How much hum is acceptable in a Fender Red Knob Twin

[jalexquijano]

Just fixed the squealing and water drop background sound by tightening the preamp tube sockets and replacing the former 47uf 500V filter caps. However, even though there is no squeal i would like to know the following:

1 - How much hum is acceptable on CHannel 2 - Overdrive when the gain is pushed to 10?

2- Can i play this amp with the gain up to and presence up to with no squeal at all?

[leydenjar]

I've been looking over your posts.

I recommend attaching the schematic, if you haven't already, and your voltage readings.

Pictures of what you're doing may be helpful at times as well.

I've repaired a red knob showman reverb (similar circuit) and it really shouldn't be a big deal.

Good data is essential for these guys to efficiently help you.

[guitician]

Just fixed the squealing and water drop background sound by tightening the preamp tube sockets and replacing the former 47uf 500V filter caps. However, even though there is no squeal i would like to know the following:

1 - How much hum is acceptable on CHannel 2 - Overdrive when the gain is pushed to 10? Under what conditions?

2- Can i play this amp with the gain up to? and presence up to? with no squeal at all?

[bob p]

1 - How much hum is acceptable on CHannel 2 - Overdrive when the gain is pushed to 10? Under what conditions?

2- Can i play this amp with the gain up to? and presence up to? with no squeal at all?

Those are tough questions to answer without a schematic, and tough questions to answer without hands-on expeirence with the amp.

Regarding hum -- any level that's obtrusive is too much and the problem needs to be addressed.

Regarding stability -- there are many high-gain amps available today that will squeal and oscillate at some settings, and many people consider it the user's responsibility to operate the amp at settings that aren't "unreasonable" in order to prevent the problem from occurring.

Personally, my philosophy is that an amp shouldn't be sold that has settings that allow the amp to oscillate, squeal, or become unstable. I think it's the responsibility of an amp designer to engineer the amp so that it's stable at all settings that a user can adjust it to. Unfortunately, there are amp designers out there that don't follow these rules (and many modders that routinely violate them), so that some high-gain amps will misbehave at some settings. When an amp does that, it becomes very difficult to know how to answer your question.

Personally, I think that if your amp is oscillating/squealing, then something is wrong with it and it needs to be fixed, so my answer is that oscillations/squeals are never acceptable at any control setting. In ohter words, my persional opinion is that your amp should be stable with all of the controls at any setting. If it oscillates then there are only two possibilities: it's poorly designed and/or malfunctioning.

Other people might answer differently.

[g1]

This is your third separate post about the squealing issue with this amp. Glad to hear you fixed it, but could you please go back to your other posts and mention how you solved the problem? This will be helpful for other people in the future who may be searching for information about the same problem.
Thanks.

[jalexquijano]

How much gain am I supposed to dial in without no squeal?? I can put the gain and presence in 10 but if I raise the volume over 4 I start to listen to a squeal that is even present with the guitar volume in zero.

[leydenjar]

How much gain am I supposed to dial in without no squeal?? I can put the gain and presence in 10 but if I raise the volume over 4 I start to listen to a squeal that is even present with the guitar volume in zero.

It shouldn't do that.

Again, data would be helpful.

Post the schematic. Post voltages.

[soundguruman]

Just fixed the squealing and water drop background sound by tightening the preamp tube sockets and replacing the former 47uf 500V filter caps. However, even though there is no squeal i would like to know the following:

1 - How much hum is acceptable on CHannel 2 - Overdrive when the gain is pushed to 10?

2- Can i play this amp with the gain up to and presence up to with no squeal at all?

How much hum? Quite a lot in these earlier overdrive circuits...it's normal...
How much squeal? It's oscillation, usually in the first two preamp stages.
Connect a 6pf 1000V cap between plate and grid of the preamp tube. That usually does the trick.
It would never hurt to replace the preamp tubes...with selected low noise types.

[jalexquijano]

I just replaced V1, V2 and V7 with brand new JJ ecc803s preamp tubes. I tried some sovtek 12ax7 lps - long plate but it did too much squealing. What about changing tube sockets?

[jalexquijano]

I just replaced V1, V2 and V7 with brand new JJ ecc803s preamp tubes. I tried some sovtek 12ax7 lps - long plate but it did too much squealing. What about changing tube sockets?

