SPP 6CW5 EL86 bias resistor value?

[Pooch]

I've been using 270 ohm, 5 watt, bypassed with 100uF, 50V as indicated in the assembly manual. I blew one of the 270 ohm resistors. Is there a better value for these tubes? I just got replacements in the mail and after I install them I'll post pin readings. Maybe just a bad tube?

 

[vinylkid58]

What's the idle current thru the 270 ohm resistor? You could be very close to the 5 watt dissipation rating.

jeff

 

[Tubelab_com]

The idle current should be less than 50 mA per tube, which results in about 14 volts on the cathode. That's under 1 watt on a 5 watt resistor. I would suspect a tube that ran away, or a bad resistor.

 

[vinylkid58]

Oops, I was out a decimal place.

jeff

 

[Pooch]

Yeah, I'm leaning towards a bad tube. The Ohmite that burned and turned black reads 277 ohms, tough little bugger. I'll check readings later, gotta head out.

 

[Pooch]

I show .006 apms across the resistor. Milliamp scale shows 0, probably over the range?
Pin 2 = .46 VAC on all 6CW4
Pin 3 = 18 VDC "
Pin 4 = 14.5 VDC "
Pin 5 = 14.5 VDC "
Pin 7 = 227 VDC "
Pin 8 = 227 VDC " Pin 9 = 230 VDC on all 4 outputs

 

[Pooch]

I checked Pin 2 for DCV and got .02 VDC on 3 of the tubes and .05 VDC on the suspect one that had the blown resistor. Earlier after I replaced the resistor and first plugged in the amp, I got red plate on the suspect tube. I switched the tubes on that channel and no red plate, but the new resistor started smoking. Then I switched around the 12AT7s and the amp ran, no smoke, no red plate. The above measurements were taken after that.

 

[Pooch]

I thought I mentioned this was an EL86, 6CW5 version of this amp. I must have left it out. Antec IT200 power transformer and Sansui 1000 tube receiver output transformers. They're 3.3k ohm.

 

[Pooch]

Rechecked the heater on AC setting and got 6.36 VAC on pin 4 of two of the tubes and 6.36 VAC on pin 5 of the other two tubes .08 VAC on pin 4 or 5 depending on what tube. V drop across heater?