Noise after mod

[hebaton]

I have an great JTM45 clone. sound excellent and very responsive. But enough is never enough so I wanted options. First installed a switch to allow going from stock to split cathodes using 2k7 /.68uf on the bright channel. I got something usable but I wanted a bit more character. So I added to that switch ( there was a pole still available ) a .68uf cap bypassing the 820ohms on V2a. Now I get a lot less bass and some noise... I believe the 50 watts had more filtering in the preamp ? Is this why I am getting this noise ?

Thanks for you support to this newbie not always sure what he's doing...

[johnnyreece]

A picture is worth a thousand words, but, in the meantime: Is it possible you've wired the cap in backwards?

[hebaton]

THose SoZO .68 are not polarized as far as I know ?

[pompeiisneaks]

They do have an 'outer foil' that, if connected correctly, sends external noise to ground or rejects it back to the previous stage instead of passing it, if not, it can add noise to the signal. I'm guessing that's what the reference is to...

~Phil

[John_P_WI]

Either your "new" bypass cap isn't ground referenced when out of circuit, or you are mixing grounds up and causing a loop.

[johnnyreece]

They do have an 'outer foil' that, if connected correctly, sends external noise to ground or rejects it back to the previous stage instead of passing it, if not, it can add noise to the signal. I'm guessing that's what the reference is to...

~Phil

Thanks for chiming in and knowing what I meant! I kinda doubt that's the issue, but ya never know...

Either your "new" bypass cap isn't ground referenced when out of circuit, or you are mixing grounds up and causing a loop.

This is a great thought, and a picture or schematic of your mod would help track that down.

[wpaulvogel]

The preamp filtering isn’t the problem. I’ll bet you’ve either created a ground loop if you randomly grounded the additional capacitors or your wire runs are long enough to get noisy. When I apply this modification, I always mount the components in the same location on the board as a lead circuit. The v2 bypass cap, I add a turret right next to the cathode turret. I mount the switches (I always use multiple switches) directly under the board and the wires drop right down to the switches. My version is noiseless. Here’s a 50 watt with the switches. It’s split/shared cathode, v2 switchable cap and bass/lead tonestack.

[hebaton]

I believe you are correct and that the placement of the parts I added and the way and length of wires are responsible for the added noise I get. I cannot fix it post pics immediately because the amp is at our practice space, I'll get it back later this week. However, there is still something I wonder about. Besides the noise, I noticed the change in tone that seems associated with the added cap on V2A cathode, make a huge difference in that so much bass is filtered out and, some low mids ( to mu ears ) that it is not so pleasing. The result was not anywhere like what I expected, witch was to get closer to the 1984 50 watt plexi response, it sounded very nasal, even after adding in a lot of bass ( turning up the bass pot over half )
Is this also related to the physical placement of the actual part ? Pics will be posted later but In short, I hade the cap right over the two first triodes, outside the board... IU was expecting a much hotter low mid response for one thing....