Fender Twin Reverb 2012 (new) - blown 10A fuse / short - arc between plate and filament 6L6

[spunko]

Hello, I got this Fender Twin reverb, the PCB says 2012 but it has six months of use. It was bought new at the store.

My friend says he was playing and suddenly the amp just went off and never powered on again.

We checked the main SLO fuse and it was ok, so I took the chasis out and found a blown 10A fuse (Not shown in the diagram. The parts list says 8A). Also some burnt marks in the first 6L6 socket where the pilot lamp is conected.
When seeing closely, I noticed the cables from pin 2 and 3 were almost touching by a hair, so I assume they indeed shorted, or at least they were so close that did arc to each other.
The 1.5K resistor reads good with my fluke tester, but is half burnt so I don't trust it.

Here is what I have done so far, the amp works and sounds good, no hum, no noises, everything seems perfect, but I have some doubts.

1) Replaced the 10A fuse with an 8A normal. Unfortunately I wasn't able to find a 10A SLO. Also the original doesn't look like a SLO fuse to me. Is this ok?
2) Changed the 1.5K with a new one 1/2 watt. The diagram says all resistors are 1/4 watt unless is specified, so I asume the burnt one was 1/4 watt. Also it is really small, and unfortunatelly I don't have 1.5k 1w at hand. If I put two in series it will be huge, and there is no room. Is this ok?
3) Cleaned the socket and placed the wires correctly.

I have tested the amp only with a current limiter (200 watts bulb in series with hot wire).

- When power on/ stand by off, the bulb ligths up almost nothing. Like from 0 to 10, it is in 0.5
- when power on/ stand by on, the bulb ligths a bit more, like 2 of 10. This is normal.
- With the amp on, volume from 7 to 10, the bulb in the current limiter lights more, and if I strum really hard the guitar, the bulb ligths up like 4 out of 10.

I have used the current limiter in other high watt amps and it has the same behavior, but this is the first time I play hard with the current limiter connected, so I don't know if its normal that the bulb gets brigther with all the volume up and hiting hard at the strings. Is this because the tubes are pulling more current? I afraid to test without the current limiter yet.

Video: https://streamable.com/5moow
*It seems the bulb brights at full in the video, but I think is because of the camera. It's as I described above, at max is 4 out of 10 in the scale of brightness
*I will post voltages tomorrow, but the current limiter will affect the numbers, without it must read higher numbers. I took some readings really quick and they were close to the schematic.

Schematic: 65_twin_reverb_RI Service Manual.pdf

Pics:

[TomCarlos]

Good start to your work.

As for the Bulb Limiter, I wouldn't use it when playing the amp. During play, the amp will be pulling more current- so of course the bulb will light up brighter. Now that you know you are not blowing that fuse, you can probably ditch the bulb limiter.

I think you are ok with 1/2 watt 1.5KΩ resistor. I have used 1/2 wire wounds as a replacement.

Yes, post voltage readings when you can.

And thanks for the video!

Tom

[g1]

Doesn't it say 8AT under that fuse? That's 8A time delay (slow-blow).
The 1/2W resistor is fine there.
Check that the hum pot is still good, they often get damaged when pin3 arcs to heater.

[vintagekiki]

[ATTACH=CONFIG]54612[/ATTACH]
R50 R51 R52 R53 = 470ohm 1W (page 5)
R46 R47 R48 R49 = 1.5K 1/4W (page 4)
XF3 FUSE = 2AT (page 3) for 220V/240V
XF3 FUSE = 4AT (page 3) for 100V/120V
XF4 FUSE = 8AT (page 3)

Tube socket is a little burned out, just in case, replace it.
Burnt wires on tube socket isolate with heat shrink tube.

[52 Bill]

Agree with the others, but if this is 6 month old amp, you should have had Fender fix it for free.

[spunko]

I should have posted the fuse pics in the first post. The PCB says T10A fuse. I'm sure my fuse is a normal 8A , no slow blow. Can I use mine?


I took some readings without the current limiter, and they are a bit higher compared to the schematic.
AC outlet: 120.5Vac

TP	AMP	SCHEMATIC
TP21	450 VDC	440 VDC
TP25	-67 VDC	-65 VDC
TP26	-48.8 VDC	-50 VDC
TP27	454 VDC	442 VDC
TP28	451 VDC	440 VDC
TP29	440 VDC	428 VDC
TP34	403 VDC	393 VDC

I'm sure the schematic I have is not the same as my amp (starting with the 10A fuse), but the voltages should be really close, and mine are higher.
Can I test the HUM pot in circuit? each side lug reads 20 ohm to ground.

