuartukguitarampguy5830]

I'm pleased you enjoyed it. All the best.

[@ertreri]

Thanks a lot. Superb video as always.

[@wsscott72]

Stuart-I've been watching your videos for some time now and enjoy your explanation and tricks of the trade. I was intrigued with the Megger, and got one off of eBay for a nice price. It's a BM100/4. So I was using it today for the first time on a 1983 Rivera Fender Concert Amp to check the caps. I was checking the Orange Drop caps and also the Dark Blue ones. There must be 15-20 of them in the circuit. I did this testing all in circuit. I set the Megger to 200 MOhms. And all of them I tested went all the way to the right side of the scale or somewhere around the middle. I understand this means they are BAD. I can't believe this is correct, ie. that all I tested were bad. So I took one of them out of the circuit, and did the same test, connecting the Megger's leads to each of the leads on the cap, and it tested perfect--all the way to the left of the scale. So can you tell me what's going on? Does this only work on certain types of caps, or only on certain parts of the circuit? Thanks for your time.

[@stuartukguitarampguy5830]

Hi the problem here is you cannot test in circuit. Think what the Megger is doing. It's providing a high DC voltage (say 500V) and then measuring for any tiny current which flows (which indicates a resistabce). Well, with the caps in circuit there are all SORTS of routes that current could take, other than the capacitor, You need to lift one leg of each capacitor and then retset.

[@AnodyneHipsterInfluencer]

I know this video is 3 years old but perhaps you might still see this comment and be able to answer a couple questions for me... - The last capacitor you tested, you de-soldered one end from the board. Is that the way to do it or can the same testing be done - and done as effectively - _without_ de-soldering one end of the capacitor from the board? - I understand and agree that with the larger, electrolytic caps (in an old Fender, the ones on the other side of the chassis where the tubes/transformers are and usually under a "dog house" type cover) the premise of "if they're older, just replace them" - but for information sake, would the testing procedure for those electrolytic caps be the same as the procedure you demonstrated here for these smaller caps? - the devide you used for testing leakage and putting a voltage across the capacitor, could you clarify what it's called? A meg/meggo I think you referred to it as? - Lastly, assuming this procedure is the same for all the caps (these and the bigger electrolytics) should I assume that if a larger cap has a designated positive(+) and negative(-) end, that I should use the corresponding +/- probes on my multi-meter and/or meg(?) devices when testing? Thanks so much, regardless if you see this comment or can answer these questions. People like you posting videos like this are extremely helpful.

[@stuartukguitarampguy5830]

Hi. Ideally you need to desolder one end. With the capacitor in-circuit there could be all sorts of paths from one end of it to the other. Don't forget you're checking for a very high resistance, usually over 1M. Second question, no you can't test electrolytic caps in the same way. The Megga puts out 500V DC and this is usually too high for an electrlytic. 3rd question, the more general name for it is an insulation tester. You can get them on eBay. You want one that puts out 500V DC not 1000V. I think 'Megger' (cabn;t recall how to spell it!) was a brand name back in the day. I still call it that out of habit. 4th question - see my answer on electrlytics. Thanks

[@motokev2727]

When applying the voltage, does the polarity of your meter need to match the capacitor i.e. positive and negative.

[@ranius7388]

I really like the amp tone on the intro 🤣 What amp was it recorded with ?

[@stuartukguitarampguy5830]

Ah, I'm not really sure as it was whatever amp happened to be in the workshop at the time!

[@jeremywatson3882]

Great info, Many thanks

[@stuartukguitarampguy5830]

Thanks Jeremy

[@TinwhistleD]

Hello Stuart. Thanks for all the freely available information you have provided. It is helping me to try and fault a 14 year old Orange 50w Mark 2 Rockerverb at the moment. Orange do not seem to like providing Schematics for their amps - so I am trying to trace out a schematic for it. I have a 50W Head at the moment which was blowing only the 2 HT fuses. It is the 2x EL34 output valve version. My main question here, looking at testing the capacitors, what voltage would you select on the Mega? I have a mega that can output 250 Volts up to 1000V. I presume you set the mega to below the stated Max Voltage on the cap? Thanks again. Keith

[@stuartukguitarampguy5830]

Hi Keith. I just checked and I don't have that schematic either. 250V DC will be fine and will show up any leakage. All the best.

