Tweaking Fender Amp AB763

[Jake1524]

I have 3 AB763 fender amps and they all exhibit the same qualities.

There is a sizzly top end that I cannot stand. The harder I strum my guitar, the more apparent this is. Is there a way to dial this out? I would much prefer a much rounder, less fizzy high end, while still staying clean.

I have built a couple amps and restored a handful of old fenders, but my knowledge is limited. There are so many different mods out there i'm not sure where to even begin.

What do you guys think? What tweaks and mods are worthwhile?

[sluckey]

The easiest thing to do is back off the treble pot a bit and turn the bright switch off. Or load the cab with some Celestion speakers. If you want to go inside the amp, put a 100pF cap across each 100K plate load resistor for V1 (normal channel) and/or V2 (Vib channel). Put another 100pF between the PI plates (V6 pins 1 and 6). That should take care of most sizzle. If too dull, use 47pF rather than 100pF.

Or just be patient. Your ears will eventually mellow out.

[Jake1524]

The easiest thing to do is back off the treble pot a bit and turn the bright switch off. Or load the cab with some Celestion speakers. If you want to go inside the amp, put a 100pF cap across each 100K plate load resistor for V1 (normal channel) and/or V2 (Vib channel). Put another 100pF between the PI plates (V6 pins 1 and 6). That should take care of most sizzle. If too dull, use 47pF rather than 100pF.

I have at least 10 different speakers. Some speakers are warmer, but they doesnt seem to change the way the amp behaves. I actually prefer the percussive nature of these amps with the treble up a bit.
I am really pushing the amp to be clean, clear, round and sizzle-less(that thats a word).


What exactly am I accomplishing by adding a cap across the plate load resistor or PI plates?

[sluckey]

What exactly am I accomplishing by adding a cap across the plate load resistor or PI plates?

You are cancelling out some of the high end sizzle.

[Jake1524]

What exactly am I accomplishing by adding a cap across the plate load resistor or PI plates?

You are cancelling out some of the high end sizzle.

Sluckey, thanks for the info. I understand what it would be doing, im more tying to wrap my head around whats happening on a functional level

[sluckey]

I thought I was talking on a functional level. Most of the Marshall amps use a 47pF cap across the PI plates. Some actually place the cap after the coupling caps but the effect is the same.

[Jake1524]

I thought I was talking on a functional level. Most of the Marshall amps use a 47pF cap across the PI plates. Some actually place the cap after the coupling caps but the effect is the same.

Apologies, I was not using the proper verbiage. I should have stated on a 'electrical/mechanical' level.

I understand the effect of the mod, but im still trying to wrap my head around why it does what it does.

[Stevem]

Another very easy thing to try is on the first gain stage.

Install a resistor before the 100K plate load resistor to drop the plate voltage at the tube down by 20 to 25 volts .

You may then need to bring the cathode resistor down to 1200k , but I find that needing such is in responce to how strong a 12AX7 you have in there.

Speaking of tubes you might want to try a 12AY7 in that socket even before trying dropping the B+ down .

[Jake1524]

Another very easy thing to try is on the first gain stage.

Install a resistor before the 100K plate load resistor to drop the plate voltage at the tube down by 20 to 25 volts .

You may then need to bring the cathode resistor down to 1200k , but I find that needing such is in responce to how strong a 12AX7 you have in there.

Speaking of tubes you might want to try a 12AY7 in that socket even before trying dropping the B+ down .

Hey Steven, thanks for the info.

Bear with me as I’m learning, but wouldn’t lowering b+ lower the headroom in the preamp? I’m afraid that would make the issue worse as the sizzle almost seems like it is a clipping thing (could be totally wrong).

[romberg]

I have 3 AB763 fender amps and they all exhibit the same qualities.

There is a sizzly top end that I cannot stand. The harder I strum my guitar, the more apparent this is. Is there a way to dial this out? I would much prefer a much rounder, less fizzy high end, while still staying clean.

Do these amps have reverb? If so, try pulling the reverb driver tube and see if the fizz goes away.

Mike

[TUBEDUDE]

I thought I was talking on a functional level. Most of the Marshall amps use a 47pF cap across the PI plates. Some actually place the cap after the coupling caps but the effect is the same.

Apologies, I was not using the proper verbiage. I should have stated on a 'electrical/mechanical' level.

I understand the effect of the mod, but im still trying to wrap my head around why it does what it does.

The signal at the P.I. plates are 180° out of phase with each other. So connecting a small value cap across them will have them cancel each other out. The value of the cap determines the corner frequency and frequencies above that that will cancel. And if used with a pot, the pot will determine how much the high frequencies are attenuated.

[pdf64]

I have 3 AB763 fender amps and they all exhibit the same qualities.

There is a sizzly top end that I cannot stand…

The query regarding bright switches hasn’t been addressed. Some models have bright caps fitted with no switch.
So, exactly which amp models? There are various differences between various models.
Which channel/s are you using? Depending on the particular model, there’s various differences between the channels.
Do the amps have any mods?
Have the amps had a safety earth connection to their chassis and 3 prong cable fitted by a competent person?
Have the amps been serviced by a competent person recently, eg within the past couple of years?

[Jake1524]

I have 3 AB763 fender amps and they all exhibit the same qualities.

There is a sizzly top end that I cannot stand…

Exactly which amp models? There are various differences between various models.
Which channel/s are you using? Depending on the particular model, there’s various differences between the channels.
Do the amps have any mods?
Have the amps had a safety earth connection to their chassis and 3 prong cable fitted by a competent person?
Have the amps been serviced by a competent person recently, eg within the past couple of years?

Hey! I have two bandmasters and a twin reverb. All 3 AB763. All the same, all serviced recapped etc.

The sizzle im talking about seems to be inherent in these amps. I hear it in my amps as well as most demos you hear. You dont hear it with two rock, dumble clones etc, but i hear it in all fenders. I love almost everything about these amps, but would really like to find a way to correct this.

[sluckey]

The sizzle im talking about seems to be inherent in these amps. I hear it in my amps as well as most demos you hear. You dont hear it with two rock, dumble clones etc, but i hear it in all fenders. I love almost everything about these amps, but would really like to find a way to correct this.

Have you actually tried any of the simple options I offered?

[Jake1524]

The sizzle im talking about seems to be inherent in these amps. I hear it in my amps as well as most demos you hear. You dont hear it with two rock, dumble clones etc, but i hear it in all fenders. I love almost everything about these amps, but would really like to find a way to correct this.

Have you actually tried any of the simple options I offered?

I have been very busy with work and have not had even a moment to work on the amp. I was planning on giving it a shot this weekend.....