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Westo091946 · 14th Mar 2021, 9:44 pm

I have started a project:-Build a leak 20 from the ground up
I did radio and tv some 55 years ago as hobby only finished year two as work got more demanding. I built the mullard amps fine bit of kit.
So though I would get my hands dirty and take on this project on in my day it was mono only one channel giving my age away, now we have 2. 2.1,5.1 and 7.1 I collected as much as I could in by way of circuit diagrams pictures and some schematics Leak 20.

I have also discovered the mullard Book (publication HiFi amps in Adobe format) and my son wanted a valve amp so I will build the 10+10 for him I have manage to source the Pot's for this amp at only £7-50 each exact match (2Meg) from a company who used to manufacture these for mullard and are still producing pot's switches (Wafer) and other items.

To continue:- thought looking at the circuit's of the leak 20 and picture I had seen on the net including the underside PCB Component layout it would be fairly easy process , but it has proved to be not so simple.

I have the underside wiring of the component pcb if you could call it that, have good photographs and can see the connections to pcb and valve bases, but my problem is under pcb wiring loom I think that this is predominately the valve heater circuit wiring split into two ? with the 6.5-0-6.5 volt split ? between the two channels given the mix of valves and seeing the configuration on the valve bases the ECC83 heaters are pins 4 &5 Linked and 9, 6.3 Volt if pins4,5,6 are connected this would need 12.6V heater voltage, El84 are 4 & 5. 6.3 V and so I am assuming that there are two runs for the valve heaters, and are parallel with centre tap to earth ? also the wiring of the input and output of the output transformers as well. I know that the EZ34 has it own 5 volt supply

I have been trying to pickup an old 20 but they are fetching quite a high price.

I am wondering if anyone could help me, with any information about the wiring harness, I have managed to get some info from another forum member which was very much appreciated , if any one could contribute
to this I would appreciated any input.

I can get off the self mains transformer but it's the 300-300 which is the very original which they switched for a 280-280 volts the higher HT value is limiting value of the ELE4, from my research mullard confirmed to Leak that this was OK (so I read) but leak anyway changed to the lower HT and changed R12 and R13 from 1Meg (300-300)V to R13 and R13 to 470K (280-280)V I do not know the the mA rating for the Mains Transformer so if some one could help here please.

We were always told, and it's in the Mullard book of amps to twist the heater wires together as this would have a cancelling effect on any emission's from the heater wiring affecting the high gain audio circuitry.

I have started to put together a schematic of the wiring with information I have I hope that I can complete this with some wider input from members.

I have the parts list for the Leak 20, Resistors and Capacitors only

Thanks

14th Mar 2021, 9:44 pm	#9
Westo091946 Triode Join Date: Mar 2021 Location: Caerphilly, Glamorgan, Wales. UK Posts: 11	Leak 20 Project I have started a project:-Build a leak 20 from the ground up I did radio and tv some 55 years ago as hobby only finished year two as work got more demanding. I built the mullard amps fine bit of kit. So though I would get my hands dirty and take on this project on in my day it was mono only one channel giving my age away, now we have 2. 2.1,5.1 and 7.1 I collected as much as I could in by way of circuit diagrams pictures and some schematics Leak 20. I have also discovered the mullard Book (publication HiFi amps in Adobe format) and my son wanted a valve amp so I will build the 10+10 for him I have manage to source the Pot's for this amp at only £7-50 each exact match (2Meg) from a company who used to manufacture these for mullard and are still producing pot's switches (Wafer) and other items. To continue:- thought looking at the circuit's of the leak 20 and picture I had seen on the net including the underside PCB Component layout it would be fairly easy process , but it has proved to be not so simple. I have the underside wiring of the component pcb if you could call it that, have good photographs and can see the connections to pcb and valve bases, but my problem is under pcb wiring loom I think that this is predominately the valve heater circuit wiring split into two ? with the 6.5-0-6.5 volt split ? between the two channels given the mix of valves and seeing the configuration on the valve bases the ECC83 heaters are pins 4 &5 Linked and 9, 6.3 Volt if pins4,5,6 are connected this would need 12.6V heater voltage, El84 are 4 & 5. 6.3 V and so I am assuming that there are two runs for the valve heaters, and are parallel with centre tap to earth ? also the wiring of the input and output of the output transformers as well. I know that the EZ34 has it own 5 volt supply I have been trying to pickup an old 20 but they are fetching quite a high price. I am wondering if anyone could help me, with any information about the wiring harness, I have managed to get some info from another forum member which was very much appreciated , if any one could contribute to this I would appreciated any input. I can get off the self mains transformer but it's the 300-300 which is the very original which they switched for a 280-280 volts the higher HT value is limiting value of the ELE4, from my research mullard confirmed to Leak that this was OK (so I read) but leak anyway changed to the lower HT and changed R12 and R13 from 1Meg (300-300)V to R13 and R13 to 470K (280-280)V I do not know the the mA rating for the Mains Transformer so if some one could help here please. We were always told, and it's in the Mullard book of amps to twist the heater wires together as this would have a cancelling effect on any emission's from the heater wiring affecting the high gain audio circuitry. I have started to put together a schematic of the wiring with information I have I hope that I can complete this with some wider input from members. I have the parts list for the Leak 20, Resistors and Capacitors only Thanks