I recently built my first fixed bias amp. It's a #183 Dumble except I replaced the output section with the 20w PP 6V6 building block from ax84.
With the ax84 design, you set the bias for the power tubes by measuring the voltage across a couple of 1R resistors at the cathode of the power tubes, and use the formula 0.7 * Pa max / Va to calculate the bias point.
My anodes are seeing about 300v under load so that suggests a bias current of about 33mA at the cathodes. However, as I approach this bias point, the amp starts to hum loudly, and when I hit 33mA it starts to splutter or motorboat. This is happening without a guitar plugged in. Raising the bias to about 37-ish mA, the hum vanishes, however that puts the anode dissipation around 80%. The received wisdom is obviously that fixed bias shouldn't go above 70%, with some suggesting even lower.
I imagine there could be a number of ways the gremlins have crept in since I've mashed different schematics together, but I just wondered if anyone with more experience would recognise this as a typical issue?
Many thanks,
Chris
Bias induced hum
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Stevem
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Re: Bias induced hum
If you back the bias down to get a current draw of 15 to 18 ma then what is the plate voltage on the tubes?
The motor boating is odd, I would try swapping the plate wires on the outout tube or the swapping the input to pin 5 on each tube a see if it gets better or worse!
In regards to the output tube heater wiring do you have pin 2 going to pin 2 and the same from 7 to 7?
The motor boating is odd, I would try swapping the plate wires on the outout tube or the swapping the input to pin 5 on each tube a see if it gets better or worse!
In regards to the output tube heater wiring do you have pin 2 going to pin 2 and the same from 7 to 7?
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Re: Bias induced hum
Thanks for the suggestions. I'll investigate those this evening when I get off work 
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Stevem
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Re: Bias induced hum
Also you my have a bad connection on the bias suppy filter , or just a plain bad filter such that when you have the tubes biased up to 33ma the bias supply largely goes un- filtered!
You may just need to reflow those two filter connections.
You may just need to reflow those two filter connections.
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: Bias induced hum
In addition to what Steve says, I'm wondering if it's the tubes. If they are badly mismatched, that might account for hum. Or it just might be something about one of the tubes. Do you have any spare tubes to try?
Mix and match preamp + power amp shouldn't be a factor. It's done all the time by home brew builders.
Remember, as you adjust bias voltage, plate voltage changes. You've got to check everything each time you make an adjustment and recalculate. For purposes of setting bias, figure that a 6V6 into an 8K output transformer is a 14W tube. 70% is 9.8W per tube. Remember that you are measuring cathode current, which includes screen current. If you aren't able to determine screen current, estimate it at ~2-3mA and subtract that from your cathode current reading. (Across a 1Ω resistor, voltage is the same as current, Ohm's law.) If there is a screen stopper, you can obtain the screen current by measuring the drop across that resistor and working the math of Ohm's law.
Mix and match preamp + power amp shouldn't be a factor. It's done all the time by home brew builders.
Remember, as you adjust bias voltage, plate voltage changes. You've got to check everything each time you make an adjustment and recalculate. For purposes of setting bias, figure that a 6V6 into an 8K output transformer is a 14W tube. 70% is 9.8W per tube. Remember that you are measuring cathode current, which includes screen current. If you aren't able to determine screen current, estimate it at ~2-3mA and subtract that from your cathode current reading. (Across a 1Ω resistor, voltage is the same as current, Ohm's law.) If there is a screen stopper, you can obtain the screen current by measuring the drop across that resistor and working the math of Ohm's law.
Re: Bias induced hum
Thanks, all good stuff to investigate. I don't have any spare tubes unfortunately, so new tubes will be my last port of call if all else fails.
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Stevem
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Re: Bias induced hum
With no spare outputs you can still yank one at a time and check if that kills the hum.
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: Bias induced hum
Is the hum frequency a line multiple, eg 120Hz?
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Re: Bias induced hum
Thanks for the replies, guys. First off, the weird motorboating was one of those infuriatingly simple problems - it went away when I disconnected the probes of my DMM. Gah!
As for the hum, I didn't realise the interplay between bias and plate voltage was so sensitive. So I continually checked the plate voltage which shifted quite a bit as I adjusted the bias, and ended up with 320 (ish)Va , suggesting a bias of 30mA. But that brought on the hum. Going down to 28/29mA killed the hum, and adjusting for screen current (something I hadn't accounted for, thanks Phil for pointing it out) that bears out about right.
Thanks again everyone. It's a learning process!
As for the hum, I didn't realise the interplay between bias and plate voltage was so sensitive. So I continually checked the plate voltage which shifted quite a bit as I adjusted the bias, and ended up with 320 (ish)Va , suggesting a bias of 30mA. But that brought on the hum. Going down to 28/29mA killed the hum, and adjusting for screen current (something I hadn't accounted for, thanks Phil for pointing it out) that bears out about right.
Thanks again everyone. It's a learning process!
Re: Curious
Will do. I'm building it into a combo, I'll post some pics and sound clips when it's done.SilverFox wrote:Be sure to let us know how this version of the Dumble sounds!
silverfox.