Well don’t quote me on that 
Hi Dan,
Thanks. I finally got round today to try and replicate with a 1KHz tone but no real new insights, i.e.: the tone generation doesn’t seem to affect the 50Hz and third harmonic.
The stickers says “Ip 38” (mA), so that would be 63 1/3% which seems reasonable.
I would need some equipment to check this. And I fail to understand why with a valve issue, the hum would disappear when removing the top lid.
This was and is the case in my setup.
I still think the hum’s origin is mechanical, rather than electrical.
-Magchiel
I found the sticker, Ip38. The output tubes also had stickers with the same Ip value. But isn’t this just the plate current matching value for the TUBES? Surely this can’t refer to a target value for the Power Station’s actual plate current? I mean, your sticker says 46 and magchiel’s said 38 (which is actually the same as mine). That’s a HUGE difference that surely can’t refer to the actual bias of these units? Tubes with such different Ip values will need to be biased very differently to reach whatever % of maximum dissipation you’re aiming for. I’m pretty sure these Ip values can’t be referring to the actual plate current when the unit is properly biased???
Sorry, but I don’t understand what you mean here? Did you measure the plate voltage? Because without knowing the plate voltage I don’t see how you could possibly calculate the % of max dissipation. Also, the number you’re quoting, Ip 38, seems to be the matching value for the tubes, rather than the actual plate current inside your unit, right? In which case it’s worthless for calculating %max dissipation. My apologies if I’m missing something here???
It’s definitely not a ground loop. I’ve mentioned several times now that I get the hum with nothing except a speaker element connected to the Power Station. That alone 100% rules out a ground loop (unless there’s a ground loop internally in the Power Station circuitry). The fact that the ground lift switch doesn’t reduce the hum significantly should also rule out a ground loop.
I still don’t think it’s mechanical though. I suspect some kind of electro-magnetic interference from the power transformer. If this was mechanical, how can it make sense that the hum gets significantly lower when removing the top lid, and that the hum increases markedly by simply moving the top lid closer to the unit, without actually touching it? That’s not mechanical, it can’t be.
So I did some tests to make sure:
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I put 2x silicone ring dampers on every tube. This had no effect on the hum whatsoever, which to me strongly suggests that the cause of the hum isn’t mechanical.
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I held the lid upside-down above different areas of the unit without actually touching it while monitoring the hum. The hum got significantly louder when I moved the lid closer to the power transformer. The hum didn’t increase nearly as much when I moved the lid closer to other parts of the unit. This strongly suggests what I’ve suspected for some time now: That the power transformer is somehow introducing hum into the audio signal path. The fact that other users are reporting the exact same issue with brand new units makes me suspect that we might be looking at a bad batch (possibly of just ONE component) rather than random faults.
Quick question to Dan:
In the attached pic, can you please confirm that the yellow wire with the spade connector on the left side of the power transformer is supposed to be disconnected?
I also checked the actual bias:
(For reference: The stock tubes as well as the chassis on my unit were labelled Ip 38).
I actually got slightly different readings when using my TAD bias-probes and my Eurotubes probe. The TAD probes are 20+ years old so I guess they may not be properly calibrated anymore. Also, the TAD can only measure the plate current, whereas the Eurotubes probe can measure both the plate current and the plate voltage.
With the Eurotube probe I got:
Left tube: Plate voltage 504 V - Plate current 32 mA
Right tube: Plate voltage 504 V - Plate current 32 mA
This means we can rule out unbalanced tubes/bias as the cause of this hum.
Dan, can you confirm that these values seem reasonable?
Like I mentioned, the TAD probes gave slightly different readings:
Left tube: 36.6 mA
Right tube: 35,7 mA
This is still within what I would consider good matching/balance. These slightly higher values are most likely not accurate, because these TAD probes are 20+ years old and probably need calibrating (or maybe replacing…).
Anyway, after doing the calculations I end up with 54% of max dissipation with the readings I got from the Eurotubes probe. This is extremely cold bias, FAR colder than I would ever bias a guitar amp. We can certainly rule out a too HOT bias, which was suggested as a possible source of hum.
