Hi. I have a Bandmaster build that has this constant hum, the only thing that affects is the bias pot, it gets louder as the the value is towards -37. I have have basically tested everything, changed parts etc. The funny thing is that when I put in just one power tube, the hum goes away, with two it comes back.
Pin three on both PT's have about 479v, on the layout is said 440v. The transformer is a Hammond 290DEX, with 325/2 v on the secondaries. I wonder if that could be the wrong transformer?
I would appreciate som tip on this.
Hum in Bandmaster
Moderators: pompeiisneaks, Colossal
Re: Hum in Bandmaster
Do you have DC on the board? Are the 68k's on the jacks?
Re: Hum in Bandmaster
Do you have a known good pair of tubes to try ? Can you try these tubes in a different amp ? Did you check the screen resistors ?
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Stevem
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Re: Hum in Bandmaster
Minus 37 is way too little for a bandmaster with 6L6 output tubes!
The schematic calls for -45 at the junction of the 220k resistors with 440 volts or so on the pin 3 of the outputs.
Your hum is the output tubes red plating .
If you keep running the amp like this your hum problem will gradually go away because you have killed the output tubes!
The schematic calls for -45 at the junction of the 220k resistors with 440 volts or so on the pin 3 of the outputs.
Your hum is the output tubes red plating .
If you keep running the amp like this your hum problem will gradually go away because you have killed the output tubes!
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: Hum in Bandmaster
I am not having it on -37, I just say that when I turn the bias pot, that's the only time the hum increase or decrease.
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martin manning
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Re: Hum in Bandmaster
How much ripple is on the bias voltage? Have you tried increasing the filtering on the bias supply? That solved the hum problem on a Bandmaster for Bombacaototal.
Re: Hum in Bandmaster
How could I do that? Increasing the uF values? Its 20uf / 70uf now?
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martin manning
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Re: Hum in Bandmaster
What does your bias circuit look like now? Same as the original or something different? It's possible that the bias setting affects the hum simply because you are increasing or decreasing the current draw through the power tubes. Was it ever quiet? Are the filter caps in good condition?
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Stevem
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Re: Hum in Bandmaster
If you only have one output tube in either socket is the level of hum the same ?
Also like Martin asked, how much ac ripple voltage do you have on pin 5 of a output tube socket?
Also like Martin asked, how much ac ripple voltage do you have on pin 5 of a output tube socket?
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!
-
Stevem
- Posts: 4576
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Hum in Bandmaster
If you only have one output tube in either socket is the level of hum the same ?
Also like Martin asked, how much ac ripple voltage do you have on pin 5 of a output tube socket?
Also like Martin asked, how much ac ripple voltage do you have on pin 5 of a output tube socket?
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: Hum in Bandmaster
Can you post some pics of your build.
Re: Hum in Bandmaster
Same as the original. It might at one point have been quieter, I am not sure. I have bought ned tubes to see if that might have something to do with it. (The amp works though.)martin manning wrote: ↑Fri May 26, 2023 6:26 pm What does your bias circuit look like now? Same as the original or something different? It's possible that the bias setting affects the hum simply because you are increasing or decreasing the current draw through the power tubes. Was it ever quiet? Are the filter caps in good condition?
The ripple on pin 5 I am not sure how to measure.
I have changed the filter caps to larger ones 47/100 uf, but no change.
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Stevem
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- Joined: Fri Jan 24, 2014 3:01 pm
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Re: Hum in Bandmaster
Ripple is a small ac voltage riding on a larger dc voltage so you set your voltmeter to ac to read it.
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!
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martin manning
- Posts: 13207
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: Hum in Bandmaster
Which Bandmaster circuit is it?