12A-series etc, preamps

RCA applications data for commonly used double triodes.

Valve and transformer manufacturers published designs using their components to both illustrate typical applications and to encourage design and construction, and thus sales.

RCA also supplied tables of typical operating conditions for popular valves which make application an even simpler matter. These will also allow you to check if the stage in front of you is reasonable or not.

Bottom view 12AU7, 12AT7, 12AY7, 12AX7, ECC81/3/4
NJ7P Database

12AX7 is shown below. Click for tables for 12AT7, 12AU7, 12AY7, 6SN7.

The most common arrangements used in guitar amps are found in the third, 300 volt, group -

• 100k anode load and a bypassed 1k5 in the cathode for a gain of 52, and
• 220k anode load and a bypassed 2k2 in the cathode for a gain of 59.

Both require a 220k grid return.

FAQ

Q: What's the difference between a 12AU7, a 12AT7, and a 12AX7?

A: Voltage gain.

12AU7	~10
12AT7	~30
12AX7	~65

Q: Are these three interchangable?

A: On the whole, yes, but the amp may not work as intended - a preamp may distort, or the tremeolo oscillator refuse to run.
Caution: other valve types such as the 6GW8 and 6X4 also use the same 9-pin base and there is a risk of damage if you exchange one of these for a twin triode.

Q: how often should I change my preamp valves?

A: Only when one gives trouble, such as jingles and microphonics. Like any valve they lose cathode emission over time, but I've seen gear with preamp valves manufactured in the 1960's still going strong.

Q: Do I need to adjust the bias or something?

A: No. Preamps are self-biasing.

Q: I've read that valve cans suck my trebles, is that true?

A: No. Valve cans are intended to shield the valve against unwanted signals getting in, and to secure the valve in it's socket. The stray shunt capacitance is only a couple of picro-farad and any loss immeasureable, much less audible. Even bats wouldn't notice. But the improvement is stray pickup can be quite noticeable. Valve cans hide the look of your pretty valves, but they are a good thing for your sound. Normally they are only really needed on the first stage or two where levels are low and amplification is high.

Q: I was thinking of putting little rubber grommets/heat sinks/cheezle-rings on my preamp valves. Is this a good idea?

A: Some anti-microphonics valve damping gizmos do seem to work, and may be of some actual benefit in a gigging guitar amp, but heatsinks are elephant-proof paint. They can improve the look of a chassis, but you certainly don't need them. If the gizmo costs more than the valve, it's probably just another Hyper-Fi rip-off.

Q: My pre's light up very brightly around the base at switch-on. Is this a problem?

A: No. It's the quick warm-up feature in these valves. They all do it, and always have - though it certainly looks alarming.

Q: My tremolo/normal/crunch channel stopped working and I found that one of the double valves seems to be lighting on one side only. Could that be the problem?

A: Yes.

Q: Is this just the valve, or could something else be wrong?

A: Yes; and unlikely.

Q: My amp is blurting/farting, and from tapping around seems to be coming from near the little valves.

A: When did you last clean your guitar lead plugs and sockets? Aside from that, the most frequent cause is poor connection between the valve pins and the valve socket contacts. At a gig, standby the amp and rock the offending valve with a gentle circular motion so the pins scrub in the socket, then pray. On the bench they need a clean and re-tension, or even (gasp!) replacement.

Q: Help! My amp is feeding back all by itself!

A: All valves (except rectifiers) are capable of turning into effective microphones and giving every sensitivity up to full howling acoustic feedback. Find and replace the offending valve (tapping carefully around the chassis works). This is most commonly a preamp problem, but it can be a power valve.

Underchassis

Q: I'm building; which is better, wiring for 6 volt heaters, or for 12 volt?

A: Unless you are running from a car battery, 6 volt. 12 volt wiring has less hum field because the current is lower, but it is generally more important to balance the cathodes to AC ground, and this can only be done with parallel 6.3 volt connection and a hum-dinger (two resistors or a pot to ground) at the supply.

Q: I want to audition different anode resistor values. Can I just change them?

A: From 47k up to whatever you fancy (1Meg is a practical maximum), but you must re-bias the section (see tables above). The value of the cathode resistor tracks the anode resistor, if one goes up the other must also. A good target point is for the anode voltage to rest half-way between the cathode voltage and the local HT supply. Apply Ohm's Law a lot.

Q: I have a stage with almost no cathode voltage, and almost full HT on the anode - what's wrong?

A: The valve isn't drawing any current and has most likely just lost emission - replace.

Q: I have full HT, but almost nothing on the cathode or the anode - what's wrong?

A: Two things to check, first that the cathode bypass cap, almost always a 25uF/25V hasn't shorted and turned the valve hard-on; secondly that the anode load resistor hasn't gone high and is starving the stage for current.

Q: Supply's normal, but the anode is low, the cathode is high, and I seem to have a couple of volts on the grid. What's happening?

A: Most likely is leakage from a preceeding stage through a leaky coupling capacitor. Lift the grid end and see if the stage voltages return to normal. Cap should test infinite ohms on your highest meter ohms range. Replace cap and check all others like it.

Q: I have a section spare (for later features), how should I wire it for now?

A: Don't leave it disconnected. Don't wire it in parallel with the other section, it increases distortion (see Steve Bench's excellent outstanding research work on this and similar matters).

Wire the anode and cathode with whatever you have in mind. If you have nothing in mind then wire them with anode and cathode resistors the same as you are using elsewhere, typically Ra is 100-220k, and Rk is 1k5 to 2k2. Don't bother to bypass Rk. Ground the grid. Connect to local HT and check for Va about half the HT, and that there is about 1 mA flowing (Vk/Rk). Now it will age in step with the working side, a bit like rotating your car tyres.

But with all this anyway, you may as well go the whole hog and wire it in as a cathode-follower driver for the tonestack, or the ultimate crunch stage, until you think of something better.

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