Henrys Cat
New member
Having read about the merits of active anode (plate) loads I searched the internet for technical information and circuit diagrams. Most solutions seemed to recommend parts that are unavailable in the United Kingdom, but then I found the Bottlehead website and the answer was there in the form of the C4S Active Load upgrade!
In 1998 I built the "Single-Ended Glory for under $100" valve amplifier from the design published in Glass Audio, Vol. 7 No. 3 1995. I recently decided to do a complete re-build of this amplifier as it was too tall for its intended new location and it would give me a chance to experiment with active loads.
Unlike the original design there is a regulated DC heater supply to the 6SN7GT driver valve and the H.T power supply is also regulated at 282V. I abandoned using the centre tap of the filament supply and fitted a 'humdinger' 50 ohm potentiometer to tune out filament hum to a level only audible by placing your ear next to the speaker. I had hoped to use DC on the 45's filament supply but power dissipation was a problem with a risk of overheating of the voltage regulator. Resistors R2 and R7 - the anode loads of the 6SN7GT - were replaced with the C4S Active Load.
As the active load is designed for use with the Single-Ended Experimenter's Kit I had to do some experimentation! After some trials with a bread-boarded 6SN7GT I found that to maintain a 2mA current through D1 and D2 of the C4S R2 needs to be reduced to 137 kilohms (75K and 62K resistors in series). R1 was reduced to 270 ohm to give an anode current of around 3mA, which is the same as the original design in Glass Audio. The driver valve cathode bias resistors R4 and R9 I increased to 910 ohms.
The pictures show the rebuilt amplifier and interior of the off-board power supply. The amplifer is mounted on a piece of beech wood with MDF plinths for the output transformers. The plain aluminium case is clad in irridescent black glass and perspex. The amplifier's output transformers were custom built in 1998 by SJS Electroacoustics and are rated at 8W with 5 Kilohm primaries and secondaries with 4 ohm and 8ohm taps. Website is - http://www.users.globalnet.co.uk/~valveamp/index.htm
The filament and heater supply transformer for the 45's and the 6SN7 was custom wound by Airlink Transformers. 230V primary, two 0-1.25V 5VA secondaries and one 0-7V 5VA secondary by Airlink Transformers. Website - http://www.airlinktransformers.com
The S.E.X C4S Active Load is certainly an upgrade for this amplifier. There is more detail in the music, an even better stereo image (this was already impressive before the upgrade) and improved handling of bass. Background noise and hum have gone too.
In 1998 I built the "Single-Ended Glory for under $100" valve amplifier from the design published in Glass Audio, Vol. 7 No. 3 1995. I recently decided to do a complete re-build of this amplifier as it was too tall for its intended new location and it would give me a chance to experiment with active loads.
Unlike the original design there is a regulated DC heater supply to the 6SN7GT driver valve and the H.T power supply is also regulated at 282V. I abandoned using the centre tap of the filament supply and fitted a 'humdinger' 50 ohm potentiometer to tune out filament hum to a level only audible by placing your ear next to the speaker. I had hoped to use DC on the 45's filament supply but power dissipation was a problem with a risk of overheating of the voltage regulator. Resistors R2 and R7 - the anode loads of the 6SN7GT - were replaced with the C4S Active Load.
As the active load is designed for use with the Single-Ended Experimenter's Kit I had to do some experimentation! After some trials with a bread-boarded 6SN7GT I found that to maintain a 2mA current through D1 and D2 of the C4S R2 needs to be reduced to 137 kilohms (75K and 62K resistors in series). R1 was reduced to 270 ohm to give an anode current of around 3mA, which is the same as the original design in Glass Audio. The driver valve cathode bias resistors R4 and R9 I increased to 910 ohms.
The pictures show the rebuilt amplifier and interior of the off-board power supply. The amplifer is mounted on a piece of beech wood with MDF plinths for the output transformers. The plain aluminium case is clad in irridescent black glass and perspex. The amplifier's output transformers were custom built in 1998 by SJS Electroacoustics and are rated at 8W with 5 Kilohm primaries and secondaries with 4 ohm and 8ohm taps. Website is - http://www.users.globalnet.co.uk/~valveamp/index.htm
The filament and heater supply transformer for the 45's and the 6SN7 was custom wound by Airlink Transformers. 230V primary, two 0-1.25V 5VA secondaries and one 0-7V 5VA secondary by Airlink Transformers. Website - http://www.airlinktransformers.com
The S.E.X C4S Active Load is certainly an upgrade for this amplifier. There is more detail in the music, an even better stereo image (this was already impressive before the upgrade) and improved handling of bass. Background noise and hum have gone too.
