Ampeg B15N burning 22k resistor

[boroman]

I need someone smarter than me.... Unfortunately Ampeg B15N is burning ONE 22k resistor... 490 on the plates, 38mA bias so all within specs... Don't know what's going on. Spec is 1K/22k - this one pictured is 2W and still melts... Same with other set of 6L6GC power tubes.

[Helmholtz]

What about posting a full schematic?

[The Dude]

......Spec is 1K/22k......

I presume you meant 1W 22k? Yes, a schematic would help. We need to know what circuit the resistor is in and which node it supplies. I see two 22k's in your partial schematic. Which one is burning up? Also, in your picture it looks like the third band is black, unless just discolored by heat. If so, that would be 21 ohms instead of 22k ohms. I'd measure it to make sure it is the correct value. If someone put the wrong value resistor there, it would certainly get hot.

[Helmholtz]

Also, I see two of them in your partial schematic.

Actually I can't even identify one 22k.

[The Dude]

Agree. I wonder if it's the correct schematic or if we're looking at the right circuit? I wasn't even looking at the other resistors- mostly paying attention to the one that's obviously been hot.

[Jon Snell]

I would suspect a leaky capacitor.

[g1]

Appears to be R38 in the diagram. You can see one end goes to a 120K (R27) whose other end connects to a 510K (R29).

[Jon Snell]

Take the 6l6s out and see if it still gets hot. If it does, replace the capacitor can (40uF 40uF 40uF 500v), it is leaky. Don't just stick another one over it!

[boroman]

Hey!

Just to reply all the Qs - sorry it should be of course 1W/22k (not "1k/22k" - a typo)

Schematic below - itys the last rev of B15N probably. 1972

Tube set was swapped and bias was set, also burns that R38.

Caps were changed by someone before (main 40/40/40 cap was changed to 22uf x 6 in pararell as I see) - maybe here's the problem... I'd change them to 47uf's

[boroman]

Solved: I just rpleaced those 6x 22uf caps by new 3x 47uf and works great now. Thanks for your input guys!