Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ledvedder on November 07, 2022, 02:09:47 pm
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I'm just wondering if these voltages are acceptable on my new 5e3 build? B+2 seems a bit high at 333V, which is also affecting pin 4 on the power tubes. Also, the v1 voltages seem a bit low. I've noted my voltages in pen on the attached image.
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Look fine to me.
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When I get that close to spec, I am extremely happy and have a beer. Since its election season... they are all within the margin of error. Play it yet?
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sounds ok. Not great yet. I think I'm having an issue. The NFB switch works in only position 1. In the center position, the sound just fizzles out. In the 3rd position, I get sound, but as soon as I whack a note, it fizzles out.
I'm, also having a hard time deciphering which side of the bias switch is stock 5e3, and which is fixed bias.
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I'm, also having a hard time deciphering which side of the bias switch is stock 5e3, and which is fixed bias.
In the stock position there will be zero vdc on pin 5 of the output tubes and some positive vdc on pin 8.
In fixed bias there will be some negative voltage on pin 5 and zero volts on pin 8.
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I'm, also having a hard time deciphering which side of the bias switch is stock 5e3, and which is fixed bias.
In the stock position there will be zero vdc on pin 5 of the output tubes and some positive vdc on pin 8.
In fixed bias there will be some negative voltage on pin 5 and zero volts on pin 8.
This is with the output tubes installed?
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fixed bias with or without tubes installed. cathode bias there won't be negative voltage on the grid but there also won't be positive voltage on the cathode without tubes installed
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I'm, also having a hard time deciphering which side of the bias switch is stock 5e3, and which is fixed bias.
In the stock position there will be zero vdc on pin 5 of the output tubes and some positive vdc on pin 8.
In fixed bias there will be some negative voltage on pin 5 and zero volts on pin 8.
This helped me determine which switch position was fixed vs cathode. Thanks!
Now to figure out what's going on with the NFB switch.
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You may need to flip the OT primary leads for the NF. With my 18 watt AX84 October I had to do that. It will make a nasty noise if it's wrong. I had a switch to turn NF on or off. I haven't looked at what your three way does.
Also had to increase tail resistor for fixed bias. It was a little too hot even with the pots dialed all the way down (I had one for each tube).
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You may need to flip the OT primary leads for the NF. With my 18 watt AX84 October I had to do that. It will make a nasty noise if it's wrong. I had a switch to turn NF on or off. I haven't looked at what your three way does.
Also had to increase tail resistor for fixed bias. It was a little too hot even with the pots dialed all the way down (I had one for each tube).
My 3 way switch is doing stock mode/no v2a bypass cap/56k NFB with no bypass cap. I've attached the diagram.
I haven't gotten to the fixed bias adjustment yet. Did you just use the instructions here, https://robrobinette.com/Tube_Bias_Calculator.htm?
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You may need to flip the OT primary leads for the NF. With my 18 watt AX84 October I had to do that. It will make a nasty noise if it's wrong. I had a switch to turn NF on or off. I haven't looked at what your three way does.
Also had to increase tail resistor for fixed bias. It was a little too hot even with the pots dialed all the way down (I had one for each tube).
My 3 way switch is doing stock mode/no v2a bypass cap/56k NFB with no bypass cap. I've attached the diagram.
I haven't gotten to the fixed bias adjustment yet. Did you just use the instructions here, https://robrobinette.com/Tube_Bias_Calculator.htm? (https://robrobinette.com/Tube_Bias_Calculator.htm?)
I can't remember if I used that since I built the amp 10 years ago. I ended up using 24ma for my 6V6GT's. I used 1 ohm in line resistors and had test ports on mine which were really handy. Whatever you measure in mv is the ma bias current.
PS: I ordered my 5E3 kit from Boothill today. My build is going to be super stretched out time-wise. I like taking my time especially since I'm retired. Hopefully it's not done before xmas. :sleepy2: