'65 Deluxe Reverb RI. Blown resitor.

[Chuck H]

The result of no bias has many possibilities that include an expensive catastrophe. The result of a blown bias winding don't seem significantly different. In other words, there is more risk in a fused bias winding than there is with a winding designed to do or die. JM2C on that.

You can find people in any profession that are ignoramuses. Even fields generally reserved for educational degrees. How many bad doctors are there?!! The guy that designed that bias wind as fused goofed up for his own reasons that we will never know. It was a bad idea. Just because it came from Fender doesn't make it a good idea by default.

[nickb]

The result of no bias has many possibilities that include an expensive catastrophe. The result of a blown bias winding don't seem significantly different. In other words, there is more risk in a fused bias winding than there is with a winding designed to do or die. JM2C on that.

You can find people in any profession that are ignoramuses. Even fields generally reserved for educational degrees. How many bad doctors are there?!! The guy that designed that bias wind as fused goofed up for his own reasons that we will never know. It was a bad idea. Just because it came from Fender doesn't make it a good idea by default.

It's not credible to me the the design engineer added this component thoughtlessly. There would have been at least a one design review and in some places the production and safety guys will carefully check every component in a new design. They probably had a more senior engineer looking over their shoulder too.

There is something else that is being overlooked. The transformer winding probably has just a few ohms of resistance since it's only 50V or so. Adding a series resistor significantly reduces the size of the current spikes. It could be that this was giving excessive noise on the output on a prototype and a series resistor was an easy fix.

I recall doing UL testing where you had to deliberately induce fault conditions. The test was passed if it didn't burst into flames or something like that.

My real point is, we just don't know. We don't know what was in his mind and if you haven't got all the information it's hard to reach a valid conclusion.

OTOH screw-ups still happen

Now, looking from a different perspective, it will take something over 100mA (100mA ~= 1/4W) say >150mA to burn the resistor up. The normal RMS current through it is ~10mA. If a fault has developed that is drawing 150mA then I expect you haven't got any bias voltage anyway. Therefore, the view that loss of bias is moot as you didn't have any.

A better design might have cut the path to the power tubes if bias was lost, as has been mentioned, but they chose not to do that. Again, I bet they had their reasons.

[Sea Chief]

Ok rwgardless of whether the design is a good idea or not, I feel best replacing it with what was there.. esp as I dont understand what you guys do re. the alternatives and why poss better etc etc at all (feel free to continue to discuss- but Im not on board!).

Can anyone point to the main culprits as to possibly -why- then this resisitor blew? The amp was simply on, with an 8 ohm load on a test bench. Little prodding other than v basic voltage checks was being done. Amp was fine. No symptoms of any probs at all up till this most incongruous happening.

Actually thinking of it, the only thing odd was the engineer disregarded the standby and flipped both on/stby switches up to turn fully on "stby switches are rarely needed tbh, defo not with a 5U4" he adamantly said.

I'm just wondering if this could be the cause. thx SC

[nickb]

Ok rwgardless of whether the design is a good idea or not, I feel best replacing it with what was there.. esp as I dont understand what you guys do re. the alternatives and why poss better etc etc at all (feel free to continue to discuss- but Im not on board!).

Can anyone point to the main culprits as to possibly -why- then this resisitor blew? The amp was simply on, with an 8 ohm load on a test bench. Little prodding other than v basic voltage checks was being done. Amp was fine. No symptoms of any probs at all up till this most incongruous happening.

Actually thinking of it, the only thing odd was the engineer disregarded the standby and flipped both on/stby switches up to turn fully on "stby switches are rarely needed tbh, defo not with a 5U4" he adamantly said.

I'm just wondering if this could be the cause. thx SC

The standby didn't cause it. Perhaps a probe slip?

On the like part the problem was we could not find one in less that qty 5000 hence the discussion. There are a few suggestions on what to use but honestly there's not a lot to choose between them. It's up to you. I would not sweat it.

[Sea Chief]

Probe slip? very possibly- it was an end of the day rush he was doing as a favour (yanked the amp out bending the facelpate slightly.. strewth) and a bit ham-fisted probing, altho a nice chap who literally grew up on tubes as Pa was a radio tech so has tube amp knowledge seeping from his pores.

But could we narrow down to what might have shorted to what.. or is that too tricky to do here?

Im just want to find out -why- this occured so it doesnt do it again, or Im worried there is a weak point I cant establish that might well cause same prob again ever time Im using the amp.

[Chuck H]

I never said there was anything wrong with the inclusion of series resistance.

The normal RMS current through it is ~10mA. If a fault has developed that is drawing 150mA then I expect you haven't got any bias voltage anyway. Therefore, the view that loss of bias is moot as you didn't have any.

