Fender Super 60 red plating and 12AT7 issue

[slaweks222]

I'm trying to fix old Fender Super 60 with loud hum and crackling after switching Standby on, causing quick red plating on both 6L6GC output tubes. Reason 0V grid voltages on pin 5 on both tubes.

I checked bias circuit from power transformer to grid pin 5 and checked all components resistors, coupling caps, everything measured and looks fine, and found fuse F5 T250mA ( its european version) blown, no other damages. Fuse replaced.

Now I have -49V on both pins 5 with no tubes -looks good.

However I also noticed very low plate voltage on preamp 12AT7 pin 1- 170VDC ( schematic says 231VDC) and only 14V on pin 7 ( schematic shows 225V) also pins 2 and 7 grid only reads 27VDC and 29 VDC, instead of 40VDC from schematic. All measurment conditions as requested in schematic, except power tubes inside.

Could somebody experienced give me a tip? , a possible reason of that situation?
I would be grateful.
Slawek

[Jon Snell]

Check the anode load resistors on the phase splitter; 100k and 91k.

[slaweks222]

Thank you very much I will.
Schematic below:

http://ampwares.com/schematics/super_60_rack.pdf

[Chuck H]

It should be noted that the bias supply fuse didn't blow for no reason. Could be a bad filter capacitor going short in that circuit. Possibly the same issue with the C node feeding the phase inverter. Check the supply voltage right at the C supply node. It may be time for a "cap job".

[slaweks222]

Thank you, I will have a look

[Enzo]

Or an internal short in a power tube putting B+ on a grid pin

14v on pin 7? You mean pin 6?

On V103, the phase inverter, pins 2 and 7 are the grids. The schematic says 40v, yes, but you cannot directly measure that with a meter to ground. Measure from the cathode or cathode resistor. Also, if one of your plates is reading really low, that means the cathode circuit is likely missing that triode current, which will alter the cathode - and grid - voltage.

[slaweks222]

Thank you for answer, yes, you are correct 14 VDC on pin 6- plate, my mistake. I actually measured power tubes with old school mutual conductance tube tester before started anything else, so I believe I can rule out short in a tubes.
By now I took off all caps in circuit and measured with Capacitance Measurement mode in DMM and all looks correct.
I will go on with all suggested measurements.

[Enzo]

Caps rarely cause trouble by being off value. What happens, especially in tube amps, is they get leaky.Your meter usually uses maybe a volt or two to test a cap. But a cap can be leaky at 400v or 200v or 100v, yet test fine at 2v.

14v on a plate? Probably not loaded down, more likely voltage not reaching it. The plate resistor is R139, you got 14v at one end of that? And what is the other end reading?

[slaweks222]

I'd like to thank you all Jon, Chuck and Enzo for your help.
Amp is fixed ,all voltages in 12AT7 circuit look very close to schematic, Jon- excellent shot, R140 was open, Chuck and Enzo I followed your suggestions and changed caps in bias circut and 12AT7 supply. Enzo thank you for measurement tips.
Slawek