Burning up dropping resistors

[moonbirdmusic]

I built my son a champ clone (6v6) from a replacement turret board i installed in an Epi Valve Jr (cathode bias). I got the kit from watts audio. He plays around 4 hours a day for about the past six months. Been working fine. However, recently the first dropping resistor in the power supply has been failing (in front of the output plates). At first I thought I needed to up the wattage of the resistor from the stock 2w to a 4 watter I had. But now that resistor looks overheated too and the amp is cutting out when it gets hot. Any ideas? thanks much.

[Wayne]

I don't know which Champ you built so I won't stick my neck way out, but a couple of things come to mind. Class A duty is hard on tubes - is your 6V6 going south? A leaky filter cap beyond that node could potentially cause this as well. All this is assuming, of course, that 2W was the appropriate size in the first place.

W

[Phil_S]

Not many parts in that section of the circuit. You've got a rectifier, a filter cap and a resistor. The resistor value and wattage are time tested in the traditional circuit, so it is reasonable to conclude upping the wattage on the resistor isn't going to fix the problem. The problem is heat caused by either excessive voltage or excessive current. IMO, that has to come from the filter cap or the rectifier.

If you are using a tube rectifier, try changing it. If you are using an axial or radial cap (not a can cap), those are relatively cheap and I'd go ahead and change it on the grounds that it has probably been stressed beyond it's rated limit or that it is the more likely culprit.

More remote in my estimation is that you've got an intermittent short. Use a magnifying glass and look for a bad solder or a whisker that is making an unintended ground.

[jjman]

Measure the voltage drop (or difference) from one end of the resistor to the other. And check it's resistance. Then do the math or post the figures.

[moonbirdmusic]

I went ahead and replaced the diodes and lytics for the first two taps with beter quality caps - so far so good.

Thanks to all for the input!! Great resource for tabe amp wannabies!!

[rfgordon]

I'm not sure I read your post correctly, but if the B+ for the plates is AFTER a resistor, you've wired it incorrectly. The B+ for the plates should come immediately after the rectifier (tube or SS makes no difference), before either a choke or first resistor. The screen supply is AFTER that resistor.

PT >> recto >> B+ plates >> choke/resistor >> screen supply >> resistor >> preamp supply

[moonbirdmusic]

The amp operated just fine for a number of months prior to failing. It IS wired as you suggest.

However, I spoke too soon. It is still burning out the dropping resistor before the screen tap. I guess it is time to measure voltages and possible suspect the PT - right?? thx again to all.

[rfgordon]

If the PS wiring is correct, then the excessive current draw is somewhere further downstream.

Sooooo, something is dumping current to ground. If the amp was operating properly, that means something failed. Candidates are:
- Shorted filter or cathode bias caps
- Shorted coupling cap from gain stages. Check for high DC upstream of the caps.
- Physical short somewhere, like debris in the amp or a loose connection.
- Bad tubes (you did try tube substitution, right?).
- If a tube arced in the socket, the remaining carbon deposit can act as a resistor to ground.

I once had an amp come in for service that had blown the fuse and burnt a dropping resistor. The amp had preamp tube sockets with the high collars for tube cans. The owner had replaced a 12AX7 and jammed the tube in such that one of the pins bent and touched a neighbor, shorting B+ to the cathode resistor. Gentle insertion is always the best policy....

[moonbirdmusic]

rfgordon -- thx for the great suggestions. I will check all and report back. thx again for your time.

[Structo]

Also be aware that a lot of resistors have a rated maximum working voltage.
Many are 350v max.

That could be leading to failure if you aren't taking that into consideration.

Usually it is best to use a wirewound if a heavy current and voltage is present.
If you use a metal oxide flameproof resistor then you have to consider the max voltage.
I think a lot of those have a 500v working voltage rating.

[tubeswell]

Also be aware that a lot of resistors have a rated maximum working voltage.
Many are 350v max..

Also those little 0.6W Metal Film resistors have a breakdown voltage of 360 (or so). But that only kicks in if you have more than ~360V dropped across the resistor, which is not a situation that typically happens in supply resistors. (However you wouldn't want to use 0.6W for most supply resistors either)