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previous: DanF Hi Cole – So changing the cathode r... -- 7/31/2000 4:38 AM

Re: 448VDC on the plate of a 6V6 (using a 6X5 rectifier tube)

7/31/2000 3:04 PM
Cole	Re: 448VDC on the plate of a 6V6 (using a 6X5 rectifier tube) Yup, upping the cathode resistor will lower the idle current, the plate-cathode voltage will increase slightly. The 12ma figure is more likely in the 35-45mA range (the cathode voltage will increase). I would go for the 470 ohm cathode resistor first, if that doesn't get you in an acceptable range, you can try lowering the overall B+ voltage with a zener diode. I had a similar problem to what you're having with a silverface Champ amp - it was running too hot with the stock configuration, even with a 560 ohm cathode resistor and new 5Y3 rectifier. So I inserted a 50V, 10W zener between the center tap of the PT and ground, which lowered the B+ down to an acceptable level, which allowed me to use the stock 470 ohm cathode resistor. The cathode resistor's wattage rating is as such: if it's idling at 50mA, and the drop across the cathode is 22V, then the dissipation is .050A*22V=1.1W. To be safe, double (or triple) that, so you're OK using a 3W or 5W resistor. I normally use the big block 5W resistors, available locally or through Mouser.

7/31/2000 3:04 PM

Cole

Re: 448VDC on the plate of a 6V6 (using a 6X5 rectifier tube)

Yup, upping the cathode resistor will lower the idle current, the plate-cathode voltage will increase slightly. The 12ma figure is more likely in the 35-45mA range (the cathode voltage will increase).

I would go for the 470 ohm cathode resistor first, if that doesn't get you in an acceptable range, you can try lowering the overall B+ voltage with a zener diode. I had a similar problem to what you're having with a silverface Champ amp - it was running too hot with the stock configuration, even with a 560 ohm cathode resistor and new 5Y3 rectifier. So I inserted a 50V, 10W zener between the center tap of the PT and ground, which lowered the B+ down to an acceptable level, which allowed me to use the stock 470 ohm cathode resistor.

The cathode resistor's wattage rating is as such: if it's idling at 50mA, and the drop across the cathode is 22V, then the dissipation is .050A*22V=1.1W. To be safe, double (or triple) that, so you're OK using a 3W or 5W resistor. I normally use the big block 5W resistors, available locally or through Mouser.

Replies:

DanF Oops... I meant to say 12 watts, 34... -- 7/31/2000 8:25 PM