1 Watt Single-Ended Output Stage

[jbltwin1]

Don't know if you've seen this one but it's got all info for construction.

[printer2]

On selecting your output tube I would look first at what you have foe an output transformer and get a tube that matches it. Then select the PT going by the data sheet. A one watt output would be fine (I built a push pull with the 6AK6 that sounds great, good tube.) and you can drop the voltage to it is you want a quieter output using a power attenuator. Also at less than 5W levels I find old hifi speakers from the 60-70's can sound pretty decent and they are less efficient than guitar speakers.

[Mick Bailey]

I've built quite a few low-wattage amps and an ECC82/12AU7 in self-split mode sounds fine - much better than SE, which always sounds like an overdriven hi-fi amp when using a triode. Take a listen to the Firefly Mk3 clips (the one with boost). Here's one to be going on with;

https://www.youtube.com/watch?v=u9_WM7Wdtys

[FalderGuitars]

Morning all,

Thanks to everyone for their responses, they've given me plenty of food for thought - alongside my morning porridge
As usual, just when I thought I knew what I wanted, a host of options present themselves. Still, we live and learn...

A couple of points I should have probably made clear at the head of the thread:

1. I like the sound of a single-ended amp, which is why I'm planning to build one.

2. I've no interest in using any kind of L-pad/attenuator etc. (I'm actually currently using one, but at the volume levels I need the tonal response goes all screwy).

3. I'm aware there are other attenuators available, but see point 2


The advantage (for me) of the circuit I drew was that I have all the parts to hand. There are a couple of single ended amps out there using (ECC82) triode output stages (the Palmer Eins and Hughes and Kettner Something-Or-Other), so I'd assumed they make a decent choice.
However, since there seems to be a consensus here that triode power amps sound like the proverbial 'bee in a jam-jar', so I'm beginning to have second thoughts.

Time to print out some pentode datasheets? Could be...

Thanks again to everyone for helping out,

Matt.

[Gregg]

Check out the schematics in the attachment. 50EH5 is basically a 5902/6P30B with a 50V filament and different cathode resistor. It doesn't get more simple than that. You get ~0.5W from one tube only. If you run it at higher voltage like 150-180V you can get maybe 1W clean. The russian 6P30B-R can take safely up to 220-230V (tested) and it costs only couple of bucks. A reverb OT won't cut it though and will saturate at that anode current so you'll need an appropriate one.

SE.zip

[Enzo]

Not to mention a 50v heater supply.

[Mick Bailey]

L-pads can be excellent in a small amp. But, if you're using one 'straight' your amp will lose a lot of apparent top-end definition when turned down. This is partly due to psycho-acoustic effects (Fletcher-Munson, etc.).

I always use a non-polarized (polyester) HF bypass capacitor placed across the input/output of the L-pad. Ideally, make it switchable - 2.2uf/none/4.7uf as a suggestion. My own EL84 SE amp uses 3.3uf. It makes a great deal of difference to the padded sound.

[Gregg]

Not to mention a 50v heater supply.

That's not such a problem. You can use a PT with one secondary only like ~50V and derive anode voltage from there via doubler or tripler but it's not even necessary because as you can see several other tube options are available.

[Mick Bailey]

The catch with doublers or triplers is that the smoothest supplies are from full-wave circuits, but these don't reference ground directly from the transformer secondary. You can end up with hum problems due to the potential difference between the heater ground reference and signal ground. IMHO this complicates a simple SE amp into having a high component count.

FWIW, the ECL82 makes a fine SE low-wattage pentode amp. You also get a triode in the same envelope. That means for a two-tube amp you have the added benefit of three preamp stages, so either switchable gain or a CF to drive the tone stack. They also sound nice at relatively low voltages. I really like these tubes - they're very much underrated.

[benito_red]

I built several SE amps, some of them with EL84 other with 6V6. The results were very good, but too loud at the "sweet spot", with problems with neighbors that you can imagine. I tried to reduce the power using different tipes of power attenuators: Lpad with resistors, Lpad with resistors and capacitors (as in VOX AC4TV), then with a voltage reductor, but I did not like the results. So I started to build 1watt SE tube amp (using 6BM8 = ECL82) but the volume was still too high and I didn't like the sound. My conclusion after all those tests was that at bed-room volume level a decent solid state amp + a good overdrive pedal does the job better then tube amp. Also amp modelling (f.i. Roland microCUBE) can be a good alternative.

[J M Fahey]

Amen brother ... although many won´t like that result

[PeanutNore]

There are a couple of single ended amps out there using (ECC82) triode output stages (the Palmer Eins and Hughes and Kettner Something-Or-Other), so I'd assumed they make a decent choice.
However, since there seems to be a consensus here that triode power amps sound like the proverbial 'bee in a jam-jar', so I'm beginning to have second thoughts.

I've built 4 low power triode power amps at this point, and I have zero doubt that they can sound good when done right. Since you're looking for single ended ones, you might be interested in this one (audio clip) that uses both triodes of an ECC99 (basically a beefed up ECC82) in parallel as a power amp after a preamp that clones the 5150 lead channel. Since posting about it here I realized that without a phase inverter it can't drive the power tube grid enough to get full volume, so I added another ECC83 after the tone stack as a gain recovery stage w/ cathode follower, with a DPDT to bypass it for a low volume mode. It's basically a 0.1 watt / 5 watt switch.

Ok, so that's a super high gain circuit where the tone is almost all in the preamp, what about a simple low gain amp and some ECC82 power tube distortion? This amp is a miniaturized clone of an Orange Pics-only head with an ECC82 push-pull fixed bias power amp, and this is what it sounds like.