What other voltages should I check? Are these numbers ok?
I noticed that when I switch the standby on, the pilot lamp flickers for a nano second, and you can barely hear a puff noise in the speakers.

**The amp was bought in the US and shipped overseas. No fender service in here. Shipping it back will cost more.

[vintagekiki]

What other voltages should I check? Are these numbers ok?
I noticed that when I switch the standby on, the pilot lamp flickers for a nano second, and you can barely hear a puff noise in the speakers.

Measured voltages are OK and are in the range +/- 5%
Barely hear puff noise in the speakers (for a nano second) indicates that filament voltage occasionally interrupted.
If pilot lamp flickers check CP13; CP14. Is there filament voltage (~6.3V) and are all tubes in amplifier lights up.
Check PCB ground (CP15 ... CP20) that is not it accidentally burned during the accident.

[g1]

Use a T10A (slow-blow) fuse there. They probably changed it from 8A to 10A due to 'nuisance' blowing of the fuse due to turn on surge.

[spunko]

Thank you for the answers.
Yes it has 6.3 Vac and all tubes light up. Without that fuse there is no filaments voltage.
I will try to find the proper fuse, and will check those ground points when I get home.

[Enzo]

First, CALL FENDER and ask for the proper schematic. The one you linked in post #1 is for the original reissue version, twenty some years ago. MY latest file is revision G. It shows a T10A heater fuse, which is what you ought to use.


If the schematic says 440v and you have 450v, that is an insignificant difference.

[spunko]

I’m glad I visualy inspected the amp. Besides the short/arc problem in the v10 power tube socket, v6 also had the cables too damn close to each other (pins 2 and 3), and pin6 has a cold solder.
Inspected the capacitor can and found two clipped capacitor leads dancing around, waiting for the sparks party to start.
I have checked the ground points, and also pocked the amp with a plastic stick trying to find more cold solder joints.
Now everything seems normal, the puff sound and the flickering persist. I don’t have another tween to try. Some of my amps do the same puff, some others don’t. I can’t recall if any of them has the pilot lamp behavior.

[vintagekiki]

Does the puff sound and the flickering persist while you play the guitar or when you switch off standby. If it occurs at the moment of standby off it is generally normal.
If the pilot lamp flickering, check it (or replace) and clean any fine oxide from it. The pilot lamp must constantly lights.

[52 Bill]

Common problem with the pilot light is an intermittent socket.

If you move the wire leads going to the socket and the lamp flickers, you need to clean the contacts.

[spunko]

Does the puff sound and the flickering persist while you play the guitar or when you switch off standby. If it occurs at the moment of standby off it is generally normal.
If the pilot lamp flickering, check it (or replace) and clean any fine oxide from it. The pilot lamp must constantly lights.

The puff/flickering thing occurs for a nano second, just when I swicth the standby ON. When playing the guitar, everything is normal. If I swicth OFF the standby no puff/flick, switch it ON again and I get the puff/flick.
With the red orb installed, the pilot lamp just seem to dim a bit.

If I don't find a slow blow fuse, what could I use? a normal 10A fuse? Unfortunatelly are not sold here, you can't find them =(

Edit:
Fisrt power up in the morning didn't do the puff/flick, from the second power up it did it again. This is the first time it didn't do the puff, but it was just once.
I made a video, the pilot lamps seems to dim a bit. Everytime you hear the puff is when standby on.

https://streamable.com/k01c6

[vintagekiki]

The phenomena you describe (puff / flick) occurring during the manipulation of swicth the standby, which do not exist while playing the guitar, are normal transient phenomena, because at that moment the capacitors are fill or empty, the tubes draw electricity ... ...
In addition, the PCB is not directly connected to the active components (tubes, transformers, switch ...) already via the tabs contact, so there is the possibility of catching fine oxide over time, which forms a transient resistance between the tabs contacts.

All tubes in F65'R for heating have consumption of order ~ 6A, so a normal 10A fuse could generally handle it.
After all, all oldstyle Fender amps did not have a heating fuse.


https://music-electronics-forum.com/showthread.php?t=39519
Fender Twin Reverb 65 RI keeps blowing 8 amp filament fuse

https://music-electronics-forum.com/showthread.php?t=49130&p=531527&viewfull=1#post531527