How to check resistors on old valve amplifiers

These older amps use carbon resistors which often drift out of tolerance.
In this video I show you how to measure them and introduce a natty new digital test meter I recently acquired!

[@Ten80pete]

Stuart, you have been an absolute delight, not to mention that I couldn't have discovered your channel at a better time. I have some experience as a "tinkerer" and can hold my own when a solid solder joint is needed, but I recently came into possession of a Kitty Hawk M5 45 Wart Combo Amp, and considering the only documentation I've found online is a single hand drawn schematic from the 80's and 3 "gut shots" of the 15 Watt variant, I've been hesitant to attempt a repair on it. It worked perfectly fine (if a tiny bit noisy) when I first tested it out, but I just HAD to dissect it after getting a lemon the last time I got an amp. I found 2 scorched 10k resistors, and immediately replaced them. To be on the safe side, I also replaced all of the tubes with brand new 12ax7s and 6v6S's. I turned it on only to be greeted by a pop and then crackling that sounded eerily similar to the Fender Hot Rod Deville that you repaired with the crackle. Anyway, my point is that although I still haven't deduced what the issue is, I have learned more from 3 of your videos than a year of reading Rob Robinette's material (nothing against the guy; just feels like his material is more for electrical engineers and people with far more expertise than I have). Thank you!

[@stuartukguitarampguy5830]

Great! I hope you are able to work out what's going wrong!

[@Tonetwisters]

After 62 years of playing, I have come to find that "frying egg" resistors are my electronic arch enemy ... got one right now (at least one!) in the NORMAL channel of my 1966 Super Reverb. Thanks to all of you repair guys for what you do.

[@stuartukguitarampguy5830]

Freezer spray is great stuff to help you narrow down which resistor is giving the problem.

[@smartquark]

Thanks Stuart for another good video. The automatic multi-meter really does make light work of testing and provides an audible confirmation of the reading which is very useful.

[@delatronics3257]

I use an LCR meter DER EE 5000 which is not only automating but also auto identifies Caps, inductors or resitors. You an also accurately test in circuit by selecting test frequencies, measure ESR of capacitors etc with very high accuracy. I don't think it's made in China and it's not super cheap, but I bought mine directly from Japan via eBay at a good price.

[@stuartukguitarampguy5830]

Ok useful info thanks.

[@Pstaines439]

The best multi meters in this part of the world (NZ) are the UNI-T range. Super cheap and great quality

[@stuartukguitarampguy5830]

It's crazy how cheap a multimeter is these days. I use one called 'White Gold' and really like it but haven;t been able to find another one!

[@SinnerSince1962]

Auto-ranging meters have been around since the early 80's. Where have you guys been? I was shocked that auto-ranging seems to be the point of a video made just a year ago.

[@stuartukguitarampguy5830]

Sure but this one is really cool! Most of my viewers won;t have ever seen an auto ranging meter.

[@theprogrammerrolandmc3039]

@@stuartukguitarampguy5830 stop shilling and repair amps or you will end up with no subscribers

[@stuartukguitarampguy5830]

@@theprogrammerrolandmc3039 Huh??? That's exactly what I DON'T do, EVER!!! On the very rare occasion I mention a producct it's because I like it. I have never received a penny piece or ANY free product of any description from this channel!

[@peterroyle7082]

Enjoy your videos , learned a lot . I am building a clone of 5F2A and have been advised to put Carbon comp 100k Resisters in the signal path to input . Does this matter or can I put in Metal resisters in this position . many thanks , Peter , Herne bay . UK.

[@stuartukguitarampguy5830]

Thanks Peter. I would use modern metal film reistors. You have been advised by a purist who believes they can hear the difference between carbon and metal film resistors. They can't!

[@peterroyle7082]

Thank you for you prompt reply. I’ll take your advice , and thank you again for the time you put into your videos. Cheers Peter.

[@rizzlerazzleuno4733]

So you get correct values with the resistors in the circuit, no need to disconnect one end? I use a Fluke autoranging meter. I am repairing a Firestone radio from 1953, all original, actually works OK, but from all I see and read I am changing the capacitors and want to correctly measure the resistors.

[@stuartukguitarampguy5830]

A lpt depends on the actual circuit of course, but most can be measured in circuit. If in doubt lift one end.

[@rizzlerazzleuno4733]

@@stuartukguitarampguy5830 Thanks. I'm doing that now. First one I tested has no resistance, 2nd one is exactly what it is supposed to be.

[@on1ytheb3st]

How do you know when a resistor is not going to be effected by other resistors in parallel when measuring with a DMM? I have a Super 112 amp and looking at the schematic it is not always obvious what is parallel with what, especially if you include pots and LDRs.

[@stuartukguitarampguy5830]

Yes you have to watch for that. Surprisingly though many resistors do not have anything in parallel with them.

[@on1ytheb3st]

@@stuartukguitarampguy5830 I read somewhere that usually failing resistors will read high while if they read significantly low that means they are being shunted via another trace to likely something in parallel. Have you found that to be true?

[@stuartukguitarampguy5830]

@@on1ytheb3st Yes that's true. Old carbon resistors nearly always go high with age.

[@on1ytheb3st]

@@stuartukguitarampguy5830 I wish mine was that simple, I'm working on and oldish (late 80's) circus board amp and it's got mostly carbon film resistors and no carbon comp resistors. Just finished measuring every single one and there were a few that are questionable, mainly one that was in spec when it settled but would fluctuate back and forth. I've already replaced everything from plate load, to power resistors but had to brake it apart again because of excess hiss on certain channels. Lastly, on the schematic of the Super 112, next to the expected voltage specs, there are sometimes circles or ovals with a number inside divided by a V or MV symbol; would you know what this references, I'm assuming it has to do with the voltage spec with reference to what comes in through the mains.

[@stuartukguitarampguy5830]

@@on1ytheb3st I thinbk the numbers in ovals might be AC voltages but not sure. Hiss is quite hard to find. You haven't really said what your main issue is with this amp which is causing you to check resistors etc??

[@pedrolourenco2707]

Never thought that Carbon resistor could drift soo much...now the question is, that magic old amps would they sound better with the exact values or does the mojo comes from this random drifting?

[@stuartukguitarampguy5830]

Ah, interesting question and alas, unanswerable even in principle.