JCA100/50 mod - heater hum fix?

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simon_d

simon_d

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Hi, very late to the Jet City modding thing, but I hope someone might still remember these!

I've just done a JCA50H mod on my JCA100H following Jason Tong's scheme he published, video here. All is working well except for V2 which is picking up a lot of hum, presumably from the AC heaters, thanks to the increased gain.

I'm surprised that V1 is not affected as pulling V1 doesn't remove any hum, but pulling V2 does. Disabling the cathode bypass mod on V2B reduces the gain and so the hum, so I suspect its mostly being picked up in V2A (and B). Tapping the board also indicates V2 is much more sensitive.

What can I try to reduce the hum?

I want to check there's a centre tap on the heater windings and that its earthed. The 50W schematic shows a centre tap, and on my board there are 4 wires coming in labeled 6.3v, PL1, PL2, 6.3v. (The 6.3 wires are both purple). PL1+2 go to the bulb (Pilot Light I presume!)

There is a thinner black wire from the secondary side of the OT going to the mains safety earth on the chassis. Presumably this is the tap. Just want to make sure it has one!

Not sure what other options there are apart from elevated heaters, or DC supply. Both may be a little beyond me at this stage though. Any suggestions?
 
Power transformers typically have a center tap on the primary side and the secondary side. One is for the heaters and the other is for the HT secondary. So the black wire you're talking about might not be for the heaters. Some transformers do not have a heater center tap. And an artificial tap is constructed. Will need an actual schematic of your exact amp to offer any more detailed advice.

That said... if V2 is a cathode follower, your heater-to-cathode voltage differential might be too high. That can cause some hum, but not always. Measure the differential and reference that to the 12AX7 tube data sheet to see if it's within spec. If it's too high, you can elevate the heaters by referencing the center tap (if it has one) to a positive DC source, or you can find the artificial center tap (if it has one) and make sure the CT resistors are good and 100% balanced with each other. The Soldano SLO uses an artificial CT, for example.

If your amp has a CT, and the noise began after you modded the amp, I don't think it would be the heaters to be honest. Check all your wiring around V2 (including solder connections) and make sure your lead dress is good. And check your grounding scheme. I built an amp a while back that had a similar hum problem on V2 and it turned out to be the physical location of where I grounded something. Moved the ground wire to another point and it killed 100% of the hum.
 
I’d double check values, IE make sure that 470k isn’t a 470ohm and other little stuff like that that bites you. Also try another tube in v2.
If that dosent work I’d disconnect the grid of v2 on both sides (one at a time) working towards the PI, Tap around to narrow the noise down to one half of the tube.

To me this doesn’t sound like like a heater issue. Every time I’ve had a similar issue it was due to me attempting to drive to much signal through an amp and or bad grounds/ cable routing.

And like 46&2 said, check your joints. I just had the same issue with a jca20 due to bad joints.
 
Can't find a schematic online for the 100w, only the 50w. I don't want to just presume everything will be identical in preamp. The 50w does show a centre tap on the heaters but who knows...

If I disconnect that black wire I can just check for continuity with it and the 6.3v wires, right?

Yeah, it might not be heater noise, I can't recall if there was that hum before, if there was it was a lot louder after modding. Definitely not a super quiet amp though. I did the usual, swapping tubes etc, no change.

Actually I didn't really like the mod, on my particular amp anyway, which was very compressed. I've reverted some things now to try to reduce compression and make the generic sounding distortion a bit spicier, by dumping more of interstage gain and trying to get more headroom... 100k plate instead of 220k on V2A, changed the 10k cathode back to 2k7 and deleting the bypass cap on V2B (it's 100k/10k, 100k/2k7 plate/cathode on V2 now). The noise is lower, still there but not as loud. Could well be worth reflowing everything BC the soldering isn't great on these. It's PCB so can't do much about wiring.

A lot happier with the sound though. I changed the PI tail resistor from 10k to 47k, and that seemed to make a difference in the right direction.

If I can confirm the heater CT I'd like to try elevated heaters just to see if I can do it. This amp is basically a learning test bed.
 
I disconnected the black ground wire and tested it and there was continuity between it and the 6.3v taps. But there was also continuity from the 6.3v to any ground point. So they are grounded elsewhere via the board I think. That was probably the PT CT (skinnier than expected). Added the schematic for posterity.

Good strategy for troubleshooting noise @Chubbs that will come in useful I'm sure.
 

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