Orange AD15 - Gain query.

paisleysoul

So, after failing mid gig a month or so ago, I fitted new valves to the AD15. It's now back to where it was when I got it.

But this reminded me of something that I thought was fairy odd, but it could just be my naivety. 

Up to about 10 o'clock, the gain knob gives me a lovely clean tone. However, pushing past 11 there is an audible jump in gain which pushes the amp straight into OD territory. I've never played another AD15, so is this a standard thing here for the gain to leap in all of a sudden, or have I got a dodgy pot? Or worse yet something more sinister?! I'd like to be able to get a bit more clean tone from the gain knob (That feels odd to type) before it starts to OD out.

Thoughts?

ICBM

I'm not absolutely certain since it's a while since I've played one, but that sort of 'step' in the pot taper is unfortunately common with the type of pots they used (and some other makers, eg Marshall on the JCM900s).

It may be possible to fit a higher-quality pot - or rather, it's definitely possible but whether you can get one that fits the PCB correctly and doesn't involve bodging it slightly is the question.

matone

Have an AD15 and it doesn`t behave like that.

shady

Would a less gainy pre amp valve help there?  Putting 5751 in the first position is certainly a cheap experiment that might get you a nicer taper...or am I talking shite...

ICBM

It might, but a 5751 probably isn't enough of a gain reduction to make a big difference. A 12AY7 or even a 12AU7 would probably be more useful.

shady

Aha, found this - I knew I'd seen it somewhere.  May be useful.

ICBM

That's correct, although it's important to remember...

The gain is not only dependent on the valve, but also the circuit. The listed gains are the maximum possible, not always what you get in practice. So for example changing from a 12AX7 to a 12AT7 might not give you as big a drop as 40%.

The tone will change too - the gain is only one factor in the valve spec. Generally the lower the gain of the valve the wider its bandwidth, ie more bass and treble, but again depending on the circuit.

The bias point and the current draw will change. *Usually* this isn't a problem and the circuit will self-adjust, but in some cases either the valve will run too hot, or it will make a component in the amp (usually the plate resistor) either run too hot or exceed its voltage rating, which can blow it. Be careful before assuming they're all plug-and-play.

There are also a very few circuit locations where you can't run some of the types because the valve has a lower voltage rating. This especially applies to putting a 12AT7 or 12AY7 in a cathode-follower position, eg V2 in the classic Marshall circuit.