[fuzzroffe]

I built this some 15 years ago and just recently got it out again. I've cleaned up some wiring and changed out the tubes and it plays great, got rid of the hum and noise despite it still looking like a rat's nest underneath It's designed for 600Ω and I've been using 250Ω DT531s with it, but I've got my hands on a couple of other phones with lower impedance so I've built an impedance matching box with transformers (testing it in front in the picture).

Right now the tubes in the signal path are NOS RCA, I've installed 6BX7s as the upper tubes since one of my 6BL7s died. Has anyone else built this? Since the article stated the resistors etc. are "ballpark" values I thought there might be some tweaking to be done.

 

[TronII]

I built this some 15 years ago and just recently got it out again. I've cleaned up some wiring and changed out the tubes and it plays great, got rid of the hum and noise despite it still looking like a rat's nest underneath It's designed for 600Ω and I've been using 250Ω DT531s with it, but I've got my hands on a couple of other phones with lower impedance so I've built an impedance matching box with transformers (testing it in front in the picture).


Right now the tubes in the signal path are NOS RCA, I've installed 6BX7s as the upper tubes since one of my 6BL7s died. Has anyone else built this? Since the article stated the resistors etc. are "ballpark" values I thought there might be some tweaking to be done.

Would it be possible to make a solid state version?

 

[fuzzroffe]

I’m sure you could build a solid state mu follower, but solid state devices have enough drive and current capabilities into low impedances so it wouldn’t really be necessary.

 

[tomb]

I never built one, but you are doing the right thing with the impedance matching transformers, or you would possibly suffer a bass loss at 250 ohm loads and below (getting worse with lower impedances).

You might try looking into solid-state CCS supplies for the tubes for a fairly noticeable improvement. Barbour already resorted to a string of Zener diodes in the power supply, so the "purity" of the tube circuit was lost from the start. Again, I don't know enough to suggest suitable current settings, but some googling might turn up some interesting possibilities.

 

[fuzzroffe]

I read up a little on mu followers on the Valvewizard site, just followed the text, and plotted some lines for the 6SN7 and 6BX7. The component values in the original schematic aren't far off, but I think I made some small optimizations. I changed a few resistors and got some more current through the amp which should help with driving headphones. The rise in current lowered the cathode voltage on the BX so I was able to lower the heater elevation from 105V (100V is max for the BX...) down to 88V.
I also added an RC stage to the power supply to drop a few volts, it was 340V, a bit on the high side, and I got it down to 320V now.
I put some extra capacitance on the output so the bass rolloff goes way lower when driving lower impedances. With 32 ohm phones on the transformers, the amp will see around 280 ohms, so a pair of electrolytics paralleled with the poly output caps gives a 5Hz rolloff.

It sounds really nice now, and has less noise than before. There's only a couple of other easy mods I'm going to try, LED bias on the 6SN7s and a bigger shunt resistor on the output so I don't waste as much power.

 

[nikongod]

It looks like you used the tube-voltage reference and skipped the zener chain.

I would still go for the LED bias on the 12SN7.

Floating the heaters 100V above ground is not a problem for the 6BX7, as the cathodes are quite high above ground, but it may stress the 12SN7 gain tubes. Good move dropping it a few volts.

The "bleeder" resistors from the output-side of the output caps to ground don't eat any power. Do not use higher impedance resistors there, especically if you have electrolytic output caps. Using higher impedance resistors only lets more voltage stay on the output side of the cap, which leads to bigger pops when you plug your headphones in. There may be very little leakage with film caps, but its good to note for any other amps.

Nice work for the little transformer box. They are super cool on amps like this.

 

[fuzzroffe]

Yeah, I’m using the 0D3s for voltage reference. I tried with zeners at first but I didn’t notice any difference in noise. Besides, the gas tubes look a lot cooler I’m also using 6SN7s instead of 12SN7s and they have a separate heater supply so there no elevation on them. I found out that the BL7 I thought had died is fine, it was just a dodgy soldering on the heater wires, so I'll just keep the BL7s as spares.

I'll just keep the resistors on the output, and look into LEDs on the SN7s. I have another project going right now (SE KT88), so it'll have to wait a bit

 

[TubeBoy]

I just completed a build of this amp, no mods just vanilla. I used AC for the filaments all at 6.3V, and zeners for the voltage reference. It sounds incredible with Sennheiser 600 HD headphones.
I'll post a picture shortly. I am very pleased with this circuit.
MK II may have DC for the filaments. With nothing connected to the input and the volume at close to max I can hear a little hum, but at normal volume settings I hear nothing.
I do have a small problem which I'll post about here if there is still interest in this topic.

 

[fuzzroffe]

Cool to see someone else trying this circuit. Although I was happy with the sound, my headphones are all quite a bit lower impedance than 600 ohms and my current favorite is Grados at 32 ohm, so I rebuilt mine into an Aikido frontend with a White cathode follower output, all point-to-point wired. I’m using the same 6SN7/6BX7 tubes and kept the tube regulated power supply. This circuit has a lot more current capability and can drive 32 ohms with no problem, but the Brute Force circuit should be excellent with 600HDs (and it’s also a great line stage!)

I used AC for the heaters too and had no significant hum issues.