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An audio buddy bought these tube monos thru Amazon less than a year ago and now they are not working right. One amp has bad distortion and the other amp blows fuses. The amp that blows I got it up to 80vac and it plays fine and everything seems ok. I just wanted to check if the tubes were an issue and they seem to be working. The distort amp I have not done any testing yet other than to verify that the power supply is not shorted and there are no obvious faults. I did power it up to about 70vac and when music is played the distortion was easily heard.
I am sorry but I could not finde a schematic for this amp.
If you are not familiar with this amp it has balanced and se inputs. So the input goes to a transformer which goes to a 12at7 input tube. the 2 sections of the at7 are paralleled which is cap coupled to a 2a3. this is then transformer coupled to the 845. Filaments for the 845 is rectified. I measured at 80vac on the blows amp, 4.5 vac and 4.4vac on the ac input to the rectifier and 6.9vdc and -1.7vdc on the pos and neg terminals. On the bridge rect for the 2a3 I measured 2.2 and 2.1 vac on the input and 46 and 45 vdc on the output. In fact i measured dc on 3 terminals of 44 to 46 volts, except the plate which is 290 vdc.
For the 845 tube I measure at 80vac, 825vdc Av, -112vdc G1, and 6.0 and -1.6 on the heater. The 12at7 has 220v Av, 0 grid, and 2.7v Kv. Filament is AC.
there are 2 SS devices. 1 IRF 840 and a Lingar? LT1083CP. On this same circuit board are 2 bridger rect. brand "ch"? 2W10M or 2WIOM . I am guessing this circuit board supplies bias voltage for the 2a3 and 845 tubes.
Right now the only thing that sticks out is the 45vdc on 3 terminals of the 2a3. But, I am unfamiliar with this tube and 2a3 amps so I have no idea if this is normal or not.
Also the Power trans primary is wired for 110vac as there are taps for only 110 and 220.
Well, if anyone can point me in some areas to check and also comment on the voltages if they are in the "ballpark" or not please let me know. cheers, Dak
Edits: 04/12/16Follow Ups:
When you say "now they are not working right..." do you mean that they did until recently?
You said the fuse-blowing amp sort of worked at 80vAC - I assume this was the power input from a variac? And it was designed for 110 volts. Presumably it blows fuses at 120v or whatever you have. Does it blow fuses at 110v input? Are you using slow-blow fuses or fast-blow?
Without a schematic, I have no way to evaluate the voltages, except for the 2A3. It seems to be cathode biased, so the filament voltage will be that between pins 1 and 4 (the slightly larger ones). At 80vAC power line, the 2A3 voltages indicate the standard operating point from the RCA manual. It is disturbing to see such a close match with the specification, given that the input voltage was only 73% of the presumed design target.
I see no point in addressing the distorting amp until the fuse-blowing amp is functional. Only then can we compare voltages between the two amps, which might enable a diagnosis.
Hi Paul, on the 2a3 I measured 45vdc more or less, on 3 pins of the tube. Is this normal bias voltage, more or less. I have no experience with 2a3s so any advice is welcome. regards, Dak
The 2A3 bias voltage is 45 volts at the most standard operating point - see the link for a typical data sheet.
What is called the filament voltage is applied across the filament, measured with one meter probe on pin 1 and the other on pin 4. Really, there is no such thing as a voltage; there are only voltage differences. Usually but nowhere near always, one of the probes is on the ground - but even then, sometimes you also need to choose between signal ground, power supply ground, and safety ground.
OK, thank you for the info. The fact that I measured +45 vdc is not a problem? Also I could not find the cathode resistor. On this amp they also take the heater wires to the interstage tranny. What is the purpose of that? thanx again, Dak
And it was designed for 110 volts
Paul that is exactly what I told him over on DIY audio.Usually the one's bought on Ebay were halved from the 220vac that China uses and what I ended up doing was series connecting 450v electrolytics as they were swelled and then I changed the supply resistor to a higher value feeding the driver and preamp tubes as the B+ was 34v high..The outputs were higher as well but I didn't worry about it as much especially since the 845s could handle much more than the 720v the Music Angel was feeding them.
They should have designed a power transformer that was for the US and Canadian markets.
A friend you get for nothing,an enemy has to be bought
Yes, they were working until a few months ago.
I have not run the AC voltage up to full yet.
The fuse I am currently using is a fast blo.
It is good to know that the 2a3 is working properly.
I will be attempting full voltage on this amp and then get some voltage readings. thank you for the confirmation. regards, Dak
Dak
Did you get the email off to them?
A friend you get for nothing,an enemy has to be bought
The link you gave was not actually the manufacturer. It is like a clearinghouse for contacting various manufacturers. Anyway, I went through the motions and filled out the contact info, but no word so far.
The amp was actually purchased on Amazon. Evidently there is no warranty left. And the seller is also being scarce. So, I am afraid i will just have to try and sort this myself and hopefully you all can help too. cheers, Dak
I asked ebay seller "listeningchina.hifi.cn" if there was a schematic.
This was his response.
"hi,sorry the schematic is available for the buyer who purchased this item."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
So, this ebay seller has the Sinovt 2a3 845? The amp comes from Amazon though. so do you think the seller would give me the schematic?
Yes. He has them listed at the link below.
I don't think he would give a schematic to anyone except the one that buys the amps from him.
But it is good to know that there is a schematic for that amp.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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