5F1 keeps frying 10k resistor in the filter section . . . any ideas????

KissTone

Silver Supporting Member
Messages
2,894
As noted in the title, I'm looking at a kit-built 5f1 Champ clone that continues to burn up the 10k (2 watt) resistor in the filter section.

I've tried other tubes after replacing the resistor, but the resistor still gets toasted within a minute or two of powering up.

The filter caps are newer Spragues which appear okay, so what would you check next? Transformer?

Thanks for your ideas . . .
 

doctord02

Member
Messages
1,061
sounds like something downstream from that resistor is a dead short, and pulling way too much current thru that resistor. Check for shorts...
 

mark norwine

Member
Messages
17,168
resistors burn because of current.

Use ohms law: measure (to ground) the voltage on either side of the resistor, and subtract to calculate the difference. Take that difference & divide by the resistor value.....there's your current.

If the current is "appropriate", then you need a bigger [wattage] resistor.

If the current is too high......which seems to be the case here......continue to trace that current & find out where it's flowing.

Don't look at this as "a champ".......look at this as an electrical circuit, and work the math.
 

Trout

Member
Messages
7,550
based on the location of the 10K resistor, odds are better than average that you have a bad/shorted output tube or bad filter cap.

You do not want to use a bigger resistor in this location, the end result will just be a burned out power transformer.

Swap the resistor with the correct value, pull the power tube(6V6) if the resistor holds, you can replace the tube. If the resistor blows, Check for shorts downwing & change the 2nd power supply cap.
 
Messages
295
I have the exact same problem with my 5f1...:confused:

Original poster, did you ever get the issue resolved?

Also, when the resistor dies and the amp is still running, are the other components being damaged?
 

Blue Strat

Member
Messages
30,194
The amp won't run for long with that resistor fried...it applies operating voltage to the whole amp.

First, try without the 6V6 installed. A shorted 6V6 or cathode capacitor will draw a lot of current. If you still have the problem there's likely a wiring problem (short circuit) down stream. Left the end of the 22K resistor closest to the preamp tube and see if the problem stops. If it does, there's a short associated with either of the 100K resistors attached to the 22K.

When in doubt, get a clean schematic and a hi-lighter and trace out your wiring...hi-lighting each connection on the schematic as you go.
 

Prairie Dawg

Member
Messages
1,928
The amp won't run for long with that resistor fried...it applies operating voltage to the whole amp.

First, try without the 6V6 installed. A shorted 6V6 or cathode capacitor will draw a lot of current. If you still have the problem there's likely a wiring problem (short circuit) down stream. Left the end of the 22K resistor closest to the preamp tube and see if the problem stops. If it does, there's a short associated with either of the 100K resistors attached to the 22K.

When in doubt, get a clean schematic and a hi-lighter and trace out your wiring...hi-lighting each connection on the schematic as you go.
+1. Basic troubleshooting theory. 95 per cent of problems are caused by the last person who was in there.

Like my old crew chief used to say "Think system, Robert" meaning don't see things in isolation but as part of a system-or circuit as Mark sez.

And, I have eaten my share of crow too-it tastes OK with salt and pepper. :bonk
 
Messages
295
I'll get the multimeter out today and check everything with and without the 6v6.

Thing is, the amp worked beautifully for a week or so, then simply didn't work one day. Could a short burn the resistor out slowly like that?

. Left the end of the 22K resistor closest to the preamp tube and see if the problem stops.


Could you elaborate on that part?
 

phsyconoodler

Member
Messages
4,305
Not really.A short will usually burn the fuse.Too much current going through that 6V6 will.
What are your voltages?If the voltage is really high and your cathode resistor is too small,the tube will draw too much current and the resistor will get hot and eventually burn up.
It sure sounds like something along those lines.A poor ground will also make resistors heat up.
 

Blue Strat

Member
Messages
30,194
I'll get the multimeter out today and check everything with and without the 6v6.

Thing is, the amp worked beautifully for a week or so, then simply didn't work one day. Could a short burn the resistor out slowly like that?





Could you elaborate on that part?

Look at the schematic. Left is toward the preamp, right toward the power tube.

Why would you suspect that it went slowly? It's impossible to say. A tube can short in 1 microsecond and the resistor will go pretty quickly once the tube shorts.

If the amp was working fine for HOURS I suspect the 6V6 above anything else.
 
Messages
295
Why would you suspect that it went slowly? It's impossible to say. A tube can short in 1 microsecond and the resistor will go pretty quickly once the tube shorts.

If the amp was working fine for HOURS I suspect the 6V6 above anything else.
That makes sense!

So what caused the 6v6 to go? The cathode resistor? Bad tube?
 

Blue Strat

Member
Messages
30,194
That makes sense!

So what caused the 6v6 to go? The cathode resistor? Bad tube?
Since I'm not telepathic (at least not with tubes brains) I don't know. :D

Another thing to watch out for. If the tube DID short, it probably took out the cathode capacitor as well. So disconnect that until things look like they're ok otherwise you'll continue the cycle of blowing 6V6s, capacitors and resistors endlessly. Also, check the value of the cathode resistor. You may have to increase it to prevent further 6V6 blow outs. Measure the idle current when things are stable. Chain reactions are common and it's often impossible to trace the source, so you replace however many 50 cent components might be culprits rather than continuing to take out tubes.

Once you get it stable...if it WAS the 6V6..replace the cathode capacitor no matter what. Once the power tube shorts it's going to stress out the capacitor even if it doesn't take it out.
 
Messages
295
Since I'm not telepathic (at least not with tubes brains) I don't know. :D
I figured if anyone was it'd be you!


I took at look at the 470 resistor (which is the cathode resistor,right?) and it LOOKS a little burnt up, but i tested it at 450 ohms. I'm still going to replace it...when i have everything that outta be replaced taken care of, I'll post some voltages.
 
Messages
295
Ok, i've checked pin 8 of the 5y3 with two different tubes (sovteks, not nos) and both are giving me around 480 VDC! this is without the other tubes installed, not sure if that''ll make a difference
 
Messages
295
ok, after some reading i've found that without the tubes, the voltage will be much much higher....is it safe to power up the amp with the all tubes with out the load of the speaker?
 

pula58

Gold Supporting Member
Messages
1,665
I had a power supply cap die. It then acted like a short and fried a power supply resistor! I happened quite suddenly.
 
Messages
295
ok, so i threw a brand new 10k power resistor in, as well as a new cathode resistor which i tested at 470 ohms (the old one tested at 448)......annnnd....it works! sounds as it should on all different volumes.

lets just see if it lasts....i'm going to get a NOS rectifier to replace the sovtek thats in there now, hopefully that'll bring the voltage down some, its still on the high side
 




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