25 years ago, I was given a present of a World Audio Design K5881 MkII amplifier kit. This was a stereo Hi-Fi amplifier with 5881 pentode output valves in Push-pull configuration, with about 25 Watts of power. The discrete components were wired on tag strips. I spent a satisfying few weeks building this amplifier, then making a few of the recommended component upgrade modifications. When finished, the open soundstage and tight bass from my valve amplifier led me to relegate my trusty Quad 404 power amplifier.
Over the years I had plans to build more valve amplifiers and collected some components, but these projects failed to come to fruition. However, I recently spotted a new model compact valve amplifier in my local vinyl record shop and decided to revisit my association with these glowing tubes. After a little web-based research, I decided to base my compact project on the Caledonian Audio Classique 6BM8/ECL82 single-ended amplifier, as I already had a small stock of ECL82 dual package triode/pentode valves.
Caledonian Audio were out of stock of their complete amplifier kits, and their transformer supplier had a backlog of orders for winding, but I wanted to use a valve rectifier power supply anyway, so I selected a suitable power transformer from Hammond. This was in stock at their UK distribution centre in Basingstoke, which is local to me, so I could pick up my order. I also chose a pair of single-ended output transformers and a small black painted steel chassis type 1441-16 from Hammond.
Caledonian Audio do, however, sell mono block amplifier kits, with the amplifier section PCBs, discrete components and a B9A valve socket, which was very convenient for my purpose. The PCB has through hole plated contacts so, apart from the valve base, the resistors and capacitors can be installed on either side of the board, to suit the amplifier construction arrangement. When I was building and testing my amplifier on a bread board, I made a few component changes, I will detail later.
Power transformer
I used a Hammond 370DAX power transformer. This is a black painted shroud style, with mounting feet and 8 inch long connecting leads. Hammond also stock a 370DAZ model for drop-through mounting with the same winding specification. The primary windings are universal, so I connected in series for 240 V. The secondary windings are 260 - 0 – 260 V @ 90 mA rating and 6.3 V @3.5 A centre tapped, for the valve heaters. I measured 298 VAC rms no-load, on each leg of the HT secondary, so rounded this up to 300 V in the following calculations. The 240 V series-connected primary winding resistance (Rp) measured 26.5 Ω. One leg of the HT secondary winding (Rs) measured 115.4 Ω. The turns ratio 👎 is about 300/240 = 1.25. The transformer equivalent internal winding resistance (Ri) is calculated as Ri = Rs + (n ² x Rp) = 115.4 + (1.25 ² x 26.5) = 156.8 Ω.
Valve rectifier
I chose a EZ80 full wave rectifier valve, as this meets the rating of the amplifier and the valve uses the same B9A base as the ECL82 valves. The EZ80 has a heater rated at 6.3 V @ 0.6 A and a typical maximum output current of 90 mA. The valve specification for the minimum Limiting Resistance for each Anode is 215 Ω at 300 V rms Anode input voltage. This is higher than the transformer Ri, so I needed to add a 58.2 Ω resistor in series with each anode feed. Given a maximum Anode current of 90 mA /2 = 45 mA, these resistors need a power rating of about 0.12 W. I used the standard value 56 Ω and chose 3 W resistors for their cosmetic appearance.
Power supply and smoothing
The EZ80 rectifier is specified for a maximum reservoir capacitor input of 50 μF. I used a dual capacitor rated at 50 μF + 50 μF 500 V with the smoothing capacitor configured as a Second Order L C low pass filter with a 10 H choke. With both amplifier PCBs connected, the HT measured 309 VDC and the total current drawn was 69 mA. This is a little high, so I added a 680 Ω and 150 Ω series resistors and a 47 μF smoothing electrolytic capacitor, to drop the HT to 252.6 V. The total current drawn by both channels also reduced to 66 mA. Shortly after switching the amplifier on during warm-up, I measured voltage across C6 peaking at 358 V before settling back at 234 V as the amplifier valves warmed up. I changed out the supplied C6 capacitors for 4.7 μF 450 V units, to give more headroom. The L C Low Pass filter has a cut-off frequency of 7.12 Hz. The 100 Hz Ripple on the HT output of the choke and the smoothing capacitor section measured about 21 mV rms.
Valve Heater supply
I decided to initially use 6.3 VAC for the heaters on all the valves, as in my existing amplifier. The transformer 6.3V secondary centre tap is grounded. In listening tests, the hum in inaudible.
I built the amplifier on a breadboard, to check the voltages and to make any adjustments.
Over the years I had plans to build more valve amplifiers and collected some components, but these projects failed to come to fruition. However, I recently spotted a new model compact valve amplifier in my local vinyl record shop and decided to revisit my association with these glowing tubes. After a little web-based research, I decided to base my compact project on the Caledonian Audio Classique 6BM8/ECL82 single-ended amplifier, as I already had a small stock of ECL82 dual package triode/pentode valves.
Caledonian Audio were out of stock of their complete amplifier kits, and their transformer supplier had a backlog of orders for winding, but I wanted to use a valve rectifier power supply anyway, so I selected a suitable power transformer from Hammond. This was in stock at their UK distribution centre in Basingstoke, which is local to me, so I could pick up my order. I also chose a pair of single-ended output transformers and a small black painted steel chassis type 1441-16 from Hammond.
