Power tube heaters

[Wasnex]

Re-seat the connectors in the amp. All the tubes should be labelled and removed. The amp should be turned on in playing mode, standby off. The heater and power supply voltages should be checked with a meter and compared to the references on the schematic.

If the heater fuse (F6) is good and the output tubes are lighting up, but the preamp tubes are dark, the connector to the DC filament supply may be damaged or unplugged.

Although a bit confusing, I believe the path from the heater taps to the DC filament supply is heater taps to J41 and J42 on the bottom of power amp sheet 2. These connect to H1 and H2 on J10 on power amp sheet 1. I believe H1 and H2 also connect to J15 pins 1 and 2 on the left side power amp sheet 1. J15 is the mate for J5 on the left side of the preamp board, which feeds the DC filament supply.

 

[No Treble]

I am so grateful for all the help. When I checked all the fuses earlier, I was hurrying too fast. It is the fuse at F6.

F6 on your schematic is a fuse in the heater circuit. If that fuse blew, all the heaters would go out. I’d look at that. might be on the circuit board.

I needed to disco the power transformer, and found that fuse open. F6, heater fuse. I must have been rocking it too hard.

Noteworthy: I had 5 or 6 folks here within minutes helping right away. Thank you for making me bonk my own head for a minute. Saved me $260 if I diagnosed to the power transformer, like a bonehead.

That said, it should have taken a lot longer to get this kind of help. Shouldn't you all be rocking out?

Additional problem: some real jerk soldered the fuse to the clip. It must have broken at some point, and to fix it, I'll have to pull the same shenanigan.

 

[micguy]

OK, so the heater fuse blew. There are 2 possibilities:

1) The fuse just blew. If this is the case, replacing it will get you back in business.

2) There’s a short circuit somewhere in the heater circuit. If so, you’ll continue to blow fuses until you fix that short.

 

[No Treble]

OK, so the heater fuse blew. There are 2 possibilities:

1) The fuse just blew. If this is the case, replacing it will get you back in business.

2) There’s a short circuit somewhere in the heater circuit. If so, you’ll continue to blow fuses until you fix that short.

I'll replace that fuse, and see if it maintains that etherial tone. If it blows again fairly quickly, I'll know to keep digging.

Thank you for all the support. All of you here have provided more help than I expected, and a ton more than any "customer service" robo switch board would be. Maintenance on this thing consists of frequent cleaning, but little else. I have a number of options to consider if it doesn't quit blowing fuses, also.

Thanks again. I'll report back when it's bothering the neighbors once again.

 

[beans-on-toast]

If the heater fuse (F6) is good and the output tubes are lighting up, but the preamp tubes are dark, the connector to the DC filament supply may be damaged or unplugged.

Although a bit confusing, I believe the path from the heater taps to the DC filament supply is heater taps to J41 and J42 on the bottom of power amp sheet 2. These connect to H1 and H2 on J10 on power amp sheet 1. I believe H1 and H2 also connect to J15 pins 1 and 2 on the left side power amp sheet 1. J15 is the mate for J5 on the left side of the preamp board, which feeds the DC filament supply.

The connector numbering is confusing, that's why they should all be reseated. There is a big connector for the power tube board. The circuits should also be probed with a meter.

The subject line says that the power tube heaters are the problem.

 

[agedhorse]

I'll replace that fuse, and see if it maintains that etherial tone. If it blows again fairly quickly, I'll know to keep digging.

Thank you for all the support. All of you here have provided more help than I expected, and a ton more than any "customer service" robo switch board would be. Maintenance on this thing consists of frequent cleaning, but little else. I have a number of options to consider if it doesn't quit blowing fuses, also.

Thanks again. I'll report back when it's bothering the neighbors once again.

If the fuse blows again, you don't really have a number of options to consider... either properly troubleshoot the problem of pay somebody who is qualified to do it for you.

 

[okcrum]

Additional problem: some real jerk soldered the fuse to the clip. It must have broken at some point, and to fix it, I'll have to pull the same shenanigan.

Why not replace the fuse clips too? It was probably soldered because the old clips lost their temper. That way, you won't have to do this again.

You should be able to find the right ones from Link Removed.

 

[Wasnex]

Why not replace the fuse clips too? It was probably soldered because the old clips lost their temper. That way, you won't have to do this again.

You should be able to find the right ones from Link Removed.

With the amp I mentioned in post #19, the resistance between the fuse and fuse holder was causing enough heat to liquefy solder and char the circuit board. When I pulled the amp out of the head case, there was a puddle of solder in the bottom. Some of the solder had flowed onto one of the fuses, so it was soldered permanently in place. This was a common problem with the amp and it was obvious that multiple failed repair attempts had been made.

I cut the charred section out of the circuit card and soldered in pig tail fuses. They pigtails get quite warm, but not hot enough to melt solder. The amp has operated without fault since 2010.

My amp uses a pair of 20A fuses. One on each leg of the heater tap. Since your amp only uses a 10A fuse there may not be enough current to produce a similar problem. I suggest inspecting the circuit card to see if there is evidence of high temperature damage near the fuse socket.

