bassman burnout

[hackinblack]

I have a Fender concert PRO (model PR-244) 1x12 combo model which is the fancy bassman channel switching version.
it has seen active service in a UK studio,but has burned out both R56 and R57 fusibles in the output stage

schematic manual here:-

is there a common fault that causes these resistors to go O/C? what, if any, is the likely chain of desruction?
p.s. it took nearly as long finding a service sheet as getting the PCB out!

[g1]

The number one cause of open screen resistors is bad power tubes.
That being said, I would have expected to see maybe 2W there rather than 1W rating.

[guitician]

The old 6G4 Super uses 1W. But those double as fuses too.

[g1]

If you're going to use them for fusing, go all the way and make them externally replaceable in the cathode circuit, like JCM900; they even add an indicator light .

[hackinblack]

i was worried that the OPT may be fried! now to save for new OP tubes...
the amp has had heavy use from new (around 1994) although it appears to never had a solder joint touched,or part,except 2 pots replaced.
the resistors are listed as 1W fusible types;now i have to find a european supplier!
unless there has been a real screw-up at fender specifying 1W types;i think 20+yrs before failure is OK
many thanks to all. now back to modding a fender greta...

[52 Bill]

the resistors are listed as 1W fusible types;now i have to find a european supplier!

I believe that the resistor specs are Flame Proof not Fusible.

[guitician]

I think Metal Oxide FP is considered fusible. If you look at the data sheets they spec that they open fast at a certain level of power.

[hackinblack]

as far as i know the FP suffix means 'fusible protection' am i correct?

[eschertron]

as far as i know the FP suffix means 'fusible protection' am i correct?

the ones I buy say 'Flame Proof' on the package. So they can be used as protective devices, since they are guaranteed to not catch themselves on fire while they overheat and open. Kind of a "last line of defense" for the designer.

[pdf64]

as far as i know the FP suffix means 'fusible protection' am i correct?

See P1 of your manual attached to post #1; 'Resistor Codes' RES FP = Flame Proof
If you can't readily source flame proof ones, I suggest that you select a 1W MO type with a flame retardant coating.

[guitician]

So, if you find a 470 Ohm 1W Metal Oxide "Flameproof" resistor, then I would go ahead and use it.

[Leo_Gnardo]

the resistors are listed as 1W fusible types;now i have to find a european supplier!
unless there has been a real screw-up at fender specifying 1W types;i think 20+yrs before failure is OK

Farnham "right next door" in Belgium I think, and if I'm not mistook Mouser has a European presence as well. I don't know what Mouser's Euro policy is, but in USA there is no minimum order. OTOH you may find a few more things in the catalog you'd like to buy.

Fender used to use 1 watt carbon comp resistors in this position. Not flame proof but they would take a pretty good beating, usually drifting off spec long before giving up entirely, plus I've never seen any sign that a failing one affected anything nearby so as to threaten spreading a fire. Perhaps Euro safety standards demand the use of flame proof and/or fusible R's.

[pdf64]

I've seen heater wiring with rather charred cloth insulation, due to an adjacent CC screen grid resistor taking a hit.