Need help on my Rat-Rod Express Please!

[johnny fontana]

Hello all,

Thanks for all the great information on this forum! While cleaning out the house a month or two ago, I doscovered I had most of what I needed to build yet another amp of some sort, I decided on a Rat-Rod Express. This was built with just stuff out of the parts bin so it isn't for the purists at all. The circuit is as Expressish as I could get while still trying to "work with I got!"

Anyway, I got it together, but it has a buzz/hum. Normally I can find em and fix em but not this one. Here's the sitch:

Not affected by the tone controls
Not affected by any chopsticking or wire moving, etc.
Is affected by the volume control
Is affected by the presence control
Still there if you remove the wire going to the right lug of the volume pot
Stops if you remove the wire on V1 pin1
When V1 pin 2 is removed, it goes crazy

I've tried a new volume pot, ruled out filter caps and tubes, with V1 pulled, shes quiet as can be, Ive tried grounding the filter caps differently (Most everything that I could is done per the Ceriatone layout at the moment) I moved everything on V1 1,2,3 to the unused 12ax7 half, and a few more things Im sure.

I cant find it, any help will be greatly appreciated!

Also, the seems to play fine and does what an Express is supposed to do, just WAY too much buzz

Johnny

[dartanion]

Try moving the NFB wire around.

Also, do you have a know good set of tubes in it? Mine are very picky about tubes.

It can also be that you are getting significant EMI/RFI since you have no cover over the open side of the chassis. Try setting a sheet of aluminum over the chassis when testing. Also, turn off any fluorescent lighting in that room.

[johnny fontana]

WOW....

preliminary wiggles look like thats it!! I tried to get too cute with my lead dress!!!

Iron is warming now, Ill report back soon!

THANK YOU

Johnny

[johnny fontana]

False alarm.

wiggling the NFB actually exposed a cold solder joint on pin 6 of one of the EL34's so it was loosing power,thus the buzz would stop. Fixed that, buzz still going strong.

When the volume is on 10, it actually gets a littel quieter, but it is constant.

I tried covering the open side, no help.

Johnny

[ampdoc1]

I think i see you have a ground buss wire from the jack across to the pots. Do you have the preamp board ground connected to this buss also? That's pretty much a must have on these amps.
Also, since the input jack is already grounded to the chassis, you might try removing the buss wire from it.

ampdoc

[johnny fontana]

ampdoc,

Thanks for your help.

Yes, I do have all the preamp grounds on the main bus, and I have tried disconnecting the input jack.... nothing.

Still buzzing.

thanks

Johnny

[Richie]

I don't see a CT for the heaters on your power transformer[may be there, i just can't see it].. try using the 2 100 ohm resistors,one off each leg to ground off the heater wires..

[johnny fontana]

Thanks,

I do have a center tap coming off the PT and it is bolted down at one of the Pt bolts. There are actualy 2 wires off the PT that are grounded. the center tap and the heater tap i think.

should I still give toh resistors a shot?

Johnny

[Richie]

what color are the CT wires you have bolted down? the CT for the heaters should be green with a yellow stripe.. but i have no idea the PT your using,so it could be a different color stripe... But the heaters are the green wires..and i didn't see the CT for it.. I do see the red yellow stripe for the PT.

[johnny fontana]

Then the orange one that is wrapped around the red wire with the white stripe is also grounded. I/m pretty sure that is for the heaters. I dont know what kind of PT it is either, I pulled it out of a Vibrolux a few years back. It's a little hot for an Express (about 450 on the plates) but it'll do.

I do know that both wires were grounded in the previous amp it was in.

Thanks

[Richie]

It won't hurt to try the 100 ohm resistors.. i don't think the orange wire is the CT for the 6.3v heaters..[just guessing here] Maybe someone else may know for sure.. but the 100 ohm resistors may help cure some of the buzz.. Alot of those fender PTs were weird.. overseas versions etc..

[doctord02]

I dont think that orange wire is related to the heaters either, its not exiting the bell in the same place the heaters are. I'm suspicious that it's a bias tap. At any rate, I'd lift it and tape it off and see what happens to your noise. Maybe see if it has any voltage on it.

[johnny fontana]

the orange is definitely not the bias tap..that one is red/blue stripe and I have it taped off..

Ill lift the ornge wire and see what happens, ill also check it for voltage an report back.

thanks

[mdroberts1243]

the orange is definitely not the bias tap..that one is red/blue stripe and I have it taped off..

Ill lift the ornge wire and see what happens, ill also check it for voltage an report back.

thanks

On Hammond transformers the orange is the shield and should be grounded... does your transformer have a copper shield?

[RB]

I had some PTs some time ago that had an orange wire that was connected to an internal/elctrostatic sheild and that wire was connected to ground. It belive they had yellow/green heater CTs though. I just have a hard time accepting the orange wire as a heater CT unless they ran out of yellow/green that day.