Hi all,
I'm planning on building a guitar amp based on Rob Robinette's simplified AB763 circuit he calls the "Blackvibe". I want to use the power and output transformers I salvaged from a Roland Bolt 60. Trouble is, the Blackvibe circuit draws approximately 2.4A at 6.3v for the heaters while the power transformer is (supposedly) only rated for 2.1A on the 6.3v winding. I have attached a pic of the transformer and schematic of the Bolt 60 so you can see what's going on.
I hooked up several 6L6 tubes and measured the voltage on the 6.3v secondary:
No load (0A) - 6.36v
2 6L6 (1.8A) - 6.15v
3 6L6 (2.7A) - 6.04v
DC resistance of the 6.3v secondary is approximately 2 ohms. There was no load on the other secondary windings. Do those voltage drops look like the transformer is being loaded down excessively? My suspicion is that the 2.1A rating was based on the fact that the three tubes in the Roland Bolt 60 would draw 2.1A, and does not actually reflect its maximum current rating. Alternatively, I could use a 500ma 12.6v transformer for the two preamp tubes in the Blackvibe circuit, and use the Roland tranny for the 6L6's alone.
I should mention I'll use a solid state bridge rectifier for the B+ supply, as the Roland transformer can't accommodate a tube rectifier.
Anyways, I appreciate any thoughts you have and thanks in advance for the help!
Power Transformer for AB763 Circuit
Moderators: pompeiisneaks, Colossal
Power Transformer for AB763 Circuit
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Re: Power Transformer for AB763 Circuit
I think the 5% regulation exhibited by the 2.7A load is a reasonable / typical full load characteristic, so I’d be happy to use it for such a project.
The 2ohm winding resistance seems higher than expected though; did you calibrate the meter first, so that shorting the probes resulted in a reading of 0ohms?
The 2ohm winding resistance seems higher than expected though; did you calibrate the meter first, so that shorting the probes resulted in a reading of 0ohms?
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Re: Power Transformer for AB763 Circuit
I would get a small separate filiment transformer just to fulfill the needs of your preamp tubes and leave the two outputs powered off what you have now.
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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: Power Transformer for AB763 Circuit
I did try to calibrate the meter, but I was using a different set of probes. I would guess the DC resistance of that winding is actually 0.1 ohms or less, but I don't have a milliohm meter to accurately determine that.pdf64 wrote: ↑Fri Jul 22, 2022 8:59 am I think the 5% regulation exhibited by the 2.7A load is a reasonable / typical full load characteristic, so I’d be happy to use it for such a project.
The 2ohm winding resistance seems higher than expected though; did you calibrate the meter first, so that shorting the probes resulted in a reading of 0ohms?
Looks like the transformer should be able to supply heater current to 2 6L6's, a 12AX7, and a 12AT7 without trouble though, so I'll give it a try. I might get a small 12.6v transformer just in case the main one under performs in the amplifier circuit.
