• 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.

New DHT heater

Hi QuangHao, New Version of Heater PCBs are being fabricated now. They will be here next week, and made available shortly after.

The new PCB is built around the circuit used since 2004, and preferred by most amp builders that tried it, compared to other heating methods.

I wanted to add a voltage-servo, to adjust current until the filament voltage achieves rated value (eg 5V for 300B). But listening test showed it was clearly audible, degrading the current drive sound! So that has gone.

The new PCB is now only 79 x 21mm (see attached dimensioned drawing).

Compared to the previous circuit, the ripple rejection is much improved, and a sealed cermet multiturn trimmer added, to fine tune the current.

But most of all, you get open, articulate current-drive DHT sound.
 

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Thanks Kevin! The higher ripple rejection & noise filtering should be just the thing for the #26.

I'm planning a configuration (with this PCB) that normally drives 250mA filaments (00A, 01A, 3A/109B, 4021 etc), but increases current to #26 level (1.05A) when a jumper (PC motherboard type) is attached. Like a 'Guest Mode', where a 26 comes to stay when you want a change from the Resident DHT!
 
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Thanks Kevin! The higher ripple rejection & noise filtering should be just the thing for the #26.

I'm planning a configuration (with this PCB) that normally drives 250mA filaments (00A, 01A, 3A/109B, 4021 etc), but increases current to #26 level (1.05A) when a jumper (PC motherboard type) is attached. Like a 'Guest Mode', where a 26 comes to stay when you want a change from the Resident DHT!


This is exactly what I need as it will allow for easy swaps between my reference 26, and the 12/112(A), 01A that I also want to run in the next version of my pre.. I have a circuit, but I'd rather use yours as it seems rather better thought out and is the result of a lot of study whereas mine is just... textbook..

Kevin
 
Hello,
I'd like 4 boards, if you don't mind. Is there a chance you'd offer a complete kit of parts?
Regards,
David

hello David, I prefer to ship with all parts as a kit - the performance depends greatly on using certain Japanese transistors. The MJE150xx series and the like really degrade the ripple-rejection and low-voltage control.

I'll probably offer them assembled too, if anyone prefers that way.
 
- the performance depends greatly on using certain Japanese transistors. The MJE150xx series and the like really degrade the ripple-rejection and low-voltage control.

This is a big red flag for me - I'd like to use them with 6C4C, but I'm wondering whether your approach has enough design margin with respect to the parts used. I want to know that the amplifier is repairable in the future without being reliant on hard to find parts (except the bottles of course!) ???