Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ncusack on March 14, 2014, 10:44:51 am
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Hey Everyone,
I'm planning a new build using some on hand parts thats based on Geezer's Little Wing bassman amp. The big constraint in this build is the power transformer which is a Hammond 261G6 which has a 6.3V winding rated for 2A. The little wing would call for 2.16A using the 4 tube setup. My idea is to scale it down and use a MOSFET for the cathode follower and a single triode for V1. This should bring the heater requirements down below the rated limit and hopefully still prove to be a decent amp.
What I'd like to know is if my layout and schematic look like they make sense. The schematic and current layout can be found here
(http://s17.postimg.org/mpk7tkojf/Chicken_Wing.png) (http://postimg.org/image/mpk7tkojf/)
(http://s17.postimg.org/4k2o1rznf/Chicken_Wing_Layout.png) (http://postimg.org/image/4k2o1rznf/)
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Looks like an interesting project. Something to consider. A little 6C4 7-pin single triode for your cathode follower. 150mA filament draw. Would that put you over the top?
Tubenit has had proven success with a MOSFET as a CF in his builds. You can confirm your configuration from one of his schematics.
Jack
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The current draw with the 3 tube setup is 1.86A just under the 2A limit. I pretty much pulled the idea from some of the ideas Tubenit had success with. I'm mainly looking for any huge mistakes I might be making with the layout.
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Your 2.16A should be no problem for a 2A winding. Hammonds are generally underrated and at worst you might get a voltage of 6.0V rather than 6.3V, which is within 5%. If you have your tubes put them in parallel and hook it up to the winding and find out what you end up with.
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Looks good. You might need an anti POP resistor across the PAB switch and the 220r on the mosfet is 1W. :thumbsup:
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Your 2.16A should be no problem for a 2A winding. Hammonds are generally underrated and at worst you might get a voltage of 6.0V rather than 6.3V, which is within 5%. If you have your tubes put them in parallel and hook it up to the winding and find out what you end up with.
This is true. You can also call tech support at hammond, they are great. They will probably insure you about this, however the MOSFET will work and we really don't need all those tubes getting in the way. They are inefficient and out of date anyway.
Kidding of course. Your idea should work well.
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Alright so it's looking like I'm on a good track so far.
I just have a few questions about the mosfet setup. I have a number of Fairchild FQP5N60C mosfets available on hand through work. Will that part sub in for the common IRF820? The FQP5N60C appears to have pretty similar specs to the IRF820 from the datasheet.
I would also like to know what value gate-source zener should be used? Is it 1/2W, 1W, 6V, 12V?
Cheers,
Neill
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So the build has started and I want to confirm a few things about my plans. I'm looking to follow Hoffman style grounding and I'm wondering if my plans make sense.
In the attached photo you can see the board build in progress. Preamp to PA goes form right to left. I plan to run a buss wire from the PI power cap in the middle of the board over to the preamp grounds then connect that to the input jack. The power ground will be done using jumper wires on the board and then one wire off the cathode ground to a chassis bolt.
I would like to know if this makes sense? I also want to know if I should run a ground buss for the pots or if I should just fly the ground connections to the main buss wire.
Cheers,
Neill
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Things are looking great, First off, I see that you have 100r resistors on the heater supply, if there is no CT then those resistors need to be grounded to form an artificial CT.
The specs on the FQP5N60C look good for a match to the IRF820.
The 12v ZENNER I think I used was 5w.
I think the grounding that you have on your layout is fine with a solid buss wire running along the side of your circuit board.
Many have different views on a buss wire on the pots.
I have found it is a lot easier to run the buss above the pots and rather than pass the wire through the lug on the pot, I snip a piece, the thickness of the wire out of the lug ,this then allows you to desolder the lug for easier removal of the pot should it have any problems (I think most would just pass the wire through the lug as the pots really don't have many defects)
This is connected to the input jack then either use the input as the ground or use a separate ground bolt.
Use one wire from the circuit board to the buss on the pots. :icon_biggrin:
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Hey mate, this is about the best pic I could find. As you can see I've bent the buss so as to clear the top of the pots. Snipped a small piece out of the loop and soldered to buss.
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Ok so I'm on to the overall chassis layout stage and would like to know what you guys think. I've included a photo here http://s24.postimg.org/89p95ugyt/IMAG0179.jpg (http://s24.postimg.org/89p95ugyt/IMAG0179.jpg) of the stuff im working with. The Chassis is 14 1/2" x 7 1/2" x 2 1/4". The board seen earlier in the thread is 13" so things will be pretty tight in this box. I plan to do the headphone trick to place the OT but id like to know if where things are in the pic make sense for a starting point. I'm mainly concerned with the choke placement and the OT placement with regards to the 6BM8 tubes. I'd like to avoid any noise being induced anywhere possible.
Let me know what you guys think.
Cheers,
Neill
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Well, you have the ac power and PT on the far right, the guitar input on the far left and your OT and choke are shielded, the PT and OT are turned 90deg from each other. You also have the PT placed so the largest amount of magnetic flux field will radiate towards the front and back of the chassis and not towards the OT and preamp/guitar input.
Looks good to me.
Brad :icon_biggrin:
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Sounds good to me. One more question though. When routing the OT wires should I keep them on the outside of the chassis for the majority of the wire run then bring it through a grommet close to the circuit connection or should I just bring it right through the chassis under the turret board?