Help debugging overdrive vintage fender amp

[StrummersOfThunder]

I have a couple older brownface amps (vibrolux and vibrosonic)
Both sound utterly lovely clea n to edge of break up
There after I run into issues.
Lets stick with the vibrolux. 30w 6l6 1x12 (new celestion type A)
It tends the get an unpleasnt sound a bit like a blown speaker. Poppy, clipping, rattle. I dont want to say fizzy cos its not that. There is a nice drive sound in there somewhere but its unusable with the other sounds.
Its had a EL recap but the coupling caps and most other board components are original.

Any pointers on where to start?

[GrantB]

Paul C mod on the Princeton fixed my same issues, but of course, different circuit. Maybe the theory is transposable?

[StrummersOfThunder]

Paul C mod on the Princeton fixed my same issues, but of course, different circuit. Maybe the theory is transposable?

something something phase inverter something something dark side something something ....

[jeremyb]

I asked Claude 3.7 Sonnet

This is a classic issue with vintage brownface Fender amps like your Vibrolux. That "poppy, clipping, rattle" sound beyond the edge of breakup is often related to specific components in the signal path. Given that you've already had the electrolytic capacitors replaced but still have original coupling caps and other components, here are some targeted areas to investigate:

Coupling capacitors - These are prime suspects. Original brownface coupling caps (often "blue molded" or ceramic types) can become leaky or change value over time. When they fail, they can cause exactly the type of distortion you're describing. Consider replacing them with high-quality modern equivalents (Sprague Orange Drops, Mallory 150s, or similar).

Phase inverter circuit - The phase inverter in these amps can be a weak point when pushing into overdrive. Check the resistors and capacitors in this stage, as they significantly impact how the amp transitions from clean to distorted.

Screen resistors - The screen resistors for your 6L6 output tubes might be drifting in value or failing under load. These can cause unusual distortion characteristics when the amp is pushed.

Bias circuit components - Even with new tubes, if the bias circuit components are original, they may not be providing stable bias under load, causing the tubes to clip in an unpleasant way.

Cathode resistor/bypass capacitor - In the preamp section, these components greatly affect how the amp breaks up. The bypass capacitor especially might be failing under higher signal levels.

I'd recommend starting with the coupling capacitors and then moving to the phase inverter components. These are relatively straightforward replacements that often yield dramatic improvements in how these vintage amps handle overdrive.

[robthemac]

What about the newbies? If it's also present in the 6g3 builds, presumably component drift isn't an issue?

[murky]

Just curious what years/circuits we talking about here? Big fan of the 6G16 and its cousins - they have great breakup (enough for Nuno)!

[StrummersOfThunder]

I asked Claude 3.7 Sonnet

This is a classic issue with vintage brownface Fender amps like your Vibrolux. That "poppy, clipping, rattle" sound beyond the edge of breakup is often related to specific components in the signal path. Given that you've already had the electrolytic capacitors replaced but still have original coupling caps and other components, here are some targeted areas to investigate:

Coupling capacitors - These are prime suspects. Original brownface coupling caps (often "blue molded" or ceramic types) can become leaky or change value over time. When they fail, they can cause exactly the type of distortion you're describing. Consider replacing them with high-quality modern equivalents (Sprague Orange Drops, Mallory 150s, or similar).

Phase inverter circuit - The phase inverter in these amps can be a weak point when pushing into overdrive. Check the resistors and capacitors in this stage, as they significantly impact how the amp transitions from clean to distorted.

Screen resistors - The screen resistors for your 6L6 output tubes might be drifting in value or failing under load. These can cause unusual distortion characteristics when the amp is pushed.

Bias circuit components - Even with new tubes, if the bias circuit components are original, they may not be providing stable bias under load, causing the tubes to clip in an unpleasant way.

Cathode resistor/bypass capacitor - In the preamp section, these components greatly affect how the amp breaks up. The bypass capacitor especially might be failing under higher signal levels.

I'd recommend starting with the coupling capacitors and then moving to the phase inverter components. These are relatively straightforward replacements that often yield dramatic improvements in how these vintage amps handle overdrive.

Yep great answer
I think I’ll lift the caps and test with new ones on place
I’d like to keep em for obvious reasons

[MikeC]

Paul C mod on the Princeton fixed my same issues, but of course, different circuit. Maybe the theory is transposable?

