The Mule saga continues and it was time to modify the 3B7 preamp and to test the 2P29L valve. This was quite easy as they both have loctal sockets. I had to modify the Rod Coleman filament regulator to set the filament current down to 120mA. Then a bit of wiring work, and in less than an hour a new preamp was ready. Job done, this is why I built the Mule:
The circuit is quite close to my original design time ago. I modified the filament resistor to use an existing Russian NOS wirewound part I had in stock and suit this preamp quite well. Added grid and screen stoppers as well:
The rest stays the same. The gyrator board is based around the BSH111BK and the top MOSFET is IXTP08N100D (with its own heatsink).
I have a series of daughter PCB boards to adapt the BSH111BK to the gyrator PCB. I will be offering these shortly as pre soldered boards with the gyrator.
The 2P29L is run hot at is nearly maximum Pa. Well, I’ve got plenty of these so I won’t worry about it. It sounds better at 20mA in my opinion. If you can lower the HT a bit may help with power dissipation of the top MOSFET. I’d go for 200V to provide enough headroom.
Frequency response is really good, mind you there is no 100K load (I forgot to add it on my test) as in normal operation. When loaded it will reduce the response. However, this is really good. Gain about 18dB is as expected and similar to other DHTs I use. Enough to feed the output stage (4P1L PSE).
Distortion is about 0.045% for 5.5Vrms and 0.12% for 11Vrms output.
How does it sound?
Well, another great DHT in my opinion. Compared to the recent version of the 3B7, the 3B7 lacks of some warmth and or strong body sound I think. Hard to explain, I liked it but not as much as the other DHTs I have been using. This one has a powerful bass and clear treble. No coloration, great sound overall.
I wasn’t wrong when I said this was a good DHT to explore. Microphonic noise is minimal, surprising. No need of special arrangements.