more volume = less bass??

[yunger]

is it normal for bass to disappear the more you crank the volume? i've noticed this in my amp and wondered if it is power starved at high volume...or if the ot isn't passing low frequencies at that level...or possibly the speaker...or is this the nature of amps with no negative feedback.
i'd just like to know if this is normal or if i have another troubleshooting issue to tackle.

[d95err]

Difficult to know if it's normal unless you tell us what amp it is.

[Chuck H]

is it normal for bass to disappear the more you crank the volume? i've noticed this in my amp and wondered if it is power starved at high volume...or if the ot isn't passing low frequencies at that level...or possibly the speaker

Yes

or is this the nature of amps with no negative feedback.

No

i'd just like to know if this is normal

Yes

With your circumstances I'd say it's a capacity issue, not voicing. Having no NFB doesn't effect this much. But adding NFB may tighten and improve bass response to some degree at the expense of some gain and a tonal change. If your speakers are generously rated in watts for the amp your using it probably isn't the speakers. But it could still be partly due to the cabinet volume if the cabinet is small. If your PT is over rated for the amps current demands it probably isn't the PT. But if you have a lot of resistance in the B+ in front of the power tubes (ie: a rectifier tube or "sag" resistor) and/or small value filters, it may still be partly due to the power supply. Likewise with the OT. If it's under rated or has a small core it can saturate sooner than it should. This effect is most appearent in the bottom end where most of the amps current is required. So...

If you were to build an amp with a PT rated right at the current requirements, use a rectifier tube and small filters in the power supply, an OT rated as needed but with a small core stack and a speaker that breaks up as the amp does loaded in a small cabinet, The bass will be very poor under overdrive. Most guitar amps sacrifice bass in some or all of these areas to some degree. It's part of the sound of a guitar amp. Deciding where and how much defficiency to allow is the tricky part for the builder.

Chuck

[yunger]

nice answer chuck! good stuff. though my question was mostly theoretical, the amp i referenced is a 15 watter with fenderish preamp, voxish power amp, bridge rectified, blah, blah, blah. given your reply, i'd suspect i have inadequate transformers (came out of a pro junior). that is, if i'd ever play that loud....probably never will though, so all is good.

[d95err]

What's size (capacitance) are your filter caps? Increasing the filter capacitance, in particular the first two stages, could improve bass response.

Other things to try:

Decrease the resistor between the first two filter stages, or even better - use a choke.

[tubeswell]

Other things to try:

Or a bigger OT.

I think it has something to do with; Bass = lower frequencies/longer wavelengths.

Therefore (how's this for a crackpot theory?) each side of the AC signal swing is increasing the amount of time that 'varying DC' (or each half of the signal swing) is being inducted through the OT iron from the primary to the secondary. If you have more iron, you have more inductance and therefore less saturation. Therefore the longer wavelengths can make it through. (At least that's how I think the theory goes. Someone is bound to correct me for crackpot theorising if I'm out of line here)

[Steve Conner]

No, that's quite right. I can't think of any better explanation that doesn't involve hand-waving about calculus, integrals and so on.

It may or may not be normal for bass to disappear when you crank the volume, but it's a good idea. A small amp can't afford to waste power trying to make bass, it needs to focus its output into the midrange to make itself heard on stage.

It always seemed to me that guitar speakers in open-backed cabinets would just fart out with high levels of bass. You can compensate for the cabinet's lack of low end by turning the bass control up, but that only works at levels low enough that the power amp and speaker have headroom to sustain the bass boost. After that it all just turns into a mush.

[tubeswell]

No, that's quite right. I can't think of any better explanation that doesn't involve hand-waving about calculus, integrals and so on.

Yay! (right for once) I hate calculus. I knew how to do it once, but thankfully I've lost the knack. If you have a better intuitive explanation I'm all ears.

[yunger]

in my particular amp, when cranked, i don't necissarily hear mush as much as an empty shell of a bass note...if that makes any sense. kind of like opening your mouth to belt out a note and only air coming out. this perception may be completely due to a 10" speaker in a small, open cabinet. i do however plan on upgrading the ot to this, which i've been eyeballing for a while.

[olddawg]

is it normal for bass to disappear the more you crank the volume? i've noticed this in my amp and wondered if it is power starved at high volume...or if the ot isn't passing low frequencies at that level...or possibly the speaker...or is this the nature of amps with no negative feedback.
i'd just like to know if this is normal or if i have another troubleshooting issue to tackle.

How old are the filter caps? What type of rectifier?

[Chuck H]

this perception may be completely due to a 10" speaker in a small, open cabinet.

By your description I think you would do well to upgrade to a 12" speaker that is bass generous and a bigger cab. You don't need a closed cabinet, but deeper is good. The combination of a bass heavy speaker and a deeper cab would probably add that low end EQ your missing. I know it may be a pain if you need to upgrade the cab, but it may well be worth it. You could try the amp through some different speakers and cabs to find out for sure if it will help.

Oh, FWIW I'm always a fan of an upgraded OT. That underrated OT/synonymous with guitar amp thing doesn't fly with me personally.

Chuck

[Steve Conner]

Me neither. I think if a guitar amp had an underrated OT, it was either to save money, or in the case of a combo amp, to constrict the bass enough that it couldn't shred its speaker.

I once tried putting a huge OT into a Champ-style amp with one EL84. The OT came from an old tube audio oscillator (also with a single EL84 output) that was rated down to 3Hz! The previous OT was tiny, but this one was the same size as the PT and weighed about a pound. I had it lying around and I just had to try it to see what would happen.

As you might guess the result was a big boost in volume and bass, to the point where it seemed in real danger of wrecking the 8" speaker. If I played a low E, I could hear the cone bashing against the baffle board.

[olddawg]

Me neither. I think if a guitar amp had an underrated OT, it was either to save money, or in the case of a combo amp, to constrict the bass enough that it couldn't shred its speaker.

I once tried putting a huge OT into a Champ-style amp with one EL84. The OT came from an old tube audio oscillator (also with a single EL84 output) that was rated down to 3Hz! The previous OT was tiny, but this one was the same size as the PT and weighed about a pound. I had it lying around and I just had to try it to see what would happen.

As you might guess the result was a big boost in volume and bass, to the point where it seemed in real danger of wrecking the 8" speaker. If I played a low E, I could hear the cone bashing against the baffle board.

I have an 18 watt Marshall clone (EL84 X 2 PP) that has a huge old Zenith OT in it that is about 2 or 3 times the physical size of the usual spec OT. I run the amp through a 4x10 cab. People are always asking me why this amp is such a monster. I has excellent low end response, touch sensitivity, and is very harmonically rich. I believe the main reason is the big old OT. Runs cool too when dimed. Some botique builder is going to catch on to this and sell them for $2500.

[Chuck H]

Workin' on it

Chuck

[jrfrond]

Simply driving the amp to OT saturation and achieving power compression narrows the power-bandwidth, making the highs and lows seem less apparent. This is NOT a bad thing, and like Steve said, is actually a GOOD thing. I have actually designed and built amps with ganged volume pots that decrease bass response as you approach "10".

If it were a high-fidelity amp, you'd have lots of iron in the OT to minimize the chances of saturation and a multiple-stage PI/driver to deliver clean drive to the power tubes. It's a different animal, whereas reduction of power-bandwidth is a no-no. In overdriven guitar amps, you actually want and need this.