55 year old component replacement

[jordan86]

So I just picked up a bunch of parts for my newly acquired 65 Showman. And still, I am just a little hesitant to get into it and start replacing things. I mean its 99% percent stock except for two filters caps. I do have some 60 cycle hum so I will start will all new F&T filter caps and then reevaluate. Will probably go through and lift one leg of the bias cap and all the other electrolytics for some additional measuring/testing and replace what is failed.

Most all the resistors measure within 10% of spec except two grid stoppers on one pair of the 6L6s. So my questions...
- Should I just replace all these? Or parallel in a second resistor to hit the correct value?
- Would it be better to stick with the stock varieties (1/2 watt or 2 watt carb comp), or replace with upgrades (metal films, oxide, wirewound in 3 or 5 watt)?

Currently measuring....

Grid stoppers
A: 1556 Ω
B: 1578 Ω
C: 1904 Ω
D: 1966 Ω

Screen Grid Resistors
A: 513 Ω
B: 509 Ω
C: 519 Ω
D: 474 Ω

I am happy to upload more photos and a picture of all measurements if any are interested or think that would be helpful.

I think I am just fighting the reality that some stuff needs to be replaced. Probably just need someone to slap me out of it. I know I can do quality work, just kills me to pull things from a working amp in its mostly unmolested condition.

Appreciate the consideration and advice.

[jjasilli]

There is nothing wrong with your plan, especially because it's your amp.  If you have to go back in, no problem; but a customer might lose faith. 


Aged components tend to fail in series. For some unknown reason, replacing some of the old parts seems to cause remaining old parts to fail in the near future.  Personally I would replace all the components in the PS, including dropping resistors, plate resistors, K resistors & bypass caps. 


However, I would measure the values of the removed parts, which may have drifted substantially.  That way you can duplicate as-is tone if you like it better than the resulting tone after a rehaul to stock values.


What types of R's & C's to use is another rabbit hole!

[drew]

This is what I would do:

https://el34world.com/schematics.htm#Servicing%20fender%20amps

[bmccowan]

Tubes, resistors, and capacitors were supposed to be replaced when they failed, or fell out of spec. If resistors are still close to spec and you love the sound, then keeping them is not a bad idea. Keeping leaky caps is a bad idea, but you know that. Keeping an amp 100% original is museum stuff IMO. With eyelet boards, its pretty easy to swap out board components, save and swap them back in again. I have 3 collector obsessions: amps, cars, and classic handmade steely bicycles. They are supposed to be played, driven, and ridden (mostly in that order.) If an amp does not sound good, it's no longer a musical instrument, it's just a curiosity.
Jordan, is that the slap you were hoping for?

[nworbetan]

Resistors typically increase in resistance when they drift.  When they fail catastrophically, they tend to open circuit.

Because of the positions of the resistors you mentioned, I personally would just let them ride as long as the amp sounds good.  Yeah, they're technically out of tolerance, but not so far that they're going to cause problems just because of the value.

One thing to consider is when and how you're going to use the amp.  Are you taking it out and playing shows where it needs to be 100% rock solid reliable because you're hundreds of miles away from your soldering iron?

[shooter]

In my field, some "parts" had a time frame, PA tube 17K swap, cryo valves 12 months, ......
the parts that didn't fall into that category, fell into the; IF it ain't broke don't fix it time frame.


here's the logic, you're good, 96% accurate, every time, the parts are good 1% fail rate, so your overall, 95% success.
If you do nothing, you're 100% successful. 

[pdf64]

My thinking is that the grid resistors mounted over the socket on Fenders etc have a pretty hard life. They get roasted due to the heat from the tube, which will be both radiated, conducted and, with upside down chassis, convected. Then there's the necessarily loose fitment of the lugs in the socket body, so the resistor legs get waggled around with tube changes, and vibration picked up by the tube will also be transferred to them.
Plus the screen grid resistors are vital to the tube functioning, if they fail (eg a crack, plate current stops.
But most significantly, if the control grid stoppers fail, the tube will pass A LOT of plate current and bad things can happen, including transformer damage.
So I view those parts as a (operational) life limited consumable, to be replaced at service by default.
1W MO flame proof/retardant 470 for beam tetrode screen grids; 1W as it's beneficial for them to blow if current gets that high, MO flame proof/retardant to mitigate for collateral damage for when that happens.
Absolutely NOT carbon type resistors, as the resistor material provides a fuel load.
1/2W MF 1k5 - 10k for the control grid stoppers, as they're the most temperature stable.
With both, when fitting, put a bend / kink in the legs, ie to reduce physical stress on the resistor, don't stretch them tight between the lugs, with no 'give'.