Unusual ceramic resistor Fender Pro Reverb

[Markorock37]

I have a 1979 Fender Pro Reverb in my shop for a cap job. It has a 30K 20w ceramic resistor going to ground that is measuring only about 10k. I can't find anything like this anywhere as I was going to replace it as well. The only thing I can find is an aluminum bolt in resistor which would work fine, would just have to screw it to the chassis instead of the tagstrip the old one is on. Sound ok to you guys or is there a better fix? Thanks.

[Leo_Gnardo]

The only thing I can find is an aluminum bolt in resistor which would work fine, would just have to screw it to the chassis instead of the tagstrip the old one is on. Sound ok to you guys or is there a better fix? Thanks.

The resistor you found will be even better than the original Fender part, as it will be using the chassis as a heat sink. For some reason Fender's design team found it necessary to dial back the clock some 40-50 years and install these as "bleeder" resistors in an effort to provide some cheap'n'dirty regulation for the high voltage supply. Curious they did this when no previous Fenders had any such thing. I've found them in old RCA PA amps from late 1940's - early 50s.

[Markorock37]

I figured that's what was going on there, never seen that before however. Thanks, I'll go the better route.

[Jazz P Bass]

That honkin 20 watter is the 'A' voltage on the preamp.

What does that point read (Vdc)?

I find it odd that a resistor value would age downward like that.
Is that reading with one end of the resistor lifted?
(parallel caps throwing the reading off? ie: high ESR)

Personally, with all the other goofy CBS stuff going on in this amp, if the 'A' voltage is within reason, I would say leave it alone.

cbs_70w_mstrvol_pullsw_super-pro-bmstr_rev.pdf

[Tom Phillips]

I agree with the points Jazz made. In addition I'll add that it would be very unusual for a wire wound resistor like that 30k/20W (wire wound is what's inside that rectangular housing) to fail as you described. (The huge change of 30k dropping to 10k)

When you look at the circuit you will see that the 30k/20W resistor is the bottom leg of a voltage divider. The top leg is the 2700 Ohm/10 W. It may very well ohm out OK if you disconnect one lead from the circuit.

[mozz]

I can see a wirewound resistor going lower in value, it would only have to short out some turns.

[Markorock37]

I didn't lift the leg, but I will tonight. The 2.7K 10W measures exactly so I just figured the other one was bad. Also fried the hum balance pot, which I added a couple resistors at the pilot light and just disconnected the pot.

[Enzo]

It would have to short a LOT of turns to drop from 30k to 10k. Resistors rarely go down. But resistors OFTEN measure low while they are still in the circuit, because there are other parts in parallel paths.You need to tack down the problems, rather than just measure every part looking for a wrong value.

Jazz linked the schematic, thanks. What does your resistor measure if you turn off the standby switch? Since your resistor is to ground from the B+, when the standby switch is ON, I see the 2700 ohm resistor to its right, plus the two series 100k bleeders across the main caps are in parallel with the 30k. So we have a 30k resistor, with 202,700 ohms in parallel. That gets me about 26k, which is no 10k. I don't spot other parallel resistors, however, any residual voltage in the caps can drown your reading. What happens if you measure this 10k but reverse your meter probes?

Do as they say and lift one end to really measure it. I suspect it is just fine, and does not need replacing.

Don't forget to include the bias filter caps, and also note they are wired in "reverse" because it is a negative supply.

[Markorock37]

Well, as suspected that 30K resistor measured...30K exactly. I should've known better,lol. Caps are all ordered plus the bias caps (standard procedure along with power resistors on the cap board).

[Markorock37]

Got all the new caps in, along with new metal oxide resistors under the doghouse. What prompted me to replace the old caps (besides age) was low voltage to V1, V2, and V4. I'm only reading 20v on pins 1 and 6 of these 3 tubes and also directly after the 4.7k resistor between the last two 20uf caps. All grounds from these caps are good, as well as all of the 100k plate resistors. Unsure what could cause this?

[Enzo]

How about the 4.7k resistor?

[Markorock37]

Brand new, measures 4.7k.

[Markorock37]

[Enzo]

So you have 20v on the downstream end of the 4.7k B+ resistor, what is on the upstream end? And is it getting hot?

Always remember, it is not about the parts, it is about the circuit. The 4.7k measures 4.7k or close enough? OK so the part is OK. Now measure it another way. The left end of that resistor is connected to four 100k plate resistors off to the left. OOps, six, sorry. Pick one of those on the end facing teh 4.7k. Now on the other side, the upstream end of the 4.7k is wired to a pair of 47k plate resistors in the PI. So measure resistance from a 100k to the junction of the two 47k. Still measure 4.7k? What we did was measure the resistor through its connections to other parts. Easy to have a looks good but isn;t eyelet connection.

I will assume that all six 100k are not bad, but for future testing, rather than check the part themselves, I'd park one probe on teh 4.7k and the other to each plate pin at a socket. That tests not only the part, but the wiring and connections of each 100k.

[Markorock37]

Upstream of the 4.7k I have 257v. Measures 3.78k in circuit, as well from each 100k plate resistor. Plate to PI Junction also reads 3.78k. Must be in the cathode?