1 Tube 1 Watt Guitar Amp

[tomsamps]

I am interested in building a small 1 tube low power guitar amp, less than a watt is fine. I know there are numerous options, but I wanted to try an ECL82. OTs are easy to find and cheap but power trannies are not.

So here is what I want to try and need some advice. There are interesting options for outputting around 190V, one of which is the NeonNixie High voltage module (have a kit which takes 1/2 watt 12-14V and puts out 22ma @ 190V), which should do the trick. Has anyone used a small split output transformaer such as the MagneTek FS12-1000 (1watt 12.6 2watt 6.3) and build a power curcuit to drive that (or a similar) tube? Maybe another option is running it off a wall wart, but those are not usually too clean.

Once I have the power worked out I will get to the amp specs, but I think this would make for a small footprint practice amp (stomp box size) that would sound reasonable.

Thanks
Tom

 

[Doz]

I doubt your nixie-tube supply will put out enough current to drive the output stage.
I built an amplifier with an identical tube, and used two back to back mains transformers to obtain the HT. Not so easy in the USA, because you have 110V mains, but you could get two transformers : 110 to 6V then 6V to 110-0 110-0 (Universal "Primary"). You can then use the 6V winding for your heater, and wire the 110-0 110-0 for 220V and your HT. Done and dusted and change out of £20 🙂

I think you are thinking along similar lines with the magnetek transformer.

I used a Hammond "Universal" SE O/P transformer. The thing had to be cheap, and mine was made entirely from junque box parts.

 

[Cassiel]

ECL82 is fine but you need more gain, one triode to drive the pentode won't be enough. Here are some ideas for the power supply:

JJS.at : Electronic Projects : Class A Subminiature Amp

freestompboxes.org • View topic - "Superfly" - submini tube version of Doug H's Firefly

 

[Doz]

ECL82 is fine but you need more gain, one triode to drive the pentode won't be enough. http://freestompboxes.org/viewtopic.php?f=2&t=7018

Very true, although running the first triode in "starvation" helps, and produces some "colourful" distortion 🙂

 

[SY]

Have you considered using a three section compactron? I'm putting together a small practice/studio amp using a 6AF11, which has a nice 5W pentode, a high mu triode, and a medium mu triode in the same envelope (thanks to rsumperl who pointed me toward that tube).

 

[Tom Bavis]

The 6AF11 and its equivalents (6BD11, 6AS11) had occurred to me as well - they have 15V versions too, but you'd probably want DC for heaters to avoid hum. A dual-primary 6V / 30+ VA transformer with ISOLATED primaries (not bi-filar wound, like some Signal parts) could be used, with one primary used as 120V secondary. Use a doubler to get 280VDC.

 

[Fuling]

I made a just-for-fun guitar amp for my brother in law a few years ago: A single ECC81 mated with a 180V power supply, a tiny 7k:4R OPT and a 4" 8ohm speaker. One ECC81 section is good for 0,3W or so, loud enough for bedroom use. A switchable diode clipper after the gain stage increased the rock `n roll factor quite a bit.

 

[M Gregg]

I still find this interesting.

OK its not one tube the use of the EF86 I find interesting.

http://www.ampbooks.com/home/amp-technology/EF86-power-amp/EF86-power-amp-bench-test.gif

I have never seen an EF86 in SE mode though!

Regards
M. Gregg

 

[Eli Duttman]

What's all the fuss and muss about? An inexpensive Triad N-68X isolation trafo feeding a Greinacher doubler takes care of the B+. A 117Z6 is perfect for this job, as it will provide the "sag" guitar pickers favor.

 

[ChrisA]

You can run a micro-power guitar amp off a wall wwort power cube. The trick is to use an AC power cube. Get one that puts out at least an amp at 12VAC. Then inside the amp you have a 12V filament transformer wire "backwards" THat gives you 120VAC and when you rectify it the filtered DC comes out to maybe 160V. You might also use a 6.3V filament transformer and get a higher B+ voltage. This sytem will only give you single digial milliamps but for a 0.25W amp that is all you need.


