I’ve received several emails from people asking me why I’ve gone quite lately. Nothing to be alarmed, I’m just simply buried on the daily grind like many out there are on these Pandemic times. My work-life balance has changed dramatically. This left me with very little time on my hands to work on audio stuff. I won’t complain as many others are actually struggling to survive, that is tough.
As my system is playing amazingly well and I’m so satisfied with my 300B-Liionidas OB setup and the low-gain 01a preamp, I do spend every single time at nighttime to enjoy music. That is what is built for in the end, isn’t it’?
Having some time to update on a long-time design, I received last week the remaining iron needed for my 845/813 SE amp. Yes, the monster one. If you see the output transformers that Alexander built for me, you will say the same. They are 40cm tall and 10.5 Kg weight each! Massive, but needed for a good performance. I originally intended to run these at low-ish operating point, like my friend DHTRob does. With a Zaa of 5KΩ you can do 10-11W with 700V at the anode. Otherwise you can increase the anode voltage to the crazy 1kV or beyond to get much more power, if you really need that.
In my case, I want to aim for a sensible HT and enough power to drive well my OBs. You may have other needs and there are so many ways to skin a cat so I’m not going to open up the debate which is pointless.
If you’re interested, here are the specifications of the OPTs:
Impedance ratio: 4.97k/4Ω 4.42k/8Ω 4.97k/16Ω Primary inductance: 75H Nominal primary Idc: 120mA Max power output at 25Hz*: 35W Primary coil Rdc: 130Ω Secondary coil Rdc 16Ω: 0.48Ω Secondary coil Rdc 8Ω: 0.27Ω Secondary coil Rdc 4Ω: 0.12Ω DC flux density at 120mA Idc: 1.12T AC flux density at 420Vrms**: 0.47T *Theoretical power output without the losses included **This primary AC voltage is equal to the maximum theoretical power output Shunt capacitance: 1.55nF Leakage inductance: 13.2mH Cp/Ls resonant frequency: 100kHz Cs/Ls resonant frequency: 54kHz Frequency response (1700R Zout ; -3dB): 4Ω 3Hz-54kHz 9Ω 3Hz-52kHz 16Ω 3Hz-55kHz
Some driver experiments
I’m looking to build this amp around the 801a valve. The topology is very similar to what I’ve been using lately. A SUT 1:8 with the LL7903 and the custom IT 1:1 directly into the 845 or 813 grid in fixed bias. Simple and yet elegant:
I get 34.5dB gain here and with a very low distortion. See the 250Vpp swing at 0.1%. That’s brilliant with nice harmonic decay:
The alternative option I will test as well, is of course the board I have to complete to replace my 47 driver in the 300B. It’s the hybrid mu-follower version:
Here is with the VT-25, but can be tweaked to run the 801a in the same way. Only challenge is wiring carefully the regulator to avoid hum being picked up as the Rod regulator is high impedance and we have a 1K (un-bypassed) cathode resistor. If you fail to get this working well you have 2 choices, you either add a bypass cap or you use a different regulator (e.g. voltage regulator) which has low output impedance.
Meanwhile, I’m waiting for the superb sockets from Jakeband Audio.
Hope you are all keeping well and safe…
Ale