Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on January 24, 2022, 11:10:49 am
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Hi all I recently built a Sluckey designed Dual Lite amp. I really like the amp especially the Vox AC15 side.
The problem I have is when switching the Brilliance knob on the 6 pos rotary switch it pops when switching.
I'm now not 100% positive the switch I received is a shorting switch.
I installed a .0047uf film cap across the input and output and it does cure the pop after a short amount of time the amp has been turned on and the switch is switched.
Doesn't have any effect that I can tell on the circuit other than stopping the pop.
I guess I should order and make sure I receive a shorting switch. I ordered this one from AES and it is not in my order history for some reason so I can't confirm the part #.
Any other ideas? different cap value maybe?
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You should be able to look at the switch wipers to see if it's make before break. Turn slowly and you'll see the wiper makes contact with the next position contact while still touching the previous position contact. That's make before break. Turn a little farther and the wiper loses contact with the previous position and will only be connected to the next position.
You can put some 5.1M resistors across the contacts (five resistors needed for a six position switch) to prevent pops. Look at this...
http://sluckeyamps.com/hammond/hammond_ao39.pdf
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I wired up a similar contour switch, but using 4.7M resistors between each contact, as illustrated in the attached image from a Trinity TC-15 build manual (page 39). https://www.trinityamps.com/support/ (https://www.trinityamps.com/support/)
It was a royal pain to do but it is quiet and works well. No popping.
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Thanks guys.. it can unequivocally say that is indeed is a pita to add things to an already populated switch but is doable. I don't have any half-watt 4.7 pr 5.1meg resistors so I would have to order them.
I guess I'll have to decide if i want a new shorting switch or to add the resistors to the current one.
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Hey mresistor, I had the same issue even with a make-before-break switch. I added the 4.7M's and it solved the problem. I used an Alpha DP6T (P-H392).
But, I didn't want to feed B+ directly into the switch, so I also have a .1uF coupling cap between the EF86 output and the switch. I measured the capacitances on the switch and the coupling cap doesn't lower the values much at all (capacitors in series). For example, the cap on my switch position #1 is a .01uF. With the .1uF in series it still measures .009uF - a very small reduction.
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Thanks guys.. it can unequivocally say that is indeed is a pita to add things to an already populated switch but is doable. I don't have any half-watt 4.7 pr 5.1meg resistors so I would have to order them.
I guess I'll have to decide if i want a new shorting switch or to add the resistors to the current one.
Half-watt is not needed. There won’t be any significant current through those resistors. 0.25w or even less would work, and probably also easier to fit on the switch, due timo smaller size.
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Thanks again for the tip, I have 1/2w 51M resistors on order, also finishing the faceplate and will post a pic when I get it on. I've got it set up to sit on feet with it exposed. Would like to design a cage or something to have it open but a little more protected. Any ideas would be welcomed.
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I have 1/2w 51M resistors on order
I hope you ordered 5.1M.
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Yes thats a typo I have 5.1M on order .. thanks for the alert.
Concerning a cage I guess a cabinet with front and back open with expanded metal front and back would be cool, then could have a handle on it.
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But, I didn't want to feed B+ directly into the switch, so I also have a .1uF coupling cap between the EF86 output and the switch.
I'm not really understanding this because the schematic and layout I used has a .1uf coupling capacitor already existing between the plate of the EF86 and the switch input. Can you enlighten me?
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I think he's referring to my AO-39 conversion and Pharmrock's diagram.
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Ah OK I didn't read the text in the diagram he posted because the pic was explanatory enough for my need.. Thanks .
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Hey mresistor, sorry for creating confusion! I did originally have the .01uF cap on the board and no cap on switch position #1.
I'm reworking this amp and decided to put the .01 cap on the switch and an additional .1uF cap on the board. :icon_biggrin:
This way, each switch position has its own cap and the switch itself is blocked from DC by the cap on the board. It may be overkill, but I have the parts on hand. The schematic detail I posted above is my new, reworked circuit.
I don't know what voltage the Alpha switch is rated for (I think AES states 125VDC), but I just feel better not having DC on any part of it.
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In case anyone is looking for this switch, you can order it on Amplified Parts as either shorting or non-shorting: https://www.amplifiedparts.com/products/switch-rotary-2-poles-6-positions-shaft (https://www.amplifiedparts.com/products/switch-rotary-2-poles-6-positions-shaft)
Just ordered one for this control as part of an EF86 layout, and wasn't even aware there was a MBB/shorting option until seeing Sluckey's schematic. Hopefully will eliminate the headache of wiring up all those resistors on the switch.
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The 5.1M resistors arrived and I got the microscope glasses out and installed them. Works like a champ - no more pop. Thanks to all.
Here are some pics of the chassis.
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front and back front panel made in the Ken Fisher tradition.
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I need to get some tube retainers for the EL84s. That thing that's attacted to the side is for the chassis to fit in my crude amp cradle. The amp has a bottom cover with feet.
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Very nice. :thumbsup:
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Thank you. :-)
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Nice work. :icon_biggrin:
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Thank you