adjusting EL84 cathode bias?

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Walker
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Re: adjusting EL84 cathode bias?

Post by Walker »

Bieworm wrote:
Thu 11/24/22 1:19 am
Could you give us the other numbers/Voltages? Here's a voltage chart, but watch out.. I think your V2 is the PI, and here V2 is the preamp of the tremolo channel.
You can notice that the Modern Classic's tube layout improves the lead dress, with less wires crossing.

files/JMPGuitars_18_Watt_Modern_Classic ... _Chart.pdf
Hey @Bieworm do you know where I can find the schematic for this amp? Filling out the voltage test sheet now but I'm not sure what "A" "B" "C" and HV1 and HV2 are? Thanks!
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Walker
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Re: adjusting EL84 cathode bias?

Post by Walker »

Haha ok nevermind, all I had to do was replace "voltage test sheet" with "schematic" in the url and lo & behold :D
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JMPGuitars
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Re: adjusting EL84 cathode bias?

Post by JMPGuitars »

Walker wrote:
Sat 12/03/22 8:38 pm
Haha ok nevermind, all I had to do was replace "voltage test sheet" with "schematic" in the url and lo & behold :D
You're welcome. 😉😉
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Walker
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Re: adjusting EL84 cathode bias?

Post by Walker »

Thanks everyone for your help. All of my voltages lined up - but there was one peculiarity....whenever I would touch pin 7 on the PI I heard a crackling/crunching noise (not through the speaker, just coming out of some component :? ). Turns out the 0.01uF cap on the PI was not connected to ground and now that I've connected it, this amp is sounding exactly as I wanted it to, it's screaming!

To understand how the heck I got here, this amp was originally a TMB when purchased, I wanted to gut the TMB channel and convert it to an 18W lite. In doing so, I removed the TMB master volume and inadvertently left that capacitor hanging in the breeze! Without the master volume, one side of the cap was totally disconnected...duh. Rebiasing the output tubes was helping the tone and volume, but apparently was not the real culprit.

Now this amp is just ruling and I'm super stoked, thanks again!
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Jon_Kayak
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Re: adjusting EL84 cathode bias?

Post by Jon_Kayak »

Thx a lot for the explanations! Very very useful!
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