And this is a very good point So, If there is likely no bias anyway, why not save the winding if possible so the circuit can be rebuilt without the need for a new PT?

[nickb]

Probe slip? very possibly- it was an end of the day rush he was doing as a favour (yanked the amp out bending the facelpate slightly.. strewth) and a bit ham-fisted probing, altho a nice chap who literally grew up on tubes as Pa was a radio tech so has tube amp knowledge seeping from his pores.

But could we narrow down to what might have shorted to what.. or is that too tricky to do here?

Im just want to find out -why- this occured so it doesnt do it again, or Im worried there is a weak point I cant establish that might well cause same prob again ever time Im using the amp.

A would hazard a guess that the -49V bias got shorted to ground. Put in the new resistor and leave it in standby. Power up and check the resistor is OK and you have around -49V on TP5. If its OK you can come out of standby. Check TP5 again now - it should not have changed significantly.

[Jazz P Bass]

All I have to add is the tech should not have been 'poking around' in the amp.

Turn it off, place a measurement lead, turn it on.

Poking is for amatuers.

[Chuck H]

Poking is for amatuers.

Guilty here!

I poke into live amps with hand held probes all the time! And I'm not ashamed to say so. And unless everyone else here decides to just look at the ground and scuffle their feet, I'll bet it's the most common way to take measurements used by members here. In fact, the only time I clamp probes to the circuit is when I need my hands free for something else.

I'm happy to take a beating for this if I'm really alone in this practice and acting unprofessionally.

[Justin Thomas]

This is why I don't claim the title "technician..."

Justin

[g1]

When amateurs do it it's called poking. When we do it it's called "probing" .

As far as speculating where the slip could have been made, it's tough as it is more dependent on the layout than the schematic. It would probably have to be from the C- point to ground, as further down the line would have the bias pot limiting the current. So my vote would be he touched the hot lug of the bias pot and the pot casing at the same time.

[olddawg]

Many times I (and so do the manufactures) replace SMD resistors with standard film resistors of the proper size for repairs. I didn't do it much for warranty stuff unless someone was howling to get the piece out the door and the manufacture/supplier was slow. You just have to be careful how you bend the leads. Also maybe a drop of non conductive glue if it's a combo. But it's done all of the time. Especially if the customer needs the amp that day and the part isn't in stock. Don't do it if you are a klutz and lift traces and such.

[pdf64]

It's not credible to me the the design engineer added this component thoughtlessly. There would have been at least a one design review and in some places the production and safety guys will carefully check every component in a new design. They probably had a more senior engineer looking over their shoulder too...

Agree, that's what should happen, but a strange thing with modern Fender tube amps is a seeming lack of consistent arrangements from model to model for this type of type of thing.
Looking at a few of the schematics / BOMs in the public areana, the DRRI seems about the only one with a series resistor from the PT bias winding, fusible or otherwise.
Some amps have silicon diodes in series with tube rectifier plates, some don't.
Few have B+ fusing, though I think that the TRRI is fitted with fuses not shown on the public schematic.
The EC Vibro Champ even has series diodes and B+ fuse.
What perplexes me is that I'd expect that a senior engineer in post for a while would have a view on good practice in this regard and try to ensure that at least new designs incorporated similar arrangements.

That DRRI schematic shows R69 the 22ohm series resistor as FP, defined as fireproof on the BOM front page; but the BOM has R69 as a metal film fuse, whereas for other FP resistors, eg R62, metal oxide types are used.

[Enzo]

There is not one little office with three guys designing amps. There are groups or teams of people doing designs. So while some designs are derivative, like the HotRod series based on earlier models, others are not. I note the "57 twin Amp has one. A lot of other models of recent vintage are voltage doublers, so they lack the part.

[DRH1958]

Guilty here!

I poke into live amps with hand held probes all the time! And I'm not ashamed to say so. And unless everyone else here decides to just look at the ground and scuffle their feet, I'll bet it's the most common way to take measurements used by members here. In fact, the only time I clamp probes to the circuit is when I need my hands free for something else.

I'm happy to take a beating for this if I'm really alone in this practice and acting unprofessionally.

I'm with you. You need to have a healthy respect for the voltages involved and not be scared of them. Think ahead about what part of the amp you are going to work in, where the high voltages are in that part and where other parts that have high voltages are that you won't be working in so as to keep your hand away from them. Know all the high voltage points and use only one hand to do the work. There is no reason to turn off the amp every time you work on it to take a reading if you know what you are doing and observe common sense. If I'm going to take multiple readings such as all B+ nodes and their corresponding pins on tube sockets, bias voltages, etc. it would take all day. On, off, on, off. Nope.