Tabletop 12au7 Amp

[tmaxwell]

Hi,
My first post, hope it's a good one!
Here's a little project amp, built for my daughter's room. First start-to-finish design and build for me, it was a lot of fun!
I wanted something REAL small, to sit on the dresser. It didn't need too much volume, though in the end it's got plenty. It's made with mostly recycled parts and I feel makes a pretty easy, not to mention cheap, first project.

About the circuit...
Power supply is based on tid-bits I've used here and there. The back to back wall-wart transformers are nice and cheap, as is the voltage doubler, given access to caps. These came from a t.v. and a computer PSU (both shot, of course). It puts out ~ 280 vdc under load. Pretty sweet for the price, if you ask me. I'll definitely use this in the future for tube stompboxes.

The amplifier is really simple. A self-driven 12au7 with grounded cathodes.
That's about it. I did fiddle with the input impedance to accommodate both headphone-out, and 1/4in guitar out (it doubles as a teeny guitar amp, and sounds quite good with a clean boost or overdrive unit in front...)
The OPT is garbage and not really ideal, but is sooo small, so it fits in the case. Bottom line is, this amp sounds WAY better with a larger speaker, and a proper OPT. There isn't a volume control, that is up to the device running through it.

SO, I'm posting this as both a potential project for others ( I feel like it's pretty unique ) as well as for critique/suggestion. This was designed pretty seat-of-the-pants style, and could almost certainly use improvement circuit-wise.

Here are my initial questions
Only 60vdc on the first plate? Is that right? Plate #2 is full voltage - 280 or so.
After about 5 minutes of duty (Prior to installing the fan) the volume dropped, and the signal became quite distorted. What in there was responding to heat in that way? OPT, PT's, tube, all seemed within temp range. Certainly, nothing way out of line, as far as I could tell. Fan takes core of it, but is a little whiny, which I hate, esp. on an otherwise dead quiet amp.

Anyway, attached are some images, for your viewing pleasure! (pictured with a truly dead technology, the Minidisc!!!)
take it easy,
tyler

[planet10]

Cool.

Any idea what an optimum OPT value would be for this?

Looks like a good use for 12AU7s

dave

[tmaxwell]

Not sure on optimum value, but this one is really low primary resistance. That is another question I have, as a matter of fact- how exactly does one go about figuring that value, both the optimum for the tube, ant the actual impedance of the transformer, given DCR.
thanks for looking, tyler

[tmaxwell]

Also, sorry for being a forum "noob" so-to-speak, but how the heck to I edit a prev. post, I need to rotate the first image!

[Cookj]

Heater supply bridge rectify is drawn wrong, top diodes should be reversed - so heater op's should be taken from a-a and k-k

Regards

john

[planet10]

Originally Posted by tmaxwell

Also, sorry for being a forum "noob" so-to-speak, but how the heck to I edit a prev. post, I need to rotate the first image!

You only have a limited time to edit a post (30 min IIRC).

I rotated the drawing for you and fixed the diodes while i was at it.

dave

[jerluwoo]

A output tranny with 20k/8 ohm or more common 10k/4 ohm with 8 ohm load will net the most power out of a single section of an 12au7 which will come in at a little under half a watt. Needs to be a good tranny with high primary inductance to get any bass lower than 100hz though. Your probably killing your output section. It shouldn't be ran at higher than 8.5mA with that voltage across it to keep it within spec. That means a voltage around 11 volts on the cathode. The cathode resistor needs to be at least 1.2k.

[planet10]

Cool... i've got some OPTs with that impedance... if it was a tweeter amp, the bottom end would be a no care...

dave

[Arnulf]

Nice, I;d change that 100K load resistor to something more sensible though if you want less distortion, that is if that 60V Va of first stage worries you. Try ~20K or so for starters, reduce cathode resistor of first stage to 680R. measure or calculate current through first stage them, measure cathode to anode voltage and report back with the results if you need more help tweaking your amplifier.

[Merlinb]

Originally Posted by Arnulf

Nice, I;d change that 100K load resistor to something more sensible though if you want less distortion,

Lower load resistances generally result in more distortion, not less.