The start of a different DHT experience with the Mule
I built the “Mule” to provide enough flexibility to test other DHTs as pre-amplifier / line stage. Using the gyrator board, the flexibility is fantastic. Can share same HT and dial the right anode voltage. The LT supply can also be shared amongst many DHTs and Rod Coleman provided me with a set of different resistors to test the list of 9 or 10 DHTs I have in mind which haven’t listed carefully on this design.
The C-299
First one of the list is the rare 99/199/299. This old DHT has thoriated filaments with low current and slightly less gain that the 01a. I push it hard to 4mA and beyond 90V. It can work well at 110-120V though. I want to minimise the slew-rate of the stage when driving the 4P1L PSE amplifier. 4mA is ok, if you’re worried you can bias it colder. I have a set of the C-299 as well as CX-299. Given I installed the UV-4 sockets, I added the C-299 with the socket adaptor.
They are slightly microphonic, but not much. I haven’t even starved the filaments. This hasn’t been a problem at all on my system.
The circuit is already popular, here we go:
The component selection is on the same lines as before. I have followed the advice from Barry French and trailing the Takman MF resistors on R5 and R7. The filament resistor is Kiwame. Russian caps, the rest is already known.
How does it perform?
It measures really well. Great frequency response from 4Hz to 250kHz without loading the preamp:
The distortion is really low (lower than 0.002%) but didn’t save the measurement plot, next time.
How does it sound?
The sound is very interesting. I can say after listening to a few records (including Giant Steps from Coltrane), the detail is very similar to the 01a. The bass is strong as it’s the detail of the treble. I like it, sadly has lower gain than the 01a.
Will listen to it for a while….
Hi Ale,
Good to see that you are trying different DHTs.
I’ve just finished a preamp with multiple options DHTs, it has separate power supply and few boxes that are for different anode current tubes.
I just finished yesterday the low current tube unit and tried the 30 and a CX199.
I can confirm that the sound experience is similar with yours. A very nice sounding tube. Actually I can say that I couldn’t find any DHT that I didn’t like.
Still 01A is my preffered one, followed by the 26 and 45.
Cheers,
Radu
Hi Ale,
if you want to minimise the slew-rate of the stage when driving the 4P1L PSE you can drag more current through the gyrator to ground with a ccs. So, say, 4mA thru the 299 and then another 16mA through the CCS to ground (in parallel to the 299) so total of 20mA through the gyrator.
best
tim
Hi Tim, yes you suggested it before. I didn’t find any improvement at least when simulating it (see 01a thread on Tony’s build). I have to listen to it thought
So far, I’m liking this c2999, the bass is strong, perhaps there is some microphonic feedback playing on, either way I love it!
Oops…..
I’m getting repetitive in my old age.
Apologies.
It probably has less effect when the bottom device is a Jfet but with mosfets the impedance and capacitance drop effectively.
tim
Hi Ale, off topic but could you please suggest a suitable cable and socket for the flexible HT supply. It looks like your using a 5 pin din, but I can’t find anything online rated for high voltage DC. Would ordinary 250vac flex be safe.
Hi Dave. For HT (up to 300v) I use the normal speakon. I use it as well for LT. for higher HV you can get a high-voltage Speakon connector. The 5 pin is a military connector