[Heathkit] 6GW8 needed

[Drew P.]

Ronnie wrote:

> the one in my newly acquired HW101 is kaput.. hep me 
> hep me..  anyone have one of these beasts they will
> part with?

I found a number of online suppliers for these via a Google search.  Prices tend to be somewhat stratospheric as the audiophiles like this tube. One supplier might be findatube.com.

After repairing an SB-101 (very similar to HW-101) I measured circuit voltages to verify that they were as per the figures given in the manual.  The 6GW8's pentode section cathode voltage measured as per spec: 4 volts. With the 100 ohm cathode resistor, the pentode cathode current figure leaped out at me: 40 mA.  With the close to 300V plate voltage and a few mA screen current, a little mental arithmetic yielded approximate power dissipated in the pentode section plate: 12 watts.  Out of curiosity, I checked several tube manuals and found that maximum plate dissipation rating for the pentode section is 9 watts. Yeah, I thought that it ran a little hot.

I fiddled with the values of the pentode's cathode resistor and screen dropping resistor (which also drops plate supply voltage for the triode section) so that the pentode plate current was reduced and the screen voltage/triode plate supply voltage stayed within reason. This also reduced the gain slightly, as the pentode's cathode resistor also forms part of a negative feedback network.  IIRC, power dissipation dropped to 6w and gain dropped about 3dB.  I could probably have made up the missing gain by altering the feedback network or adding a bypass cap around part of the cathode resistance, but for a meager 3dB I figured it wasn't worth worth the effort. I will post values here if anyone is interested.

Has anyone experienced short life of the 6GW8 in the SB-100/101/102 or HW-100/101 as a result of what appears to possibly have been a design error (excessive pentode plate dissipation) on the part of Heath?

Drew