18-07-2020, 08:49 AM
Something is wired wrong. Changing the transformer won't have that effect.
The two red wires (AC HT) go to pins 1 & 2 of the EZ81, order isn't important.
The HT centre tap goes to the can / -ve terminal of the main smoothing cap (red/yellow wire?). There should be a common earth / 0V point usually the can -ve or the chassis nearby.
The black pair are for a rectifier that's not isolated, so they go to EZ81 pin 4 and 5, the heater.
Pin 3 of the EZ81 is the DC out (cathode). It goes to + connection on the smoothing cap. Normally there are two caps or two sections in one can. The smaller value is from Pin3 and then the larger value is the HT rail and there is a resistor between them. If it's wired wrong you can put too much load on the EZ81 or not have enough smoothing (filtering). The EZ81 can be connected differently, but perhaps it's derived from an older design. So one side of the heater supply for the EZ81 connects to the EZ81 HT out (Pin 3 & Pin 4 connected). It's not the only way to do it, but works and is safe. The EZ81 heater supply can't be floating, one side must connect to pin 3 (HT) OR the earth / OV point. This PSU is intended to use one side of the EZ81 heater supply connected to Pin 3.
The rest of the valve heaters are fed by the green pair of wires. Either the white centre tap OR one green wire must go to 0V/negative/Chassis. There will be hum if it's floating.
With all the valves unplugged and no power, verify there is:
More than 1M Ohm between either black wire and either green wire.
About the same resistance between pin 1 or pin 2 of the EZ81 to the HT centre tap and chassis. Twice this resistance BETWEEN pin 1 and pin 2 (across the red wires).
Zero resistance from chassis to centre tap of HT and negative of smoothing can. Zero resistance from negative of smoothing capacitor (usually the can or a common tag, some cans for USA sets are isolated) to chassis. Almost zero resistance to all the heater pins except the EZ81 from negative of smoothing can and chassis.
The resistance of the two 6.3 windings (between black OR between green) will be less than an Ohm. Very low.
There are several mutually exclusive schemes to avoid the valve heater circuit generating hum. Only ONE scheme can be used. You should use which ever scheme was originally in use:
1) Earth (OV/Chassis, smoothing can etc) ONE side of the heater supply. This is the oldest scheme. Only at the common earth point, either the smoothing can outer or the chassis nearby.
2) A centre tap on the heater supply is earthed instead of either heater wire. Only at the common earth point, either the smoothing can outer or the chassis nearby.
3) A 1K or similar pot is across the heater supply and the moving contact goes to the main 0V/Negative/Chassis/Earth common point. You adjust for minimum hum and perhaps reverse heater connections to the first preamp valve to see if any better.
4) If there is a Cathode follower stage the valve there might not have so high cathode / heater insulation, so one side of the heater supply is connected to a cathode of an output valve. There is no earth/0V connection.
5) DC is used, referenced to 0V. This a mad option except on maybe an electrostatic microphone or industrial amplifier 50 to 70 years ago.
If there is no separate heater for the rectifier only valves like the EZ80 and EZ81 with isolated cathodes can be used. If pin 3 and 4 are connected on an EZ81, then it MUST be on a separate, isolated heater supply. Rectifiers like the GZ34 can ONLY use a separate supply. The EZ80 and EZ81 date from about 1956 and were specially designed to use the same heater supply as the rest of the valves, like in a radio. Most earlier rectifiers either had a direct filament cathode, or one side of the indirect heater connected internally to the cathode or poor cathode / heater insulation. The early exception was rectifiers for operation in a series chain as they needed an isolated heater. Often Instrument amps and PAs used "cut down" versions of earlier designs that used more powerful rectifiers that had no heater cathode insulation, so often the they wired the EZ81 the same way, though there is no need below 500V HT.
The two red wires (AC HT) go to pins 1 & 2 of the EZ81, order isn't important.
The HT centre tap goes to the can / -ve terminal of the main smoothing cap (red/yellow wire?). There should be a common earth / 0V point usually the can -ve or the chassis nearby.
The black pair are for a rectifier that's not isolated, so they go to EZ81 pin 4 and 5, the heater.
Pin 3 of the EZ81 is the DC out (cathode). It goes to + connection on the smoothing cap. Normally there are two caps or two sections in one can. The smaller value is from Pin3 and then the larger value is the HT rail and there is a resistor between them. If it's wired wrong you can put too much load on the EZ81 or not have enough smoothing (filtering). The EZ81 can be connected differently, but perhaps it's derived from an older design. So one side of the heater supply for the EZ81 connects to the EZ81 HT out (Pin 3 & Pin 4 connected). It's not the only way to do it, but works and is safe. The EZ81 heater supply can't be floating, one side must connect to pin 3 (HT) OR the earth / OV point. This PSU is intended to use one side of the EZ81 heater supply connected to Pin 3.
The rest of the valve heaters are fed by the green pair of wires. Either the white centre tap OR one green wire must go to 0V/negative/Chassis. There will be hum if it's floating.
With all the valves unplugged and no power, verify there is:
More than 1M Ohm between either black wire and either green wire.
About the same resistance between pin 1 or pin 2 of the EZ81 to the HT centre tap and chassis. Twice this resistance BETWEEN pin 1 and pin 2 (across the red wires).
Zero resistance from chassis to centre tap of HT and negative of smoothing can. Zero resistance from negative of smoothing capacitor (usually the can or a common tag, some cans for USA sets are isolated) to chassis. Almost zero resistance to all the heater pins except the EZ81 from negative of smoothing can and chassis.
The resistance of the two 6.3 windings (between black OR between green) will be less than an Ohm. Very low.
There are several mutually exclusive schemes to avoid the valve heater circuit generating hum. Only ONE scheme can be used. You should use which ever scheme was originally in use:
1) Earth (OV/Chassis, smoothing can etc) ONE side of the heater supply. This is the oldest scheme. Only at the common earth point, either the smoothing can outer or the chassis nearby.
2) A centre tap on the heater supply is earthed instead of either heater wire. Only at the common earth point, either the smoothing can outer or the chassis nearby.
3) A 1K or similar pot is across the heater supply and the moving contact goes to the main 0V/Negative/Chassis/Earth common point. You adjust for minimum hum and perhaps reverse heater connections to the first preamp valve to see if any better.
4) If there is a Cathode follower stage the valve there might not have so high cathode / heater insulation, so one side of the heater supply is connected to a cathode of an output valve. There is no earth/0V connection.
5) DC is used, referenced to 0V. This a mad option except on maybe an electrostatic microphone or industrial amplifier 50 to 70 years ago.
If there is no separate heater for the rectifier only valves like the EZ80 and EZ81 with isolated cathodes can be used. If pin 3 and 4 are connected on an EZ81, then it MUST be on a separate, isolated heater supply. Rectifiers like the GZ34 can ONLY use a separate supply. The EZ80 and EZ81 date from about 1956 and were specially designed to use the same heater supply as the rest of the valves, like in a radio. Most earlier rectifiers either had a direct filament cathode, or one side of the indirect heater connected internally to the cathode or poor cathode / heater insulation. The early exception was rectifiers for operation in a series chain as they needed an isolated heater. Often Instrument amps and PAs used "cut down" versions of earlier designs that used more powerful rectifiers that had no heater cathode insulation, so often the they wired the EZ81 the same way, though there is no need below 500V HT.