New Amp Builder question

For those building and repairing amps on their own. Learna and share ideas.

Re: New Amp Builder question

Postby Unit_1 on Wed Jan 15, 2014 3:08 am

NICE GLASS!

again, i am unfamiliar with VOX amps and el84's and am not even certain if yours is a cathode bias or fixed adjustable

if i were you i'd start a new thread in the vox section "how do i bias my vox"
:cheers:
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Re: New Amp Builder question

Postby black_dog on Wed Jan 15, 2014 5:18 pm

That's exactly what I did.

Vox are fix bias and there is a bias resistor that manages the bias. Next I need to learn how to correctly calculate and set the bias.

However, wondering if hotter is better ? (up to what the tubes will tolerate or wallet will tolerate as the result of burning through output tubes).

Thanks Unit_1!

-James
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Re: New Amp Builder question

Postby pdf64 on Fri Jan 17, 2014 5:20 pm

Vox are fix bias

Where did you find that info?
It is false, AC30 and lower tube amps are cathode biased.

To confirm that the static dissipation is appropriate, measure the Vdc of the el84 plates and cathodes, with respect to ground.
That means put the black meter probe on to the circuit ground point.
Be careful not to let the red probe slip on the tube socket terminal and short something out.

For the above to be useful, the tubes have to be fairly well matched, as both el84 share the cathode circuit.
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Re: New Amp Builder question

Postby black_dog on Tue Jan 21, 2014 7:19 am

Yes of course the Vox is cathode biased. Got the two confused (newbie)...

Thanks Pete for the good information on how to check to see if the bias is correct...

-James
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Re: New Amp Builder question

Postby pdf64 on Tue Jan 21, 2014 12:48 pm

James,
apologies I must have been distracted when posting earlier, as I didn't include the full details regarding the calculation.
Please see http://www.webervst.com/tubes1/calcbias.htm and give the bottom calculator a go.
Ask if something doesn't make sense.
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