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repycte 18th January 2013 06:20 PM

Direct heated triode (dht) cathode bypass: different options
Hi, I recently started building a tubeamp, based on a pair of 6c4c direct heated triodes.

Looking at how cathode bypassing is done most of the time (Option 1 in the picture), I thought why not using the proposed option 2. Using this kind of bypassing cancels the degeneration caused by the humpot.

What do you think? Why isn't bypassing done like that?
Neglecting the degeneration caused by the humpot is not a reason not to use option 2.

MrCurwen 18th January 2013 10:21 PM

If you heat with AC, as I assume you do by those schematics, then the option 2 would have both filament connections at the same AC potential, hence no current thru filament and no tube operation.

With DC filaments (smartest choice with regards to fidelity and also hum) you could use option 2.

andyjevans 19th January 2013 08:38 AM

The best option is to use no cathode bypass. Bypasses do degrade the sound audibly, and there are other options:
1) Fixed bias
2) Filament bias
3) Ultrapath

Fixed bias needs a very clean supply. Rod Coleman has suggested using his filament boards to give a voltage across something like a 47K resistor. I haven't tried this but intend to. I never got ultrapath to sound that good so discarded that.

My solution was filament bias, but 6C4C will need a herculean DC supply. So I used a pair of 4P1L instead. Details of how to do this are in the 'yet another 4P1L SE' thread.

Personally I consider cathode bypass caps to be unacceptable and I'd do anything to not use them.


repycte 19th January 2013 10:39 AM

I already built a DC filament supply based on a LT low drop regulator. Using this supply I can only switch to fixed bias.

I will have a closer look at filament bias. Although I already used very big film caps for bypassing.

I chose the 6C4C for testing and later replacing by 300Bs, as I have no experience building tube power amplifiers, only preamps.

yagoolar 19th January 2013 01:12 PM

If you have about 126 x 2 cheap red LEDs you can use it for biasing 6C4C. (lot of soldering and space used) - I tested it in my 6C4C amp (search for C3g + 300b thread). Replaced RC cathode combo. Improvement is huge.
Get familiar with Steve Bench's article on DHT bias.
Effects of AC Heating Power Applied to Directly Heated Triodes

andyjevans 19th January 2013 04:09 PM

I forgot about the red LEDs or indeed other things like diodes. Some report good results with these. No bypass cap.

yagoolar 19th January 2013 06:42 PM


That is approx. 2h soldering. But I think worth the effort :)

repycte 19th January 2013 08:16 PM

I will try some LEDs. Used them only in preamps, but bypassed them with filmcaps. Will look for some that can take the 40-60mA needed for the 6c4c.

I prefer SMD types, as this large amound of LEDs is soldered faster using a reflow oven.

yagoolar 20th January 2013 07:16 AM

I used cheap red LEDs measured Vf= 1.84 V @ 29mA. 4 rows 26 LEDs each.

cotdt 20th January 2013 07:30 AM

Ultrabright blue LEDs make the task easier as fewer are required, though they are ridiculously bright.

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