Fender The Twin Resistors?

[misterc57]

Recently picked up a The Twin that needs some work. I was told one of the output tubes, V5 was red plating.

I found one bad output tube at v10, also two bad preamp tubes.

On V5, I notice that the 1.5K 1/4 watt resistor at pin 5 appears burnt although it reads ok on a DVM. The 470 1 watt resistor at pin 4 is cracked but also reads ok on the DVM. I plan to replace both of these resistors.

On V11, the 470 resistor at is reading around 660 ohms, while all the others are close to 470. I will also be replacing this resistor as it is out of spec.

I was wondering if I need to stay with carbon resistors for the 470 1 watt? I only found a ceramic replacement doing a quick search on Mouser.

Thank you. MC

[DRH1958]

Is this the red knob The Twin? Since this amp is about 20-25 years old, I would replace all screen resistors with 470 ohm, 1 watt, metal film resistors. You don't want to take the risk of these failing. I use these guys. Most popular parts for decent prices. Easy to find, not 1000 of each item to sort through. https://www.tubesandmore.com/

[drewl]

Those resistors being burnt or high in value indicate an output tube pulling too much current or going bad.

I dont know why they use two or five watt resistors there as I'd rather a ten cent resistor burns up than a transformer.

[Leo_Gnardo]

Practically every "The Twin" I see has these resistors looking frazzled or just plain burnt & drifted well off value. Of course, you replace all of them. The options are "Do I put in a replacement that will act as a fuse, 1 watt part whether carbon comp or metal film or goat cheese, OR put in something that won't break say 5 watt wirewound, or even go further and increase the value to 1K to avoid beating those screen grids so badly?" Me, I'd put in 1K 5W WW plus a kit of matched 6L6GC whatever brand you like and be done with it. Might lose a couple watts of power but that's a hardly noticeable tradeoff. The choice is yours.

Whatever you do, you don't want to wreck the power transformer. I've seen a couple from The Twin gone bad, the only replacement is big big $$$$ from Mercury. Owners opted to scrap their amps rather than see their repair bills soar beyond $500-600 when you count in the tranny, new output tubes, whatever else plus labor.

[drewl]

Which is the point I was making.
Use a 1 watt that is enough to handle the power but will burn open before your granny is cooked.



Edit:
Ha,ha, granny cooked.
Transformer cooked!
Damn auto-correct.

[misterc57]

This is the red knob The Twin LL 66385. Schematic I found says 470 ohms 1 watt.

Picture attached. I can see that someone had previously replaced the v10 1.5k resistor.

[Leo_Gnardo]

Which is the point I was making.
Use a 1 watt that is enough to handle the power but will burn open before your granny is cooked.

That's why I leave it open "it's your choice."

The Twin is spec'd to have a whompin' big AC line fuse, 7 amps, with a second "dummy" fuse 8 amps inside. How the XXXX do it need that kind of current, ever? Without a second thought I'd dial that down to 4A slo blo at most, put some better safety factor there. It's all good, like granny's apple pie.

[DRH1958]

So misterc57, there are two school of thought and both are valid. 1. Use a larger resistor maybe both wattage and ohm value but wattage for sure, to have less possibility of it failing. Higher ohm value(1k ohm) will make life easier for your tubes(6l6s) 2.Use just enough(1 watt) so if something goes wrong, the resistor burns up and acts like a fuse. If it opens, this provides protection for your output tranny(expensive).
You also might want to closely inspect those 1.5k grid stoppers on the sockets. Any resistor mounted to the socket is subject to high temps with those tubes hanging upside down. One has already been replaced, so that should tell you something. It might not be a bad idea to replace those too. You are already working on it anyway.
I didn't know about such a large fuse(7A) and Leo brings up a great point. Too large fuse size really doesn't provide much protection, just against a short in the AC primary. A lower value will be better.

[misterc57]

Help me to understand the component safety concept

That's why I leave it open "it's your choice."

The Twin is spec'd to have a whompin' big AC line fuse, 7 amps, with a second "dummy" fuse 8 amps inside. How the XXXX do it need that kind of current, ever? Without a second thought I'd dial that down to 4A slo blo at most, put some better safety factor there. It's all good, like granny's apple pie.

I need clarification.

Regarding lower amp fuses;

If something goes wrong (like what? bad tube, bad resistor) the circuit can start to draw too much power which can damage expensive components? Changing the fuse to 4A Slo Blo (internal one, does it matter which one?) will shut it all down before causing too much damage.

