Help with cheap mono SE

[nightanole]

Ive been gathering some scrap and i think i have enough to build a simple SE amp for my desk at work. It only needs to power 1 book shelf in cubical land and they dont like it if they can hear it 30 feet away over the pc fan noise.

Anyway heres what ive gathered.
1 grubdac with wolfson 2vrms output
1 365-0-365 power transformer with 6.3vac tube heaters.
150 ohm 5 henry choke
lots of 50-80uf 450v caps
1 china 50k stepped pot
0.047 μF PIO input cap
100nf 716P series Orange Drops


Biggies i still need to buy.
tube
socket
output transformer
stereo summing circuit


If i go the edcor route i can get output transformers for about 20 bucks. I can also get stereo summing transformers for 10 bucks. What i can also get is 10:1 stereo summing transformers.
EDCOR - WSM6415

Would 10:1 be enough to drive a el84 spud? Or should i just copy 1 channel of the tubelab simple se and use an el34 with a 12u7?

[Joshua_G]

Before deciding about OP transformer and tubes, you have to have a schematic, which will determine what you'll need. Most likely you'll need at least 2 tubes: driver and OP. The Tubelab SE is an option, however I haven't tried it and I have no idea about its' sound quality.

[nightanole]

I have several schematics on the el34 and el84. All have driver tubes, but some el84 hint that they are not needed with a good preamp. Right now my preamp is a amb mini3 with a swing of 10vpp.
If i can get away with a input stepup transformer and reduced output el84 (2 watts) I would rather do that then complicate things with a driver tube.

If i need a driver tube i might as well do the driver/el34 route.

As far as sound quality, its a noisy enviroment, so revealing quality isnt really necessary, just clean 1 watt bass would be enough. Im just tired of my retrofitted 1 watt console amps with open back speakers and fuzzy output transformers that are as small as a golf ball.

[Joshua_G]

For your requirements, most SET amps will do. You may choose any schematic that is reported to work well.

[kevinkr]

Originally Posted by nightanole

I have several schematics on the el34 and el84. All have driver tubes, but some el84 hint that they are not needed with a good preamp. Right now my preamp is a amb mini3 with a swing of 10vpp.
If i can get away with a input stepup transformer and reduced output el84 (2 watts) I would rather do that then complicate things with a driver tube.

If i need a driver tube i might as well do the driver/el34 route.

As far as sound quality, its a noisy enviroment, so revealing quality isnt really necessary, just clean 1 watt bass would be enough. Im just tired of my retrofitted 1 watt console amps with open back speakers and fuzzy output transformers that are as small as a golf ball.

Where are you using the "fuzzy output" sounding console amps and open back speakers? If in your office cubicle then there might be something going on - I suspect these console amps are good for a couple of watts anyway and in that environment you do not need a whole lot of bass anyway. So I am not sure that the direction you are headed in is necessarily an improvement in practical terms. Why not fix the fuzzy sounding amplifiers, convert them to triode mode, use better components (or replace suspect original parts) - at low powers most console amplifiers were capable of pretty decent performance (there were plenty of exceptions I'm sure and you may have some of those)

Also are you sure the fuzziness you hear is not due to low bit rate mp3 encoded music? Or are you using lossless (flac?)

Finally simple resistive summing is all that is required to convert a stereo source to mono for this application.. I would however recommend using a simple driver stage, it would cost no more than the fancy mixing transformer you were talking about using.

Also anything beyond an 6BQ5/6AQ5 power tube is gross over kill in this application, also your proposed power transformer is far too large for the application. Save it for something more ambitious and pick up an Allied or similar.. George (tubelab) could advise as to specific models that might be suitable.

I'm surprised your employer actually lets you use tube gear in your cubical, my last few have been adamant that nothing without a UL/CSA approval shall be used even in one's cubical.

[nightanole]

My amps are a converted portable zenith radio with an open back speaker and no isolation transformer, and a hammond ho44 organ amplifier that i tried to convert to audio use with the help of eli. Unfortunately the hammond uses an expensive ECL86 triode/pentode combo tube, its modified, and has a problem with the gain increasing with frequency. Its so complicated that i dont know how to service it since its a funky PP which i dont understand.

Its got a good power transformer and a good power supply in it that i can tune for 300-400vdc. And its got NOS tubes that i can sell for $50 to fund the SE project.

I was just wondering what people thought of using either:
1: el84 with a 2vrms in
2: el84 with a stepup audio transformer
3: standard issure 12aXX and a el34

The audio transformer just seemed like a good idea because; i have to make an edcor purchase anyway, it eliminates the input cap and only cost 7-$10, and i would think it would be a more proper way then a couple of 1k resistors that attenuate 6db and be basically a 1k load on the source.

Hmm... On that note couldnt i just tie the outputs of the 50k pot together?

[Joshua_G]

If you have a schematic for EL84 with stepup transformer that you have reports that it works well, it should do and satisfy your needs.

[mashaffer]

6EM7 are $10, 6BM8 about $15-$16, 6LR8 are less than $3 at AES $10 at the tubestore.

All of these should do what you need with only one tube (a genuine spud in you case since it is one channel).

The 6EM7 is a small signal triode and power triode in one package using standard Octal sockets. There are several schematics out there or you could do your own.

The 6BM8 is a small signal triode and power pentode using standard 9 pin miniature sockets and is in current production. The output is much like a somewhat lower powered 6BQ5 (EL84).

The 6LR8 is a small signal triode and power pentode of somewhat higher power. It uses a 9 pin compactron socket that is less common but still available. It is electrically the same as the 6LU8 that Kegger over at AK uses in his famous $250 amp so you could lift that schematic basically unchanged.

Any one of these could be used in a dead simple SE topology with cathode bias and capacitor coupling with maybe a touch of FB in the case of pentodes run pentode or UL mode. You could also triode strap the output pentodes and run them open loop as well.

I bet any one of them would have adequate power for your application assuming any reasonable speaker. The 6EM7 would be the lowest power and the 6LR8/6LU8 the highest though.

If you decided to go the Kegger route you could save some money by going LCRC... instead of LCLC... but the radiodaze chokes are only $10 or so.

[nesha]

How about this one: file:///D:/hi%20-fiiii/AMPS/DIY%20AMPS/EL84/MagneQuest%20Forum.htm

[nightanole]

Looks pretty good but i dont have access to your hard drive