Preamp mod help

[schatman]

Hi,

I just built a DIY MOD 102 as my first amp build. It went together really quick and sounded pretty good as is. The only problem was a slight hiss (peaks between 2800 and 3300 Hz) when the volume was above 5 or so. I wanted to do a couple mods as a first timer, so I started playing with the tonestack. It had a tonestack like the BF Deluxes, Princetons, and similar to my SF Champ which I find a little low in mids, so I switched out the mid resistor for a 25k linear pot and I was very impressed with the amp.

After that I decided to try to put in a more tweed like preamp using a dpdt to make it switchable. I chose the Vol/Tone combo from a Deluxe, used alpha 1M audio pots (like those that came with the kit) and used CBE (Mallory) propylene and one silver mica cap. When I wired this up with the the dpdt switch so I can switch between circuits. Using this the original preamp still worked perfectly but all I got from the tweed preamp position was noise. Then I simplified it to a tweed champ style with one Vol pot and still the same thing. Then i simplified it even more with just the 0.02 mfd cap, so theoretically like a tweed champ at full volume, but still just noise. The switch is wired in properly, as the original postion works fine with no noise. Any ideas? Any help would be greatly appreciated!

Thanks!

[Firestorm]

Schematic? Pictures?

[JazzGuitarGimp]

Just on the off-hand chance you've got a DPDT switch where the common (wiper) connections are at one end, rather than in the middle (where we all expect them to be), you might try ringing-out the switch with an ohm meter, just to make sure it's behaving the way you expect it to. You may also discover one pole of the switch is bad in the Fender position.

[schatman]

Thanks for the quick replies.


"Schematic? Pictures?"

I was going to upload a couple but had to run to a meeting. Should hopefully have them up soon.


"Just on the off-hand chance you've got a DPDT switch where the common (wiper) connections are at one end, rather than in the middle (where we all expect them to be), you might try ringing-out the switch with an ohm meter, just to make sure it's behaving the way you expect it to. You may also discover one pole of the switch is bad in the Fender position."

That is something I hope to check tonight. I should have more info soon. Thanks for taking the time to help. I was shocked how good the kit sounded. With the mid control boosted, played through a 2x10 with cheap 80s G10S50 celestions with a Les Paul Junior bridge p90 I got some tone similar to the Whole Lotta Love solo tone.

[Firestorm]

Cool. You gotta watch the signal chain and make sure it's doing what you think. It should be a simple fix.

[schatman]

If this attachment works right, it should show my intention.

[img581]https://tubeamparchive.com/files/mod102_mod_999.jpg[/img]

[schatman]

Figured it out. I had bypassed the anode resistor. It works alright now. The "Tweed" channel has a huge amount of gain. The original MOD102 design has higher gain in both stages to compensate for the signal loss in the tonestack. When I turn up the tweed volume it goes from zero to almost max in a short amount of sweep, then at high volume I get an odd helicoptering sound, but it works reasonably well, perhaps just a little tweaking with resistors! Thanks for taking your time to reply!