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Old 5th April 2011, 07:31 PM   #1
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Default 300B with interstage

Hi,

I'm looking around for a new amp project and I found this one. While I have a few scratch built amps under my belt, I am still learning when it comes to circuits. What is the purpose of the relays in the B- and C+ arms of the PS? Do I need them?

Thanks

Jonathan
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Old 5th April 2011, 07:48 PM   #2
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Hi!

The relais in the B+ of the output stage will be triggered only when the bias voltage is there. This prevenst the B+ from being present to early in case bias is not settled yet and acts as a safety mechanism in case the bias fails, B+ will also be interrupted and thus avoid over current on the precious 300B.

For my taste the 417 is not a good choice as driver for the 300B since it will not have a lot of headroom. It will work however.

Best regards

Thomas
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Old 5th April 2011, 08:40 PM   #3
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Most designs avoid this complication by using solid state rectification in the bias circuit. Tube bias is certainly much cooler.

Not liking relays much i would examine the possibility of using a directly heated diode for the bias instead.

Last edited by analog_sa; 5th April 2011 at 08:43 PM.
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Old 5th April 2011, 08:45 PM   #4
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Thomas,

I think I understand now. The switch on procedure for this amp would be: a) turn on main switch; b) wait for the heaters to come on, manually turn on the GZ37 relay which would then eventually automatically trigger the 24V relay in the 2nd EZ81.

Jonathan
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Old 5th April 2011, 08:51 PM   #5
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Hi Johnathan,

no, it is much simpler: just turn on the main switch, the rest comes on automatically. With the mains switch on, both EZ81 start to heat up. Driver B+ and output stage bias come up. Once bias voltage exceeds a certain level, the relais for the outputstage B+ gets triggered

Thomas
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