Trying to understand valves, phase inverters, etc. (...watt the hell?)

imalrightjack

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!

Misterg

The power output only depends on the power valves - EL84s in each case:

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...

imalrightjack

Aaah, that makes sense! Schematic here:

https://www.dropbox.com/s/3bpqm6ro80engrt/E300-schematics.pdf?dl=0

Misterg

OK, so it's true:

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.

imalrightjack

What does the phase inverter actually do? I assumed it did a very specific job in a singular way but clearly not...

ecc83

imalrightjack said:

What does the phase inverter actually do? I assumed it did a very specific job in a singular way but clearly not...

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.

ESchap

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.

bandmaster188

If you are interested in what's happening inside your amp, these books are well worth a read. They do lean towards fender style circuits a lot but all the basic principles are there. http://www.amazon.co.uk/The-Guitar-AMP-Handbook-Understanding/dp/087930863X/ref=sr_1_1?ie=UTF8&qid=1408863005&sr=8-1&keywords=guitar+amplifier+books http://www.amazon.co.uk/Guitar-Amplifier-Essentials-Gerald-Weber/dp/0964106027/ref=sr_1_9?ie=UTF8&qid=1408863218&sr=8-9&keywords=guitar+amplifier+books

imalrightjack

Cheers, chaps. As I read through these posts, I thought, "I'll ask for some book recommendations." And there you have it! Really interesting stuff that I would love to get understand better. Thanks!

imalrightjack

PS - what led me to this question was whether changing the solitary 12AX7 in the Engl might have a pleading effect or whether, because there's only one where it seems people often just change V1, it might be too drastic (the amp breaks up too early for my liking and it pretty much doesn't have a proper 'clean' channel).

ICBM

I would think it will have a significant effect. While there's only one valve, it's used in the configuration which most affects tone (both halves in series, both plate-output) and the first stage is set for very high gain (330K plate resistor).

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.

imalrightjack

I think I'll try a valve change before I do anything with commitment to buying a new amp.

ICBM

Good idea. You can almost certainly even sell a valve on if it doesn't work out, so you have very little to lose.

The valves to try first would be the 12AT7 (ECC81) - you can still get really decent NOS ones of these quite cheaply, particularly the JAN/Philips version - which will reduce the gain probably by about 50% and give a brighter but sometimes 'colder' tone - or the 12AU7 (ECC82) which will reduce the gain drastically and give a full, clear tone - and are not too expensive either. In many amps the 12AU7 is too big a reduction in gain, but in others it works perfectly so it may be worth a try and will at least give you a 'lower limit'.

In case you ask, yes the valve will affect the clean sound as well - both channels go through it even though it produces the distortion - the extra gain and volume controls are switched out of the circuit when you're on the clean channel.

imalrightjack

Thanks, @ICBM. What does NOS mean though? New old stock? Just had a look on HotRox and they have an NOS Mullard ECC81. What's the benefit of this over, say, a JJ gold pin which is nearly £10 cheaper?

ICBM

Yes. It really means unused old-production US or Western-European valves (pre-mid 80s, when production ceased in the West) which were all what was 'old stock' when the term started to be used - and it really should be used only for those.

Unfortunately it's now also sometimes used for old *used* US/European valves - which it shouldn't apply to because they are not "new" old stock, although they can still be good valves if you know what you're getting - and possibly even worse, to unused East-European or Chinese valves from around the early 90s - which are certainly now "old stock", but are also some of the worst valves ever made and have not improved with time!

imalrightjack

For testing purposes, I could get a JJ ECC81 and an ECC82 for £20. One question is, if these don't work well in the Engl, could I pop them into the Laney and run a combination of ECC83, ECC82 and ECC81? Also, would any rebiasing be required?

imalrightjack

Be useful if anybody can answer my last question, please

ICBM

You must not put an ECC81 in V2 in the Laney (it's a cathode-follower position with a cathode voltage of 145V; the ECC81 is only rated for 90V there), but otherwise yes.

No rebiasing is necessary with any preamp valve in any amp - they're almost only ever run cathode-biased. Phase inverters in a tiny number of amps are the only exceptions I know of, and even those are not adjustable.

imalrightjack

ICBM said:

You must not put an ECC81 in V2 in the Laney (it's a cathode-follower position with a cathode voltage of 145V; the ECC81 is only rated for 90V there), but otherwise yes.

So the ECC81 would be okay in V1 or V3?

FunkWizard

Please excuse bumping an old thread just wondering if anyone still has the schematic? I have a poorly one in my lap. Cheers

ICBM

Have you tried a Google search for 'Laney LC15R schematic'?

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.