swapping OT primaries

[phillyhudson]

hi there all - never had this problem before - hoped I might get an answer from a more experienced builder - when you connect the OT primaries the wrong way round you either get a squealing noise or the amp reacts very strangely with bad sounds - this new build is working but acting strangely with oscillation on the tone stack and first gain pot so I swapped the primaries over and its exactly the same - works but with oscillation when you turn first gain pot past about 2 o'clock - [ new ]OT measures fine - never had swapping the primaries do absolutely nothing before.....
any ideas ?

[pdf64]

Does it have a global NFB loop around the power amp?
Is there a gain control within the loop? eg type 1-4 master vol. If so, turn it up full and reassess.
Beyond that, a schematic is needed really.

[TUBEDUDE]

You have an oscillation aside from the positive feedback from the reversed primary.
Do you have an O'scope? Yes, schematic please.

[phillyhudson]

thanks for reply - I'll try what you suggest tomorrow when the neighbours are out of the house

yes it has NFB [havnt finished schematic but its a usual marshall set up] - its a kind of plexi going into dumble type bluesmaster lead

amp.pdf

[phillyhudson]

sorry - forgot to say - dont have scope

[TUBEDUDE]

I'd add buildout resistors to your cathode followers, make sure your grid resistors are soldered against the socket pins, and watch lead dress. Generic anti- oscillation measures but no scope makes it a tougher job.

[phillyhudson]

not familiar with build out resistors - where would they go on the cathode follower , and what value ?

[10thTx]

its a usual marshall set up] - its a kind of plexi going into dumble type bluesmaster lead

Have you seen the D'Mars OverDrive schematic and layout? Has some similarities to it. Marshall topology into an D-style overdrive channel.

https://el34world.com/Forum/index.php?topic=14302.0 Hoffman amps carries a board for it.

There are several versions with either 12AX7 or 5879 tubes in the OD section with and without onboard FX loops.

Some soundclips:

https://soundcloud.com/dageezer/dumars-noodles1-0

https://soundclick.com/r/s71th8

https://soundclick.com/r/s71g2i

With respect, 10thtx

[phillyhudson]

thanks 10thTx - nice clips - not seen your schematics - just liked bassman/plexi front ends and bluesmasters and wanted to see how it all sounded

[TUBEDUDE]

The buildout resistors are tied to the cathode and are typically 100- 1kOhm. They help isolate the tube from the driven circuit reactive component, (capacitance, inductance), and can eliminate instability/oscillation. Like the resistor to protect the control grid.

[TUBEDUDE]

How goes the oscillation problem? You can short the P.I. input to ground. That should show the problem to be in the pre stages or not. Do you have a DMM?