Hi all,
I am confused by valves. In fact, I am confused by most things guitar and amp related, other than just playing the bloody things. Anyway...
I have an Engl Gigmaster 30 combo and a Laney LC15R. The former uses one 12AX7 in the pre-amp stage and the latter uses three. What throws me there is that the Laney despite being half the wattage uses three times as many valves. Why? What does all this mean? From what I can gather, the phase inverter in the Laney is the third 12AX7 but in the Engl it's the first EL84.
Can anybody help me understand the differences and what effect it might have changing different valves?
Thanks!
PS - my next question relates to speakers!
Comments
The LC15 has one pair producing ~15W, the Engl has 2 pairs producing ~30W.
The Laney uses valves for all the amplification and the PI (but not reverb, FWIW).
I doubt that the EL84 is anything to do with the PI in the Engl - I strongly suspect that it uses solid state circuitry for the PI and also for extra gain to boost the single ECC83, but I can't find any schematics in a brief attempt at searching.
Dunno if that helps...
https://www.dropbox.com/s/3bpqm6ro80engrt/E300-schematics.pdf?dl=0
My band, Red For Dissent
There's at least two solid state gain stages (around IC1A and T1) and a solid-state PI (IC2b, T3 & T4).
What matters is how it sounds, though, whatever the innards are made of.
My band, Red For Dissent
The PI DOES do a specific job (but more on that later) but it also can do extra duty. The stage is often used as the "injection point" for negative feedback and by that fact also used for extra tone shaping, usually called "presence". The feedback can also be controlled/switchable and be labelled "resonance".
The PI (usually) provides some gain and has a very high input impedance (with low capacitance) and is thus often the stage that the tone stack dumps into.
But yes, the primary job of a "Phase" Inverter is to provide two signals of nominally equal amplitude but of opposite "phase". In " " because the signals are strictly speaking POLARITY inverted. But WHY we need such a pair of signal is harder to explain!
For all its apparent complexity, PIs, fancy transformers, the push-pull output stage is really just a means of putting pairs of valves in "series". Now, to understand why we need the valves connected that way needs an understanding of basic series and parallel circuits and some Ohms Law.
Like SO much about technology, if you jump in in the middle of the development (as we all did with computers!) you find it hard to grasp concepts based on first principles you don't know!
But maybe ICBM or Frank will do better than I?
Dave.
Very simple explanation, missing out a thousand details but hopefully the pictures will help you understand the basics.
The PI takes the sine wave signal coming from the preamp and in effect makes two copies, one in an inverted form to the other, which is as per the original. The PI may amplify the signal as well. Each copy is passed to each power output valve (or set of paralleled valves if there are more than two).
This is the long tailed pair inverter which is the most commonly used in valve amps:
The power valves then do what they do and amplify the signal passed to them. However, the valves are driven with identical but out-of-phase grid signals. When one grid goes positive, the other goes negative i.e. they will only start to conduct as the signal approaches positive (above the line). When one valves conducts more, the other conducts less: one pushes, the other pulls. The output signal developed in the transformer is the difference between the two AC currents.
My band, Red For Dissent
My band, Red For Dissent
It follows the Lead gain control and precedes the Lead volume control, with no solid-state gain stages or clipping diodes in that part of the circuit, so it will almost certainly be providing most/all of the distortion. It also directly drives the tone stack which again is most likely to have a large effect on the tone.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
My band, Red For Dissent
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
My band, Red For Dissent
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
My band, Red For Dissent
My band, Red For Dissent
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
My band, Red For Dissent
First result...
http://www.thetubestore.com/lib/thetubestore/schematics/Laney/Laney-LC15-Schematic.pdf
Or do you mean the Engl? They're harder to find and Engl are less than helpful.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein