Test resistors in amp?

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334
My old Fender has all the important electrolytic caps replaced (filter, cathode bypass, bias), but I was wondering if there is anything else I should check?

Should I be checking other resistors in the circuit with my multimeter?

How about capacitors? Some of the coupling caps have been replaced.

There is nothing glaringly wrong and I think the amp sounds really good, but wondering if there is anything 'maintenance' I ought to do just to make sure it sounds/works its best.
 

Diablo1

Member
Messages
620
It doesn't hurt to check resistor values to see if they match the color codes and/or schematic. If any have drifted way out of value, you might consider replacing them. For instance, if the schematic called out 5% tolerance on the resistor, and the value is out of that tolerance, that would be where I would change it out. Old carbon composition resistors tend to drift up in value.
 

icr

Member
Messages
2,855
All the carbon resistors are suspect in an old amp. I have replaced perhaps more open circuit carbon resistors in old amps than bad capacitors.
 

pdf64

Member
Messages
7,926
As he thinks the amp sounds really good, perhaps any resistor drift to date has been beneficial?
It may be useful to check and document every resistor value, eg in case they go noisy in the future and need replacing, then the replacement can be selected for a value close to the original's drifted value, from a time when it sounded good (it may drift further!).
I think the above process may be known as blueprinting.

All the carbon resistors are suspect in an old amp
Yes, or at least carbon comp types. I've found 100k plate resistors drifted to nearly 1M!
 
Last edited:

pula58

Gold Supporting Member
Messages
1,665
IMHO, check the output tube socket grid resistors. In most Fenders they are nominally 1.5K carbon comp. Carbon comp R's tend to develop internal cracks over many years of warming/cooling. So, if those R's measure a whole lot more than 1.5K they might be developing a crack. And you don't want that because the output tube grid will then float and the tube will suck downs tons of current - FAIL.

A Pro Reverb I just worked on had a grid resistor with a color code for 1.5K that measured as 5K. Amp was blowing fuses intermittantly.
 

donnyjaguar

Member
Messages
4,194
Haha! on the play guitar comment. :)
I would check the resistors myself, and yes, particularly the Sgrid ones on Fenders.
For me its easy as I know the resistor colour codes, and pretty much every one of them can be checked with the amp powered unplugged and powered down.

FWIW, I'm building another amp right now and have found some seriously out-of-whack resistance values in new 2Watt resistors. Also found whacky-jacky 1MΩ pots with 30% tolerances. Hmmm...
 

J M Fahey

Member
Messages
2,940
1MΩ pots with 30% tolerances
That's normal on audio use pots :)

They can be made to closer tolerances but process is slow (involves carefully baking the raw tracks in an oven) so usually not done unless specially asked (and paid extra).
 

Chris_M

Member
Messages
233
IMHO, check the output tube socket grid resistors. In most Fenders they are nominally 1.5K carbon comp. Carbon comp R's tend to develop internal cracks over many years of warming/cooling. So, if those R's measure a whole lot more than 1.5K they might be developing a crack. And you don't want that because the output tube grid will then float and the tube will suck downs tons of current - FAIL.

A Pro Reverb I just worked on had a grid resistor with a color code for 1.5K that measured as 5K. Amp was blowing fuses intermittantly.
Why would a 5K grid stopper cause the fuse to blow?
There are amps with 100K grid stoppers on the output tubes. More than enough current flows into the grid, the large resistance is just having an effect on the high pass filter caused by the tube's internal (Miller) capatence.
 

pdf64

Member
Messages
7,926
Why would a 5K grid stopper cause the fuse to blow?
Surely pula58 explained his hypothesis adequately in the 1st paragraph quoted?
To paraphrase, a resistor so wildly out of spec on value may be unstable, ie intermittantly open circuit, when subject to thermal cycling.
 






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