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My 6922 mu-follower didn’t pan out. It’s power supply has far too much ripple, and rather than build a new PS, I decided to add the cathode follower to my 4S. It went pretty well, but it was annoying trying to fit everything into the limited space I have available. I have a couple of questions, and some observations...
With respect to my voltages while using a 12AU7 in both positions, my B+ before the 100K plate load resistor is 266 Volts, and there is 68.5 Volts on the plate after the 100K plate load resistor. The plate of the 12AU7 used for a cathode follower is at 266 Volts as well. The schematic shows 300 Volts B+. Is this okay? If I was intending on using only 12AU7s in this preamp, would there be a plate load resistor and cathode resistor that would work better with the lower B+?
In my system, with the Amp-1 as well as with my Decware SE84CS, I get a hiss through my speakers, which are La Scalas. It is easily audible from my listening position when using 12AU7s, but when I put a 12BH7 or a 12AX7 in the first position, the hiss is much louder, and it rises with volume until it gets close to full volume, at which point it drops off substantially, but is still louder than the 12AU7 at any volume position. There is always a hiss, even with the volume turned all the way down, regardless of tube, and it is in both channels. I added the cathode follower, and included the switch that would allow me to choose between a .22uF or 2.5uF output capacitor so that I could use the 4S with either tube or SS amps respectively, but omitted the optional cathode bypass capacitor on the first tube. I realize that my speakers have a very high sensitivity, but the 4S, before adding a cathode follower, was very quiet with the La Scalas, with only some tube rush sound at full volume. Are there any strategies or techniques that I can apply to eliminate or reduce this hiss? I followed the schematic in this thread exactly, but can make any changes suggested. Also, if the hiss is being created by the volume pot at the input, a 250K Alps Blue Velvet, I can put it on a switch to take it out of the circuit as needed if that will fix the problem. The hiss was present with both amps that I tried the modded 4S with, a Decware SE84CS and the Amp-1.
These are my listening impressions so far, and they are premature as it’s only been a short time, and the components added were all new and could probably use some running in.
With the Amp-1:
I was expecting the 4S with cathode follower to give me a bit of tube warmth with the Amp-1, and that I would prefer it with the 4S+CF. I was hoping to get a combo that sounded like a high power SET amp. In fact, I prefer the Amp-1 on its own. The combo dulled the exciting, crisp and crystalline sound of the Amp-1. This was a very surprising outcome for me. I will try the 4S+CF with my pair of Pass Amp Camp Amps and report back on that as well.
With the SE84CS:
As this is already a SET amp, I thought that the 4S+CF wouldn’t make much difference. It did however, and the difference is like a seasoning that I will want to use depending on the music that I am listening to. The SE84CS is very unforgiving of mediocre quality listening material, of which I have a lot. The 4S+CF and SE84CS together make everything more listenable, regardless of quality. There is little lost in the way of resolution although the sound is warmer and slightly softer overall with the combo. I don’t recall this being as much the case with the combo of the original 4S and SE84CS, although this quality was also there a little prior to adding the CF.
With respect to the hiss, I know that my speaker’s high sensitivity is part of the problem, but this sensitivity is required for the SET amps that I build and use regularly, and it hasn’t been an issue with any of these amps or the solid state amps I normally use. Any advice on eliminating the hiss would be much appreciated.
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