Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mike_J on March 28, 2021, 12:13:55 pm
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Looking through the leather binder of honor to see what is in it. Some of it is Hoffman related. The attached is a copy of a comment from correspondence related to a California builder's amp. Pretty sure I made a copy of this amp. I call it the ODS 100 - HRM. It sounds real nice but I am pretty sure it can be made better. Will be using Hoffman's layout program to design the board or maybe boards for it. What it won't include is the ODS JFet input stage. Waste of valuable amp real estate.
Notice how secretive this is. Apparently this builder would take or threaten to take anyone to court who would use his name which the document is cloaked in such secrecy. He even covered his boards in this gunk so nobody could tell what values he was using . Obviously only partially successful.
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What I was really looking for is evidence of what he did in the phase inverter section. Looks like he used a 2K presence pot with a 1Uf capacitor and some resistor in parallel to ground. For the 5f6a rebuild I am planning on using a 2.5K presence pot with a 2.2K resistor across it as an experiment. Was planning on using the standard .1 cap from the wiper to the CCW lug. Could experiment with the 1uF cap as well as I have one that I would never probably use otherwise.
Think what the amp builder was trying to do was balance his PI, or at least find a balance he considered satisfactory maybe for his entire output stage. Was known to use trimmer pots which are probably unnecessary with the calculator we have today if you know all the factors to include or at least control the factors that could impact the balance we are trying to achieve.
This has to be solved in the 5f6a build. Makes no sense to go forward until the balance in the amp is understood.
Thanks
Mike
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Anyone see a potential problem with using the 1uF cap I have on hand versus the .1uF/250v Xicon next to it in the picture below?
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You would have to build your amp around that thing. It is much larger than a 22uF/500v ecap.
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Anyone see a potential problem with using the 1uF cap I have on hand versus the .1uF/250v Xicon next to it in the picture below?
Does anyone see anything I can use these for? Tubenit seemed to like the Musicap coupling caps in the V1 coupling stage. What would happen if I used 1uF caps as coupling caps for V1? Only 50 times the size of a .02uF cap, both literally and figuratively. I can't think of any way I would use these in a million years. Ordered .1uF/400v caps and this is what I got. It was many years ago. Have no idea where I even got these.
Thanks
Mike
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Could anyone explain why the amp builder would want to use a 1uF cap on the presence pot instead of the standard .1uF cap that is commonly used?
Thanks
Mike
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Depends on the resistance in the network, don't it? And you are describing several difference resistances. So.... it depends?
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Depends on the resistance in the network, don't it? And you are describing several difference resistances. So.... it depends?
They were describing a 2k presence pot with a resistor across it from CW to CCW meaning it would need to make the pot less than 2K. Dumble also used trimmer pots between the 82K and 100K plate resistors in the PI. My guess is he was doing all that to balance the PI. The entire tail resistor can only be 8K to balance it. The trimmer pot is there to fine tune the PI. Would bet the farm on it. Maybe because the presence pot is so much smaller, as would the entire PI tail resistor, the 1Uf cap is preferable. Just a guess on my part.
Also noticed Dumble was using two ecaps in series for every stage, even the preamp stage. Used 470K resistors across every cap. Merlin explained why that was advantageous both from the point of using a 470K resistor instead of the 270K resistor Fender commonly used and also the ecaps in series throughout but I can't remember his explanation. When I look it up again I will post what he said.
Thanks
Mike
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What it won't include is the ODS JFet input stage. Waste of valuable amp real estate.
Maybe yes, maybe no. There are pictures of David Lindly, lap steel player (many recordings with Jackson Brown), showing him plugged into the Jfet input stage of his Dumble. He sounds pretty good to me. :dontknow:
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What it won't include is the ODS JFet input stage. Waste of valuable amp real estate.
Maybe yes, maybe no. There are pictures of David Lindly, lap steel player (many recordings with Jackson Brown), showing him plugged into the Jfet input stage of his Dumble. He sounds pretty good to me. :dontknow:
Thanks Willabe. I was so sure of what I was going to do. Now I don't know. Will re-evaluate the plan when I get to that point.
Thanks
Mike