Single ended power amp - weird current draw

[mlabbee]

I'm breadboarding a single ended power amp.  The tube is a 6201 - the first stage is set up as a preamp: 220k plate r, 2k2 cathode r.022 cap and a 500k pot on the preamp output.  The second stage has: preamp output (from 500k pot) into the grid, a 3k3 cathode resistor to ground and a 3.5 watt output transformer (25k ohm primary, ohm secondary) hooked up in single ended mode (8 ohm side through speaker and 25k ohm side hooked up to B+ and the plate).

When I turn it on and increase the B+ everything seems to work fine until I hit around 50 volts - the then current pegs the meter (200 mA) and I shut down.  Is it supposed to do this?  Doesn't seem like the current should spike like that - any thoughts on what I might be doing wrong?

[sir_modulus]

Hmm...I'm no amp wizard...but maybe a component is not rated for more than 50 volts? I dunno...maybe something is shorting, or giving a low resistance path to ground due to overvoltage.

Just my 0.02F, Cheers!

Nish

[R.G.]

Georg Ohm to the rescue.

You have two current sensors in there - the two cathode resistors. If I were doing that, I would watch the voltage across those resistors as the shoot-the-moon current happened to see which tube is getting you.

If it's not one of the tubes, sir_m is right - something is breaking over.

[mlabbee]

I got it - and discovered qa new way of debugging circuits.  Just wait until a cap explodes . . .  I had my bipolar power supply plugged into my breadboard and it was shorting the high voltage supply.  After a few seconds at high current while I was measuring voltages all over the circuit to pinpoint the problem, one of the caps on the low voltage supply started smoking.  D'oh!  Unplug unused circuits!!!  

Thanks for the advice, gentlemen!

I think I like class A, single ended.  Lots of charachter to the sound.

[puretube]

you sure got an adequate output-transformer in there? (SE vs. PP...airgap...)

[R.G.]

I think I like class A, single ended. Lots of charachter to the sound.

So do I - a lot.

My favorite amp sound is what you hear on old blues recordings - a low wattage amp being pushed beyond its capabilities and going into distortion with a lot of treble loss. I get that from my favorite practice amp.

The amp contains a fairly standard Fenderish preamp with a switchable two-12AX7-stage over drive running into a single ended 6AQ5 pentode. The output tranny is ripped from an old 1950's furniture-style stereo console amplifier. It's not loud - maybe 5W max - but man, does it sing!

Single ended is the bee's knees...

[mlabbee]

Puretube - the OT should be powerful enough. It's rated for 3.5 watts and the 6021 puts out about 1 watt maxed out. I've read that rating your OT for a minimum of twice your output for single-ended operation is a good design point.  Does that sound right to you?  It seems to be working pretty well . . . but you never really know until things start smoking

R.G. - I'd love to hear that practice amp of yours - seems like a very cool set-up.  I've only played with triodes so far - do pentodes have a very different sound?  The 6021s sound awesome, but are very low power. I'd like to try something with a bit more oomph.

[puretube]

ask in an amp-forum about the neccessity of airgaps in SE OTs...