211 SET Interstage Monobloc

211 SET Interstage Designed Monobloc Power Amp


This project was completed by Brendon in January 2004.  Brendon built a pair of monolock valve amplifiers using 211B+ Valves in its output stage.  This provides 20 watts of power to the speakers.

This project page gives photographs and descriptions to the main features of the amplifier build.  It was origionally published on Brendan’s web site hosted on Geocities.  Brendon has generously allowed the article to be re-published here in order to preserve it beyond the closure of Geocities free hostings.

Overview:                                                              Completion Date: January 27th, 2004

211 B+ : Valve Art, Heaters are AC
6c45pi B+: 170VDC, Sovtek, Heaters are DC (Schotkky Diodes)
6AU4GT rectifiers RCA (nos)
Interstage: Lundahl 1660 (40ma) version
Toroidal Transformers: Custom low flux wound made in Australia (thanks JOE)
Output Transformer: from "www.handwoundtransformer.com" BEWARE that this outfit takes a LONG TIME to send his orders out (if they ever send) . I won these transformers off Ebay and it took almost 3 months to ship within the US.
Output Power: 20 watts Class A
Hum/Noise: 12mV AC


All aluminum chassis made from 3mm thick stock.

211_Rubber_1 All components are isolated from the chassis with these rubber washers and rubber grommets to protect the wires.


Rubber feet for the chassis. No special spikes around here!211_Inside_3

Hybrid full wave rectifier design. 1N4007 & 6AU4GTA TV damper diodes. I really enjoy the slow voltage rise of these tubes at power on.


Rear of unit. I've made allowance for an extra IEC main power plug. Really a thing of the past but VERY handy for power other sources (like the other monobloc)


I've purposely left plenty of space for future modifications. Possible extra LC stage to go on the HV B+

211_Inside_3 You can see the Lundahl 1660 (40ma) interstage transformer. Originally I had tested the cheap Allied Elec IT with bridging capacitor with good results. I went with the Lundahl because it had more high frequency response. But for those on the cheap, the Allied IT does the job.

211_Front Separate transformers for each monobloc. The big donut is for the 211 B+ supply. The smaller donut supplies all the filaments and 6c45pi B+.
211_PairYes the 211 tubes glow nicely. They also give off a lot of heat. Much like a 100 watt light bulb.