Recent JMI / VOX AC30 6 TB with idle noise

[pdf64]

...JMI is not responding to the mails I sent them. It's a shame for such an expensive amp / brand...

I'm under the impression that JMI and Hiwatt are ventures of the dreaded father and son criminal duo Richard and Justin Harrison, of Music Ground infamy Page 459

[John G]

Hi,
Yes !! I applied additional good quality solder to every joint, heating tag and applying solder to both component as well as the tag. A bit of a tricky job in some areas as the components where tightly packed.
Have you considered that one or more of the carbon anode resistors could be noisy as well ??
John

[GibbyLover]

I tried to resolder the points closest to the PI, but no luck so far. While I was there already did all the ground points around the pots. It will a job to do it all, it's packed.
What carbon resistors do you mean?

[Mick Bailey]

Referring back to post #10 - did you check how the heaters are wired? Very early AC30s had 'floating' heater wiring and reissues are usually either ground-referenced through a centre-tap, hum-cancel preset, or 2x100R resistors, or power the output tubes off AC and have a bridge rectifier/smoothing capacitor to provide DC tho the rest of the circuit. I don't know what the JMI uses. A fault here could cause the amp to hum badly with just the PI, rectifier and output tubes installed.

[GibbyLover]

Mick, I really don't know how it's wired. I don't have the schematic and I'm no tech

John, If I posted some pictures, would you be able to tell where those carbon anode resistors are? Can I measure them out?

Thanks guys, I'm trying to troubleshoot this one myself, but maybe I'll have to pay a tech to get it right.

Picture from JMI:

[pdf64]

I think that the carbon film plate resistors used don't tend to become noisy, a different kettle of fish to the carbon comp type.
The simplest 'for sure' test is substitution.
If you have a test meter, try to measure the resistance between each side of the heaters and chassis 0V; the readings may be very low <1ohm, so a good contact is needed to discern if they're truly different. But even if you can't resolve any difference, the worst case (no 0V reference) would be eliminated.

[GibbyLover]

I measured them and you said a good contact was needed. They were all the same (0,1ohm on one side and 0,2ohm on the other).
When I find some more time I will try to resolder more points, or I'll try an attenuator, that might work as well.

[edit] I had some sparetime, so I removed the wood panel and "L profile" from the TB on the back.
I already resoldered everything on the bottom and the top boost (where it's so crowded I can't reach it all).
When I test again the ticking is gone, but first I had a loud 60Hz noice. I figured I had the "L" from the TB isolated from the frame. When I made connection again it was a lot better, but often there is a softer woosh sound (+-60Hz). I guess it is also due to the not optimal grounding of the TB.

[Mick Bailey]

On original TB amps with the piggyback circuit I've sometimes had to experiment to get the best grounding arrangement, as sometimes they can be a little more noisy than later amps with the integrated TB. Nothing you would perhaps notice in a band situation, but more annoying for a home player.

To get any further you would need to eliminate the heaters as a noise source. From your measurements you have the heaters referenced to ground and it looks like a centre-tapped arrangement. It pays to make sure the centre-tap grounds to the chassis are secure. Take a look at the power transformer and you'll see where the leads come off that (possibly) two are bolted to the chassis (could be ring terminals on your amp).

[Steve A.]

I don't know if it is applicable or relevant here but in my Mission Amps 5E3 kit Bruce Collins had us connect the 100R filament supply resistors to the cathodes of the power tubes rather than chassis ground. Doing that reduces one source of noise in preamp tubes.

Steve A.

P.S. Although I never tried this myself the preamp tube filaments could be connected to a battery of appropriate voltage to see if a DC filament supply would eliminate or reduce hum. A good idea to try that before actually wiring up a DC filament supply...

[Mick Bailey]

The 5E3 is usually grounded of one side of the heater winding, which means that it's imbalanced and the dual resistors improve hum. It's applicable in that sometimes a centre-tapped supply doesn't always give the best results. Sometimes I lift that connection and install a 'humdinger' pot and get the best balance and it can be surprising how far off centre the pot ends up for minimum noise.

There's another source of noise I've encountered with heaters where the heater string is not wired same-pin to same-pin and gets crossed over. This doesn't always result in noise, but when it does can be an elusive source. Those amps have often been noisy from new.

[GibbyLover]

It took some time, but here an update.

I can't follow all instructions, because I'm not that used to working on amps.
I checked the connection of the heater cables to the transfo and they were tight.

Then I found the courage to resolder all joints I can reach. I fired her up and I have the impression that it got better. The noise is still there, but for now it doesn't bother me. I wonder it is the amp, but maybe it's me, after all that time not playing it, got used to noise, bon't bother anymore?

[Mick Bailey]

Maybe you have got used to it. Sometimes I get amps in for repair where I consider them to be noisy but the owner never even noticed. Some amps are just noisy as a characteristic and do not have an outright fault. The wording on one Fender service bulletin to fix a noise issue reads "This will reduce but not eliminate the noise".

[TimmyP1955]

Don't use a felt tip pen as a tapper - at least don't use a Sharpie. Sharpies are apparently a bit conductive, as in some amps I can induce a hum/buzz without the sharpie touching anything - change to a dowel rod and no hum/buzz.

[GibbyLover]

Well, excuse me for the late reply.
It seems that I got used for it, at least some days, maybe I was tired or something.
Now the noiselevel bothers me again. I'll try to find onother AC30ri with the same circuit and try if the noise is the same. They are loud animals

[GibbyLover]

The weather is bad and I'm thinking about this one again.
Maybe I should start searching for the cause of the 60Hz popping. The noise is around the PI. Any idea where to look?

EDIT:
When I tap the cable between the 2 jacks of the top boost channel I hear quite a noise. I resoldered everything round the jacks and the idle hum changed, but it's not gone. Is it normal that you hear the tapping on that cable in the amp? The same happens when I wiggle the input cable of the input most to tha back(TB).

When I tap on the cap closest to the power switch I hear that one too inthe amp. It's 0.047mF +-10% 400V. Should that be normal?

At the back of the top boost there is also 1 side of a component that makes noise when I tap it. Resolder doesn't help.

Observation: I can attenuate the hum with an attenuator between the amp and the speakers, but the 60Hz ticking remains the same volume?