Vacuum tube amp help

I just bought a lot of 125 vacuum tubes on ebay for 30 dollars including shipping (the seller didn't list what tubes were there, just said a crate) I got the following (if the same name is listed 2 times in a row, then I got 2): 10JA8 12AD6 12AE6 12AF6 12AU6 12AX4GTA 12BD6 12BE6 12BG6 12C5 12CN5 12CU6 12J8 12K7GT 12RS 12X4 17AX4GT 17BR3 18FW6 18FX6 18FY6 1B3GT 1G3GT 1G3GT 1S4 1S4 1S5 1T4 1U5 1U5 1X2B 21LR8 2BN4 2BN4A 2CW4 2CY5 32ET5 3S4 3S4 3V4 4BQ7A 4BZ6 4BZ7 4BZ7 50B5 5AT8 5AT8 5AT8 5AT8 5BR8 5T8 5T8 5T8 5T8 5U8 5U8 5U8 5U8 654A 6AB4 6AF4A 6AG5 6AG5 6AL5 6AL5 6AM8 6AQ5 6AQ5A 6AS8 6AT6A 6AU6A 6AU6A 6AU6A 6AV6 6AV6 6AX4GTB 6BA6 6BA6 6BA6 6BA6 6BA6 6BE6 6BE6 6BJ6 6BK7B 6BN4A 6BN4A 6BN6 6BQ7A 6BQ7A 6BQ7A 6BQ7A 6BQ7A 6BU8 6BZ7 6BZ7 6BZ7 6BZT 6CB6 6CB6A 6CF6 6CG8A 6CN7 6CY5 6CY7 6CY7 6CY7 6DQ6A 6DQ6B 6EA8 6FM7 6GN8 6H6GT 6J6A 6J6A 6J6A 6S4A 6SA7 6SJ7 6SK7GT 6U8 6U8A 8FQ7 9A8 I'm thinking about making a small amp (below 5 watts) for my ipod, which one of these would be best?

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asdterror8 years ago
Don't knock TV tubes-- a lot of them have a great frequency response plus they have the added bonus of being off the audiophile's radar, so you can still find them for under $4.
Oh and if you don't like your pentodes send them to me :)
gmoon already said that USA tubes start with the filament voltage-- some of the tubes listed above are the same as other, more commonly used tubes. For instance the 5U8 listed above is the same as a 6U8, but because most of the available power transformers marketed as tube PTs use 6.3v or 12.6v heaters, a web search for "5U8 amp schematic" won't produce many useful hits but a search for "6U8 amp schematic" will. The only thing that would need to changed in the uncovered schematic is the heater supply.
Go to Duncan Amps and you can search for tube data sheets. With their setup you don't have to download and read through a data sheet just to find it's a rectifier. On the home page for every tube they list max gain, max voltage, and close equivalents.
Have fun with your random tubes! Feel bad for the poor smuks with a lifetime supply of 12AX7s.
73lonestar8 years ago
has anyone ever used 6em5s i have a bunch use them in push pull rated at 250 volts kinda like 6973 or 6cz5 and also 6dl5 el95 2 watt beam tube all good for lower dc voltage dont forget about 6av5 larger cathold but a good cheap tube heres something headroom thd total harmonic distortion can you put more current from your main dc supply go higher in mfd 450v 540mfd vs 450v 80mfd also aimed at lower voltage projects thanks. mark
gmoon8 years ago
Cool, I love this stuff.

Quite a roster. But a good percentage of those tubes are probably RF or TV tubes, and not so useful for audio. There are quite a few rectifier tubes on the list, but I'd recommend a solid state rectifier...(cheap, less voltage loss, no filament voltage/current required.)

-- There are lots of dual triodes on your list. Those make great preamp tubes. Some of the (non power) pentodes, like the 12AU6 also make decent preamps.

You'll probably need at least one preamp stage per channel...

-- If you want more than 1/4 watt output, you'll need a power tube (generally pentodes.) The 6AQ5 is on your list, and should work well. For a single ended Class A amp, approx. 2 watts @ 180V, 4.5 watts @ 250V.

With tubes, two 4.5 watt channels would be quite loud with good output trannies and speakers...

The 12C5 and 50B5 are also power tubes. The low-V radio "battery" tubes (1S4, 3S4, 3V4) others have mentioned would be fun to play with, though not very powerful...

(You know with USA tubes the first # is the filament voltage, right?)

Don't want to buy a power transformer? (expensive, don't know what to buy.. etc.) They usually have all the required filament voltages included. But there are other options...

1) An isolation transformer, something like the Triad N-68X. With an SS rectifier, you should get about 160V DC. That's a very acceptable voltage for ~3 watts with a 12L6, 50L6, 6DG6, etc. type of power stage.

I've even heard of wiring those backwards--using the 230V AC primary (euro mains) as the secondary instead (~320V DC.) It's a 435mA capacity wired forward, should be able to supply 150-200mA backwards...(more than enough for two 5 watt channels.)

