When you started this or the other postings about this amp you said you had a electrical engineer buddy, does he have a Scope to view this new issue with the amp?
It sound now like you have a that hum issue and the amp may also be oscillating!
coupling cap question
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Stevem
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Re: coupling cap question
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: coupling cap question
Remote amp diagnosis can be challenging. I'm not trying to get in Stevem's way here and I'm not disagreeing with anything he's said. I just thought I'd give a bit of a different perspective now that you've gotten past some of the basic things.
Pulling V2 kills the hum. Pulling V1 does not kill the hum. (Is this correct?) That means it is about 99.9% certain the problem is somewhere beyond the cathodes of V1 and the plates of V2.
I'd go for a secondary proof of the problem location at this point. Do this by bypassing the preamp section between V1 and V2. Use a simple jumper wire with gator clips. Apply one clip to the coupling cap on the plate of V1 (the side closer to V2) and apply the other end to the grid of V2. I'm not sure which schematic is the right one here, so follow the one that applies. If this kills the hum, you've confirmed the source. If it doesn't then the problem is very likely something about V2.
Let's take a step back and look at the typical list of things that cause 120Hz hum:
Loose connections including cold solder joints or loose hardware.
Bad tube.
Bad or shorted cap (both coupling and filter).
Open resistor.
Faulty ground.
You also report 60Hz hum. This is sometimes masked when 120Hz hum is also present. The most likely source of 60Hz is the filament supply. I might be tempted to remove the artificial CT and ground the actual CT from the transformer. (Use one or the other, not both!)
On the 120Hz, the one tell-tale thing you've reported is that it sometimes works fine. Electricity if a funny thing -- the rules of physics apply 100% of the time. If there is any intermittent operation, that tells us there is a loose connection or a faulty solder. Because you reported that it has worked properly, this is what you should be looking for first. It can be really hard to find this sort of thing. If you know to confine your search to the stuff between V1 and V2 you can continue to chopstick this, moving each wire and tapping each solder. I'd also get a magnifying glass and closely inspect every connection. Or I might just reflow every solder joint in that part of the amp. You might also meter across each coupling cap for DC voltage. If DC is flowing across a coupling cap, the cap is leaky and needs to be replaced.
Pulling V2 kills the hum. Pulling V1 does not kill the hum. (Is this correct?) That means it is about 99.9% certain the problem is somewhere beyond the cathodes of V1 and the plates of V2.
I'd go for a secondary proof of the problem location at this point. Do this by bypassing the preamp section between V1 and V2. Use a simple jumper wire with gator clips. Apply one clip to the coupling cap on the plate of V1 (the side closer to V2) and apply the other end to the grid of V2. I'm not sure which schematic is the right one here, so follow the one that applies. If this kills the hum, you've confirmed the source. If it doesn't then the problem is very likely something about V2.
Let's take a step back and look at the typical list of things that cause 120Hz hum:
Loose connections including cold solder joints or loose hardware.
Bad tube.
Bad or shorted cap (both coupling and filter).
Open resistor.
Faulty ground.
You also report 60Hz hum. This is sometimes masked when 120Hz hum is also present. The most likely source of 60Hz is the filament supply. I might be tempted to remove the artificial CT and ground the actual CT from the transformer. (Use one or the other, not both!)
On the 120Hz, the one tell-tale thing you've reported is that it sometimes works fine. Electricity if a funny thing -- the rules of physics apply 100% of the time. If there is any intermittent operation, that tells us there is a loose connection or a faulty solder. Because you reported that it has worked properly, this is what you should be looking for first. It can be really hard to find this sort of thing. If you know to confine your search to the stuff between V1 and V2 you can continue to chopstick this, moving each wire and tapping each solder. I'd also get a magnifying glass and closely inspect every connection. Or I might just reflow every solder joint in that part of the amp. You might also meter across each coupling cap for DC voltage. If DC is flowing across a coupling cap, the cap is leaky and needs to be replaced.
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JoshBernstein
- Posts: 289
- Joined: Tue Jan 27, 2015 1:31 am
- Location: Key Largo, FL USA
Re: coupling cap question
My buddy does have an oscilloscope but i dont currently have access to it.
Will try all the stuff reccomended in the last post
Also, would rectifying the heater voltage help at all? If so how would i do it?
Will try all the stuff reccomended in the last post
Also, would rectifying the heater voltage help at all? If so how would i do it?
Not really
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JoshBernstein
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- Location: Key Largo, FL USA
Re: coupling cap question
Update: i bridged the coupling cap of v1 to the grid of v2 and the hum decreased to about half of its original volume. The kind of sputtering buzz type thing dissappeared when i did this.
Not really
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JoshBernstein
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- Location: Key Largo, FL USA
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JoshBernstein
- Posts: 289
- Joined: Tue Jan 27, 2015 1:31 am
- Location: Key Largo, FL USA
Re: coupling cap question
Ive also just discovered that pulling v2 doesnt kill the hum. It makes it very quiet but its actually still there.
Not really
Re: coupling cap question
I'm thinking you've got multiple problems. If the worst of the hum is cured by either pulling V2 or jumping over the tonestack, you know where the worst problem originates. There is a lot of territory there. First, I'd focus on the grounds. Second I'd look for a cold solder. You might reflow all the solder joints in that area.
You can also use the jumper to bypass only certain parts of the tonestack. Eventually, you are likely to find the culprit.
You might have a bad filter cap. I'm not really sure how to diagnose that. I think the procedure is to tack another one along side in parallel. I know you said no ripple, but maybe that isn't right?
After you cure the big hum, you can look for the little hum.
You can also use the jumper to bypass only certain parts of the tonestack. Eventually, you are likely to find the culprit.
You might have a bad filter cap. I'm not really sure how to diagnose that. I think the procedure is to tack another one along side in parallel. I know you said no ripple, but maybe that isn't right?
After you cure the big hum, you can look for the little hum.
Re: coupling cap question
Now how did I know there was a filament problemThat's fine, but if that where a issue it would be 60 hz hum you would be dealing with, not 120 like you have posted about!
Re: coupling cap question
Hopefully You already fix all Your problems. I had a crackling sound in my amp for 2 years. I tried to fix it many times and today thanks to your topic I've found what's was wrong. I have a wire put to a pin socket that was never soldered 
. Check all Your connection if everything looks fine try chopstick test. Good luck and stay safe
. Check all Your connection if everything looks fine try chopstick test. Good luck and stay safe