Magnatone 440 bias type

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Magnatone 440 bias type

Postby Turkeyboy on Sun Jan 01, 2017 3:49 am

Hello, I just got a magnatone 440 and want to change the power tubes. Does anyone know for sure what bias type it is? I've read that the 213 is cathode bias and the 440 is based on that but I haven't found a confirmation that it retains the cathode bias. I'm not good at reading schematics which I assume could answer it but I don't know enough to get that from it. Can anyone confirm the bias type of the 440?
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Re: Magnatone 440 bias type

Postby Baron Von Machinenmann on Mon Jan 02, 2017 5:22 pm

The two schematics I found online show it to be cathode biased, BUT if you are going to use new production power tubes the bias point will need to be verified by a competent tech as the new tubes have very wide tolerances compared to the original glass used back then.
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Re: Magnatone 440 bias type

Postby Turkeyboy on Mon Jan 02, 2017 9:04 pm

Thanks for verifying that for me. I will be using old USA tubes so I should be good to go. Thanks again!
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Re: Magnatone 440 bias type

Postby stratele52 on Tue Jan 03, 2017 10:49 am

You will have to take some reading of the 280 ohms 7 watts resistor wired on output tubes.
With 345 volt ( maybe more with modern house outlet voltage, check ) you may not have more than 36 milli-amp on each 6V6 tubes. Total of 72 mA on the 280 ohms

Bias check is a must with expensives old tubes. I often see wrong bias resistor on many old amps
Check if fuse is the right one to protect your amp. Over years some people put too powerful fuse.

Shematic here;

http://www.magnatoneamps.com/schematics ... ne_440.pdf
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Re: Magnatone 440 bias type

Postby pdf64 on Tue Jan 03, 2017 12:31 pm

I think that the shared cathode resistor looks to be 250 ohms on the schematic (thanks for the link!)?

With shared bias arrangements (no matter whether it's cathode bias or a fixed negative voltage applied to the grid), it's beneficial for the tubes to have reasonably similar cathode currents at idle.

Note that's a different thing than the 'matching according to gm / manufacture date / batch #' that's often offered on ebay etc by vintage tube vendors.
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Re: Magnatone 440 bias type

Postby stratele52 on Tue Jan 03, 2017 1:26 pm

Yes it is a 250 ohms.
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