A separate filament transformer would be needed.

2) Voltage doubler on the isolation trannie would work too. Univox tube amps in the 60's used a voltage doubler, here's an example, though you wouldn't get (or need) 650V... (that's a center-tapped PT; yours will likely be a more conventional doubler.)

3) Xellers refers to wiring two low voltage transformers back-to-back. For instance, 115-12 === 6-115. The output of the second transformer would be stepped up to 230V AC.

If you're going to run power tubes, you'll need a hefty transformer for the step up. It's typical to tap the intermediate voltage (12, in this case) for the filaments. So a large transformer would be needed there, too.

You can rectify directly off the mains, but I wouldn't recommend it.

All the usual safely warnings apply for all the above comments...
gmoon gmoon8 years ago
Here's a schematic for one channel of a single-ended, cathode-biased 6AQ5 stereo amp. Consider it for baseline values for the cathode-bias resistor and the load resistance (OT impedance.)

It has a single preamp stage (expected), and a negative feedback loop from the OT (optional, will reduce gain and distortion.)
guyfrom7up (author)  gmoon8 years ago
thanks gmoon for all of the information! I can't look at all of the links right now, but looks good (stupid english, I have to write 40 pages by monday >:( I think I might just rectify direct off mains (with a fuse), I mean that's safe if nothing is exposed (well, besides the tubes and if they were broken, but that wouldn't be safe anyway) and it'd be much smaller, lighter, and cheaper. Is there a way of telling if somethings a triode just by looking at the name? I don't want to to find every data sheet to figure out what each one is, and some of them require a bit of searching for a datasheet to find out it's for TVs.
Sure, you're welcome, no biggie. Enjoy your english comp ;0)

I wouldn't recommend coming right off the mains. Although it's a less critical if you're not building a guitar amp. Guitars tend to have the strings gounded, using your body a shield. If the plug (Hot / Neutral) is reversed, easy to do with non-polarized plugs, then the guitarist's body is "Hot." Touch anything grounded...ZAP.

Fact is, lots of radios, amps, recordplayers ran off the mains at one time. They were called "AC/DC radios" or amps. They could be run off AC or batteries without any modifications. Here's an old Harmony amp (half-wave tube rectifier), sans transformer.

For an example of SS rectification, user Ohm posted a schematic for his stereo amp on my tube rebuild project. The comment is here (he uses a couple "glow tubes" as a voltage regulator--a purist approach; usually not necessary.)

It is possible to incorporate a little bit of protection. For instance, Ohm wired the Neutral and Ground wires together. If an electrician miswired the outlet (swapped Neutral and Hot), then the house breaker would trip. It might be safer to add a fuse to both the incoming Hot and Neutral, just in case. Still, running directly off the mains isn't considered safe practice. Yes, I know...but it had to be said.
guyfrom7up (author)  gmoon8 years ago
how about this for protection (still not using a transformer) I use a 120 volt relay hooked up to neutral and ground (using a 3 prong, polarized plug) so that if hot would be applied to the relay, it'd flip and it would automatically flip hot to what's suppose to be hot.
I'm offering information simply because it's still done occasionally (Ohm, for instance), and it's interesting. But I still wouldn't recommend it.

However, given the frequency of HV topics here (plus the size of capacitors often discussed) there are plenty of dangerous project being pursued... So I don't have any illusions that safety is sometimes .. umm... underemphasised.

NachoM's GFCI suggestion is a good one, but GFI's, GFCI's don't react fast enough to prevent electrocution--not if your body itself is the source of the leak (I.E., if you touch an external earth ground.) Of course, they continue to improve such devices and it's much better than nothing.

Incidentally, the 6AQ5 schematic I linked above uses a transformer, yet it's not a safe power circuit, either (I posted it for the 6AQ5 stage.) It includes a "death cap" between the Neutral wire and the chassis ground. But caps have a limited life span and often short out.

I have an old amp with that very schema--the "standby" switch has dual polarity, and half of the switch connects the cap between the Hot and Neutral wires. The idea was to set the switch to which ever side was quieter, since the plug isn't polarized (nor were the outlets then.)

Nowadays, we have three-prong plugs with an earth ground. The correct way is to connect the Green ground wire to the chassis, and leave the incoming current-carrying wires completely isolated, connected only to the primary coil.

Seriously--I know power transformers are kind of expensive, but the iso trannie I mentioned isn't. It's worth the investment. Also search ebay for "power transformer," you'll find lots. Or look for cast-off electronics (my amp build used an old TV transformer, 142VAC that yielded 190VDC... It cost nothing.)

Finally, I know intelligent, knowledgeable persons can break the rules somewhat. However, I suspect that the most likely to be electrocuted are both the least experiencde and the most experienced (knowledge and experience nullified by complacency and overconfidence...)
gmoon gmoon8 years ago
Oh, yeah...can you, as author, move this thread into the Vacuum Tube Group?

It's a good discussion.
guyfrom7up (author)  gmoon8 years ago
how do I move the topic into the group?
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