[@remco613]

Hi Stuart, thank you for all the great videos, I watch them regularly. I have some questions about this one, are there any benefits of using this method over using an ESR-meter? It's the first time I see someone using a megger for testing capacitors, isn't the voltage too high for some capacitors or is it too momentarily to do any harm?

[@stuartukguitarampguy5830]

Hi. Actually I've never owned an ESR meter. Also, I hardly ever test capacitors. If one is suspect I just change it. The Mega produces a volttage of about 500V DC and so it's perfect for testing any leakage on a 500V or 600V cap. I wouldn't use it on a lower voltage cap.

[@biggstile]

ESR is a completely different 3rd type of test that capacitors should also pass.

[@Scooter110]

Esr is a different measurement. In an esr test an A.C. voltage of known frequency is applied and the impedance of the capacitor is measured.

[@nickbengtsson6518]

Very helpful, thank you.

[@russellcrea9701]

Can I check capacitors, including leakage with a standard multi tester? Any tips appreciated, Thanks

[@stuartukguitarampguy5830]

Hi Russell. Only oif a cap has completely failed (eg short circuit) will a multimeter show anything amiss. For other caos you need to put a decent voltage across them (close to their working voltage) to see if they have any leakage.

[@toubabou1956]

Stuart - I need to replace an original electrolytic capacitor on a 1963 Danelectro DM10 amp, but I can't find a replacement with the same specifications. The original cap is a Sprague 0.47MFD at 200V, but the closest I can find is a Vishay 0.47MFD at 450V. Is it okay to replace an electrolytic with a new cap that has a much higher voltage rating like this?

[@stuartukguitarampguy5830]

Yes higher voltage rating is fine (better) you just can't go lower.

[@pabloadolfolopezalegria7390]

Hello there great channel 👍🏼, yo know I have a Mesa Boogie Stiletto Ace combo but is cutting out, I just changed preamp tubes using Chinese tubes as recommended by mesa and changed the rectifier tube and the power tubes, even though the amp Is still cutting out, I cleaned the loop fx jacks too, I think could be a capacitor but I'm not sure. Any advice thank you 😉

[@stuartukguitarampguy5830]

Hi Pablo. It's very unlikely to be a capacitor - posible but unlikely. If it has FX send and return jacks, try cleaning them. Other than that, unfortunately it could be a lot of things.

[@_Planeview_]

Why did you lift one leg on the second capacitor for the testing?

[@stuartukguitarampguy5830]

I can't remember without rewatching the video but it's obviously better to disconnect the cap before measuring as all sorts of things could be going on with it in circuit.

[@mikeault]

Is there a modern day Mega tester to test the cap from passing DC voltages?

[@stuartukguitarampguy5830]

Hi Mike YEs these you can buy a High Voltage Insulation tester quite cheaply on eBay. Stuart

[@strangequark420]

Fantastic, and thank you!

[@stuartukguitarampguy5830]

Glad you enjoyed it Tim!

[@michaelevans3852]

Wonderful! Is there a newer tool to measure leakage? I have some resistors that measure lower values than they should; is this an issue? They are in an old early Fender Concert 6G12. Also a question about wattage; how do you determine resistor wattage so they can be replaced with correct resistors?

[@stuartukguitarampguy5830]

hi Machel. No I don;t think there's anything newer. Any insulation tester' should do the job as long as it gives out about 500V. It's very unusual for resistors to be LOWER in value, they often drift higher. Make sure you are measuring them out of circuit! On wattage, no I'm afraid it's just eyeballing and experience. The size is the give away. Send me pics if you have particual resistors you want identifying.

[@michaelevans3852]

@@stuartukguitarampguy5830 I have found the value on the schematic. It is on the bottom of the chart. Thanks for your input. I am enjoying learning this immensely.

[@michaelevans3852]

@@stuartukguitarampguy5830 where do I send pictures?

[@stuartukguitarampguy5830]

@@michaelevans3852 Contact me via the email address on the splash screen at the end of any of my videos. I'll reply and then you can send them.

