Elementary questions on gain.

[tonequester]

Greetings to ALL.

I hope that it's o.k. to ask some pretty basic questions here. Apologies if I am in the wrong place.
First, is the numeric assigned to a pre-amp tube viewed in this manner : "a tube with a gain of 100 is twice the gain of a tube that has a gain of 50 ?
Second : Does a single 12ax7 produce enough gain to overdrive an "average" 6V6, or does the 6V6 have a maximum signal input Voltage ?
Third : If one is not using high output pickups, can a gain control(generally) be turned up enough to overdrive a power tube, WITHOUT causing pre-amp distortion ! I truly appreciate ANY attempts to instruct me here, and the time involved in doing so !

tonequester

[d95err]

1. The "gain" value you're talking about is basically the theoretical max gain of the tube. The design of the gain stage will limit the actual gain. I.e. substituting for a higher/lower gain tube does not necessarily mean higher/lower actual gain.

2. It would depend on circuit design and and input signal level. For a typical guitar pickup signal, you would probably need to use both triodes in the 12AX7 cascaded to overdrive the power tube properly.

3. It all depends on circuit design. Old non-master volume amps will typically overdrive the power tubes first, then the phase inverter and then the preamp stages, as the volume is turned up.

[Stevem]

You also need to conceder how good the basic tube is itself even within a basic type of tube itself.
For example if one 12ax7 is putting out more current drive than another one and the output of the tube is feeding a tone stack and not just a simple treble roll of type of tone control, than the tube with the better current drive will push a good amount more signal thru and out the tone stack and the controls themselves will have a bit wider control range.

[matt h]

(deleted)

[gui_tarzan]

All of which begs the question - where in the signal path does the Fender "chime" or Marshall "bite" come from?

[matt h]

(deleted)

[LeftyStrat]

Too add to what Matt said:

When I first learned of the Trainwreck Express, what stood out to me the most was the un-bypassed cold-biased third stage. As an experiment I replaced the second stage 820R/.68uf in my Marshall jtm45/50ish clone with a 10k resistor. It resulted in an increase in sustain, with a complete loss of the Marshall crunch.

Grab a copy of Duncan's Tone Stack calculator, click 'Snapshot' while looking at the Fender tone stack, and the switch to the Marshall tone stack. The Fender has greater losses, and a bigger dip in the midrange (as Matt describes). Snapshot the Marshall tone stack and replace the 33k slope resistor with 56k (Bassman and early Marshalls). The cathode follower certainly contributes, as the Fender Bassman doesn't really sound like the later BF Fenders.

NOTE: If you are on a Mac, you can use Wine to run Duncan's TSC. It's a great tool for visualization, but somewhat imprecise.

[JazzGuitarGimp]

you can use Wine to run Duncan's TSC.

Primarily, I use Wine to turn off my left-brain when playing in public!

[EtherealWidow]

Lefty, that's super interesting. I always wondered what that stage did. Although it's super cold biased, it takes a LOT to overdrive the stage because the cathode resistor is huge and unbypassed so there's tons of internal feedback making the output impedance huge. The gain of that stage is only 8. It's really cool to find out what that stage does though in terms of tone.

[pdf64]

In most cases, what people are claiming to be power tube distortion is actually PI distortion (typically in an LTPI arrangment. By the time most power tubes are distorting (thd rather than ear figure, which is much earlier), the earlier stages are audibly distorting to such a degree that the power tube's contribution is minor

Matt, my findings differ (or at least my interpretation of them).
Would you mind outlining your position, or directing to where this has been (largely) resolved previously?
Pete

[matt h]

(deleted)

[pdf64]

Thanks Matt, I'll get my head around that and also look into the thread in the trainwreck forum mentioned.
Pete

[xtian]

The long thread in the Trainwreck forum went into this in great detail with stage-by-stage analysis.

I'm interested to read. Can you point it out?

[vibratoking]

http://ampgarage.com/forum/viewtopic.php?t=15694

[pdf64]

Thanks!
No kidding about it being a long thread.
Pete