One other very good power source for micro-power amps is batteries. It is good to not have to plug in to mains power but then you need a switch mode power supply

 

[tomsamps]

OK - This is what I feared most - 6 months of ideas! Thanks
I was thinking of using a Stepdown Transformer backwards hooking 115V to the secondary and getting 230 out of the primary (but I have no idea if that could or should be done). The Greinacher doubler that Eli mentioned seems the saner move and a little heater tranny.
Doz - I think anything with the Hammond name is about $40 now (that’s worthless American greenbacks), but Weber has some cheap single end "firefly" trannies.
Cassiel - thanks for the "Micro" link I have some Russian 6N16B-V tubes that need a home.
I have also looked at the 6AF11 but know only of the Squirrel Monkey which looks like it uses 2 trannies, just need to part something out (have not seen many other builds or schematics.) Those NOS tubes are real cheap though.
Fuling - do you have a schematic of your little 12ax7 amp, sounds like a perfect guitar headphone/small speaker practice amp and I think the NeonNixie and a small 6.3 would drive that for sure. The Weber OT is 4K in and 4 and 8 ohm out @ 3Watts way more than enough.
Thanks to all again - I finish my two-stroke this weekend and will start getting a power supply together.

 

[tomsamps]

Pursuing the backwards Stepdown Transformer idea I have found a smaller transformer which provides 20VA @ 117V for under $15. Wired backwards I should get around half. There are also $10 6.3V 1.2amp transformers available. I would think this would do a 6BM8 with a little room left over for a 12AX7 or similar if needed. Open to ideas or criticisms on powering it this way.

Also looking for some decent schematics for the 6BM8 single tube configuration.

 

[Printer2]

I would use a 12AU7 and use a 120V-120V transformer with a voltage doubler. Get a 10W 70V audio transformer and do a push pull as in the Firefly.

 

[Tubelab_com]

Triad N-68X isolation trafo feeding a Greinacher doubler takes care of the B+.......Pursuing the backwards Stepdown Transformer idea I have found a smaller transformer which provides 20VA @ 117V for under $15. Wired backwards I should get around half.

The N-68X is good for 50VA and costs $11.20 at Mouser. You can use it as an isolation transformer and run an entire guitar amp heaters and all on the secondary. I have used them when restoring the old series string "shock box" guitar amps from the 1950's that used 50C5, 35W4, and 12AX7 or 12AU6 tubes.

You could run the secondary into a voltage doubler or you could just turn the transformer around wire the primary (now the secondary) for 230 volts and run it into a bridge (you still get 50 VA). I would tend toward the bridge and that is what I have used in the past with the N-68X. There are several 6.3 volt transformers available for under $10.

There are likely as many opinions on the circuit choice as there are circuits. My choice would take 2 tubes. Either a 12AX7 feeding a 6AQ5 or a 6AU6 feeding a 6AQ5 both derived from the appropriate Fender Champ schematic.

 

[Eli Duttman]

George,

I've wondered what the turns ratios for Triad's N-68X and N-77U really is. A 1:1:1 setup would be nice for a bridge, as you stated. However, a distinct possibility exists that the single winding designated as the secondary may have a few extra turns to deal with losses. FWIW, I always design for a doubler, which allows for use in both "120" and "240" VAC mains zones.

 

[andrew_k]

Also looking for some decent schematics for the 6BM8 single tube configuration.

While designed as a power amp, I found the attached a good starting point for guitar. 6AU6 or similar in front will give you plenty of colour.

 

[bparnell57]

I believe that quoting back to the reverse step-down transformer, you could also use a dual primary transformer, such as 120 or 240 to 12 volts to get 240 volts DC center-taped... this could simplify your power supply some and allow for more interesting designs that require slightly higher voltages. The triad N-68X is a great iso tranny for the money, and I have had success using one with a 1950's danelectro guitar amp with a 12au6 preamp, 50c5 output, and 35w4 rectifier.

 

[tubelectron]

I am interested in building a small 1 tube low power guitar amp, less than a watt is fine. I know there are numerous options, but I wanted to try an ECL82.

Look at the Magnatone schematics : the Magnatone 411 used an ECL86, and I attest that it works well. Forget the tremolo stage and a single ECL86 is OK.

A+!