Latest home build :(

[496dart]

Im finishing up my latest amp. Its pretty similar to a 2204 marshall. It does have an extra gain stage in front of it. The problem im having is the VERY, VERY loud BUZZZZZ thats going on. The amp does sound ok--i can play through it.

The amp is Star grounded. EVERY ground point has its own wire to the ONE ground point . Ive checked all the grounds. I used shielded cable between stages where i could. The shields even go to the start ground.

When i pull the PI out - the amp is silent. Ive tried tracing it back by grounding the grids of each pretube. The buzz goes away when i do this at every point--even right at the Input jack .

Im stumped for now...

[dai h.]

the star grounding is probably the problem. Possibly other things as well.

[496dart]

The star grounding fixed all the noise on my other home builds--thats why i did this one with it. But this one is buzzing SUPER loud.

[Tom Phillips]

I'd look for a prblem near the input

I suggest looking near the input because you said "I've tried tracing it back by grounding the grids of each pretube. The buzz goes away when i do this at every point--even right at the Input jack". This indicates that the buzz is probably originating in the first stage. If it's a buzz rather than hum it sounds like something is not properly grounded. Does it sound like when you grab the tip of the guitar cord that's plugged into the input?

Another thing to check is if your heater circuit is properly referenced to ground. If you have a meter, check to see if each side of the heater circuit measures about 3.15 Vac with respect to chassis ground.

Good luck,
Tom

[496dart]

it is similar to the sound of touching a tip of a cable.

i will check the 3.15 v tomorrow

[Enzo]

The problem has to be at the first stage. If grounding the input jack stops the hum, then the shunt contact is obviously not grounded. You do have a grounding jack on the input, not just a plain old vanilla jack?

You apparently have a star wire from the ground of the input jack back to the star point. Is the input jack insulated from the chassis? For true star ground it should be. On the other hand, if it is insulated, try grounding it to chassis to see what happens. Who knows what ground currents are flowing through your chassis

You added a gain stage in addition to all the ones a 2204 has to begin with? There is a limit to how much gain an amp can have before it turns into a radio.

When troubleshooting, any point you can ground that kills an unwanted noise is AFTER the source of that noise. By grounding things back to the input, you have in the process narrowed the problem down to that area.

[496dart]

The problem has to be at the first stage. If grounding the input jack stops the hum, then the shunt contact is obviously not grounded. You do have a grounding jack on the input, not just a plain old vanilla jack?

You apparently have a star wire from the ground of the input jack back to the star point. Is the input jack insulated from the chassis? For true star ground it should be. On the other hand, if it is insulated, try grounding it to chassis to see what happens. Who knows what ground currents are flowing through your chassis

You added a gain stage in addition to all the ones a 2204 has to begin with? There is a limit to how much gain an amp can have before it turns into a radio.

When troubleshooting, any point you can ground that kills an unwanted noise is AFTER the source of that noise. By grounding things back to the input, you have in the process narrowed the problem down to that area.

i will check the jack tomorrow--its a marshall style plastic jack.


ive added the gain stage to a lot of these--and i havent had this noise problem.(very similar to a soldano preamp..)


what the hell is a vanilla jack ??? never heard that one enzo.

[dai h.]

he means non-switching (as in a jack with only hot and ground = vanilla). Normally amps have (mostly I think) switching (hot is grounded out when no plug is connected--floating hot can pick up hum so the idea is to ground an unused input).

[TD_Madden]

even when wired correctly, a switching (to ground) jack can be faulty, even when new (I had a couple)...check that the jack indeed grounds itself when not in use....if more than one jack, check all of 'em..

[496dart]

i played with the jack and shield . grounding the unused hot did help on the buzz, but its still there fairly loud. i tried moving the 1st wire going to the first gain stage- over to the 2nd stage--that helped A LOT with the noise. attaching the first wires shield to ground doent make an audible difference.

Somethings out of wack here: when testing my filiments to ground , i get 145 VAC (on all the tubes). across them i get 6.5 vac . looks like i have the filiments wired wrong ?

[Enzo]

Aha. If there is a center tap for the 6v heater winding, ground it. If not, then add a pair of 100 ohm resistors to ground. One from each side of the 6v winding. Just like all the Fenders in the world. Before doing that, with power removed, check resistance between the 6v wires and all the other transformer wires. Make sure the 6v is not shorting to another winding.

[496dart]

you are the man Enzo !!

That was it--i grounded the CT. the amp is almost dead silent now
3.2vac from fil. to gnd. Much better than 145v

when i was early in the build i tried grounding that ct but it would pop the 1/2 amp fuse . thats why i left it un-grounded .