My Ab763 Has Gas

[pdf64]

'some will be attenuated 5db others more
the lower the frequency the more attenuation'
I don't see it like that; it's a shelving filter, so with the regular 100k plate / 1k5 cathode resistors, 5dB is the most attenuation there is.
Just to point out that the potential for freqency shaping from varying cathode bypass values is limited.
Reducing the value of coupling caps is the way to make significant changes to low frequencies.
Pete.

[soundguruman]

What's the best way to reduce the lows that make the speaker fart without reducing the "depth" of the sound? I don't want to lose audible lows, but I want them very tight and no speaker farts. Already changed the last two .1's that feed the output tubes with .047's. It's better but I still want it a little tighter.

The last thing that deserves mentioning is the BIAS supply filter capacitor. If the value of the cap is increased, the amp will have more bass response.
If the value of the cap is decreased the amp will have LESS bass response.
AND this capacitor has quite a bit of affect over the amp "farting" in low frequencies, when the output tubes are over-driven.
If the cap is "too large" in value, the output tends to go farting into DC.
Very seldom mentioned, but important!

[olddawg]

Sealed cabinet? Different speakers.

[ajeffcote]

The last thing that deserves mentioning is the BIAS supply filter capacitor. If the value of the cap is increased, the amp will have more bass response.
If the value of the cap is decreased the amp will have LESS bass response.
AND this capacitor has quite a bit of affect over the amp "farting" in low frequencies, when the output tubes are over-driven.
If the cap is "too large" in value, the output tends to go farting into DC.
Very seldom mentioned, but important!

I didn't think of the bias supply filter. I installed a larger cap, 100uf if I remember correctly. Think Fender used a 50uf. I will try swapping it out. Thanks.

[soundguruman]

I didn't think of the bias supply filter. I installed a larger cap, 100uf if I remember correctly. Think Fender used a 50uf. I will try swapping it out. Thanks.

I think the highest is 70.
50 is middle of the road "conservative" fender kind of bass response...
I thought about your original question for a while, and finally realized the bias cap.
Although, we never really think about the bias filter affecting the tone of the amp, it really does a lot.

[Chuck H]

Although, we never really think about the bias filter affecting the tone of the amp, it really does a lot.

I never think of it.

[Chuck H]

...Well?

[pdf64]

My view is that for something to be regarded as significant (eg the bias cap value affects the tone of an amp a lot), then it has to be measureable somehow.
My guess is that the bias cap of an AB763 could be changed form 4u7F to 470uF without affecting any output characteristic of the amp >1dB.
Is there a mechanism proposed by which the bias cap affects bass response?
Pete.

[Rick Erickson]

Pdf - I was thinking the same thing as I read this thread. I have never heard a bias supply cap affect tone. I have heard a bad one introduce hum into the output stage, but it is not a tone shaping capacitor. Even if it was, it would stand to reason that a larger value would pull bass response out of the signal since it is referencing the 220K feed resistors to ground through the bias cap. I typically use 100uF/100V here. Only two ways I can think of to reduce "farting". Either turn down the Bass or the Volume. The relationship of power supply filter/bypass caps to tone should be noted however. A larger supply bypass cap in the power supply will increase low freq response. For that reason, one should be cautious when replacing the preamp filters not to increase their stock value by much, unless you compensate by reducing the preamp bass response, via coupling cap values or cathode bypass caps.

RE

[Tom Phillips]

...The relationship of power supply filter/bypass caps to tone should be noted however. A larger supply bypass cap in the power supply will increase low freq response. For that reason, one should be cautious when replacing the preamp filters not to increase their stock value by much, unless you compensate by reducing the preamp bass response, via coupling cap values or cathode bypass caps. RE

Rick,
Every change affects something but I don't understand how increasing the value of a class A preamp's power supply node bypass cap, assuming that the amp was designed properly to start with, will increase the bass response. Can you elaborate about the origin of this information?

Regards,
Tom

[Rick Erickson]

