crazy noises in tmb inputs

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twl
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crazy noises in tmb inputs

Post by twl »

the normal inputs work fine but the tmb inputs squeal like crazy now i attached my buus to the back of the tmb pots could this be the result or is it the ot primarys
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GibsonGM
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Post by GibsonGM »

Hmm, TWL,

I'd search for "squeal" and "TMB Squeal" and see what you come up with. Try plugging in and turning the TMB channel volume down and Master up (decrease the gain), see if that removes the squeal. If so, I'd suspect it's a matter of lead dress, if all else works fine. OT wires should be twisted together and as short as is practical, but let's start with the obvious...

Having the ground buss on the pots is ok as long as you're not creating big ground loops somewhere (search for "grounding" for more info). Your pots are isolated, I hope? Many TMB's squeal (annoying dog whistle) at max volume/max gain, and require some creative chopsticking to eliminate. A very popular cure for the squeal is to move the 470K R coming off the TMB volume pot directly to the socket of V3 using shielded cable, going under the turret board.
Including a pic or diagram of what you have going on could help debug this, too...mainly, how are your grounds run.
As always, whatever you do, use all the normal safety procedures when working on your amp, there are potentially lethal voltages present in there!! This will get you started, post back with more info...
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kedsinger
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Post by kedsinger »

I've found that the TMB channel needs to be tamed from the schematic I used. I replaced V2 with a NOS 12au7 for my liking. The second 1/2 of V1 is the first gain stage for TMB so that gives it enough kickstsart and then coupled to a 12au7. Keep your PI 12ax7. You might want to this...I like it, but then again I'm not into real high gain like I used to be. Also you could play around w/ cathode R and C values along with plate R. Grid leak R plays a role in gain as well, it's like a release valve for a faucet(letting signal pass to ground). I was psyched when I recently learned this. It's tough to learn all this stuff without any electronic eng. education. I learn more and more everyday.
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leslie
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Post by leslie »

yes kedsinger, it is meant to be like that,learning process :!:
Even the EE educated people have to learn the behavior of the given circuit on the practical lenght.
Theres no better education than making practical mistakes ,and learning about things you need to debug,focused.
(It's amazing to see experienced service man/tech who recognise faulty component from sound of amp,in 10 minutes the job is done.)ouch
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First thing I will do with a TMB is to apply shielded input wire/isolated input jacks,second I'll check ground buss/ground loops.
third i'll try to fix oscillaton with shopstick,and it fails i'll make sure that all wires are short,and not parallel to each other.
Tap the preamp tubes to find microphonic/oscillated one,put that tube in PI ,swap tubes ...
After that i'll swap OT primaries if it helps.
I'll check OT output jack ,this should be non isolated type.
I'll make sure that OT wires are not close to preamp wires/ stage.
Be sure that you dont have cold solders,check the underboard wires if you have ones.
Try not to modify amp till is running healthy,for me is important to notice and remember original sound and go from there...fine tunning.

It should be fun and learning process always,the amp which is running perfect from the beggining,will actually teach us nothing :!:


Be carefull with high voltages and have fun.
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