Why did this resistor burn?

[stratavox]

Good morning...Traynor YGM-4. I replaced the filter caps, re-tubed, and had no reverb. Poked about a bit and found R-26 (10k, 1/2 watt) burnt, and having scorched the board. In the attached schem, it's near the jack feeding the input of the rev tank. I know resistors drift with age, but with no other obvious issues...thanks for your time.



http://www.0rigami.com/vb/traynor_guitarmate_ygm3.pdf

[Willabe]

Looks like the plate load for the verb tube?

Tube shorted or filter cap (C-21) shorted? Took out R-26.    

Did you have verb _before_ you re-tubed and changed filter caps?
                    

                           Brad      

[Jack1962]

I would have say it probably burnt due to it's age or perhaps a voltage spike from the tube.

[PRR]

Could be drift. Could be shorted V5. Could very well be shorted C20, which pulls V5 full-ON, which will put about One Watt in the 1/2W part, which would slow-roast itself and the board (not explode like if V5 were shorted).

Since it's back to hand-labor, the difference 12 cents or 50 cents is negligible. Put in a 1W or 2W part.

[archangele]

This is your plate feed resistor and in many amps, they used 1/2 watt resistors in this position. This is barely adequate when the reverb is cranked. Now, if the tube (V5) is off and drawing more then normal plate current, this resistor will go out. The discoloration around this resistor is common in a situation where a resistor has been getting hot on a regular basis. The circuit is probably fine, it is just that this resistor should be at least a 1 watt, preferably a 2 watt to keep things cool. Another issue could be the reverb tank having been replaced with one with an input side of the  wrong impedance. This would cause the reverb circuit to be inefficient and force you to really crank the reverb to get even a minimal effect. Again causing a higher then normal load on that tube and it's plate feed resistor. Since this resistor is between the tube's plate and the filter capacitor, a bad filter capacitor would not cause this problem. This is one of the reasons why I always replace plate feed resistors in guitar amps I service and refurbish, along with power supply capacitors, bias rectifiers and coupling capacitors. Oh, speaking of coupling capacitors, any coupling cap going to the grid of V5 that leaks any DC voltage would cause a very high current draw across that resistor and would blow that resistor. Much like an output tube red plating when a  coupling capacitor from the phase splitter to the output tube starts to pass DC.

[tubeswell]

What PRR said. The cathode bypass cap shorting would mean the driver tube lost its bias, and would conduct flatout thereby smoking the plate resistor.

The load line for that parallel 12AX7 is crazy idling at 330V x 2.4mA! (0.8W) - and only making 2.4mA!. It'd make more sense to put a 12AT7 in there  getting 4.5mA @ 280V, (but you'd still want a 3W resistor), or a 12AU7 (again with a minimum 3W resistor) getting 8mA @ 230V.

[stratavox]

Can I just say "Thank You" to all you folks. I put up a question, and in no time get all the info I could hope for...I truly appreciate all the
time and info you've given. Problem on it's way out....