CC Resister drift in old Fenders and marshalls

[renshen1957]

Were the resistors you baked Allen Bradley CC resistors?

I kind of expected bigger changes before and after.

From the results from the Hi-Fi collective test, baking causes the values to drift upward, which is what people have written about CC and their exposure to heat for years.

Steve

[Tone Lover]

Yes and maybe he lived in Arizona were its dry, and the resisters have lived in desert country all there life.
But there really wasn't much drift on those resisters to start with.
Funny it looked like some of them actually went up in value after being baked (LOL).
\You know I have never used CC resisters yet and it looks like I don't need to start, since I live in the evergreen state, Washington were its wet all the time. Funny this has been a great summer without much rain.
I will just build it per spec and tweak from there thanks guys.
Bill

[Roe]

there's few CCs in marshalls (most resistors are CF) and CCs are mostly used in positions where there is little voltage drop across them. But the pots tend to read 30% high

[Richie]

Were the resistors you baked Allen Bradley CC resistors?

I kind of expected bigger changes before and after.

From the results from the Hi-Fi collective test, baking causes the values to drift upward, which is what people have written about CC and their exposure to heat for years.

Steve

They change value or go up when heated in a circuit. Fender use to wax the boards, I don't know if this was more for the board or resistors, or everything,to keep out moisture.

Maybe one reason people say their amps sound different at the later set.
I wonder how much heat is needed for the value to start changing. Might be able to use a thermo laser gun,and apply heat and check value changes of the resistors.
Coating them may help keep out moisture, but I think they also are ment to not have anything on them, so they can have an air flow or a way to give off the heat.
Also if you notice in most old fenders the 470ohm and 1.5k on the tube socket change in value and usually get brittle from the heat.
In some later model fenders that use reverb, they changed one resistor on V3 12AT7 to 680 ohm where they were 2.2k or 2.7k You can usually tell because it runs/sets the bias on that tube really hot. Even though that resistor has a low voltage, I see many that are burnt or cracked.

Just thinking on some of this stuff, schematics may tell you a value, but looking in an amp, and seeing the layout means a lot too. Just things like not putting a cathode bypass cap on or right next to the resistor. Caps also have temp ratings.Look up some of the design problems of the newer marshalls and read some of that. If heat causes a resistor to change value, would you put the bias resistors next to a large resistor that gets hot?

[renshen1957]

Were the resistors you baked Allen Bradley CC resistors?

I kind of expected bigger changes before and after.

From the results from the Hi-Fi collective test, baking causes the values to drift upward, which is what people have written about CC and their exposure to heat for years.

Steve

They change value or go up when heated in a circuit. Fender use to wax the boards, I don't know if this was more for the board or resistors, or everything,to keep out moisture.

Maybe one reason people say their amps sound different at the later set.
I wonder how much heat is needed for the value to start changing. Might be able to use a thermo laser gun,and apply heat and check value changes of the resistors.

Coating them may help keep out moisture, but I think they also are ment to not have anything on them, so they can have an air flow or a way to give off the heat.

Also if you notice in most old fenders the 470ohm and 1.5k on the tube socket change in value and usually get brittle from the heat.
In some later model fenders that use reverb, they changed one resistor on V3 12AT7 to 680 ohm where they were 2.2k or 2.7k You can usually tell because it runs/sets the bias on that tube really hot. Even though that resistor has a low voltage, I see many that are burnt or cracked.

Just thinking on some of this stuff, schematics may tell you a value, but looking in an amp, and seeing the layout means a lot too. Just things like not putting a cathode bypass cap on or right next to the resistor. Caps also have temp ratings.Look up some of the design problems of the newer marshalls and read some of that. If heat causes a resistor to change value, would you put the bias resistors next to a large resistor that gets hot?

Hi,

"Fender use to wax the boards, I don't know if this was more for the board or resistors, or everything,to keep out moisture. " I thought Fender waxed the boards either to keep the moisture out of the boards or to prevent them from becoming conductive.

Temperature coefficient of resistance is a factor with heat that effects the sound.

"Coating them may help keep out moisture, but I think they also are ment to not have anything on them, so they can have an air flow or a way to give off the heat. "

Carbon Composition resistors are made of carbon clay composition covered with a plastic case. Although sealed in plastic, resistance to moisture and humidity is an issue. Part of my reason for a hermetically sealed, epoxy filled case. CC are inexpensive to build, so which company would go to the additional expense to address the issue. Simpler to use a Metal Film resister.

"Also if you notice in most old fenders the 470ohm and 1.5k on the tube socket change in value and usually get brittle from the heat."

That was a design choice. the resistors could have been placed on the circuit card, however extra cost and labor would be involved. Book keepers (or MBAs) still have a major influence on circuit design.

"In some later model fenders that use reverb, they changed one resistor on V3 12AT7 to 680 ohm where they were 2.2k or 2.7k You can usually tell because it runs/sets the bias on that tube really hot. Even though that resistor has a low voltage, I see many that are burnt or cracked."

Traynor used lower resistance on their earlier Guitar mate/ Studio mate reverb 12AT7 tube, 68 ohm, 1 Watt. Although the voltage is low in the Fender examples, the heat generated would require a larger size (larger Watt) resistor for dissipation. But that would cost money. Later models of the Guitar/Studio mate (same circuit different speaker combination) went to a 470 ohm resistor.

There is a Tweed Bassman (picture) with an untouched board which has a large 2 Watt 10k resistor as the tail in a long tail PI. Most likely the assembler ran out of 1/2 watt 10k and since the higher Watt 10k CC was used in the power supply, they may have substituted the oversize resistor rather than stop assembly to retrieve the 1/2 watt from the stockroom. Just a hunch, but all the Tweed Bassman I've worked on or were sent gut shots of the board, all have 1/2 watts.

"Caps also have temp ratings. Look up some of the design problems of the newer marshalls and read some of that. If heat causes a resistor to change value, ...."

The Newer Marshalls have the same problem, circuit boards are designed for quick assembly ($$$) rather than sound electrical engineering practices or heat management. The MBAs (Masters of Bean Accounting) are more concerned with saving a penny that paying the extra cent to do it right.

Best regards,

Steve

[Richie]

There is a Tweed Bassman (picture) with an untouched board which has a large 2 Watt 10k resistor as the tail in a long tail PI. Most likely the assembler ran out of 1/2 watt 10k and since the higher Watt 10k CC was used in the power supply, they may have substituted the oversize resistor rather than stop assembly to retrieve the 1/2 watt from the stockroom. Just a hunch, but all the Tweed Bassman I've worked on or were sent gut shots of the board, all have 1/2 watts

I've worked on a few stock ones that had the 6.8k tail that was 2 watt.
Some of the silverfaces also went with 1 watt CC for plates etc, and they too change.

Yeah so much for the wax helping on the boards becoming conductive. I have a Twin/R here now with one of the worst I have come across. Not to stray from the OP, but other amps besides Fender and Marshalls using CC do the same. As was posted,early Marshalls had them but were not usually all through the amp.
All the amps back then used them. And it also happens in some older CF. Or again maybe a certain brand more than another. Some used in the Ampegs. Not as much or to the degree, or who knows why they drifted, but it happens.