Who cares about old resistors?-one example on Fisher X-100-C

[oldman55]

Originally 330k ohm resistor, thru 1k ohm resistor to 7868.

My 1K ohm resistors soldered to tube socket have drifted to 1.1k. Since I am opposed to desoldering and tying in new components to tube sockets, and both resistors have the same drifted value (within 10% tolerance), does it really matter if the the 1.1Ks remains?
I have already modified the coupling capacitor to .068 uf (a somewhat arbitrary value) and the 330K ohm resistors to 220K ohm (a somewhat arbitrary value) so who cares? If amp don't care, I don't care.

Applies to other selected circuits but it would save time, keep the appearance more in line with original and I don't worry about the delicate pins on the socket. And after 50 years, it is likely to survive my short time left on the planet.

Oldman taking a shortcut

 

[larryderouin]

Oldman55: Did you measure these with the tubes IN or OUT? If in you'll get some additional circuit resistance. Pull the tubes and measure again if needed. 1.1K is 10% high, and you're at the threshold of the tolerance band on those resistors IIRC. You could replace them by cutting the one lead at the resistor body and creating a "J" hook from the lead, and connecting a new resistor to it.

As for the 220K and the .068uf coupling caps values. These are far from arbitrary. These were chosen to match as close as possible the R/C timing constant of the original .047uf and 330kΩ parts while keeping the maximum grid return resistor values below maximum in "FIXED BIAS configuration(ie: 400, 500b/c, 800b/c receivers)". Cathode bias which is how Fisher biased pretty much all of their integrated and stand alone amps incl. the X-100-C, grid max value for the 7591/7868/6gm5 tube family is 1 MEGΩ. (It's in the tube Data Sheets). No need to change the original values, unless they've drifted more than the specified tolerance.

 

[oldman55]

Oldman55: Did you measure these with the tubes IN or OUT? If in you'll get some additional circuit resistance. Pull the tubes and measure again if needed. 1.1K is 10% high, and you're at the threshold of the tolerance band on those resistors IIRC. You could replace them by cutting the one lead at the resistor body and creating a "J" hook from the lead, and connecting a new resistor to it.

As for the 220K and the .068uf coupling caps values. These are far from arbitrary. These were chosen to match as close as possible the R/C timing constant of the original .047uf and 330kΩ parts while keeping the maximum grid return resistor values below maximum in "FIXED BIAS configuration(ie: 400, 500b/c, 800b/c receivers)". Cathode bias which is how Fisher biased pretty much all of their integrated and stand alone amps incl. the X-100-C, grid max value for the 7591/7868/6gm5 tube family is 1 MEGΩ. (It's in the tube Data Sheets). No need to change the original values, unless they've drifted more than the specified tolerance.

Tubes out.
Don't know the net affect on the voltage change it would cause but wouldn't think it is significant.

 

[larryderouin]

Probably less than a volt. Check the 3rd band on the resistor. Gold is 5%, Silver is 10%. If it's a 5%'er replace, if 10%'er up to you, but I'd replace.

 

[billyz]

I’m with oldman. Leave em. Its entirely possible they are not even drifted as they are old carbon comp. they seem to be matched .

 

[gadget73]

where those resistors are, they flow next to no current. Matching is also not that critical for the same reason. The value is not that critical, however they could add some noise to the mix.

if I had to desolder the connections to it, I'd change them but otherwise I would leave it alone if this were my amp.

 

[maxhifi]

Originally 330k ohm resistor, thru 1k ohm resistor to 7868.

My 1K ohm resistors soldered to tube socket have drifted to 1.1k. Since I am opposed to desoldering and tying in new components to tube sockets, and both resistors have the same drifted value (within 10% tolerance), does it really matter if the the 1.1Ks remains?
I have already modified the coupling capacitor to .068 uf (a somewhat arbitrary value) and the 330K ohm resistors to 220K ohm (a somewhat arbitrary value) so who cares? If amp don't care, I don't care.

Applies to other selected circuits but it would save time, keep the appearance more in line with original and I don't worry about the delicate pins on the socket. And after 50 years, it is likely to survive my short time left on the planet.

Oldman taking a shortcut

Leave them alone, those are just grid stoppers and are not critical regarding value or any other specs.

 

[Aerobat]

I find that many of the old carbon resistors drift high, sometimes by a lot. I replace all of them to bring the whole circuit back to original spec, just because why not.

 

[regore]

They drift a little high before they drop in value. In old TV's I"ve seen 68k and 47k two watters drop to a couple hundred ohms. Also in Fender tube amps. Leave them in but keep an eye out if they start dropping in value. No fourth band means 20% tolerance. Common in older gear.

 

[gadget73]

I've never had that happen to me, all of them have just gone high. Had a couple go open but thats less common.

 

[oldman55]

I find that many of the old carbon resistors drift high, sometimes by a lot. I replace all of them to bring the whole circuit back to original spec, just because why not.

Potential damage to tube socket, visuals and a little time.

The circuit has already been modified by the new accepted rule of thumb of swapping the 330k resistors to about 220k. Being the case, the small difference in the 1k resistors are of no real consequence.

old and keeping the old carbon comps

 

[mcgjohn]

would agree, leave them in place. I have wrecked the sonics on a few pieces of gear by replacing those carbon comps with carbon film or metal film. the carbon comps are part of the sonic signature on that amp. and the 1.1k may not be drift, they may just be at the top end of the tolerance spec. if the value changes 2x or 3x the color band value, then will search for a NOS carbon comp to replace it with.