HERE IS THE SCHEMATIC:

http://www.blueguitar.org/new/schem/fender/the_twin.gif

Okay this is what i did to stop the squeal that was present after turning the gain past 6 and the volume past 3 with the presence in 10 on channel 2:

1. reinstalled the former original made in japan 9 pin stereo plastic jacks and resolder everything.

2. Changed the 47uf 500v filter caps

3. Retensioned preamp tube sockets (V7, V1 and V2)

4. Sprayed contact cleaner on Gain pot which is 1M. I measured the pot and it is perfect.

I want to be able to eliminate the squeal completely even with GAIN at 10, presence on 10 and volume past 7. Is this possible???? Also would like to reduce the Hum in this channel.

I havent changed C21 and C16 which are Orange drop capacitors both .68 250v? Will this help also?

[bob p]

i don't want to sound like a broken record, but here goes:

squeal = oscillation = problem

i'm thinking that the best way to address the problem is to identify what it is, where it's occurring and why it's occurring. then once you know exactly what's causing the problem, you can make a directed effort to deal with it properly, instead of just guessing at fixes.

do you have access to a scope?

[soundguruman]

Just fixed the squealing and water drop background sound by tightening the preamp tube sockets and replacing the former 47uf 500V filter caps. However, even though there is no squeal i would like to know the following:

1 - How much hum is acceptable on CHannel 2 - Overdrive when the gain is pushed to 10?

2- Can i play this amp with the gain up to and presence up to with no squeal at all?

Remember the plate to grid capacitance on V1A. You can twist the wires together, plate and grid. You can just use a 6pf silver mica 1000V between plate and grid.
The plate to grid cap was stock on some Marshalls and Fenders. It worked just fine.
The cap method works the best, out of all the methods. I have tried them all.
It's probably not the socket that is causing oscillation. But changing old oxidized sockets is not a bad idea.
My previous post describes all the common methods of eliminating this type of oscillation....it's more of a design and layout problem than a socket or tube problem...

This is a lot of it: (many secrets explained)
The tube itself could short plate to grid at any time, however very unlikely.
The silver mica cap can short, but again rated 1000V in a 300 volt circuit is very unlikely.
(you can use 2 caps in series, like 2X 15 pf, 500V)
Both Fender and Marshall have used that method in several amps, and none have shorted.
There is another method if you prefer.
The plate and grid wires can be twisted together, forming capacitance between the two.
This method is used in all early Fender amps. As you notice, the plate circuit has the grid wire running directly under it.
Ever wonder why? That's why.
Alternate: a shielded cable. The grid is the center conductor, the plate is connected to the outer screen shield.
No, the shield is not grounded, it's HOT.
This creates capacitance between the grid and the plate. The wire should be rated 600 volts or higher.
the longer the wire, the more capacitance between the two.
Stupid idea? hardly, Marshall used this method on many production amplifiers.
So, if you look closely, you will see this in scores of examples. You just did not notice before I told you.

The plate to grid capacitance creates negative feedback at high frequencies.
This stops (mostly) the High Frequency microphonics and kills the hiss.
Yes it works, try it. No denial.

Other methods include:
A 39 pf cap between input jack and ground, Peavey Method. Kills a lot of highs.
A small capacitor, across the plate resistor, limits the frequency response of the stage. Kills all response above the selected frequency.
Mesa Method: small cap between plate and cathode. That's what they do, in addition to all the others I mentioned. BUT to me, this method makes the amp sound muddy. These caps are never shown on the Mesa schematic. BUT they are for sure, installed.
A cap can be a ceramic disk, or, 2 circuit tracks closely spaced together on a circuit board will form a capacitor also.
(mesa did not invent ANY of these methods)
My Method: Use Metal Film Resistors for plate, grid and Cathode. Much lower noise.
Bulk Metal Foil has noise about 1/10th of a carbon resistor.
Carbon resistors are THE noisy-est. Despite what vendors claim.
Silver Mica Capacitors, again much lower noise and much less inducted Hum from heaters. (compared to ceramic disk)

The design of 5150 is poor. Poorly laid out, cheap parts, poor serviceability, poor documentation, unmarked circuit board.
So, takes a lot of patience to get the bugs out.

But as I mentioned, the source of noise originates in the first preamp stage, typically.
And this noise is amplified thousands of times.
So, lower the noise of the first stage, and get a better result, overall.

Hum? DC heaters for the first 1-2 stages will give you the added advantage.
Humbucking pickups, forget single coil.
Effects pedals: noise city. Don't blame it on the amp. I don't use them, ever.

