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RE: TU-882R Mods - Details Part 2

Posted by Mike C on March 22, 2016 at 17:09:39:
Having changed the op point of the first valve, some other changes were appropriate as follows.
The circuit uses some global negative feedback via R9 and R7. This will increase the subjective 'drive' of the 1st stage and also tighten the bass; however, global NFB is widely disliked as having an adverse effect on some aspects of sound quality e.g. tonality. I avoid it, but I'm not totally against it, I accept there are cases where it works. Now I have increased the 'drive' capability of stage 1 so less NFB should be needed, but I was reluctant to remove it totally. I have kept R9 the same but reduced R7 to 30 ohms to reduce the amount of feedback. My intention was to try removing R9 to see the result, but results to date are so good that I have left it as it is. I may try this some time but I feel no need to mess with it. So this mod is recommended along with the higher current in the 1st valve.
I looked into C7, 220pF to ground giving some rolloff of high frequencies. I am not sure why it is used but speculate as follows.
Perhaps some noise was generated in the 1st stage and C7 was put in to remove some of this? But I have increased the current in stage 1 which should reduce any such noise, in which case less of this filtering should be needed.
Perhaps C7 is there to remove RFI? However I added a grid stopper to do this, so again a lower value cap should be OK.
Perhaps C7 is there for general filtering, allowing say a less perfect CD player to sound good by removing any 'edge' from hf distortion? Well I have good quality CD players (several ... too many) so this tailoring is not needed.
As a result I reduced C7 to 150pF, raising the hf cutoff by about 20kHz IIRC. Again I intended to remove or adjust this cap but I feel no need to do so; the high treble is all there, bright shimmering but without harshness, so it's pretty good as it is (i.e. excellent!).
R13, the grid resistor of stage 2; 1 mohm as standard. I generally use 500kohm as max, and I've seen this value quoted for the 5670 and considered going for this. But stage 1 is still marginal re current and drive, and a high value of R13 will avoid loading it down. So I compromised and went for 750kohm.
Grid stoppers:
Stage 1 has R3 as a grid stopper but I considered it could be improved (again, in conjuction with higher current in stage 1 making it less critical). The value 3.3kohm is high for this function, 100 ohms is more normal. In case the cct needs more I chose 360 ohms. I use carbon composition as that's normal here.
The fitting was improved to put the resistor body close to the valve base pin. The pcb track from R3 output to the valve base was cut near the valve base (chisel or craft knife does it) and the resistor was soldered to the R3 input position and to the valve base. I'll take photos of my next build.
A 100 ohm grid stopper was added to stage 2, again carbon composition. You have to identify the pcb track, from R13 non earthy side to valve base pin 7; cut the pcb near the valve base, and solder the resistor from that end of R13 to the valve base.
The grid stoppers were a bit tricky, it's not nice messing with PCBs but the mod is recommended as removing rfi gives a bit of purity to the sound; not major, but worth having!
Resistors: all the important resistors (i.e. in the circuit path) were changed to Takman carbon which I find give good results at a very reasonable cost.
Mains lead: it is not easy to add an iec socket to allow different mains leads to be tried. I used high quality Belden mains cable (see Cable Asylum), easily available on ebay; soldered in position much as standard. OK, it's a bit tricky; the hole in the chassis needs enlarging and protection MUST be added to prevent the cable being chafed through, but it's possible and easier than adding an iec. Next time I'll use something even better such as Furutech FP-S022N, highly recommended.
I think that's it.
I believe these mods have had a significant effect, adding some purity and tonality; increasing transparency and detail; increasing transients, speed of leading edges and dynamics. OK, as I didn't build it as standard I can't swear to all this, but my experience with valve gear allows me to have much confidence in what I say and I try to give reasons why and how the mods work. I hope people will trust me and try these changes. Anyone in the UK can come and listen for themselves (I'm Essex).
Further thoughts:
I'd like to try a separate power supply for stage 1 for several reasons. Mainly, a higher voltage will enable the stage 1 valve to run at much higher current, say 3 to 5 mA. Also, having separate PSUs for each stage has significant benefits; and I'd try a low dcr unreg PSU. But I feel no real need for such a radical change; it's really making very good music indeed.