PaulC mod works (to a degree) for PR Cathodyne phase inverter

[jeremyb]

I asked Claude 3.7 Sonnet

This is a classic issue with vintage brownface Fender amps like your Vibrolux. That "poppy, clipping, rattle" sound beyond the edge of breakup is often related to specific components in the signal path. Given that you've already had the electrolytic capacitors replaced but still have original coupling caps and other components, here are some targeted areas to investigate:

Coupling capacitors - These are prime suspects. Original brownface coupling caps (often "blue molded" or ceramic types) can become leaky or change value over time. When they fail, they can cause exactly the type of distortion you're describing. Consider replacing them with high-quality modern equivalents (Sprague Orange Drops, Mallory 150s, or similar).

Phase inverter circuit - The phase inverter in these amps can be a weak point when pushing into overdrive. Check the resistors and capacitors in this stage, as they significantly impact how the amp transitions from clean to distorted.

Screen resistors - The screen resistors for your 6L6 output tubes might be drifting in value or failing under load. These can cause unusual distortion characteristics when the amp is pushed.

Bias circuit components - Even with new tubes, if the bias circuit components are original, they may not be providing stable bias under load, causing the tubes to clip in an unpleasant way.

Cathode resistor/bypass capacitor - In the preamp section, these components greatly affect how the amp breaks up. The bypass capacitor especially might be failing under higher signal levels.

I'd recommend starting with the coupling capacitors and then moving to the phase inverter components. These are relatively straightforward replacements that often yield dramatic improvements in how these vintage amps handle overdrive.

Yep great answer
I think I’ll lift the caps and test with new ones on place
I’d like to keep em for obvious reasons

Probably worth having a capacitor tester given how many builds and mods you're doing, at least that way you could keep as much of it as original as the parts condition allows

This is the one I got, took about 7 days from China: https://www.aliexpress.com/item/1005007871693433.html

[Cdog]

I asked Claude 3.7 Sonnet

Yep great answer
I think I’ll lift the caps and test with new ones on place
I’d like to keep em for obvious reasons

Probably worth having a capacitor tester given how many builds and mods you're doing, at least that way you could keep as much of it as original as the parts condition allows

This is the one I got, took about 7 days from China: https://www.aliexpress.com/item/1005007871693433.html

Get the caps from Ali too, while you're at it?

[jeremyb]

Yep great answer
I think I’ll lift the caps and test with new ones on place
I’d like to keep em for obvious reasons

Probably worth having a capacitor tester given how many builds and mods you're doing, at least that way you could keep as much of it as original as the parts condition allows

This is the one I got, took about 7 days from China: https://www.aliexpress.com/item/1005007871693433.html

Get the caps from Ali too, while you're at it?

Still too wary to do that!

[MikeC]

Probably worth having a capacitor tester given how many builds and mods you're doing, at least that way you could keep as much of it as original as the parts condition allows

This is the one I got, took about 7 days from China: https://www.aliexpress.com/item/1005007871693433.html

You typically need high voltage in a tester (i.e. tube amp voltage) to identify DC leakage in a capacitor. It's best & easiest to test them in circuit but it can't be done that way if one end of the cap is tied to ground. I use a megger in those cases.

[Cdog]

Probably worth having a capacitor tester given how many builds and mods you're doing, at least that way you could keep as much of it as original as the parts condition allows

This is the one I got, took about 7 days from China: https://www.aliexpress.com/item/1005007871693433.html

Get the caps from Ali too, while you're at it?

Still too wary to do that!

Not in a vintage fender please

[StrummersOfThunder]

I have a bunch of decent replacement caps but I really don’t want to rip out the blue caps if I can avoid it.

Yep cap testing would be the ticket. I might look into that
What’s a megger?

I don’t mind lifting the ground leg tbh

[MikeC]

I have a bunch of decent replacement caps but I really don’t want to rip out the blue caps if I can avoid it.

Yep cap testing would be the ticket. I might look into that
What’s a megger?

I don’t mind lifting the ground leg tbh

A megger generates high DC voltage to test "insulation resistance". The old one's were hand cranked! My one looks like a multi-meter, runs off batteries and you can select testing voltages of 100V, 250V, 500V or 1000V. You don't exceed the cap's voltage rating. You test the cap out of circuit and if it's resistance breaks down the meter swings and gives a measurement in ohms. A perfect capacitor would have infinite resistance at it's rated voltage. It's handy for power filter caps which are usually connected to ground.