What is Transformer Saturation?

[R.G.]

I understand that the original Bassman amplifiers had bandwidth restrictions in the preamps that didn't let much if any bass fundamentals through to the power amp, not least because the speakers in an open cabinet count not deal with the power and excursions that low. Probably the OT couldn't either, and so the same solution worked for both.

I believe it about the ST70. Nice amps. I have a couple of ST35s that really sound great as well, and from the size of the OTs they probably have the same performance.

[g1]

The brain will fill in the fundamental anyway, probably as good or better than the real one.

[Malcolm Irving]

It is quite difficult for a tube amp to drive its OT into saturation. The transformer has a primary inductance (also called the magnetizing inductance) which is effectively in parallel across the reflected load. This inductance needs to be large to allow the OT to have good bass response, because the impedance of this shunting inductance reduces with frequency. (The 3dB point is where the magnetizing impedance equals the reflected load impedance, and for a guitar amp this should probably occur somewhere around 80Hz or so.)
Because the magnetizing impedance is a result of the primary winding acting on the iron-core, it is certainly subject to saturation. It is therefore a nonlinear inductance.
It’s interesting to note that the nonlinear magnetizing inductance is in shunt with the load and so the distorted current it demands from whatever is driving the OT, would not have any audible effect if the driving impedance was zero.
However, the output impedance of tubes is anything but zero. For pentodes particularly it is very high. But to saturate the OT we need a low frequency and a high voltage. The high output impedance of the power tubes means that as the frequency goes lower, the magnetizing impedance gets lower, so the voltage being supplied to the OT collapses (it drops across the output impedance of the tubes). The amp can certainly ‘wimp out’ but it’s not usually due to saturation, it’s due to the inability to properly drive the OT at low frequencies.
At this stage, the output tubes are easily pushed into clipping, leading to a weak distorted sound output – but that’s coming from the tubes, not necessarily from OT saturation.

[J M Fahey]

I just had in a Dynaco St70 amp.

What a beut.

In testing it, the output went clear down to 35 Hz!

Below that it got ugly.

What I find of interest is amps like the Fender Bassman.
They don't do well below 50Hz.
Which isn't even the low E.

And that is the least of your problems.

Sound must be reproduced through 4 wimpy 10" speakers , small coil small magnet **resonance 110Hz** mounted in an open back shallow enclosure ... I doubt it reproduces anything below 100 Hz ... and maybe I'm being generous.

[Malcolm Irving]

One of the best bass amps is the 300W (tube) Ampeg SVT with 8 x 10” speakers. Someone will correct me if I am wrong, but I think I read somewhere that the big 8 x 10 cab has partitions inside which make it almost like eight 1 x 10 cabs?

There is a ‘theory’ (not saying I necessarily agree with it) that it is better not to reproduce the fundamentals from a bass guitar for the following reasons:
1. The ear/brain ‘hears’ the fundamental anyway (as a kind of audio illusion) provided all the right harmonics are really there.
2. The fundamentals play havoc on stage with making everything rattle (especially the snare drum).
3. Really loud bass fundamentals are uncomfortable for the audience (makes your stomach vibrate etc.!)

Not sure if I agree with all that, but it’s an interesting theory.

[Leo_Gnardo]

One of the best bass amps is the 300W (tube) Ampeg SVT with 8 x 10” speakers. Someone will correct me if I am wrong, but I think I read somewhere that the big 8 x 10 cab has partitions inside which make it almost like eight 1 x 10 cabs?

There is a ‘theory’ (not saying I necessarily agree with it) that it is better not to reproduce the fundamentals from a bass guitar for the following reasons:
1. The ear/brain ‘hears’ the fundamental anyway (as a kind of audio illusion) provided all the right harmonics are really there.
2. The fundamentals play havoc on stage with making everything rattle (especially the snare drum).
3. Really loud bass fundamentals are uncomfortable for the audience (makes your stomach vibrate etc.!)

Not sure if I agree with all that, but it’s an interesting theory.

There is something to #1. Fake bass can 'give the illusion.'

#2, EVERYTHING makes snares rattle.

#3, that's where the fun is! You can get your eyeballs to lose focus with a nice 50 Hz signal. All part of Rock n Roll.

And the SVT cab was four 2x10's, solid baltic birch ply. Now they're made of lightweight lauan in the Far east, more suited for an 8-way community outhouse seat.

[Gregg]

Usually you don't want to hear the fundamental of the bass guitar and those Ampeg cabinets have a resonance at least twice above it. Also most bass amps have it attenuated to some degree. 30Hz on stage is not something pleasant.

[Mike Sulzer]

I suppose one man's illusion is another's clever analysis. Determining the frequency of a note by measuring the frequency differences between harmonics is extremely robust and fast, more so than measuring the frequency of the fundamental of a bass note.

Using 10" speakers for bass seems crazy, but on the other hand 8 tens will do a good job of directing upper bass straight ahead and not produce much of anything else. The bass really only needed to be a lot better than produced by a transistor radio to sound impressive.

There is something to #1. Fake bass can 'give the illusion.'

#2, EVERYTHING makes snares rattle.

#3, that's where the fun is! You can get your eyeballs to lose focus with a nice 50 Hz signal. All part of Rock n Roll.

And the SVT cab was four 2x10's, solid baltic birch ply. Now they're made of lightweight lauan in the Far east, more suited for an 8-way community outhouse seat.

[Raybob]

What I find of interest is amps like the Fender Bassman.
They don't do well below 50Hz.
Which isn't even the low E.

FYI: Been playing P copy bass, fairly large room on Saturdays, using a 69 Bandmaster Reverb head with Bassman OT, running through 2x12 closed back cab. I tune the bass 1 step low, for that low D & Eb. I haven't seen wave on scope, but I don't hear any problems or notice any volume variations with freq range of my bass, with basic Fender circuit, very close to Bassman circuit. VTB set 7,10,7~8. Had 64 Bassman before... tones were same as BandmasterReverb normal channel.

[R.G.]

I haven't seen wave on scope

[Raybob]

Quick hypothetical question: If anyone in an official capacity suspected that YOU had numerous emails containing top secret information on YOUR computer, what is your best estimate of how many minutes you would remain outside a jail cell?

A better question is, what is your best estimate of how many minutes you would remain at all?