Regarding larger wattage resistors;

If I was to increase the 470 resistors to 2 watts or higher, will this protect the circuit? But if the voltage is getting too high, wouldn't we want these resistors to blow, and if it is a higher wattage resistor it would not blow?

[misterc57]

Thank you for the explanation, that helps me to better understand the concept now.

So here is what I am thinking of doing.

Replace the 470 1 watt with 1k 1 watt, to protect the output tubes, how will this effect the performance design of this amp? Also will this effect the external bias and balance control in any way?

Replace the 1.5k 1/4 watt with 1.5k 1/2 watt, to protect the resistors from the heat. I did notice that the one replaced looks like 1/2 watt where the originals are 1/4 watt, and that the replaced one is mounted further away from the output tube, whereas the original ones are mounted pretty much inside the tube socket.

Replace one of the fuses (does it matter which one?) to 4A slo blo to protect everything.

Thank you, MC

[DRH1958]

Thank you for the explanation, that helps me to better understand the concept now.

So here is what I am thinking of doing.

Replace the 470 1 watt with 1k watt to protect the output tubes, how will this effect the performance design of this amp?

Replace the 1.5k 1/4 watt with 1.5k 1/2 watt to protect the resistors from the heat. I did notice that the one replaced looks like 1/2 watt where the originals are 1/4 watt.

Replace one of the fuses (does it matter which one?) to 4A slo blo to protect everything.

Thank you, MC

Your watt rating for the 1k ohm needs to be 5 watt rating. The higher value will drop the screen voltage a few more volts giving it a little easier life. This results in your output tubes generating a little less power, maybe a few watts, so they run a little less hot and should extend the life of your tubes. If, for example, you go from 100 watts to 95 watts, the loudness will drop only by .2dB which really isn't enough to even hear. I don't know exactly what wattage you will end up at but you won't hear he difference.
It's a smart move to replace those 1.5k grid stoppers with higher in watt rating since they get hotter. Good move.
It really won't matter electrically which fuse to replace since they are in series, but I would replace the one on the outside of the cabinet since this one will blow first. You have easier access to it if you need to replace it.

[misterc57]

I edited my last post a few times to add more info and questions. Your snipet is from my original draft.

Good explanation, I understand the concept of going from 470 to 1k lowering the voltage and helping extend the life of the output tubes. However I am confused about going from 1 to 5 watts on these resistors. Wouldn't we want to stay at 1 watt so it acts like a fuse, at 5 watts it may never blow and don't we lose some protection?

thanks!

[Jazz P Bass]

To add, ideally the grid stopper resistors should be as close to the socket as possible.

Preferably, soldered directly to the socker tab, with as short a lead length at the socket as is possible.

[misterc57]

To add, ideally the grid stopper resistors should be as close to the socket as possible.

Preferably, soldered directly to the socker tab, with as short a lead length at the socket as is possible.

Good to know, but why?

Actually I just read "why" about these online at http://www.aikenamps.com/index.php/g...-are-they-used

[DRH1958]

Usually, something in the power supply or power output section will cause the worst problems. It could be many things, but yes, it's designed to shut down the amp if too much current is drawn by a part of the amp due to malfunction thus minimizing loss of components. Lowering the fuse size to closer to what the amp draws but still higher by a reasonable amount will provide proper protection. And slo blo will protect against nuisance blowing due to inrush current at a start up when first turning on or when turning the standby switch to play.

The whole idea of the screen resistor burning up as a sort of protection was never the intent of the resistor. It became an observation later as a secondary function. It's primary purpose is to drop the screen voltage to a proper level at all operating points of the amp. At higher wattages, the screen current increases, thus the voltage drop increases on the resistor and keeps the screen voltage where it needs to be. This dynamic operating quality keeps the screen voltage at a proper level no matter what volume the amp is at. A higher watt rating will protect the resistor from failing which is really what it was designed to do. A 2 watt will protect the circuit, just in a different way, the resistor not failing but keeping the designed value. If it is a higher watt resistor, you are correct, it probably will not blow. You could go that route, 470 ohm, 2 watt.
Personally, I like the idea of a 1k, 5 watt so I don't have to go in and fix it and make life a little easier for the tubes. This also protects the screen more because it keeps the voltage on it lower. The screen grid is the leading cause of power tube failure. But it is a valid point of view to use the rating as spec'd and use it as described for secondary protection. It's one of those things among many in the amp world where personal preference is what drives the decisions.

EDIT: That Aiken Amps site you looked at is loaded with very good info. He really knows his stuff!