Hi all, this is my first post on this forum. I would be really grateful for some help please with the heater wiring on a cheap Chinese SE amp I have just purchased off Ebay. The seller has sent me the schematic for the amp which I have attached. He has highlighted in red some changes he has made to the PSU involving addition of a thermistor (good) and 4 x 100 ohm resisters added in the heater supply circuit. I can understand the reason for the two 100 ohm resistors added to the 6.3 volt (3.15-0-3.15) centre tap supply to the 6n9p valves, but I am unsure of the impact of adding the two resistors to the heater supply to the EL34 valves where there is no centre tap on the transformer. The way they have been added means that two resistors in parallel produce a reduced resistance of 50 ohm and additional current output. I wonder whether it would it make more sense to just remove one of these resistors in this section so that one end of the 6.3v winding is then grounded by that one remaining resistor. Many thanks indeed. Russell
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I'm no expert on power supplies, but I believe the two 100R resistors on the EL34s are to create a virtual centre tap. So that's not uncommon. On the other hand I can't see the point of the two 100R resistors on the 6N9P if there is already a centre tap. So one supply has a centre tap and the other doesn't - presumably you've checked that.
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Hi Andy, thank you very much for your reply - very helpful. I will check it out the transformer wiring when it arrives. There are other tasks I need to undertake on it as well including for example changing the mains switch and installing an ALPs volume pot
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+1 to what andy said
I would just check everything before power up, especially safety-wise to ensure the chassis is earthed to the mains earth reliably, maybe check the wiring to see that all the joints are solid, no wires going through chassis holes without protective grommets etc.
I would just check everything before power up, especially safety-wise to ensure the chassis is earthed to the mains earth reliably, maybe check the wiring to see that all the joints are solid, no wires going through chassis holes without protective grommets etc.
For both 6.3ac, no need resistors if you have no hum. The one with CT, either uses CT or resistors. I got hum with CT, so I tried the 2 resistors to get rid of the hum. Then tried both CT and resistors together didn't change anything so I removed CT where I have seen someone mentioned to use one of the option.
Lots of good advice already, but I'd just add: While you're in there, you might move the heater windings' center taps from signal ground to one of the EL34's cathodes. This minimizes leakage currents from heater to cathode sleeve.
All good fortune,
Chris
All good fortune,
Chris
Quite true, until you replace the tube with another of higher heater leakage.
These are 12 cent parts. It is not worth the trouble to leave them out.
