Her Majesty the Mighty 851 - Triode's Triode - Thomas Mayer's new project

Thomas Mayer is at work, in parallel with my own WE 437/LCR/801A preamp and other projects for other friends (for example a 10Y driving a 300B amp...), with a one-of-a-kind super-power amp based on one of the most exotic and mighty triodes ever built...

A tube whose size and data-sheet is able to give fearful shivers to any DIYer worldwide...

Thomas, like Daniele (High Tension) Ansaloni, is among the few able to respectfully manage such a monster...

Here below follows some Thomas' writing on the matter... a quite technical dissertation, with the management of soft-start to tame the HUGE thirst of power this very tube needs and the like.

A very intriguing niche topic from a braveheart builder I'm proud to know.

"I'm actually in the process of planning and building an amplifier with the 851 triode. It will be a design with two towers, one housing the amp and one for the power supply.

The amp tower will consist of 2 subchassis, the bottom one carrying the output transformer and last LC-filter stage for the 851 B+. The upper chassis will hold dirver stage, bias adjustment and DC-filament filters.
The 851 will be mounted vertically between those two chassis.

I choose the 211 as driver which will be transformer coupled to the 851. Autput transformer is a Lundahl 1688 PP which will be parafeed coupled.
The amp will be adjustable so that the smalle 849 can also be used.

Power supply will consist of 3 subchassis: One for the DC supplies (both the 211 and 851 will be DC heated with a LCL filter arrangement). next is one for the HV supplies, separate bridge rectifiers with 4 866As each for the 851 and 211. Last is the chassis for bias supply.

I just finished design of an on/off sequencer which will turn the amp on in 7 steps:

1. rectifier filaments
2. triode filaments
3. bias supply
4. driver B+ soft start
5. driver B+ full on
6. 851 B+ soft start
7. amp full on.

The amp will be switched off in reverse order. Sequencer control is done in analog fashion with relais and RC timeconstants.

B+ will be adjustable in 6 steps to allow various op point settings.

I plan to monitor everything with vintage style panel meters:

211 filament voltage and current
851 filament voltage and current
211 and 851 B+ plate volatges and currents
Bias voltage

All power transformers and chokes are custom made and are already available except for a few. Especially the chokes for the 851 filaments are huge, about 10kgs each.

Construction of the amps will progress over the next 3-6 months. If you are interested I will post pictures as things are progressin.

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The amp will consist of 4 chassis, separate amplifiers and power supply for each channel. The amplifier section will consist of two chassis vertically arranged above each other. The 851 tube will be placed in between them. Driver tube 211 and interstage transformer Tango NC 20 on top.

The chassis will be made out of wooden frames with metal plates on top and bottom. The 4 towers will be 40 by 40 cm and about 1 m high each. (ndt - YES: ONE METER TALL!)
The power supply rack will consist of 3 sub chassis: HV supply holding 8 866A tubes arranged in a cricle, filament supplies and bias supply.

The sequencer is done in analog fashion, mainly with relays. The time delays are obtained by LM317s with an RC timeconstant at the adjust pins. The time between the steps can be adjusted by resistors. The turn off sequence is independent from the turn on sequence and is done in pure passive fashion, also settable by resistors. At the end of the turn off sequence the circuit switches itself off, so it does not need any stand by power.

The sequencer provides trigger signals for each step which can be used for example to control the soft start of orher circuits. In parallel for each of the power supplies the mains for the various supply transformers is already switched by realys directly on the sequencer.

I also included 6 independent sense inputs which can be used to monitor filament and bias voltages. Is any of those not present, the amp will shut down. Also over current conditions can be checked.

I designed a PCB for the sequencer which is currently been manufactured. I expect to get it next week.

Another important feature is the high voltage soft start. I will place a TV damper tube in the ground return of the 866A rectifier bridge. When the HV AC is applied to the bridge, the TV dampers heater is turned on at the same time. Once the TV damper is fully conducting, it will be bypassed by a relay and turned off. For this the softstart circuit receives trigger signals from the sequencer.

Next ist a selector for different high voltage secondary taps of the power transformer. So the tube can be run at lower voltages as well. I found special high voltage relays. To avoid accidental switching of the high voltage taps while the amp is on, I added some protection circuitry. The HV selector is connected to the sequencer. At turn on it reads out the selector switch at step two the selection is stored. And at step three the selection circuit gets disconnected from the rotary switch. Again old fashion style with relays only.

For Softstart and HV selection also PCBs have been designed and are also expected from the manufacturer next week."

WOW! Have a look to the pixes, please... Gary Kaufmann with an 851 (Ed Sawyer's) in hand(s)... better in arms, and Thomas comparing 851, 849 and... the "smaller", at left side, not an ECC83, BUT a 211!!!