[jalexquijano]

Remember the plate to grid capacitance on V1A. You can twist the wires together, plate and grid. You can just use a 6pf silver mica 1000V between plate and grid.
The plate to grid cap was stock on some Marshalls and Fenders. It worked just fine.
The cap method works the best, out of all the methods. I have tried them all.
It's probably not the socket that is causing oscillation. But changing old oxidized sockets is not a bad idea.
My previous post describes all the common methods of eliminating this type of oscillation....it's more of a design and layout problem than a socket or tube problem...

This is a lot of it: (many secrets explained)
The tube itself could short plate to grid at any time, however very unlikely.
The silver mica cap can short, but again rated 1000V in a 300 volt circuit is very unlikely.
(you can use 2 caps in series, like 2X 15 pf, 500V)
Both Fender and Marshall have used that method in several amps, and none have shorted.
There is another method if you prefer.
The plate and grid wires can be twisted together, forming capacitance between the two.
This method is used in all early Fender amps. As you notice, the plate circuit has the grid wire running directly under it.
Ever wonder why? That's why.
Alternate: a shielded cable. The grid is the center conductor, the plate is connected to the outer screen shield.
No, the shield is not grounded, it's HOT.
This creates capacitance between the grid and the plate. The wire should be rated 600 volts or higher.
the longer the wire, the more capacitance between the two.
Stupid idea? hardly, Marshall used this method on many production amplifiers.
So, if you look closely, you will see this in scores of examples. You just did not notice before I told you.

The plate to grid capacitance creates negative feedback at high frequencies.
This stops (mostly) the High Frequency microphonics and kills the hiss.
Yes it works, try it. No denial.

Other methods include:
A 39 pf cap between input jack and ground, Peavey Method. Kills a lot of highs.
A small capacitor, across the plate resistor, limits the frequency response of the stage. Kills all response above the selected frequency.
Mesa Method: small cap between plate and cathode. That's what they do, in addition to all the others I mentioned. BUT to me, this method makes the amp sound muddy. These caps are never shown on the Mesa schematic. BUT they are for sure, installed.
A cap can be a ceramic disk, or, 2 circuit tracks closely spaced together on a circuit board will form a capacitor also.
(mesa did not invent ANY of these methods)
My Method: Use Metal Film Resistors for plate, grid and Cathode. Much lower noise.
Bulk Metal Foil has noise about 1/10th of a carbon resistor.
Carbon resistors are THE noisy-est. Despite what vendors claim.
Silver Mica Capacitors, again much lower noise and much less inducted Hum from heaters. (compared to ceramic disk)

The design of 5150 is poor. Poorly laid out, cheap parts, poor serviceability, poor documentation, unmarked circuit board.
So, takes a lot of patience to get the bugs out.

But as I mentioned, the source of noise originates in the first preamp stage, typically.
And this noise is amplified thousands of times.
So, lower the noise of the first stage, and get a better result, overall.

Hum? DC heaters for the first 1-2 stages will give you the added advantage.
Humbucking pickups, forget single coil.
Effects pedals: noise city. Don't blame it on the amp. I don't use them, ever.

Thanks for your advice but i want to keep the amp stock! I had one of this amps in the past and it never had any squealing even with the gain at 10 and volume at 10. Something needs to be fixed. Can you take a look to C21 and C16 (Orange Drop Caps) .68mf 250V each? Replacing them will stop the squeal? THis amp has 20 years of age.

[jalexquijano]

Could you turn the gain up to 10, pull the treble boost, and raise the volume over 4 with no squeal at all in the Dual Showman Red? My twin starts squealing with this setting after volume 4. I have already tightened the tube sockets and have controlled the squeal at some point. I have also replaced the brand new 9 pin stereo input jacks with the former original input jacks made in japan that have the transparent plastic on the bottom and resoldered the points. All the filter caps are also brand new.

Where should i look then? Should i replace all the tube sockets specially V1, V7 and V2? What about the orange drop caps C21 and C16 which are 0.68 250V just beside the gain pot on channel 2? I have no oscilloscope and can only rely on your guidance.

Please see schematic:


http://www.blueguitar.org/new/schem/fender/the_twin.gif

[jalexquijano]

Guitician,

1- Gain is dialed at 10, Volume over 4 and Guitar volume at 0 just to prove that it is no the normal guitar feedback.

2- Gain up to 10 and presence up to 10?