[@michaelevans3852]

Thanks again for this instruction. I came upon a Megger just like yours today and was able to pick it up at a good price. Model BM7, it looks the same. It's great to find such useful tools. 👍

[@Juliana_So_Unique]

Excellent!

[@stuartukguitarampguy5830]

Thanks LMB - or can I just call you L? :)

[@Juliana_So_Unique]

Whatever will do. Keep up the good work!@@stuartukguitarampguy5830

[@Matan2222222]

Is there anyway to check for leakage with a multimeter? Checking if there is any voltage when one leg of the capacitor is disconnected from the circuit and the amp is turned on?

[@stuartukguitarampguy5830]

Unfortunately no, the multimeter only puts a volt or two across the capacitor and that's not enough to check for leakage.

[@Matan2222222]

@@stuartukguitarampguy5830 Can you send me a link of the same product that you are using in the video or something else that can complete the job?

[@stuartukguitarampguy5830]

@@Matan2222222 Hi If you search for insulation test meter on eBay you'll some at about £50 UK

[@Matan2222222]

@@stuartukguitarampguy5830 Thanks for the great tips!

[@poormanselectronicsbench2021]

Some older Fluke Handheld and bench DMM's, along with newer higher priced multimeters, like the Fluke 83, 87, and the Brymen 869S have a "conductance" measuring function that can indicate testing down to the equivalent of 10 ( for the older Fluke meters) or 100 Gigaohm resistive values, while using lower voltages. These will find very low level leakage without using the higher voltages of a "megger". Some older capacitor testers, like the Sprague TO-4/5/6 models also have a "insulation measurement" function that also uses lower voltages and can indicate resistive faults to 50 Gigaohms as well. You can take your chances with getting an older Fluke or Sprague on an auction site, or buy a newer meter with that function. If you do a little research you may find one that suits your needs for a reasonable price.

[@skaboosh]

I wonder why my 65 Park 45 sounded huge, warm, creamy, in reality... nothing like any other jtm45 I've tried, which have all been more recent....... I don't know.. but it was a great amp....

[@stuartukguitarampguy5830]

There is a really superb video (forget where) of a chap who experimentally confirmed that the MAJOR difference between one amp and another is the configuration (order) in which they place gain stages, tone stacks etc. He completely eliminated any discernible difference beteen caps, valve makes and many ither parameters. Something I've doubted for years is that vintage caps sound better than new caps etc etc. It's all smoke and mirrors!

[@skaboosh]

@@stuartukguitarampguy5830 what's strange is that the amp did sound different, to any jtm45, so could it be that the values in the amp had changed over time?

[@stuartukguitarampguy5830]

@@skaboosh The problem is that without proper blind A/B testing it's impossible to get a genuine handle on the objective difference. It just become subjective "I sort of feel the amp kind of sound different to how it did" etc etc. All the best

[@skaboosh]

@@stuartukguitarampguy5830 It has indeed, it's become a religion , and perhaps it partly came from the vintage amp and guitar market but I'm a serious player and in this case it's very real, so I'll file you as not really aware that my story could be real?

[@skaboosh]

@@stuartukguitarampguy5830 Or are the transfomers, all 3 , responsible, and they can't replicate them...the difference was similar to the difference between the two channels, in a way...one on a 45 is fat and the other skinny....my Park skinny channel sounded fat, in comparison to any reissues I've tried....a very big difference

[@moorec1053]

I understand lifting one end of the caps to measure capacitance. Assuming you did not need to do that............. the amp puts high voltage across the cap. There should be no DC across it or on the grid of the following tube. Why can we not just measure from ground to tube grid or from ground to the Non-DC side of the cap.? Thank You

[@stuartukguitarampguy5830]

The thing is, without poring over a schematic you never know what each end of the capacitor is connected to, so it's easiest just to lift an end. But you're right, if you KNOW that one end goes only to a grid of a valve, then you don't need to lift it.

[@stuartukguitarampguy5830]

Yes that's right, if you know exeactly what that cap is doing (looking at the schematoc) you could make the measurements you suggest.

[@moorec1053]

@@stuartukguitarampguy5830 Thank You 🙂