Caledonian Audio do, however, sell mono block amplifier kits, with the amplifier section PCBs, discrete components and a B9A valve socket, which was very convenient for my purpose. The PCB has through hole plated contacts so, apart from the valve base, the resistors and capacitors can be installed on either side of the board, to suit the amplifier construction arrangement. When I was building and testing my amplifier on a bread board, I made a few component changes, I will detail later.
Power transformer
I used a Hammond 370DAX power transformer. This is a black painted shroud style, with mounting feet and 8 inch long connecting leads. Hammond also stock a 370DAZ model for drop-through mounting with the same winding specification. The primary windings are universal, so I connected in series for 240 V. The secondary windings are 260 - 0 – 260 V @ 90 mA rating and 6.3 V @3.5 A centre tapped, for the valve heaters. I measured 298 VAC rms no-load, on each leg of the HT secondary, so rounded this up to 300 V in the following calculations. The 240 V series-connected primary winding resistance (Rp) measured 26.5 Ω. One leg of the HT secondary winding (Rs) measured 115.4 Ω. The turns ratio 👎 is about 300/240 = 1.25. The transformer equivalent internal winding resistance (Ri) is calculated as Ri = Rs + (n ² x Rp) = 115.4 + (1.25 ² x 26.5) = 156.8 Ω.
Valve rectifier
I chose a EZ80 full wave rectifier valve, as this meets the rating of the amplifier and the valve uses the same B9A base as the ECL82 valves. The EZ80 has a heater rated at 6.3 V @ 0.6 A and a typical maximum output current of 90 mA. The valve specification for the minimum Limiting Resistance for each Anode is 215 Ω at 300 V rms Anode input voltage. This is higher than the transformer Ri, so I needed to add a 58.2 Ω resistor in series with each anode feed. Given a maximum Anode current of 90 mA /2 = 45 mA, these resistors need a power rating of about 0.12 W. I used the standard value 56 Ω and chose 3 W resistors for their cosmetic appearance.
Power supply and smoothing
The EZ80 rectifier is specified for a maximum reservoir capacitor input of 50 μF. I used a dual capacitor rated at 50 μF + 50 μF 500 V with the smoothing capacitor configured as a Second Order L C low pass filter with a 10 H choke. With both amplifier PCBs connected, the HT measured 309 VDC and the total current drawn was 69 mA. This is a little high, so I added a 680 Ω and 150 Ω series resistors and a 47 μF smoothing electrolytic capacitor, to drop the HT to 252.6 V. The total current drawn by both channels also reduced to 66 mA. Shortly after switching the amplifier on during warm-up, I measured voltage across C6 peaking at 358 V before settling back at 234 V as the amplifier valves warmed up. I changed out the supplied C6 capacitors for 4.7 μF 450 V units, to give more headroom. The L C Low Pass filter has a cut-off frequency of 7.12 Hz. The 100 Hz Ripple on the HT output of the choke and the smoothing capacitor section measured about 21 mV rms.
Valve Heater supply
I decided to initially use 6.3 VAC for the heaters on all the valves, as in my existing amplifier. The transformer 6.3V secondary centre tap is grounded. In listening tests, the hum in inaudible.
I built the amplifier on a breadboard, to check the voltages and to make any adjustments.
Good work!
A magnetic steel chassis might give you more hum at the output tap, than the breadboard version has.
Some judicious spacing off of a magnetic steel chassis may be needed.
I use aluminum chassis whenever possible.
Have you tried both Pentode and Triode Wired operation of the amplifier?
We want to hear how your amplifier sounds.
A magnetic steel chassis might give you more hum at the output tap, than the breadboard version has.
Some judicious spacing off of a magnetic steel chassis may be needed.
I use aluminum chassis whenever possible.
Have you tried both Pentode and Triode Wired operation of the amplifier?
We want to hear how your amplifier sounds.
I've read this about using a steel chassis, but after building a dozen+ amps using a steel chassis and having no hum issues, I don't worry about that any more. I think how the transformers are spaced and aligned has more of an impact than the chassis material.
I will trade you my one magnetic steel chassis for your aluminum chassis.
That steel chassis amplifier has probably had over 10 versions of amplifier circuits on it, both single ended and push pull.
Hum on that chassis has typically been from 400uV to 2mV.
I do know about angular orientation and spacing of both power transformers and filter chokes to the orientation and spacing and distance to interstage and output transformers.
And I also know about various kinds of offensive ground loop hum.
My many aluminum chassis amplifiers hum is less than 100uV.
Your Mileage may vary.
That steel chassis amplifier has probably had over 10 versions of amplifier circuits on it, both single ended and push pull.
Hum on that chassis has typically been from 400uV to 2mV.
I do know about angular orientation and spacing of both power transformers and filter chokes to the orientation and spacing and distance to interstage and output transformers.
And I also know about various kinds of offensive ground loop hum.
My many aluminum chassis amplifiers hum is less than 100uV.
Your Mileage may vary.
I'm interested in a Caledonian Audio kit but their site seems to have disappeared. Anyone have any intell? Thanks
@wanindra it doesn't work like that 🙂 When you have made enough quality posts on the forum you will be released from moderation.
If you receive a pm from the person you want to contact then you will be able to reply to it whilst still under moderation.