I've searched threads here on the forum but can't find any answers to what my problems are and I lives in Sweden and main voltage is 230V.
Now it is that I finally got my ST-35 up and running, but not without problems, even though this is the third ST 35 and a number of ST 70s behind me, it hums steadily at 50 Hz (I would think) but without finding the reason. I has also built couple others tube and ss amplifier and pre amps.
I have tried all possible troubleshooting, also checked online for something similar and as usual there are different variants of solutions.
When I got home (flea find) with the amplifier, I disassembled it for repair and found an error that the previous owner had made when they built it, the error is that one of the cables to one 115V winding that is connected to the 240V version was pinched under mains transformer there was also a burn mark in the chassis, I then tested the mains transformer and it was correct with all the correct voltages and current.
This I have done, the power supply is handled by an EFB by D.Gillespie new home-etched pc13 circuit board all resistors are original or replaced till others from Allen-Bradley. The tubes are original 7247 and new Russian EL84
But when I start it there is a hum that gets louder when the tubes warmes up, when I remove 7247 it is quiet I have gone through the wiring diagram a number of times but I think it looks right no cold solder I have been detected nothing. All tubes are glowing so that's not the fault (I think) maybe the 7247 are the fault. I have non tube tester but a friend have but he lives i a other part of Sweden.
From my search online there could be two faults but one involves other transformers with 4-8 ohm tap but mine are original and it was feedback the other talked about, that 7247 were broken now I don't know if they are broken I have none opportunity to try right now but I'm wondering if the pinched cable might have caused them to be broken. The hum or oscillates on both channels, possibly a little more on one channel.
I hope someone has a good idea how I should go about it, because right now I'm just have no clue about the problem.
Cheers Anders
Now it is that I finally got my ST-35 up and running, but not without problems, even though this is the third ST 35 and a number of ST 70s behind me, it hums steadily at 50 Hz (I would think) but without finding the reason. I has also built couple others tube and ss amplifier and pre amps.
I have tried all possible troubleshooting, also checked online for something similar and as usual there are different variants of solutions.
When I got home (flea find) with the amplifier, I disassembled it for repair and found an error that the previous owner had made when they built it, the error is that one of the cables to one 115V winding that is connected to the 240V version was pinched under mains transformer there was also a burn mark in the chassis, I then tested the mains transformer and it was correct with all the correct voltages and current.
This I have done, the power supply is handled by an EFB by D.Gillespie new home-etched pc13 circuit board all resistors are original or replaced till others from Allen-Bradley. The tubes are original 7247 and new Russian EL84
But when I start it there is a hum that gets louder when the tubes warmes up, when I remove 7247 it is quiet I have gone through the wiring diagram a number of times but I think it looks right no cold solder I have been detected nothing. All tubes are glowing so that's not the fault (I think) maybe the 7247 are the fault. I have non tube tester but a friend have but he lives i a other part of Sweden.
From my search online there could be two faults but one involves other transformers with 4-8 ohm tap but mine are original and it was feedback the other talked about, that 7247 were broken now I don't know if they are broken I have none opportunity to try right now but I'm wondering if the pinched cable might have caused them to be broken. The hum or oscillates on both channels, possibly a little more on one channel.
I hope someone has a good idea how I should go about it, because right now I'm just have no clue about the problem.
Cheers Anders
Hi Anders! So if the amp had been run with the pinched cable, could that have meant that the full 235VAC was applied to a single 120VAC winding? So potentially doubling voltages?
A few years ago I bought a Knight kit amplifier that was for US supplies but had been connected to Swedish, and the tubes were not in a very good state, actually zero emission.
You have a slightly tricky problem to work out if the unit was left running for long with the problem and the previous issue has damaged something. I suppose the heaters could be the most vulnerable, The fact that it is quiet without the 7247 is a sign there could have been some damage. Reading some links, maybe you could sub a 12AT7 as a very short term measure to eliminate the 7247 as the source of the hum?
How are the 7247's heated? Just a winding with a centre tap, or is it rectified? If it is elevated, how is that done?
Cheers, Richard
A few years ago I bought a Knight kit amplifier that was for US supplies but had been connected to Swedish, and the tubes were not in a very good state, actually zero emission.
You have a slightly tricky problem to work out if the unit was left running for long with the problem and the previous issue has damaged something. I suppose the heaters could be the most vulnerable, The fact that it is quiet without the 7247 is a sign there could have been some damage. Reading some links, maybe you could sub a 12AT7 as a very short term measure to eliminate the 7247 as the source of the hum?
How are the 7247's heated? Just a winding with a centre tap, or is it rectified? If it is elevated, how is that done?
Cheers, Richard
Don't assume the tubes are the problem.
Try unplugging only one input cable from the ST35, does that remove the hum?
If not, does unplugging both input cables remove the hum?
Try unplugging only one input cable from the ST35, does that remove the hum?
If not, does unplugging both input cables remove the hum?
Probably it has been as you describe, but the amplifier was like new when I bought it so I suspect that the person who built it has not used it at all but it has been running for short moments as they have not been able to solve the problems with pinched cable.
Separate windings for each channel see the diagram I have added.
Yes, this will be fun, but I already have a couple of suspects, but I don't want to buy new tubes if the error persists.
Is the heater supply elevated? Violating that Vh-k gives a nasty buzz that increases as the tubes warm up...maybe you need to tweak the elevation voltage.
Do you have a two wire or three wire power cord?
If three wires, is the chassis grounded?
Post #4 shows heater AC voltage across the cathode resistor, obviously an error. What is the actual amplifier configuration?
All good fortune,
Chris
All good fortune,
Chris
Please post a PICTURE of the inside of the amp. the Hum is 99.9% due to a ground loop (somewhere). This is a very established old-school design that does NOT hum.
I do have a st35 and am equipped with proper instrumentation.
Where do you live in sweden ? I can have a look at it .
One thing that others has experienced is the RCA inputs. The constitute the signal grounding, and if not properly done it will cause hum.