Dan, can you confirm that this is actually how these Power Stations are supposed to be biased?
When using the plate current readings I got with the TAD probes for the calculations I get 60% of max dissipation, which is still rather “cold”, at least for a guitar amp. But I trust the readings I got with the Eurotubes probe more. That probe is only 2-3 years old.
But I suspect that the Power Station is indeed supposed to be biased significantly “colder” than a typical guitar amp, so I’m guessing these values are actually normal?
Still, isn’t a 504 V plate voltage a bit on the high side for 6L6GCs? I checked the specs for the JJ 6L6GC that came installed in the Power Station and it seems to me that a 504 V plate voltage is exceeding the specs for these tubes? This probably wouldn’t be a problem with certain types of more robust vintage/“NOS” tubes, but with current production JJs? Can this be right??? Or am I reading the JJ 6L6GC specs wrong? I’m definitely no electronics engineer…
I guess the plate voltage should drop a little bit if the bias had been adjusted “hotter”, with a higher plate current, but as far as I can tell there is no bias pot inside. The bias would of course also become “hotter” (and the plate voltage lower) if I had installed a pair of matched output tubes with a higher Ip-rating and NOT actually adjusted the bias in any way.
Maybe the factory bias is set wrong, and possibly this can be part of the picture here? 54% of max dissipation and 504 V plate voltage does seem a bit extreme…?
Dan, can you please confirm if the plate voltage and plate current readings I’m getting are within normal operating specs for the Power Station?
@Trerik and @magchiel I think the best solution is to send them to service workshop in Germany, they can take a look at the noise issue. Do you agree this is a better option than sending it to USA?
Well, that depends on whether the warranty will still apply if I send it to USA…
The service workshop in my case is a relatively small luthier/guitar store in The Netherlands. They sell their own brand of guitars plus a nice selection pedals, amps and pickups, typically “high-end” brands that the big stores don’t focus much on. They’re not a huge store like Thomann or Sweetwater. They just happened to be the only place where I could find a Power Station in stock anywhere in Europe at the time.
For all I know their amp tech guy might be highly competent, but based on the communication I’ve had with them so far I have a feeling that the chances of a successful repair, without any further delays, will be better if the Power Station is returned to the USA, to the people who actually designed it, and where original spare parts are (presumably) available. My past experience with amp techs at small guitar stores (not to mention the waiting list…) has been a bit hit-or-miss… to put it kindly.
The shipping costs would be higher to the USA, but I would be willing to pay for this if the warranty still applies. I just need this to be fixed ASAP. The problem with the shipping costs is that I will need the shipment to be fully insured, and the utterly useless Norway Post don’t offer insurance for ANY international shipments. Which means I will have to use one of the FAR more expensive express couriers. Probably DHL Express as they’re the only courier I really trust, (UPS and FedEx are shockingly poor in my area). This means the shipping costs would be insane even if I just shipped it to my neighbor. The difference between The Netherlands and USA wouldn’t actually be that bad, it would be really expensive either way, and both would require the same amount of customs paperwork. But unlike other couriers, DHL Express actually deliver what you pay them to do. The shipment should be at your door within 1-2 work days.
So I guess it all boils down to one question:
Is the warranty still valid if I ship it to Fryette in the USA, rather than dealer in The Netherlands? Because I’m definitely not paying for the repairs here.
Hi Trerik,
Fryette has a repair tech they work with in Germany, I was suggesting this as an option. You are of course welcome to seek out your own repair locally or even ship to the USA for a repair if you want. Please let me know what you want to do!
As for warranty – talk with the dealer about that because it is not a direct sale. The warranty exist between you and the dealer.
Regards,
Dan
Ok, I understand. I will return it to the dealer then. Paying for a repair for a product that’s still under warranty is obviously not an option, whether the repair shop is in Germany or USA.
I doesn’t look like we’re gonna get any further with this here, so I just want to say thanks for your time, it’s very much appreciated! Hopefully the dealer will sort this out.
Hey,
OK, let us know how it goes, we are here if you need us. Yes, there is only so much debug that can be done without the actual device. But I did have fun with this post! Keep in touch.
Regards
Dan