Rocket noise problem...help me!

[Luca]

Hi guys. My first message here.
I'd like to ask an help from you, because I have a "big" problem in my two Rockets type amp. I don't be able to quite them as much as I want.

I don't know if my problem is a ground problem or something more, but in two weeks I discovered and solved 50% of this hum but it still remains too high.
My interventions was:

- Twisted AC cords (input, mains and 6.3v)
- Twisted all heaters cable
- At first I rectified 6.3v for pre tubes, after I put 2 x 100Ohm resistor and result are the same, so I keep with resistors.
- I use shieled cable on many crucial points, like input, tonestack, ecc.
- I wired all ground to a common ground star point, including earth point.

As i said I built two IDENDICAL amp, the only difference are one is el84 based, the other 6V6. I have same problem (and same intensity) in both. This make me though that I miss something or for sure I'm doing something wrong.
Amps sound amazing, tone to me is unbelievable, but I'm very frustrated because of this hum. I followed Amp Garage schematic, the only thing I added are a non-fixed 10k for mids, I put a 25K pots. There is also a selector switch for select two different B+.

I used Classic Tone transformer for this one (express/vox type), Mallory M150, Sprouge Atom and metal film resistor.

I'd like to say that my idea was to not make a 1:1 clone, but to have a really good Vox based amp to improve at my likes. Last week I tried an original '63 JMI AC30 and they sound really similar but mine has this incredible noise.

Here some pictures and a small video, so you can hear the problem..the amp by itself sound amazing to me, there is no problem with the signal path, the only thing is this frustrating HUM.


https://www.youtube.com/watch?v=wZ1qeQf34vk
Thanks a lot.

[Mark]

What happens when you remove the P.I. tube or ground the A/C input to the P.I. is the hum affected?

I assume you've used a garolite board.

[rp]

You tried the basics first? Could just be a bad tube, it happens. Or a cold or missed solder joint.

[Luca]

You tried the basics first? Could just be a bad tube, it happens. Or a cold or missed solder joint.

Yep. I found really strange that same hum came from two different amps. This make me thought that I made some mistake in both heads.
I forgot to say heater and secondary are center tapped, and of course I solder them with earth point and audio ground in same star ground bus.

In five minutes I'll try Mark's suggestions. (@Mark, I use garolite board, supplied by TubeTown).

[Luca]

Mark, without PI but with V1 and V2 amp was silent, with PI and no V1 and V2 same hum. I think problem is in PI section..i tried to put another 12ax7 but it's not a bad tube since I have same result with new tube.

[martin manning]

Could it be environmental? Do you have fluorescent lights or dimmer switches nearby?

[Luca]

Could it be environmental? Do you have fluorescent lights or dimmer switches nearby?

No. Plus I have played dozens amps in my basement, always with no hum problem. I think it's my fault this time.

[M Fowler]

Isolated input jack?

Preamp grounding all goes to one spot near the input jack with an isolated input jack.

PI section grounds with filter caps B+1 through B+3.

[Luca]

Isolated input jack?

Preamp grounding all goes to one spot near the input jack with an isolated input jack.

PI section grounds with filter caps B+1 through B+3.

What I have to change?
Thanks

[rp]

Did you follow a standard layout? Search through rockets builds here, it'll take time as you'll get a million hits but everyone likes to show off their babies so there's lots of pictures of successful builds. Try and copy the KF layout as close as you can, or at least lay it out like an early JTM45 Marshall which should work fine. There's also layouts on the trainwreck files forum.

Your picts aren't opening for me, and the vid only shows you twiddling knobs, not the insides, but looks like you have too many too long wires. You shouldn't need any shield wire here, and it kills tone. What are those wires sticking up in the air? Are you testing with the PT outside the amp or are you using a heater trannie outside the amp? Just a few feet of wiring here will really drop your heater voltage, I discovered that after three days of going crazy.

Have you tried testing the amps with a shielding bottom plate attached?

You should ignore the 6V6 one for now and get the EL84 one working right first.

[Mark]

As the noise is focused around the P.I. Stage I would forget about the input stage for the time being and try this.

[img766]http://i149.photobucket.com/albums/s69/ ... 7ee78e.jpg[/img]

The three turrets attached to the board are sitting free and the connections are floating in the air and not attached to anything. The picture is a little deceptive.

[img766]http://i149.photobucket.com/albums/s69/ ... ab4332.jpg[/img]

You want to know if there is any DC present on the board that shouldn't be there. I had a very similar fault that was the cause.

[rp]

I had a very similar fault that was the cause.

WOW 7V! How does that happen? I knew old Fender board could do that, but Fender board looks like it could do that but not the phenolic turret board stuff. I've some old Keystone board I've had for 30 years I'm going to use soon, hope I don't get some nonsense like this. Good to know if I'm ever inside an old Marshall again.

[rooster]

"At first I rectified 6.3v for pre tubes, after I put 2 x 100 Ohm resistor and result are the same, so I keep with resistors."

"I forgot to say heater and secondary are center tapped, and of course I solder them with earth point and audio ground in same star ground bus."




....If you are using the filament center tap, you can't use the resistors. You can only use one filament grounding method, not both.

[rooster]

Mark, that is unreal.

[lumox0013]

Are you using the trainwreck transformer mounting and if so are you using the bottom plate? thats about what mine sounds like without the plate and with the plate its deadly silent. cheers