More Preamp Headroom?

[camarofreak]

Finally got to looking at the data sheet for 12ax7s just to add some info to my overcrowded cranium. I understand that headroom is limited by a whole lot of factors (preamp, P.I., power amp design/wattage). The data sheet had a bunch of value lists and one of them had a 400 volt b+ with a total distortion of 1.1%. Am I correct assuming that at v1 this would result in less preamp dirt? And could with those values could v1a be cascaded into v1b via a switch for a "dirty channel/gain boost"type of thing? Any insights would be appreciated folks.

[tubeswell]

Generally speaking, the higher you can get the High Tension voltage for the pre-amp supply, the more headroom you will have. (This means smaller value supply resistors and any way you can of getting higher B+ supply)


After that it comes down to operating point (bias and load) of the gain stage. Setting up the stage so that its centre-biased, with a load such that the stage idles with a plate sitting at about 2/3 of the HT voltage (and having the HT voltage as high as you can get it), will generally make the stage as clean as you can get it.


After that it its sort of a combination of things. I would rank them like this: (YMMV)


1 cleanness of the input signal and the p-p voltage of the input signal w.r.t. the bias of the stage.


2 having the CC cap feeding the stage as small as you can whilst retaining an appropriate balance of the entire bandwidth of frequency spectrum of the instrument you are playing, whilst having the input impedance of the stage as high as you can to preserve a good S/N ratio.


3 the output impedance of the stage relative to the impedance bridge of whatever stage follows it (you generally want the output impedance to be a maximum of about one-fifth of the input impedance of the following stage to preserve a decent amount of bandwidth)


4 how much low-end flab you can get out of the signal by carefully choosing the Ck and the CC values. (Fully bypassing V1 gives the best S:N ratio for the whole amp, but you then might want to decrease the CC after V1 to cut out low-end flab, because you can always accentuate low end in later stages).


Only peeps may have other suggestions

[VMS]

Hi,

I found this slo100 preamp analysis to be very helpful when designing my own circuits:

http://www.ampbooks.com/home/classic-circuits/soldano-slo-preamp-1/

Ampbooks site has also other circuits where the input sensitivity is calculated.

[HotBluePlates]

... 400 volt b+ with a total distortion of 1.1%. Am I correct assuming that at v1 this would result in less preamp dirt?

When you play through a professionally-designed and commercially-produced amp, the first thing to distort will be the output tubes. To do otherwise would educe the amp's clean output power and make the amp seem "weaker".

The exception to this is a master volume amp, in which case when the master volume is set low, the gain stage immediately before the master volume is the first to distort.

The 1st gain stage will almost never distort noticeably unless you're playing a CD player through a guitar amp (which has an average signal level 10 times as big). Unless you cannot get a clean signal for any setting of amp volume controls and have to resort to turning down your signal at the guitar/pedals, then distortion in V1 is likely not your issue.

So then a general rule for a clean preamp might be to make sure successive gain stages can handle bigger and bigger input signals, that way as the signal is amplified through the circuit, no gain stage distorts. The cleanest tube amp I've heard (a Standel 25L15) uses a 12AX7-12AT7-12AU7 preamp, in part because each successive gain stage can handle a bigger input signal before overloading so the signal stays clean throughout the amp.

400 volt b+ with a total distortion of 1.1%. Am I correct assuming that at v1 this would result in less preamp dirt?

Back to your original question. A higher supply voltage could allow the stage to make a bigger clean output signal. But the tube has to be biased appropriately; if you took a tube at any supply voltage and biased it at saturation or cutoff, it would be impossible to get anything other than a distorted output signal.

Desired input & output signal level, desired function and tube type are also factors that would impact how you would set up a gain stage.

You should know now that amp designers do not typically design from input stage towads the output; they start at the output stage and work backward towards the input, given a knowledge of what characteristics are needed from the output stage and the characteristics of the input signal going into the front of the amp.

... could v1a be cascaded into v1b via a switch for a "dirty channel/gain boost"type of thing?

Yes. But with no other work, it would sound the same as simply turning the volume control from 5 to 10. In other words, you'd get a lot more volume along with a lot more distortion, unless everything after these stages have already run out of steam.

What I mean to say is there is more to consider. If you point to a specific amp or schematic you're interested in analyzing/tinkering, then it would be easier to show how this stuff works.

[jjasilli]

Finally got to looking at the data sheet for 12ax7s just to add some info to my overcrowded cranium. I understand that headroom is limited by a whole lot of factors (preamp, P.I., power amp design/wattage). The data sheet had a bunch of value lists and one of them had a 400 volt b+ with a total distortion of 1.1%. Am I correct assuming that at v1 this would result in less preamp dirt? And could with those values could v1a be cascaded into v1b via a switch for a "dirty channel/gain boost"type of thing? Any insights would be appreciated folks.

A 12ax7 has a mu of 100 and, in typical operation, gain of about 60.  200 mV in : 12 V out. (typical 1st stage with single coil PU's).  1 volt of signal in : 60 volts out.  That's plenty of clean headroom for every gain stage.  Clean headroom is easy to achieve and is assumed in the tube charts.


"Dirty" is an ambiguous term.  It could mean: harmonic distortion = warped waveform; clipping = top (or bottom) of waveform chopped off; crossover distortion; saturation / compression, etc. These things have been designed-out of the tube charts.