Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: punkykatt on February 11, 2018, 05:59:03 pm
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Hello all, a friend of mine asked me to install a pentode/triode switch in his VHT 36 watt head. In the triode mode I get a low pitch hum, in pentode its fine. The switch is located on the back panel right between the four EL84`s and all wire runs a short as possible. Attached is a schematic how I have the switch wired. Has anyone else had this problem? Thanks in advance for any help or comments.
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Swap the two wires on the center lugs of the switch.
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Thank you Steve. still have the hum in triode mode.
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Just thinking out loud here but maybe the noise in triode mode is because b+1 node has less filtering than B+2 node :dontknow:
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Maybe also just hardwire triode mode for testing to eliminate wire noise :dontknow:
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VMS, I tried clipping in a 33uf/450v over the 1st filter cap to make 66uf no change in hum. I have put Pentode /triode switches in other amps but they either had 6V6`s or 6L6`s with no problems.
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Found the source of the hum in triode mode. The quad of el84`s were not matched very well one pair was drawing almost twice the currant of the other pair. I put in my test quad of JJ`s , hum gone. Im glad I built bias checkers for el84`s. Thanks to all for your replies.
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Yep it helps to have quads that are reasonably closely matched (same with sextets :huh: )
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Swap the two wires on the center lugs of the switch.
Did you leave these wires swapped? I mentioned it because your schematic shows them crossed.
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Steve, I have to correct that schematic. The screens are connected to the wrong plates in that schematic.
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Corrected schematic