SF Twin Reverb no sound

[JAelec]

Hi!

I'm having a go at a SF Twin Reverb that is totally silent. Pilot light is not functioning and tubes are cold when the amp is on, so I suspect the fuse for the filament winding.

I will have to check that fuse tomorrow...however I have a question about safety when testing an amp with a blown fuse...

Is it safe to just pop a new fuse (of the proper spec) in and turn the amp on?

Or do I need some kind of current limiter as I've seen many amp techs suggest; the one with a 230V filament light bulb in series with the amp. This would require me to get a hold of an old style filament lamp as I suspect a LED bulb won't work for this application. Perhaps there are other methods?

Suggestions are welcome!


Schematic:
https://www.bustedgear.com/images/sc...er_100W_SF.pdf

[The Dude]

There is no filament fuse in a Twin- only a main fuse. Do you have any power at all to the unit, or is it entirely dead? Did you check the main fuse at the back of the amp?

[pdf64]

If you’ve not got an old incandescent / halogen100W bulb tucked away at the back of a dusty shelf or cupboard, ask family, friends, neighbours etc to check too.
If not, search for specialist bulb types, eg rough service, where incandescent is still allowed eg https://www.lamps2udirect.com/standa...018%20By%20Cat

[Jon Snell]

As there is no filament fuse, hopefully it is the mains fuse, thermal trip or a socket/lead issue otherwise it maybe the power switch and hopefully NOT the mains transformer, (assuming there is no output from the transformer).
Power OFF; If the HT winding is fine, look for dry joints in the heater wiring from the transformer and check for continuity by removing all of the valves and the indicator bulb then measure the resistance between pins 2 & 7 of any output valve. You are looking for <0.5 Ohm accross the heater winding. If O/C check the wiring.

[JAelec]

There is no filament fuse in a Twin- only a main fuse. Do you have any power at all to the unit, or is it entirely dead? Did you check the main fuse at the back of the amp?

This is a European export model and sometimes they were fitted with additional fuses by the company importing them. This is the second SF Twin I have had in with additional fuses and no standby switch. In total there are four fuses, three at the back of the amp and one inside.

Looking inside the amp I noticed these things:

- the pilot light has simply been removed, so that explains that.
- there seems to be a fuse for the filament winding, located inside the amp, by the pilot light. (see pic below)

- there are extra fuses are for the secondary: one for each side of the HT winding. They are 2x630mAT
- there is a mains fuse after the power switch so the power switch lights on even when the fuse is blown.
- mains fuse is 1.6AT

- Both the main fuse and one of the HT fuses on the secondary were blown.
- when removing the power tubes, the plastic alignment thing on the bottom of two of the tubes fell off. These tubes look like they are from the 90s.

I did not have any new fuses at hand so could not test with new fuses right away. Either way I'd like to know if it is safe to just fire up the amp with new fuses or if there are any risks with this approach?

[Jon Snell]

I would fit a new T1.6A mains fuse and (6.5v 150mA) pilot lamp. Power it up and check for voltages. If you refit the output valves, use your speaker or summy load!

[g1]

- Both the main fuse and one of the HT fuses on the secondary were blown.
- when removing the power tubes, the plastic alignment thing on the bottom of two of the tubes fell off. These tubes look like they are from the 90s.

Any time the HT fuse is blown, power tubes are suspect. Suggest first powering up with them removed, and verify power tube socket voltages.
Replacement 'guide keys' for the power tubes can be used if the tubes are still good, or you can glue them back on.
https://www.vivatubes.com/6-pieces-v...repair-keyway/

[Jon Snell]

Have you checked the main smoothing capacitors for age and deteriation under the dog house?

[JAelec]

Have you checked the main smoothing capacitors for age and deteriation under the dog house?

Checked today... these are the original caps. One of the caps was loose at one side and needed resoldering.

I have a cheap Chinese ESR and capacitance meter that I can test them with... a BSIDE ESR02 Pro which is supposed to have a range between 25pF~100mF.
My multimeter also has a capacitance function...

Not sure if these options would be appropriate for a large smoothing cap... I've used the BSIDE for smaller caps with good results.

Any time the HT fuse is blown, power tubes are suspect. Suggest first powering up with them removed, and verify power tube socket voltages.

After resoldering the cap I put in new fuses and measured voltages with the power tubes out:

B+ 511V
All four plate pins: 511V
All four screen pins: 508.2V
All four grids: -50V
Two bias caps: -51 and -62 V

I could not get a reading of the filament voltage, but I'm not sure why. It just said 0V. Will have to redo tomorrow.

The preamp tubes where glowing and got warm.

Transformer winding resistances:

PT: 4.6 Ohms
HT: 25.6 Ohms
Filament: 0.2 or 0.1 Ohms (jumping between these values)

After getting the voltages I put the old tubes back and switched on the amp...one of the HT 630mA fuses and the 1.6 mains fuse blew right away.

[JAelec]

Hmm...

Shouldn't the B+ and plate voltages be different because of the voltage drop across the OT center tap to plate?

[Helmholtz]

Hmm...

Shouldn't the B+ and plate voltages be different because of the voltage drop across the OT center tap to plate?

Not with power tubes out.

[JAelec]

Not with power tubes out.

Why is that so? I would have thought the resistance is still there which would cause voltage drop

[Helmholtz]

Why is that so? I would have thought the resistance is still there which would cause voltage drop

No tubes - no current - no voltage drop.

[g1]

After getting the voltages I put the old tubes back and switched on the amp...one of the HT 630mA fuses and the 1.6 mains fuse blew right away.

Sounds like at least one of the old tubes is bad. Or possibly mounting wrong if the guide pins are still broken/missing.

Voltage drop across a resistance requires current. With power tubes removed there is no current through the OT.

[JAelec]

I see...could you say there has to work done for the voltage to drop?