Fender vintage amps - out of spec resistors - How much is too much?

[spunko]

Hello friends,

This Fender Super reverb '69 has all 100Ks reading between 130K to 180K, some are 106K which is good, but how to know what is too much? when to change them?
I'm sad to test almost every single resistor on the SV '69 is out of spec by more than 30%.
Do I change them all? 

[Dave]

Does it sound good?


Dave

[spunko]

820ohm is reading 1.1k
2.2K is reading 2.7k
2.7k is reading 3.8k
Every single resistor is out of spec. There are just a few that read fine.

Yes it sounds good, I just want to be safe. But I’m afraid to change them all

[Dave]

I can't think of anything that would be unsafe about it. If it sounds good I say (opinion only), leave it alone.
Most of those 100k's you are talking about will be plate load resistors. If they are super out of spec, it will change the gain structure of the amp. Once again, if you like how it sounds, so what? Will it hurt anything, no.


In short, if it ain't broke, you could elect not to fix it, or fix it, its really a matter of taste.


Dave

[Dave]

I just thought of something that could make it unsafe... at least for the amp.
You should make sure that the bias circuit is functioning properly. Out of spec resistors could be a serious problem in that circuit, but in the signal path, not so much.


Dave

[Lectroid]

In short, if it ain't broke, you could elect not to fix it, or fix it, its really a matter of taste.
Dave

I cannot argue with "if it sounds good, leave it alone." 

Amps are designed to a basic spec, but I've often wondered how closely all examples of a particular amp model conform to the factory's image of that model. IOW, does every Deluxe Reverb ever made perform to the same standard?  I don't know for sure but I would bet there is a fair degree of variation even between amps made at the same time in the same factory, and even by the same assembler.

Any working amp is a complete, dynamically-balancing standalone system in its own right.  It seems to me that once it leaves the factory every amp "burns in" a little differently and will develop its own personality, if you will.

Hopefully some of the moderators might post their thoughts on:

1) how far out of spec is too far, assuming that an amp sounds 'good enough' otherwise?

2) if a resistor is still working but is still moving steadily out of spec, what sorts of symptoms might the amp display?

[Dave]

I didn't have the chance to try out 25 different brand-new deluxe reverbs in 1965. I wasn't quite born yet, but I was born before Silverface Twins had a master volume. But, I would bet a plug nickel that each one sounded close but not exactly alike. Too many variables. But, I would bet that same nickel that they all sounded fantastic even if they were a touch different. I think that those ever-so-subtle differences is where that magical voodoo lives. I also suspect that new-production "Blackface Deluxe Reverbs" all sound much closer to exactly the same. No voodoo.


Dave

[bmccowan]

I do not know what the "right" thing to do is. Or even if there is a right thing. But, I replace them if:
They are more than 10% out of spec.
If they look tortured.
If they vary more than 10% from a parallel resistor that they should match.
If they are a hiss culprit in that part of the circuit.
Part of the reason is that I do not want to open up the chassis any more times than I need to, and those resistors are not getting any younger.
But some factors:
I do not care if the circuit looks original - it is a music amplifying machine - not an ornament.
My goals are good sound and reliability.
Most guitar players - me included - should spend more time practicing, and less time worrying about resistor mojo, if they want their rigs to sound better.

[tubeswell]

Some variability in circuit component values is one of the great attributes of tube amps, especially vintage amps from Fender Musical Instruments. Gives each amp individual character. I echo the sentiment about leaving well-enough alone. Tube amp electronics is a 'ball-park science'.

[PRR]

... I'm sad to test almost every single resistor on the SV '69 is out of spec by more than 30%.

Did you find a fresh new carbon-film resistor and verify that your meter is reading right?

If a "1K" from 2022 reads 1.4k, suspect your meter.

Tip: low battery in meter can make all readings a little "off", even if the BATT icon has not come on yet.

[HotBluePlates]

... how to know what is too much? when to change them?
I'm sad to test almost every single resistor on the SV '69 is out of spec by more than 30%. ...

Does it sound good?

Yes it sounds good, I just want to be safe. ...

If the amp sounds good, then leave it alone.

It's the novices (me included, when I started) that are unsure of their ability to evaluate an amp's performance, and so assume there is something sacred about "the spec" and also assume the amp will somehow improve by being "returned to spec."

    - Page 580-581 of RDH4 talk about mismatching of output tubes.  They actually run through a scenario where one of the tubes is replaced with a completely different type that is only 50% "as strong" as the other tube.  This -50% difference results in about 5% distortion, which the author notes is barely audible.

    - People often plug 12AT7, 12AY7 or 12AU7 into a 12AX7 socket.  The -80% change of amplification factor doesn't sound bad.  In fact, some wouldn't notice the change if they hadn't immediately changed from 12AX7 to 12AU7 to hear the difference.

So much bigger differences can exist and sound fine.  Unless & until you can point to a specific sonic or performance failing (and then correlate that to a resistor-value problem), I would recommend you ignore whatever resistor value readings you happen to get.

[spunko]

Thank you for all the responses.

My 3 meters have new batteries (well like 3 or 4 months old). Reading new resistors, they read well in spec, so meters are good.

I have been using the amp without any work on it and it sounds good. Then I recaped the thing, and still sounds really good. Only observation is weak tremolo, but that can be fixed changing the caps I think.

All voltages seems fine, even the OG power tubes are really well matched, but too sensitive and microphonic.

I will keep testing the amp, but for now, everything seems fine.

[Dave]

I think it's Sluckey who advocates replacing the cathode resistor of the LFO tube with an LED for stronger tremolo. I have tried it and it definitely works.

[spunko]

I think it's Sluckey who advocates replacing the cathode resistor of the LFO tube with an LED for stronger tremolo. I have tried it and it definitely works.

I'm sorry to ask, but where I can find this info? I google it, but there are mixed results.
I think I watched an Uncle Doug video where he ecounters a weak tremolo. Can't remenber.

[sluckey]

https://el34world.com/Hoffman/files/Hoffman_AB763_Mods.pdf

[spunko]

https://el34world.com/Hoffman/files/Hoffman_AB763_Mods.pdf

Thank you very much.

I just replaced some resistors that were way out of spec like cathode bias resitors, dog house resistors, screen resistors (were 680/720ohm), grid stopers, negative bias resistor and all electrolytics. So far so good.

[Platefire]

After reading PRR's post about checking your MM battery, I checked it and it was weak but no low battery notification.I just changed it not long ago, so I thought it was fine. So I'll keep a closer watch on that.

I personally don't like de soldering and pulling a leg of a resistor to get an accurate reading. I'm just now learning how to check if a resistor is in parallel with the resistor being measured and how it can effect your "in place" reading(thanks sluckey). Something good to know or you could be replacing resistors that are fine.

I'm personally fine with letting resistor slide a little bit as long as I can tell they are not causing a noise problem or there is not a noted worsening in amp performance. I have
been known to let caps and resisters run way past their recommended life span:>)