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30 November 2021 

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20W All Triode Amplifier

Amplifier Circuit Diagrams - PLEASE NOTE

To build these circuits, you should be an EXPERIENCED ELECTRONIC TECHNICIAN.

The circuit diagrams etc are believed to be good quality and correct. However, please note that several different people have built amplifiers for us in the past (mainly in the 1990s), using valves like S11E12, TT21, E180CC etc.

Since most of those people are now retired, you are free to use this data as a free gift, in good faith for you, an EXPERIENCED ELECTRONIC TECHNICIAN, to study and aid development of YOUR OWN IDEAS.

The data you see in the following pages are therefore intended to be a GUIDE ONLY - not step-by-step instructions.

We try to be helpful but advice, available by email only, is limited. We cannot advise about amplifiers by telephone.

Parts List

Miscellaneous Components

Wire Ended

Output Transformers

F1 250 m/A FUSE
V1 ECC 83/12AX7
V2 E180CC
V3 6550B
V4 6550B

Resistors Capacitors
R1 1K 0.5W C1 47µF 25V ELECT
R2 2.2K 0.5W C2 0.1µF CERAMIC
R3 1K 0.5W C3 150pF S/MICA
R4 2.2K 0.5W C4 10µF450V ELECT
R5 1K 0.5W C5 0.1µF POLYESTER
R6 471K 0.5W C6 33µF 450V ELECT
R7 1M 0.5W C7 0.1µF 1KV
R8 18K 1W C8 1.0µF 1KV
R9 47K 1W C9 0.33µF POLYESTER
R9a 220K 1W C10 0.47µF 1KV
R10 47K 1W C11 1µF POLYESTER
R11 22K 0.5W    
R12 3K3 2W    
R13 471K 0.5W    
R14 1K 0.5W    
R15 100K 0.5W    
R16 100R 2W    
R17 100R 2W    
R18 1K 0.5W    
R19 470K 0.5W    
R20 100K 0.5W    
R21 100R 2W    
R22 100R 2W    
R23 100K 0.5W    
VR1 100K CERMET    
VR2 220K CERMET    


To set the quiescent current in the output valves, remove the 250m/A fuse and connect a meter in itís place, set VR2 to give 110m/A.

To balance the output valves, connect a voltmeter between the anodes and adjust VR1 for zero volts.

Printed Circuit Board Notes

The prototype was constructed on double sided fibreglass PCB. The heater pins on the valves have plated through holes or through connections and all the heater wiring is offboard twisted PVC covered hook-up wire.

The valveholders are mounted on the same side as the print, opposite to all the components. The valveholders are aligned with clearance holes in the chassis. Connections to the power supply, input, feedback and output transformer are by PCB pin connectors.

Power Supply Notes

The prototype amplifier uses a wire ended output transformer with the ratings shown above, using solid state bridge rectifiers for both HT and bias supplies.

Diagram of 6550B All Triode Amplifier

Diagram of Prototype Power Supply Using MT2 Transformer

Diagram showing Print Side(Valve Holders This Side) Valve Holder Holes Through Plated

Diagram showing Component Side of PCB (note tracking on this side of board)


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