[FalderGuitars]

Morning all,

Since I'm gearing up to start a new build (more of that elsewhere), I thought it only fair - after all the advice you gave - to let you know how I got on with the build.

The answer is, I didn't...

Before you all have me (rightly) stoned for wasting your time, I'll quickly point out it was a suggestion made by nosaj - namely that I try a less efficient speaker - which solved the problem.
I'd been using a Celestion G12M, simply because I had one. I knew in principle that it was too much speaker for a so small an amp, but I hadn't realised just how much difference changing down to a smaller, less efficient speaker would make.
Printer2 also suggested trying hi-fi speakers, which I'll admit sounded like a terrible idea, before I heard how it actually sounded (does that make any sense?). I found an old Telefunken hi-fi speaker hiding in the dustiest corner of the workshop, after removing the blown tweeter and cross-over, it's found a new lease of life as a 6" guitar cab.



The bottom end is a little flabby for humbuckers, but it takes my Strat wonderfully. I'll probably upgrade to a Jensen Alnico when finances permit, but for now I'm a happy camper, and can hit the sweet spot at 'bedroom' levels - let the 4th rate Hendrix impersonations commence...

Thanks again to everyone for helping out,

Matt.

[nosaj]

Morning all,

Since I'm gearing up to start a new build (more of that elsewhere), I thought it only fair - after all the advice you gave - to let you know how I got on with the build.

The answer is, I didn't...

Before you all have me (rightly) stoned for wasting your time, I'll quickly point out it was a suggestion made by nosaj - namely that I try a less efficient speaker - which solved the problem.
I'd been using a Celestion G12M, simply because I had one. I knew in principle that it was too much speaker for a so small an amp, but I hadn't realised just how much difference changing down to a smaller, less efficient speaker would make.
Printer2 also suggested trying hi-fi speakers, which I'll admit sounded like a terrible idea, before I heard how it actually sounded (does that make any sense?). I found an old Telefunken hi-fi speaker hiding in the dustiest corner of the workshop, after removing the blown tweeter and cross-over, it's found a new lease of life as a 6" guitar cab.

[ATTACH=CONFIG]44279[/ATTACH][ATTACH=CONFIG]44278[/ATTACH]

The bottom end is a little flabby for humbuckers, but it takes my Strat wonderfully. I'll probably upgrade to a Jensen Alnico when finances permit, but for now I'm a happy camper, and can hit the sweet spot at 'bedroom' levels - let the 4th rate Hendrix impersonations commence...

Thanks again to everyone for helping out,

Matt.

You are not wasting our time if you are experimenting and reporting back. You become part of the community when you shares your trials and tribulations. great that it worked out for you. Sometimes better is cheaper or simpler.

nosaj

[Gnobuddy]

Time to print out some pentode datasheets? Could be...

I went tilting at the same windmills you're headed for. Inspired by Printer2's 2W 5e3, I designed and built a 2 W push-pull amp using a pair of 6AK6 pentodes, and added an L-pad. It turned out to be loud enough for jam sessions with half a dozen other acoustic guitarists and a fiddle, but sounded as lifeless as a solid state guitar amp when dialled down to midnight neighbour-friendly levels with the L-pad.

Then I designed a new preamp for it, using a 6JW8 triode-pentode valve. You could think of the preamp itself as a milliwatt-power Champ, if you like, triode input stage, driving the pentode (preamp) output stage, with the tone stack after the pentode, rather than between triode and pentode. There is a gain control between triode and pentode, and is also a master volume after the tone control, and before the 2W power amp.

This idea worked out much better than the L-pad speaker attenuator. The preamp generates genuine single-ended pentode distortion, because that's exactly what the last stage of the preamp is! I can dial the SPL down with the master volume to fairly low levels, and still get a somewhat "valvey" sound out of it, probably 99% of it generated from the preamp. Remember, this is not the annoying, fizzy 12AX7 preamp distortion you're probably hearing in your head - there are no 12AX7s in this preamp!

If you decide to just go with your original plan - a small single-ended pentode driving your speaker through an output transformer - the tricky part becomes finding a suitable transformer, with a high-enough primary impedance (and inductance) to work with a flea-power tube. The Fender reverb transformer (or Hammond equivalent, etc) is about as high as you can find off the shelf - 22.5k or so primary Z with 8 ohms loading the secondary side. There seem to be a number of low power valves that can work with this sort of impedance, examples being the 6AK6, the even smaller 6AK5, 6DX8 triode-pentode, 6LY8 triode-pentode, et cetera.

You will probably be on your own with the design to a considerable degree, though you may get lucky and find spring-reverb driver circuits that can easily turn into your power amp design. For example, draw a 22.5k loadline - say crossing the voltage axis at 450 volts and the current axis at 20 mA. For single-ended operation, the anode will then need to sit at half that voltage and half that current - 225 V on the anode and 10 mA quiescent anode current. That's 2.25 watts dissipation on the plate, so look for a valve that can handle a bit more than that (6AK6, for instance). You are now well on your way to a practical small SE design, using a transformer that actually exists.

One of the nice things about the "small pentode in the preamp" approach I ended up taking, is that you don't have to scour the planet for tiny output transformers with improbably high primary impedance and inductance. You can just use an ordinary 1/2 watt anode resistor instead!

Oh, one more thing. I once asked Printer2 about the SPL levels from his little 2-watt 5e3 amp. At full blast into a 12" guitar speaker, it was over 100 dB @ 1 metre. (Which makes perfect sense if you have a 97 dB/W@1m speaker, and feed it 2 watts, which is +3dBW.)

For reference, a typical home vacuum cleaner - the loudest sound most neighbours will tolerate during the daytime - is around 70 dB SPL @ 1 metre.

-Gnobuddy