Does this amp ever need biasing? Diagram included.

[tedzepplin]

Does this amp ever need biasing?
What is this amp type called? cathode bias, fixed bias? other?

I'm not a technical person. I'd bring it to my amp tech if it needed biasing.

It's a Ken Watt's Model 24 amp based on a Supro 6424 schematic.

http://img162.imageshack.us/img162/4723/kitmodel24layoutzd6.jpg

http://img162.imageshack.us/img162/7996/s6424schematicdr2.jpg

[jbltwin1]

Cathodes are tied together and out through a 10 watt resistor. It's cathode biased and will not require re-biasing unless you want a different sound.

[tedzepplin]

Thanks for the speedy reply with the info.

and +1 with the question about changing resistors

[SatelliteAmps]

Using a 250Ω resistor will sound good, but you may want to change it to get a hotter or colder sound out of the amp. If this is being built as a homebrew experiment, I would recommend trying a few different resistors, just to see how the difference will change the overall sound of the amp.

[teletroy]

Weber sells a 500 ohm 25 watt rheostat, I've seen them on ebay before too. You can hook that up instead of a resistor and adjust to taste. Once you decide you like it, just measure and replace with a resistor. Or keep it in there and have an adjustable cathode biased amp.

[donnyjaguar]

If the output transformer is big enough you could modify it for more output power. Not sure if you want to achieve this though.

[5992]

While it isn't strictly necessary to rebias the amp every time you change the power tubes, it is a good idea to check the bias to make sure that the tubes aren't dissipating too much wattage. Tubes that are dissipating wattage over their maximum will wear out sooner than those biased within a range under the max.

[krell]

First, the speaker tap from the output transformer to the 1/4" speaker jack shows the wrong color wire.

The green wire that you see at the connection labeled FROM OT should be yellow by default...that's the correct wire for driving an 8 ohm speaker. As far as I know the green wire on this kit's Heyboer transformer can still be used but it would be only be correct for use with a 16 ohm driver.

Also, the 1/4" bass and treble input jacks should be physically swapped in the layout.

Specifically: the 1/4" BASS input jacks are the ones that are supposed to have three terminals with a tiny jumper wire.

I contacted Ken Watts back in 2007 and I did it again today...he confirmed for the second time that the yellow wire should be used for connecting an 8 ohm speaker.

As far as the input jacks go, you can see the mistake for yourself by comparing the input jacks in the color layout directly to the ones depicted in the black and white schematic shown below it in this thread.

The original Valco / Supro 6424T schematic also shows the little bridged wire connections on the BASS INPUT jacks....so the Watts color layout shown here is definitely wrong.

Ken's version of this Supro circuit is otherwise very faithful to the original Valco schematic...only quite a bit safer because it incorporates a grounded AC cord.

Hopefully these important corrections to the layout will help anyone who owns the Watts kit or anyone who wants to use the layout to build a 6424T Supro clone. If you've already built the kit, be sure to check your connections! I contacted Ken Watts earlier today because the erroneous color layout was still available for download on his site - he didn't offer much of an explanation but I assume that he'll be revising the layout promptly.



Good luck to all + best regards! ~Arno