Reforming resistors?

[Helmholtz]

I don't even change CC resistors in vintage amps that have drifted less than maybe 15%, except in the bias supply if required.
Influence on performance is insignificant.
Leo allowed for 20% tolerance in voltages and tube parameters as well as mains voltage can vary by more than 10%.

[Chuck H]

It's been speculated that the secret sauce for especially good sounding examples of these vintage amps could be due in part to tonally fortuitous circuit differences caused by things like drifted resistors, semiconductive board areas, etc. Maybe, maybe not. But my point is just if an amp sounds good and is operating safely you can do more harm than good replacing parts. With one concession...

Blueprinting a particularly good sounding amp might be a smart move. That, to me, would include identifying drifted resistors and replacing them with 1% metal film in the value they've drifted to. The logic here being that if you don't do this while an amp sounds great and then it fails later some drifted resistors could be replaced arbitrarily, possibly including one that finally drifted out of spec too far but was also responsible for the sound. Then the rebult amp might sound good, but no longer special. Then it's a head scratcher wondering where the mojo went. We've actually seen this happen here.

[Pixel]

There are only 3 CC left in this amp that had drifted less than 20%. If I apply the 15% rule then one of these three can stay...LOL. (Actually there are 2 more left that had drifted more than 20% but I kept them because they are on tone settings that I never use.) Unfortunately I don't play well enough to ever know if there is any mojo present.

[Chuck H]

Unfortunately I don't play well enough to ever know if there is any mojo present.

FWIW I now play about as much in a year as I use to play in a day. And I'm not kidding. So whatever I say is stricktly rote experience and not qualified in any modern sense when that comes up here sometimes. That said, I still think I can hear the difference. Otherwise I wouldn't bother with amp designs or building them.

If an amp sounds good to you then it's good. (<period)

[Mick Bailey]

I=Blueprinting a particularly good sounding amp might be a smart move.

I have used this method just once to build a 5E3 for a customer, based on an original and superb sounding example I had in for repair. The new amp did indeed sound excellent, but in a different way to the original. It used the same thickness yellow pine etc for the cabinet and the same value caps and resistors. The speaker was brand new. It just illustrated that a good sounding amp is the sum of its parts and cannot be fully replicated - subtle things like ESR and resistor value changes under operating conditions affect the tone. Carbon comps can also change value with voltage, so a static measurement isn't the same as a dynamic one.

Unless a resistor is faulty, or is causing a fault, I never check values in vintage amps if the amp a) works properly and b) sounds good. A lot of amps have been spoiled due to stripping out good parts and replacing them with modern equivalents. I've had vintage Fenders where not an original component has been present - metal film resistors and film caps (usually orange drops) having been substituted. Even all the wiring replaced.

[Pixel]

I have a spreadsheet that gets all the expected and actual values of components. I measure everything I can. If something is too far out of range the cell colour in the spreadsheet changes. Sometimes things get flagged for other reasons such as with paper caps. All changes to components are recorded in the sheet with date and reason for change. Sometimes the notes say why the component didn't change even though it is flagged. I use spreadsheets for just about everything I maintain like computer systems or even oil and filter changes on my car. (For a while I was cutting open used oil filters but stopped because I never learned anything useful.)

[Helmholtz]

I have a spreadsheet that gets all the expected and actual values of components. I measure everything I can.

I assume that includes measuring all caps out-of-circuit using a good LCR meter?

DMMs are not reliable with caps having some leakage, will read higher than actual.
Especially ecaps can have high tolerance and if e.g. the capacitance of a cathode bypass ecap is high by 20%, the effect on frequency response will be the same as if the cathode resistor has drifted by 20%.

[Pixel]

Right now I have Blue ESR meter that I built and a PIC-based lc meter. I just received parts to restore a Solar Exam-eter capacitor tester, that thing kind of scares me though.

[Helmholtz]

Unless a resistor is faulty, or is causing a fault, I never check values in vintage amps if the amp a) works properly and b) sounds good.

Same here.
Much more important to measure circuit voltages (especially at plates and cathodes).
So only when a voltage is wide off the mark I see a reason to measure resistors and capacitors.

[tedmich]

recapped and/or re resitor'd amps sound different than the original which is perceived as worse, humans are like that. Familiarity breeds, full stop.

[The Dude]

Slightly off topic, so please excuse:

As an extreme example of this, one of my first tech jobs was doing TV service calls back in the old CRT days. I was called to this older couples house to repair their TV that had a horizontal scan problem. I got the TV fixed and noticed that the picture was entirely green. I don't mean "leaning green", I mean very close to an old monochrome monitor. I got out my signal generator and readjusted brightness, contrast, and color balance. I got the thing looking really good thinking I was going the extra mile to give the customer a better picture. The lady came out of the kitchen to see the results of my work and nearly had a fit. She had gotten so used to the green picture over time, that she was actually disappointed. So, I readjusted color balance, made it green again, and she was happy. I think the same thing happens with a lot of amp repair. You make an amp sound "new" again, and the customer doesn't want "new".

[Chuck H]

I've experienced that. From the old couples perspective that is. When I was young I was forced to use amp rigs for a time that were not ideal or my first choice but "good enough" to get by. After learning all the in's and out's of the crappy rig I was using, and actually learning to exploit any musical properties offered, I found myself dissapointed with what would be considered much better amps and rigs because they didn't do what I was familiar with to make my music. Completely understandable and I think we've seen it here more than a few times and also supported by the odd choices of professionals we see from time to time.