Small 1W hifi amp

[jeppix]

Hello to everybody!!!

I'd like to build a small 1W hifi tube amp.
I have at home this material: 2 Fender 3,5Watt output transformers and some Talema toroidal transformers.

Now I need some schematics and some suggests on what kind of valves to use.

The amp must be very small, it must be portable because I want to use it as ipod amp with 2 small speakers in my office.

Thanks!!!!

Andrea

 

[yagoolar]

Maybe you can use 6c45 and single stage amp
6c45 amp < 2 W

Or e.g. ECL86.

 

[Sal Brisindi]

Here is my little 1 watt per channel stereo amp. I fit it on a small radio shack enclosure that fits in your hand. The only thing is you need to plug it in a isolation transformer for safety. I notice you are from Italy so you can use a 220v to 120v isolation transformer since the filaments add up to 120 volts.

Sal Brisindi

Front View

Rear View

Schematic

 

[jeppix]

Hi Sal,

very interessing amp!!!!

It's what I'm looking for.
Could you give me more details?

A more complete schematic and all the bill of materials the build it!

It will be my first tube amp... I made other type of circuits but nothing with tube... so it will be my first time

Thanks.

Andrea

 

[Eli Duttman]

Good old AC/DC AA5 Sal. 😀 Half wave rectification puts a "standing" DC voltage on the isolation trafo, which it is not designed to tolerate. 🙁

Still, that design provides the seed for a project that will work in either "120" or "240" V. zones. Double the tube complement. The heaters can be wired as a single "220" V./150 mA. string or as 2X paralleled "120" V./150 mA. strings. Hybrid bridge rectify the O/P of a Triad N-68X isolation trafo using 2X UF4007 SS diodes to form the connection to ground and the 2 50DC4s forming the B+ connection. The N-68X has dual primaries that allow it to work with both common AC mains voltages.

"Steal" the signal topology used in the AV8. A common cathode gain stage DC coupled to a "concertina" phase splitter that drives push/pull O/P pairs. The 'HB8 data sheet shows the triode to have both high mu and reasonably high gm. 🙂

The caps. at the I/P, which provide some safety in a transformerless PSU design, are no longer necessary. Inexpensive 100 KOhm log. taper pots. are all that's necessary.

 

[planet10]

Here is another... this one needs a bit of drive to hit full power (1.4W in pentode)

http://www.t-linespeakers.org/tubes/SEP_50EH5.html

dave

 

[Sal Brisindi]

Hi Andrea,
The value of the parts are on the schematic, only one channel is shown. Just double the parts for the amplifier parts and connect it to the power supply.

Its amazing how loud 1 watt really is.

Regards,
Sal

 

[jeppix]

Hi,

I've a couple of Talema trafo: primary 2x115VAC; secondary 2x7V.
Could I use them in the "upside down" method 😕 that means connect a wall trafo 15V to the secondary (that became primary) and I obtain 100VAC in the secondary (that was primary) (perhaps????).

Thank you.

Andrea

 

[Sal Brisindi]

I just noticed there is a difference with my schematic and my actual amp, I did not put a volume control on my amp, I was going to but never did.

Sal

 

[Eli Duttman]

Hi,

I've a couple of Talema trafo: primary 2x115VAC; secondary 2x7V.
Could I use them in the "upside down" method 😕 that means connect a wall trafo 15V to the secondary (that became primary) and I obtain 100VAC in the secondary (that was primary) (perhaps????).

Thank you.

Andrea

What's the VA rating of the Talema trafos? It's easy enough to work out the RMS current available at "115" V. from the VA rating. If you bridge rectify the "115" V. approx 1/2 of the RMS current will be available as DC.

BTW, if you use a 15 VAC "wall wart" with adequate current capability, "12" V. heater tubes could be employed. That would open up all sorts of interesting possibilities. 😀 Even with a substantial heater draw, not much heat will be generated in the dropping resistance.

 

[jeppix]

Hi,

the Talema are 10VA.

I also find this: Class A Subminiature

Bye

Andrea

 

[ray_moth]

You might be interested in this thread

 

[Geek]

The 6AH4GT is a spectacular tube for 1 watt amps 🙂

Circuit

 

[595ss]

http://www.roehrentechnik.de/html/kolibri.html

In German, but the scematic is all the same😉

 

[tubesguy]

I'd suggest a spud amp with an E55L (8233). One tube, B+ voltage of 150v, which means you can use an isolation transformer for B+. 3.5k output transformer, a resistor or two, and a cap or two, and you're done. Good luck with the project. - Pat

 

[Eli Duttman]

Hi,

the Talema are 10VA.

I also find this: Class A Subminiature

Bye

Andrea

Andrea.

While interesting, the submini tube circuit is a distortion making guitar amp. You did say HIFI.

When reverse connected, 40 mA. of B+ from each of your Talema trafos (total 80 mA.) is about right. That put's us back at a variant of Sal Brisindi's circuit. To get genuine HIFI performance, loop NFB will have to be added. 18HB8 tubes will be used, as a "simplification". The B+ will be SS rectified. Resistance will be used to drop the 18 VAC from a "wall wart" to the 15 V needed as I/P to the Talema trafos you already own.

You will have to buy or build a 2 A./18 VAC wall wart. A quick check did not show a suitable "off the shelf" product. I believe that Talema is now Amveco. The 35 VA Amveco model 62074 is a suitable trafo to construct the "wall wart" around. The Amveco catalog is here.

 

[jeppix]

I'm little bit confused because all the schematics are very good and I don't know what to make...

I'd like to use what I already have so the Talema trafos, the 6021 tubes and the Fender output trafos.

I also need something "portable".

The only schematic that use these kind of tubes is the Class A Subminiature Tube Guitar Amp but in the description there is written "...in the current configuration the preamp has a lot of gain but is not capable of delivering loud clean tones..." so I'm not sure and I don't like the power supply section, I prefer to use my Talema trafos, if it's possible.

I don't need tone control but only the volume and I need a stereo version so probably I could use two Talema trafo....

Just little modifications....

Could someone help me with these modifications of the schematic?

Thanks.

Andrea

 

[jeppix]

Thank you Eli for the suggestion but I find lot of difficult to find 18HB8 tubes in Italy , so i must something else.... 🙁 🙁

 

[Eli Duttman]

I'd like to use what I already have so the Talema trafos, the 6021 tubes and the Fender output trafos.

Andrea,

You also said you wanted 1 WPC. A single ended 6021 will yield (SIC) approx. 200 mW. That which you ask for does not compute. 🙁

Yes, the 6021's plate can dissipate 1 W. However, 20% efficiency is what you get from Class "A" triodes, in the real world.

PP 6021s operating in Class "AB" should yield approx. 1/2 W. If you can make do with that little power, 3X 6021s will combine with the Amveco trafo I previously mentioned. What sort of drive voltage does your ipod generate?

 

[jeppix]

You are right Eli I'm making some confusion but probably what I'm looking for doesn't exist... 😡 (only one was be able Zvex's Impamp)

It's difficult to combine a small amp and a decent hifi output probably I've to re-think the entire project.

The problem is that you and others give me much ideas but some kind of tubes are difficult to find out and I don't have the experience to try to redraw a schematic.

Thanks.

Andrea