'65 Deluxe Reverb RI. Blown resitor.

[Sea Chief]

Hi- Ive got a simple resisitor to replace, due to someone poking around & stupidly shorting x to y whilst trying to be helpful and giving my amp a quick check/ voltage & tubes bill of health (which was kind, and a pro he is.. but rather unbelievable! I thought you took in a broken amp > fixed amp comes out.. not ok amp in > broken amp comes out!).

The tubes started red-plating & soon discovered after much head scratching, that the bias circuit (my hunch) was cause s'how. Sure enuff yes I do we found the metal film resistor R69 in the bias circuit was blown & reading open (but no physical sign of burnt as its a "fuse" type of thing.. right?). There is no evidence at all that anything was responsible -other- than poking around, taking CP1 and CP2 off and mixing up putting back on again & not noting which way around (confusing both of us) & a shorting due simply to this. It happened immediatly after poking the CP1/ CP2 areas, & little else was poked at all so 99.8& likely a short occured here IMO.. besides, the adjacent cap to R69 was wiggled: fine, tubes checked: fine, any wires or preamp areas: fine so no sign of any cap, wire short in situ, preamp component etc etc was to blame.

Its R69 on the 1st google result here, a pdf which alas I cant link directly to it seems apologies (but once on the pdf, its pg 6 for the schematic, pg3/ last down is exact description of R69).

https://www.google.co.uk/search?q=54...Vv9og4Zo8ouPQA

Can someone help me find a repalcement? Im confused over this "fuse" description added to it/ never heard of such a resisitor.

Thanks SC

[pdf64]

See PHOENIX PASSIVE COMPONENTS - NFR25H-22R-5 - RESISTOR, FUSIBLE, 22R | Premier Farnell CPC UK
Unfortunately that's 1/2 watt rather than 1/4, which seem more difficult to source.
However, it seems nuts / bizarre / perverse to fuse in the bias supply whilst leaving the HT unprotected!
I suggest that a regular type resistor is used to replace it.
I don't think that the value is critical, any >1/2 watt 22-220R that will fit should be fine.

[Sea Chief]

Hi pdf64- but Im of the understanding that the metal film properties of it act like a fuse, as an added protection of sorts.

I just want to replace it, rather than consider an alternative (which may or may not be a good idea).

Thanks for the help, SC

[Jazz P Bass]

The parts list calls for a 1/4 watt resistor.
That is what I would install.

Through Hole Resistors | Farnell element14

[Sea Chief]

Hi JazzP- that correlates to what someone else says too, so I'll go with that.

whats the betting its 15p for 5 resistors + £10 post tho..

Thanks, SC

[Sea Chief]

Actually Im also told these good too: can someone second these would be ok?

0.25W 1% Metal Film Resistors 10R-1M (E12) x5 x10 | eBay

[Jazz P Bass]

Actually Im also told these good too: can someone second these would be ok?

0.25W 1% Metal Film Resistors 10R-1M (E12) x5 x10 | eBay

They look good from here.

[Sea Chief]

ok gone for those.

Amazing and alarming that one 3p resistor is all thats between it going all monster red-plating amp horrorshow.

Much obliged chaps. SC

[g1]

Amazing and alarming that one 3p resistor is all thats between it going all monster red-plating amp horrorshow.

However, it seems nuts / bizarre / perverse to fuse in the bias supply whilst leaving the HT unprotected!
I suggest that a regular type resistor is used to replace it.

Hi pdf64- but Im of the understanding that the metal film properties of it act like a fuse, as an added protection of sorts.

pdf64's point was that it is not a good idea to fuse (or use a fusible resitor as a fuse) the bias supply. What is it protecting?
It is much more important to maintain bias at all times. Otherwise you can end up with red-plating tubes and even damaged transformers.
I have no idea what Fender is trying to protect with that fusible resistor. They have been making bias circuits without any fusing for decades.

[Sea Chief]

But what would have happened in my scenario then with a std resistor in there instead?

Scenario, afaict: the Blk and Wht 240v and 0v CP1 and 2 leads coming from the xfmr are in fact -opposite colours- to the schematic Blk and Wht (because my xfmr is a HAMMOND 291 BEX). So whichever way round I had put them (correct/ opposite or actually I didnt see these wires from the xfmr as opposite and just went with the schematic).. we both found ourselves confused and putting them back on this way then that way round during tests.

Whether they make any difference whichever way round- I dont know, the point is whilst doing this the short must have occured beteeen them causing the failure of R69 > sudden unexpected red-plating horrorshow.

So what would have happened here then.. if I had -not- had this 'fuseable' resisitor in, but a standard one there instead??

Thanks SC

[g1]

No red-plating horrorshow?

[Sea Chief]

Or..?

[Sea Chief]

.. perhaps a fried Xfmr? the fuse surely did some protecting, as it was meant to do. Without it as a fuse wouldnt shorting the xfmr's Blk (240v) and its common Wht wire produced an xfmr-monster-horrorshow?

[nickb]

.. perhaps a fried Xfmr? the fuse surely did some protecting, as it was meant to do. Without it as a fuse wouldnt shorting the xfmr's Blk (240v) and its common Wht wire produced an xfmr-monster-horrorshow?

Using a plain carbon resistor seems like a bad idea to me. After all, they specified flameproof for a reason. If it's your own amp then it's your problem if it burns the house down but what will your insurance company say?

How about a a plan B? A thermistor AKA a resettable fuse. I think you get the transformer protection (it probably uses very fine wire to optimize winding utilization) and the possibility of restoring the bias once the high current problem is addressed without having to replace the part.
RXEF005 - TE CONNECTIVITY / RAYCHEM - POLYSWITCH, RADIAL, 0.05A | Farnell element14

[52 Bill]

Without it as a fuse wouldnt shorting the xfmr's Blk (240v) and its common Wht wire produced an xfmr-monster-horrorshow?

Shorting the two primary side wires together would blow the mains fuse.

The resistor in the bias supply should not be damaged by such a short. I don't know why it is listed as a fuse. In fact I would agree with the others in that it shouldn't be a fusible element at all.

My personal guess would be that it being a part of the power supply, it was spec'd as a metal film to meet some sort of CE or UL specification.