Tom - I had a great response written to this yesterday. but because I forgot to login first - it disappeared.
Guitar amps are intentionally designed with limited frequency response. Feed an electric guitar into a flat response full spectrum amp and it won't sound very good, at least it won't sound like we expect a guitar to sound. This was explained in the original Alembic Bass Preamp manual published in the mid or late 70's. Before that, Jack Darr published "The Electric Guitar Amplifier Handbook" which explained the interaction of the power supply and the various amplifier sections. As such - you can design a guitar amp around a power supply with large filter & bypass caps, but you will need to limit bass response, usually through low value coupling caps. (.001-.022uF) Fender took the opposite approach. His amps had large value coupling caps (.047-.1uF) and small supply bypass caps. (16-20uF). If you increase the value of the bypass caps in the preamp supply without decreasing the coupling cap(s) you will usually end up with a very "flubby" (sorry for the technical jargon) sounding amp. The bias supply cap(s) in question do not function the same way as the supply bypass caps. They provide a path to ground for the signal on it's way to the power tubes, via the bias feed resistors - typically 100K-220K. In this configuration a very small cap size will let more bass signal pass to the power tubes. Because of the high resistance of these resistors it doesn't take much capacitance to ensure a low impedance path for any audible frequency. Anything over 2uF should be plenty. More capacitance might make for less ripple on the bias voltage, but it will not change the frequency response. This is why bias cap values are typically between 10-50uF. You can go larger - I typically use 100uF/100V caps in a Fender amp, but this is more for the physical size of the cap than anything. Too large of a bias cap can be a bad thing. If it takes too long to charge then there might not be full bias voltage on the tubes when the standby switch is engaged. Just one of many factors to consider when designing, or changing an amp.

RE

[Bruce / Mission Amps]

... Too large of a bias cap can be a bad thing. If it takes too long to charge then there might not be full bias voltage on the tubes when the standby switch is engaged. Just one of many factors to consider when designing, or changing an amp.

RE

Unless the standby switch is in the high voltage center tap, I think all the bias voltage is developed before the standby switch is thrown on anyhow....
It might be something to mildly consider but I'm thinking only if a or the "current limiting resistor" feeding the diode is a very large resistive value and thus a long time constant is developed.
But in reality.... I think this is possibly just a red herring.

[pdf64]

'If you increase the value of the bypass caps in the preamp supply without decreasing the coupling cap(s) you will usually end up with a very "flubby" (sorry for the technical jargon) sounding amp'
Rick, thanks for your thoughts on that, I hadn't considered that possible design rationale when analysing the 'classics' previously.
Is it something that can be measured? Probably not by basic scope / sig gen, as it may be a dynamic response type of thing.
Have you derived a design 'rule of thumb' for balancing B+ bypass time constants to coupling circuit -3dB frequencies?

' Fender ..amps had large value coupling caps (.047-.1uF) and small supply bypass caps. (16-20uF). '
I guess if that approach is taken too far, positive feedback via the power supply could start to take effect (especially as electrolytics age and develop increased ESR), resulting in some strange sub sonic rumblings; just as well for open back cabs.

One point that comes to mind is that rather than the actual value of the caps having significance per se, it's the impedance in the circuit around them (and the resultant time constants / -3dB points) that create the potentially significant system elements.
eg a 12AX7 plate feeding a 50k (trem intensity) pot via a 0.1uF cap has a reasonable -3dB point of 18Hz, whereas a 12AX7 plate feeding a 3M3 mixing resistor via a 0.022uF cap is -3dB at 2Hz, which is getting close to the -3dB at 1Hz of it's B+ supply (16uF fed by 10k for a BF DR). The analysis is of course incomplete and has ample room for improvement but I think I've got the key stuff.
I guess it could be better analysed using control theory / poles and zeros, but that was a tough module and a long time ago.
Pete

[wizard333]

All those things reduce low end.

When you turn a vintage Fender up, the bass needs to be reduced. Simple way that has worked for decades: Turn the bass knob down as the volume goes up. Ta-da! When volume is way up there, the bass control needs to be at 0 or nearly 0. Big, tight, Marshall-esque sound.

Want to get trickier? All these things reduce bass. If you are saying "well I want to reduce flub but not reduce bass" sorry, you're SOL. That is an oxymoron.

Some other things: You could wire a dual pot at the volume so as volume goes up, you run your tone stack output through a passive high pass filter at the same time. Use a dual 1M pot, wire side 2 so that as the volume goes up, your signal is passed through a very small cap across that resistance, which increases as your volume goes up. You'll probably have to experiment with cap values. Also, the pot taper may not roll off bass at the rate necessary, but it would help.

Cathode bypass caps can be dropped to 4.7uf from 22uf without much perception of thinning up the sound. You'll lose some frequencies below almost all the fundamentals on guitar, but meh. The sound will tighten up but still be plenty full. Not enough to crank the volume without still turning down the bass, but it will help. I like non polar electrolytics. You can get 4.7uf non polars from mouser or even your local Radio Shack. I do that on some amps that *Big amp company* puts 47uf Cks in, because *big amp company* wants to impress little kids at Guitar Center volume levels with how much bass there is, but doesn't care that the amp gets massively farty at gig or rehearsal levels.

1uf was suggested above, but at that point you are noticeably pushing high frequencies more than bass, and the amp will thin up at any volume level.

As noted elsewhere, you can reduce the value of coupling caps. The input cap to the PI is the place to start. Socket it, try various cap values and see what works.