1990 red knob fender Twin

[nandrewjackson]

Hello everyone,  I'm doing some tech work for a friend for gear trade. I've received a red knob fender "the twin" that needs attention.  We've decided to go ahead with a full power cap electrolytics swap, no problem there.


The date stamp on the preamp board says Oct 1990.


The bass pot on ch2 has shaft broken off, so that's an obvious fix. 250k push pull dpst


The heater supply resistors are melted, schematic says 47ohms each. All builds I've done I've used 100 ohms each. I plan on replacing with 47 ohms, although it seems odd to me. Is this because the amp has 7 12ax7 and 4 6L6? That's s lot of heaters.


Another question,  there's a big 10W 27K resistor on the board, all fried and crumbled,  R129 10W 30K according to schematic.  That resistor goes from 12ax7 DC power node to ground, apparently doing nothing more than burning 5w of heat continuously for no reason.  Any thoughts?


Power tube sockets are loose on the pins, no problem. 
Most everything else seems ok.








------Praise in Jesus name---------

[nandrewjackson]

Grill cloth is deceptively awful,  rest of amp is in good shape

[shooter]

appears to be both a Vdivider + bleeder (cap discharge), but that's a quick stab from a short shcemo 

[mresistor]

Maybe your amp experienced a grid to heater short on a power tube or something similar.    Here's the schematic shooter..    I think he meant 2.7k 10w.


https://el34world.com/charts/Schematics/files/Fender/Fender_the_twin.pdf

[PRR]

> 47 ohms, although it seems odd to me.

Maybe the factory ran out of 100r? Maybe a truck spilled hundreds of 47r in the road? (Happened to me: gutter full of 68r 1W resistors. I scooped my pockets full and used them series/parallel for everything for a while.)

33r runs too hot for a cheapest-size 1/2W resistor. 330r may not swamp bad heater leakage. In between, use what you got too many of. Maybe Fender had a run of bad tubes and 47r made more of them useful than 100r did.

[glass54]

Hi nandrewjackson,
I have worked on a few of these BUT not recently. As Shooter said "Vdivider and bleeder"
I have also had the 47R blown in the same amps.
Please note that if R129 goes o/c, your new voltage at node A will be closer to full rail voltage [nominal 480V -(Ipreamp x 2k7)] which is close to the Vrating of C110! On my filed schematics, I have a note to check/suspect C110 (22u450V).
I also noted burn marks around fuse F306 and please check output tube sockets for any burn marks/flashover. (I have replaced some)
Can I just mention that Fender did use a 30k 20Watt resistor in the Super Twin (same function as R129) and in many other Twin variations (105W and 135W) and likewise they crumbled and went open. I always replaced R129 with 2 ten watt resistors in series eg 15k.
I don't actually remember finding any specific faults regarding the 47R Heater resistors as it was always with the above faults.
Trust this helps.
Kind Regards
Mirek

[nandrewjackson]

Ok, thanks everybody. 


Ill stick with 47 ohm heater supply center tap Rs, that's what was in there.


I managed to find a 30K 15W that will fit in the space where the 30K 10W was. 


I checked tube sockets, glass54, nothing looks burned there.