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Date: Sat, 1 Jun 2002 12:59:36 -0700
From: "IronMan Mike Curtis"
Subject: Re: Preamp tube subs revisited (was: Bassman '59 RI)

Keep in mind that these are DUAL triodes, and the gain factor in
preamps using both halves will be affected by the square of change in
the amplification factor for each tube.

For example, a 12AX7 has a mu of 100, which equals a total gain of
10,000. Swapping to a tube with a mu of 20 (which I believe is the
12AU7) results in a total gain for the two stages of 400.

Changing preamp tubes will have little effect on distortion in the
finals. If you deliver an X volts RMS signal to the grids of the PA,
it really doesn't matter how you've arrived at this - the PA will do
exactly the same thing with it because it's exactly the same. Any
distortion increase must necessarily be in the preamp. The only
exception would be if the signal become assymetrical. The most you
could pick up on this would be a hardly significant -3 dB, if you
completely eliminated one half of the waveform.

On 1 Jun 2002 at 13:53, Howard Young wrote:

>
>
> ----- Original Message -----
> From:
>
> >>
> > Something just doesn't jibe here. Just because the volume knob is
> > set to a higher number doesn't mean the power tubes are receiving
> > more input signal strength. the amount they receive, and thus the
> > degree to which they are saturated, depends both on that setting and
> > on the amount of signal outputted from the preamp stage. That
> > amount will depend on the gain of the preamp tubes.
> >
> > There has to be some other explanation for why some lower-gain
> > preamp tubes are reported to increase loudness before feedback. My
> > favorite one is that the player has better volume-knob control in
> > the critical, pre-feedback range. Another explanation might be the
> > one proposed by fjm, that different preamp tube types and makes
> > accentuate different frequencies preferentially.
> >
> > John Thaden
>
> As I have learned and experienced through hours of playing with tube
> swaps on 4 different amps, the preamp tubes will effect the output
> volume at which final-power-tube overdrive is achieved. So will the
> microphone, depending on how hot it is. It has to do with the
> amplitude of the signal that reaches the final power tubes. The
> hotter, or higher, the combined gain of the input signal to the
> finals, the louder that signal is going to be when run through the
> amplification factor of the finals. Start with a lower input gain
> signal and you will get lower output volume at any given point on the
> drive curve of the final amplifier(s). And, with a lower input signal
> the finals will reach a state of overdrive at a lower output volume.
> It's not just the setting on the volume control that changes, but the
> level of final output volume.
>
> In the real world on one of my amps in particular, with 12ax7 tubes in
> the preamp stages, I can not obtain final tube overdrive before
> feedback threshold. The amp is very loud and clean at "3" on the
> volume control and can't be turned up any further before feedback sets
> in. It's not a bad sound per se, just loud, clean, crisp. No preamp
> breakup and no final breakup.
>
> Subing in 12ay7 tubes drops the overall preamp gain factor by
> approximately 60%. Now the amp can be turned up to about a "6" on the
> volume control before feedback is achieved. And, the finals have to
> work harder to get the 60% lower input signal up to that feedback
> level. Overdrive breakup now begins to occur at around a "5" setting,
> prior to the feedback -- and the amp is still not as loud as it was
> set at "3" when using the 12ax7's. The feedback does begin to occur
> at about the same output volume, but the tube breakup now sets in at a
> lower volume -- before feedback. I'm sure there is also preamp
> breakup occuring.
>
> Using rediculously low gain 12au7's (80% less gain than 12ax7's) drops
> the input signal to the finals so low that I can turn the amp all the
> way up to a "12" without feedback. The finals tubes do overdrive, and
> so do the preamps, because they are working at their maximum levels to
> put out the signal, but the overall volume level is low and I cannot
> get feedback unless I put the mic right in front of the speaker. But
> I get lots and lots of overdrive. Very dirty, overdriven, low-volume
> output. Not all that great sounding, I might add.
>
> Actually, on this particular amp, my favorite setup is a 12ay7 in the
> first preamp stage and a 12au7 in the second preamp (inverter) stage.
> This allows me to turn the amp up to around a "7" to "9" range on the
> volume and hit a real sweet spot in the balance of final tube and
> preamp tube overdrive breakup FOR THIS AMP. If I want a cleaner
> output signal with less "dirt" and more punch, I run two 12ay7's. If
> I really want more volume without feedback, I line out or mic to the
> p.a.
>
> Every amps a little different and every set up varies somewhat
> according to unknown variables and the taste of the the player. So
> experiment and find your groove. Just my 2 cents worth, YMMV, etc.,
> etc.
>
> Regards,
>
> Howard
>
>
>
>

- -IronMan Mike Curtis - harmonica, guitar, organ bass, vocals
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