• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Switchable 45/VT-52 Amp

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Switchable 45/VT-52 Amp

Hi, I am about to start building (still collecting some parts) a switchable SE amp that could work in following configurations (driver/output tube): 26/45, 10/45 and 10/VT-52.
It would probably used above 500 Hz to run my horns (my 211 amp would run the woofers), but, I want to have it able to work full range in case I end up preferring the sound of passive crossover between woofers and horns.
Objective is to stay with all DHT, have a total gain of around 4, stay with 2 stages and to be able to experiment with above tubes, so I would like to make it easily switchable between those configurations.

I attach 3 images, all same schematic, but with different tubes combinations and switches in different positions to accommodate them.
It is design inspired by Thomas Mayer amplifiers as well as Electraprint DRD.
There is 1:4 input transformer to get the right gain, filament biased first stage, directly coupled to output stage.
To get different B+ voltages used in different configurations I am changing value of first C in CLCL B+ filter; voltages and values are estimated based on PSUD2 simulations and might need to be adjusted once I get to breadboard stage.
For filaments I am using Rod’s regulators, again, the input voltage adjusted by changing C value in CLC filter ahead of regulator.

I would welcome any comments/suggestions as I am just starting and could still adjust the design!


  • 26-45 DCC.jpg
    26-45 DCC.jpg
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  • 10-45 DCC.jpg
    10-45 DCC.jpg
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  • 10-VT52 DCC.jpg
    10-VT52 DCC.jpg
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Hi, thanks for your comment; will possibly try zener string during breadboard stage, but my idea we to stick with more "classic" parts; avoid batteries, LED's, etc....
Do not really know why, but just wanted to go that way...

One thing that I would appreciate some comments is on capacitor sizing in B+ supply as well as "Ultrapath" cap.
Obviously, larger caps will reduce the ripple, but, then, many advocate smaller caps to sound better.....is there really some hard rule to follow?
Gluca, you are right about bugs in filament regulators :) ; could go Thomas's way with LCL filters on those, but big inductors are pricey and let's consider filament regulators not being directly in signal path...
With Zener string replacing 45/VT-52 resistors AC signal still has to travel trough zener string and then trough last B+ cap, so what is the advantage to "ultrapath" where it goes trough "cathode-to-B+" cap?
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