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I just upgraded the tubes on my Fosi T20 amp to a matched pair of Voskhod 6J1P-EV and I love the improvement in sound. There's just one problem: when I switch the amp to 'off' the tubes remain illuminated.
I don't want to have to unplug the power cord every time I want to turn off the amp, but having the tubes remain lit concerns me safety-wise.
Has anyone else had this issue and if so, how did you resolve it? Suggestions?
So the tubes you put in looking at pictures look like they are more open to see the heater vs what the Fosi comes with. Some amps leave the heaters on which is okay if the high voltage is off. If you are talking about the dull orange glow of heaters maybe it did it before, but you didn't notice? Easy enough to swap the original tubes in and look at it in the dark to see if maybe it did this previously, but you didn't notice it.
Depending on the capacity of the filter caps and the lack of a way to drain them when you switch the power off, those little valves could stay lit for quite a while
If they are staying on, in a warm up mode when you flip the switch back on, it might be a safety feature for switch-flip off/on.
A tube killer is an inrush of current. It's one of the reasons for a soft start circuit in the first place. Your unit probably has both.
Those things are running at a VERY low voltage, to begin with. The valve inners will outlast the pins on the outside probably.
The second idea is, that is their way (in leu of a dumping/shorting resistor) that is activated by moving the ON/OFF switch to the off position as a way
to drain the filter caps. Pretty simple, OFF completes a circuit to a resistor hooked the filter caps drain resistor. 10 to 1 they go dim within a few minutes.
You could just check the voltage with a DMM which will cause a quicker drain but it will show if there is a steady voltage drop at the heater or the HV side.
They are probably just using the valves to drain the filter caps.
Just put one of the old valves back in and do a side-by-side. Won't hurt a thing. You can also check if the new valve really dose change the
sound as much as you think.
Thanks for your responses, all. Based on those I have taken comparison on/off photos. The amp was relocated to a different cabinet and it was there that I upgraded to the Voskhod bulbs. Previously it sat on top of a Kallax shelf next to my turntable and not once did I notice any lit bulbs when I turned the amp off. Now, I don't know what to think.
It's been a couple of decades since I've seen a tube but I never saw one light-up blue. The heater usually glows orange like a dim incandescent light bulb. The heater also doesn't snap-on. It slowly gets brighter. If the blue light snaps-on when you plug-in power, it's an LED.
Or can you tell if they are hot without actually touching them? (If the heater is on, they will burn your fingers.)
According to Fosi website this amp uses the TPA3116 class D amp. Tubes in the preamp ? There's a video review on the site, it looks like there are blue led lights under the tubes in the socket. At the end of the review one of the things the guy says he didn't like was the power switch didn't turn the lights and I think he said Bluetooth off.
6A2 is a heptode, and 6J1 is a pentode. The latter has a couple of pins which are wired together, and if the blue LEDs didn't constantly glow when the original tubes were installed, perhaps the difference in the way 6J1 is wired is completing a circuit, causing those rascals to light up. I doubt that this represents a safety concern so much as an annoyance.
It's been a couple of decades since I've seen a tube but I never saw one light-up blue. The heater usually glows orange like a dim incandescent light bulb. The heater also doesn't snap-on. It slowly gets brighter. If the blue light snaps-on when you plug-in power, it's an LED.
Or can you tell if they are hot without actually touching them? (If the heater is on, they will burn your fingers.)
There used to be tube TVs with quick power on as a convenience feature that did this in the 1950s/60s, but it's obviously neither good for the tubes nor for your power bill in the long run. It's also decidedly in disagreement with the EU's prescribed standby power consumption limits (0.5 W max), since I can't imagine that heating two tubes with 6.3 V @ 0.6 A combined (plus lighting some blue LEDs) uses a whole lot less than 4.5 W on the mains side.
Also, running the heaters without any HT will result in cathode poisoning and the consequent early failure of the tube. What was done with high power transmitters on standby that would otherwise take a long time to heat up was to keep the heaters warm, but well below the temperature that would result in cathode poisoning.
In this case, if the heaters are visibly on, and it's not just an LED, then it'll be both the risk of cathode poisoning and violating rules about standby power consumption.
Overall, a very daft decision by somebody that didn't think it through, or was sufficiently cynical to think marketing outweighed engineering.
With a recent purchase of a Wiim Ultra and a subwoofer to complete a modestly priced home system update, I'm not financially able to look for a different amp right now.
I really like how everything sounds to this point. I've made due with sub-optimal for so long that I am pleased more than you can imagine with the upgrades in sound quality. I guess I will likely just add an external on/off switch to the Fosi until such time as I sell my vintage turntable to fund a quality replacement amp.
