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 Post subject: Plexi MkII questions
PostPosted: Mon Apr 17, 2017 10:40 am 
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I recently purchased and built the MkII kit. Everything works pretty darn good except one small thing. Through an 8 ohm 4x12 cab it is perfect. When going through a 16 ohm 2x12 it squeals when the volume is about 8 on both channels. Sometimes fiddling with the EQ will tame it. But if I remember reading correctly if I switched the phase of the OT transformer inputs at the power tubes it should solve the issue. Is that correct? Because I can't find where I read that exactly.

Also I was messing around with a 1p2t switch to toggle out the two 47k(or 470k I don't have the schematic in front of me) on V2 to add a little TMB extra gain. Would I still need the grid stop resistor on pin 3 of V2? I would obviously need a different switch to achieve that. Just curious.

Next I add a cascade switch and tidy up some wiring that is bothering me.
All in all what a fun time assembling and finally playing this bad boy.


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 Post subject: Re: Plexi MkII questions
PostPosted: Mon Apr 17, 2017 5:58 pm 
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Holy Ghost
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Joined: Sun Mar 13, 2005 4:10 pm
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18wattandlife wrote:
I recently purchased and built the MkII kit. Everything works pretty darn good except one small thing. Through an 8 ohm 4x12 cab it is perfect. When going through a 16 ohm 2x12 it squeals when the volume is about 8 on both channels. Sometimes fiddling with the EQ will tame it. But if I remember reading correctly if I switched the phase of the OT transformer inputs at the power tubes it should solve the issue. Is that correct? Because I can't find where I read that exactly.


You can try reversing the OT leads. Explanation from the manual Troubleshooting section "Squealing/Feedback", read that for more details.
"5. If all that doesn't work reverse the output transformer leads on pin 7 on V4, V5.
Explanation: One of the primary leads is in phase with one of the secondary taps. In a high gain amp, this phase relationship needs to be maintained."

If that doesn't work, other steps may need to be taken. One thing to do it to move the wires around with a chopstick . Separate them well away from each other & shorten where possible.


18wattandlife wrote:
Also I was messing around with a 1p2t switch to toggle out the two 47k(or 470k I don't have the schematic in front of me) on V2 to add a little TMB extra gain. Would I still need the grid stop resistor on pin 3 of V2? I would obviously need a different switch to achieve that. Just curious.

The two 470K resistors form a voltage divider to effect changes to the gain. You can experiment with them if you like.

Next I add a cascade switch.[/quote]
You wont need to change the voltage divider above if you do that. Check unimind's post on his Mk. II mods. He built in a cascade switch.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps


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 Post subject: Re: Plexi MkII questions
PostPosted: Tue Apr 18, 2017 9:14 am 
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Joined: Mon Apr 17, 2017 10:27 am
Posts: 2
Thank you for the response. I tacked in the cascade switch quickly to see how I liked it and holy smokes this thing is an absolute monster. 10/10. I would recommend this to everyone. I'll permanently fix it in next week when I get more wire and have more time. What a bad ass amp.


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 Post subject: Re: Plexi MkII questions
PostPosted: Tue Apr 18, 2017 5:21 pm 
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Holy Ghost
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Location: Canada
18wattandlife wrote:
Thank you for the response. I tacked in the cascade switch quickly to see how I liked it and holy smokes this thing is an absolute monster. 10/10. I would recommend this to everyone. I'll permanently fix it in next week when I get more wire and have more time. What a bad ass amp.



:thumbsup: :thumbsup:

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Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps


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