 

[CharlieMac]

I don't have anything to say about your amp but ain't it great how generous and helpful these TB'rs are. This is a great place to hang out
Happy Holiday everybody

 

[SactoBass]

I don't have anything to say about your amp but ain't it great how generous and helpful these TB'rs are. This is a great place to hang out.

Biiiiiiiiiig +1

 

[agedhorse]

When designing fused circuits, it's important to derate for a continuous load like tube heaters, as well as sourcing adequately rated AND tested parts. This means testing the temperature rise of the soldered joints as well as the spring clip contacts themselves.

Since the heating is purely a function of the current (as a function of the square of the current), attention to detail is essential.

 

[beans-on-toast]

Wouldn’t it be nice if a fuse circuit had an LED build in to serve as a visual monitor. Maybe positioned under the fuse so as to illuminate it. When the fuse blows, the LED goes out.

 

[agedhorse]

Wouldn’t it be nice if a fuse circuit had an LED build in to serve as a visual monitor. Maybe positioned under the fuse so as to illuminate it. When the fuse blows, the LED goes out.

I have done this in the past, red LED turns on when fuse blows.

 

[beans-on-toast]

I have done this in the past, red LED turns on when fuse blows.

I’ve had a green LED when the fuse is operating normally. An inexpensive feature. With tiny surface mount components today, it makes it even easier. They could build it into the fuse.

 

[agedhorse]

I’ve had a green LED when the fuse is operating normally. An inexpensive feature. With tiny surface mount components today, it makes it even easier. They could build it into the fuse.

It depends very much on the circuit that the fuse is controlling. While it seems simple in concept, it may be anything BUT simple in practice for some circuits (especially if safety certification issues are involved).

 

[Wasnex]

It depends very much on the circuit that the fuse is controlling. While it seems simple in concept, it may be anything BUT simple in practice for some circuits (especially if safety certification issues are involved).

Trace Elliot V8 has fuses between the output tube cathodes and ground. A red LED and a 47K resistor are arranged in series and strapped across the fuse. When the fuse blows, the LED comes on.

 

[beans-on-toast]

It depends very much on the circuit that the fuse is controlling. While it seems simple in concept, it may be anything BUT simple in practice for some circuits (especially if safety certification issues are involved).

I’ve found that regulatory reviewers are demanding. They can also be inconsistent. What passes with one can be red lined by another a few months later.

 

[okcrum]

With the amp I mentioned in post #19, the resistance between the fuse and fuse holder was causing enough heat to liquefy solder and char the circuit board. When I pulled the amp out of the head case, there was a puddle of solder in the bottom. Some of the solder had flowed onto one of the fuses, so it was soldered permanently in place. This was a common problem with the amp and it was obvious that multiple failed repair attempts had been made.

I cut the charred section out of the circuit card and soldered in pig tail fuses. They pigtails get quite warm, but not hot enough to melt solder. The amp has operated without fault since 2010.

My amp uses a pair of 20A fuses. One on each leg of the heater tap. Since your amp only uses a 10A fuse there may not be enough current to produce a similar problem. I suggest inspecting the circuit card to see if there is evidence of high temperature damage near the fuse socket.

As @agedhorse implied, that's a flat out design error. I've used resistors [Invalid or Expired Link Removed] in 1" and larger diameter fuse clips, in high power applications. The saving grace with those giant clips is that you can get screw down clamps for the top of the clip to maintain the connection even when the clip's temper is long gone. You can't make them too tight or the resistor body can shatter from thermal expansion.

 

[Wasnex]

As @agedhorse implied, that's a flat out design error. I've used resistors [Invalid or Expired Link Removed] in 1" and larger diameter fuse clips, in high power applications. The saving grace with those giant clips is that you can get screw down clamps for the top of the clip to maintain the connection even when the clip's temper is long gone. Can't make them too tight or the resistor body can shatter from thermal expansion.

Yes it is obviously a design error. How often have you seen 2" fuses like this in an instrument amp?

I don't think the internal resistance of the fuse was the problem. My theory: A bit of oxidation on the fuse and fuse holder, and the resistance increases resulting in excessive heat.


Some amps were fixed by simply soldering in some heavy gauge wire across the fuse holders. I chose to solder pigtail fuses to stand offs so the oxidation is not a problem. The fuses still get too hot to touch, but not hot enough to melt solder.

 

[okcrum]

Yes it is obviously a design error. How often have you seen 2" fuses like this in an instrument amp?
View attachment 3243624


I don't think the internal resistance of the fuse was the problem. My theory: A bit of oxidation on the fuse and fuse holder, and the resistance increases resulting in excessive heat.


Some amps were fixed by simply soldering in some heavy gauge wire across the fuse holders. I chose to solder pigtail fuses to stand offs so the oxidation is not a problem. The fuses still get too hot to touch, but not hot enough to melt solder.

Sometimes that's all you can do without resorting to heroic measures. Most of us would only see the big fuses at work, or in a hand-held multimeter in the current measuring circuit.