Could be the device but that is a recording studio preamp (with tubes! 😀) directly into a motu 16a audio interface. As stated the mic was an akg c414 condenser mic.
Maybe the led-magnifying lamp, which was still on, was the cause but I needed some light 🙂
Maybe the led-magnifying lamp, which was still on, was the cause but I needed some light 🙂
Here's the somewhat puzzling reply from Ming-da:
"And regarding the power/mains transformer, if usually the amp can be turned on, and the tubes can work, then the transformer won't be with problem.
But you can also check if the AC (Alternating current)windings touching each other, for example, AC 5V and 10V, if the 5V and 5V , 10V and 10V if their filament touches each other or have shortcut, then the Power/mains transformer is WITH problem.
Please check if the filament windings of transformers contact/touch each other, if they do, then it will be with the hum and the rattling noise like the one you sent to us. Then you can change a new mains transformer. If it is not under conditional, it is not the problem of mains transformer and it won't solve the problem if you change a new transformer."
"And regarding the power/mains transformer, if usually the amp can be turned on, and the tubes can work, then the transformer won't be with problem.
But you can also check if the AC (Alternating current)windings touching each other, for example, AC 5V and 10V, if the 5V and 5V , 10V and 10V if their filament touches each other or have shortcut, then the Power/mains transformer is WITH problem.
Please check if the filament windings of transformers contact/touch each other, if they do, then it will be with the hum and the rattling noise like the one you sent to us. Then you can change a new mains transformer. If it is not under conditional, it is not the problem of mains transformer and it won't solve the problem if you change a new transformer."
I've been in further contact with Ming-da and they suggested I replace the bridge rectifiers for the heaters.
Can anyone explain to me why they are using twin 50A bridges per 805 tube? That's 100A rectification while the tube only draws ~3A heater voltage? Could I use 4 SB1020 diodes instead, just for testing? Or 4 MB1045?
Can anyone explain to me why they are using twin 50A bridges per 805 tube? That's 100A rectification while the tube only draws ~3A heater voltage? Could I use 4 SB1020 diodes instead, just for testing? Or 4 MB1045?
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50A bridges are rediculous. Something more than 3 Amp. can be expected though, since cold cathode filaments draw a lot exta current.
Regards, Gerrit
Regards, Gerrit
even at only 3A, each diode has to deal with some heat dissipation. Count on derating if it's in the chassis of a hot tube amp chassis. When they get hot, the reverse current goes up.. This will show up as noise.. The cost of a 50A bridge isn't worth not using 50A devices.
A small post-christmas update: I replaced the diode bridges for the heater supply. Unfortunately now the buzzing (not hum) coming from the transformer is much louder and constant and the power light flash on/off. Wiring is ok and this happens only after I insert the 805 tubes.
I think this narrows it down to a broken/faulty power transformer.
I think this narrows it down to a broken/faulty power transformer.
Tiny update: the problem only occurs with both 805 tubes installed. With one left out there is still hum but no cracking and the amp is stable (also checked both channels, no difference).
It's been a while. I came to the conclusion that the only thing left to do was replace the power transformer. I contacted Ming-da for a new one and the first gave me a price of $ 130. But when I wanted to order the price suddenly increased to $ 330 excluding shipping (a whopping $ 190!) and import fees... So that would be about $ 600 + ion the end whichj was waaay too much.
So I contacted a local excellent transformer builder, AE-Europe, and they could build one for € 140 euro's with shipping being € 10... That's more like it!
It took a while to arrive but I can happily say that after replacing it the hum is now completely gone! I need to do some more tweaks and fine-tuning but the main problem is solved.
So I contacted a local excellent transformer builder, AE-Europe, and they could build one for € 140 euro's with shipping being € 10... That's more like it!
It took a while to arrive but I can happily say that after replacing it the hum is now completely gone! I need to do some more tweaks and fine-tuning but the main problem is solved.
Unfortunately I was celebrating too early... The amp worked a while but then developed some new problems. Firstly after a while I noticed the new power transformer was getting very hot. Checking everything it turned out one of the zener diodes was broken. And there was 0V on the grid of the 300B.
The I had an issue with a broken preamp tube and lastly a leaking coupling cap which caused +158V at one of the 300B grids.
Now I still have one weird problem: if I turn the amp on everything is fine but after a minute or so I again get +157V on the grid of a 300B tube. I already replaced all the coupling caps.
The tube is a new EH 300B gold. If I swap this for the old Chinese 300B everything is fine. Could this be a bad tube or is the EH tube more sensitive to biasing? The grid voltage is around -32V...
The I had an issue with a broken preamp tube and lastly a leaking coupling cap which caused +158V at one of the 300B grids.
Now I still have one weird problem: if I turn the amp on everything is fine but after a minute or so I again get +157V on the grid of a 300B tube. I already replaced all the coupling caps.
The tube is a new EH 300B gold. If I swap this for the old Chinese 300B everything is fine. Could this be a bad tube or is the EH tube more sensitive to biasing? The grid voltage is around -32V...
I have no experience with these tubes, but I see you’re really having a hard time getting this amplifier to do what it’s supposed to do.
If you detach the grid coupling capacitor, what is the grid voltage? Is it becoming +157 V again or...? If it’s -32V and stays that way I still fear the capacitor is bad and leaky.
Regards, Gerrit
If you detach the grid coupling capacitor, what is the grid voltage? Is it becoming +157 V again or...? If it’s -32V and stays that way I still fear the capacitor is bad and leaky.
Regards, Gerrit