Help a newbie understand

[Tahoebrian5]

I'm in the process of studying circuits and getting up to speed on amp craft in general. So far I've read valve amplifiers by Morgan jones which was very detailed but a bit over my head, also read most of what is available n Merlin's website and plan to buy his book soon. I've built a ceriatone dumble clone and have a trinity 18w plexi clone ordered.

Anyway my dumb question..

I understand pretty well how the triode works in standard configuration and studying the anode characteristics chart for ecc83's it is apparent that you can input a grid signal of a few volts peak to peak and get a 100 volts or more peak to peak out. Ok now on to the next stage. You have a signal that is a hundred volts peak, I've not seen anywhere that shows how the grid of a second gain stage deals with such a high signal. All the charts I've seen show maybe 5 volts tops for a grid signal. What am I missing here?

[jazbo8]

The voltage level you use are perhaps 10x higher than usual, but anyway still way more than what a typical second stage can handle - therein lies the magic of tube guitar amps - the excess voltage is clipped, that's how distortion happens and sweet harmonic tones are made.

[big_teee]

If I understand your question?
How about coupling capacitors.
The signal of stage one is riding the Plate voltage.
The coupling cap only allows the signal to pass, It blocks the high DC voltage.
T

[R.G.]

The answers you have are right. But there are big drops in signal at the controls and mixing reverb.

Just to kibitz a bit: The input signal to an amp is about 100mV peak, 200mV peak to peak, from most guitars. A 12AX7 stage generally has a gain of 30 or less (this is a very crude rule of thumb). So the first gain stage gets it from 200mV to 6V pk/pk maybe. Then it hits tone and volume controls, which often drop it by 10:1 to 100:1, so you are back down in the 100mV ranges, and have to amplify again. Feeding a reverb needs amplification even higher, and the voltage back from the reverb is again back down in the 100mV range. So you have to amplify it back up.

Output stages need something like 100-180V pk/pk, so the phase inverter has to amplify the signal after the reverb mix again.

Beyond that, more stages let you go for tube distortion.

[Tahoebrian5]

Thanks RG, that helps quite a bit. So is there a rule of thumb on a max signal typically applied to a grid?

[eschertron]

From where I sit, you've pretty well nailed the basics of reading the tube characteristics chart. In general terms the voltage peak-peak that can be impressed on the grid before clipping is called the "input headroom" and the available output swing, along the load line, is called (not surprisingly) the "output headroom".

By controlling the load line and the grid bias point, the designer can massage the signal going through the gain stage to achieve his or her goals, in terms of amplification factor as well as amount of distortion (compression and clipping).

If there's a thumbrule for input signal, it would depend heavily on the type of distortion desired and the personal taste of the designer. However, there seems to be a consensus that more than about 6-9 dB of clipping is 'too much' if it is desired that the character of the guitar's tone come through. 6 to 9 dB of signal above the input headroom is a signal of 2 - 3 times the headroom voltage. More than that and the signal goes from sine wave to square wave without any subtlety.

Having said all that, careful study of existing designs can reveal how the designers 'broke the rules' to get the sound they wanted. A good one to look at might be the Marshall 2204 preamp, where a cold-clipping stage is used to get the notable distortion sound.

As R.G. said, the circuits between stages can reduce the signal level a lot, so that most amp designs won't get the signal to a clipping level before the third stage or so (without a pedal in front, anyway!). I've used the Duncan Tone Stack Calculator to predict gain reductions between stages, pairing those results with the gain increases calculated from drawing out tube characteristic lines; that's what I do to analyze what's going on in a particular amp design. Some schematics will include typical signal levels at the test points on the amp. This also is a good way (read: easier way) to come up with the gain changes for the signal through an amp.

[Tahoebrian5]

Thanks for the replies, sometimes all it takes is a bit of a nudge in the right direction and everything comes together. I could really use some help in my thread in the mods sub forum regarding my Mesa amp if anyone cares to take a look

[Tahoebrian5]

Also thought I would mention I went ahead and ordered Merlin's book. After reading the stuff on his website I was very impressed with his presentation of the concepts in a interesting way that is not too hard to grasp. I have read some of the sections numerous times and its starting to stick. I'm still having a hard time grasping the concept of impedance matching between stages but I'm sure it will click at some point. Good times!