Ampeg B15N 1961 cathode bias?

[boroman]

I have never worked on early 60s B15N, but have this baby on my workbench right now. Just a quick question - tubes get 60mA (per tube) when in fixed bias B15N it's around ~35mA range.

Isn't it a bit on the high side? I know that cathode biased amps would have higher bias but 60mA seems super high. Amp sounds good and no redplating. The bias cap (and all of the power supply caps) is new, resistor is 250ohm (measures 259, so in specs).

 

[JimmyM]

I have never worked on early 60s B15N, but have this baby on my workbench right now. Just a quick question - tubes get 60mA (per tube) when in fixed bias B15N it's around ~35mA range.

Isn't it a bit on the high side? I know that cathode biased amps would have higher bias but 60mA seems super high. Amp sounds good and no redplating. The bias cap (and all of the power supply caps) is new, resistor is 250ohm (measures 259, so in specs).

Since I'm no Tech, feel free to take this with a grain of salt, but if it sounds good and there's no problems, biasing at the spec that Ampeg recommended is the way to go. You could try biasing lower and see what happens, but my thought is they specified 60 for a reason.

 

[LetItGrowTone]

A schematic I have of "MODEL B-12NA & B-15NA" (I don't know that this is what you have) shows 6L6GC plates 470V, screens 450V, and 36V on the cathodes, sharing one 250r resistor. That implies 72mA through each tube. Yeah, very hot. [Extra info: if the tubes aren't matched one of them could be pulling more than half of the total current. This is one reason I use separate cathode resistors, each bypassed with its own capacitor.]

Another example is in the RCA Manual for 6L6/6L6GC, "Push-Pull Class AB1", shows plates 450V, screens 400V, -37V on the grids (fixed bias) and 116mA (plates) + 5.6mA (screens) total at idle, or 60.8mA each.

If the above corresponds to what you have, then you're ok.

 

[beans-on-toast]

Ampeg mucked around with the cathode component connections in the early 60’s. There were changes while they were figuring out what to settle on. Use a 250 ohm 10W resistor in parallel with 50uF@50v capacitor (capacitor [+] goes on the cathodes, not ground side).