Aikido loses power?

[lazyfly]

Hi,

I've built an Aikido 6SN7 based on Broskie's boards and some parts from 8 Audio such as:

Transformer:
Output:
0-250-280V (100mA)
0-6.3-15V (2A)
0-6.3V (2A)
0-10V (0.3A)

Choke:
5H 100mA

PS:
B+ and up to 3 sets of independently regulated filament power source to tube heaters.

The problem is that the heaters just dim out and the music fades away after around 5 minutes of play time.

I believe 2A shared among the four heaters is a little under rated but thought it just wouldn't work to its full potential and not actually just fade out.

Any suggestions would be greatly appreciated.

Thanks for your time.

Cheers
Kendal.

[koolatron]

2A is insufficient to heat four 6SN7s, which are rated at 600mA per tube. You're underspecced by almost half an amp AC.

If you're rectifying the 6.3V winding and feeding your heaters DC, you're looking at even steeper requirements. I had this exact same question when I built my Aikido, and John kindly responded to my email with an explanation.

Basically, the upshot is that the current rating on the filament winding is for AC operation. If you're rectifying it, the rule of thumb to use is to divide the AC current rating by 1.8 to arrive at its DC current rating. 2 / 1.8 = 1.11A, almost enough for two 6CG7s.

All that said, is your power transformer getting hot? I expect that it is; you may be exacerbating some type of thermal instability in your transformer by wringing too much power out of it. I'd check the AC voltage across each of your 6SN7s - if it's not within 10% or so of 6.3V, you're probably damaging your tube's cathodes anyway. My suggestion is to purchase and use a separate filament transformer with at least 3A current rating. I'm relatively certain Rat Shack has something serviceable if you're looking to get it playing, like, tonight.

--k

[kevinkr]

Sounds like in addition to the under rated winding issue your filament regulators are under heatsinked, (if I understand your comment about regulated filament supplies) and going into thermal shutdown - this might actually be saving your grossly overloaded power transformer from burning out its filament winding.

Use both sets of heater windings if possible, consider ac heating, or add a separate 4A 9V transformer (Do not double) to run the regulator.

[Freecrowder]

I feel your pain! Let me give you my version. I've been bench testing filament PS for the last week and a half. I'm running 4, 6sn7's and it's a heck of a load with DC. I've ended up using a trannie with 2 secondarys rated at 12vac 2.33amp. I rectify with a 10 amp bridge and then drop the voltage with a string of 5w power resistors rated at .2 ohm. I filter this with a 5000uF electrolytic. All of that is to feed the input of a LM338k, TO-3 variable voltage regulator. I adjust the reg to 6.3vdc and feed all the heaters in parrallel. The regulator is mounted on a 3"x 5" heatsink. I drop the voltage to the reg with the resistor string to limit how much the reg has to dissipate. It only has about 9.5 volts input and limits the heat buildup. I can run this for days w/o any problems from any component.

freecrowder

[Geek]

Hi Kendal,

Easy fix!

Have one 6SN7 as the Aikido bottom tube, the other as the top tube. Same for the other channel.

You have two windings each with 6.3V @ 2A. Use one secondary to power the bottom tubes, the other the top tubes. Each secondary only needs supply 1.2A.

Cheers!