Running 12AX7 or EC88 Tubes in Parallel for 5-10W DIY Vacuum Tube Amp Build

To start off, I know almost nothing about tubes, but want to learn, so I figure this is a good starting project.

I want to built a 5-10 watt amp using tubes. I've searched around, and found a few schematics, but most of them are only for a few watts of power.

Main question I guess is, can I run tubes in parallell? I've noticed you can run one or both 'triode sections' of a 12AX7. Running them in parallel gives more output power. Not an issue there, I can just use 2, one per channel. I've read about the whole 'matched sections' thing, but for a first tube project, not too worried about it.

Can I run power tubes like an EC88 in parallel as well?

Also, do I really need to use a 12AX7 to run a EC88? I've seen some schematics where only a 12AX7 is used. Can't I just run the power tube? Input would be from a cd player/ipod, so it wouldn't need preamplification.

Things would be simple if I could just run 2 or 3 or 4 12AX7's in parallel per channel, or could just use 2 or 3 power tubes in parallel. My goal is to keep the parts count as low as possble.

I've also seen ignition coils used as output transformers, and besides the higher risk for shock, see nothing wrong with this?

You didn't say why you are trying to build a tube amp. Is it because you want to learn or play around with these old things, or is this a audiophool religious thing?

In any case use a pair of 6L6 in push-pull configuration with a output transformer. With the right transformer, those can handle the power. The positive supply (a few 100 Volts, check datasheet) would be tied to the center tap of the primary and a separate 6L6 can pull down on each leg. The secondary should have about 8 Ohms impedance to match the speaker.

Now you have to amplify the input voltage and then invert it. Each 6L6 must be driven opposite of the other. This job can be accomplished with 12AX7 and 12AU7-A. The former is higher gain and is usually used in the front end. The latter for higher level internal signals. However, you can use all the same type if you're careful with the circuit. Getting tubes nowadays is going to be tricky (expensive) so there may be advantage to reducing the total types in the design. The three types I'v listed so far were quite common back in the day, and if anything is still available today I'd expect those to be.

Think of the 12AU7-A and 12AX7 as small signal FETs and the 6L6 as large power bipolar transistors. Remember to provide plenty of ventilation. The 6L6 in particular are going to get hot and need air flow.

The reason I want to do is is mainly to learn. I need a 5 watt or so amp for another project. I'm starting to build an alarm clock, with nixie tubes, and figured a tube amp section for audio would be somewhat better looking, giving it the whole classic look. Depending on how far I want to push this, I'm debating a mercury rectifier tube as well instead of diodes.

I asked this question elsewhere as well, and was given a variety of answers, ranging from 'stick with transistors or you'll kill yourself' to 'a single 6V6 or EL84 could do 3-4 watts'.

I'd love to be able to find a cheap/free tube radio/tv to study and salvage parts from, but haven't had any luck there yet.

I'm still reading up on tubes, but as a rough guess, how much would 2 12AX7's in parallel output, or a 12AX7 and 6V6 setup?

Less tubes is better, since as you mentioned they are hard/expensive to find.

There were tubes that can do a few Watts. The reason I mentioned the 6L6 is because it was very popular and if anything is still available today that would be high on the list. It is probably higher power than you need, but does that really matter?

The reason for two of them is this makes a better amplifier since even harmonics of the distortion get largely cancelled out. Tubes don't come in N and P flavors like transistors do, so transformer coupled push-pull stages were common. Tubes also operate at higher voltages and higher impedances that a loudspeaker, so you need a transformer in there anyway.

You can use two lower power tubes to drive the transformer in push pull mode if the power is sufficient. You have to check the datasheets.

As to how much you can get from specific tubes, that's something for you to look up. The 12AX7 is meant for signals, not power though. I just looked up a 6V6 and that looks like a slightly higher power version of the 6L6. Either should be more than enough for what you need. I would chose which one you can find a more reliable source of.

Thanks. I'll start looking at the 12AX7/6L6 setup in detail then.

Joe, a great site on DIY guitar tube amplifiers (one of the few places where there is still active interest and development in tube amps) is AX84.com

There are several designs with several power outputs. Be careful though, guitar tube amps are design to distort once you start adding gain. This is because tubes "break-up" in a musical way (the harmonics caused by the ampplified signal clipping sound "nice"), so you may have to change the input and output gain in those designs to use them for generic audio.

The ECC88 is the same as the 6DJ8. This tube in push-pull service can run about 1 watt of audio power. It was designed to be used as a cascode VHF amplifer and with a very high transconductance of 12,000 uMho (micro Siemen's these days) it is very quiet. The noise with a 50 ohm input termination is about 1 nV/rt Hz.

The 12AX7 is a very low current tube. It can run only about 1 mA of plate current at zero volts on the grid. This is not suitable for power amplification. The 12AU7 is the one to use. I once built a push-pull audio amp using this and a Radio Shack audio line transformer.

Yes you can parallel tubes. If they are the same type, manufacturer, and data code they should share current well enough. I have used cathode degeneration resistors to ensure better current sharing.

For audio power amplification a good currently produced tube (since 1936) is the 6L6. This will require some grid drive - more than a CD player will produce - and you will need a gain of 20 between it and the 6L6. This gain can be obtained with the ECC88 (no longer made), the 12AX7, and my favorite the 12AT7.