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RE: TU-882R Mods - Details Part 1

Posted by Mike C (A) on March 21, 2016 at 17:23:08

In Reply to: TU-882R Mods - Results, Details posted by Mike C on March 20, 2016 at 18:03:06:

Here are details of my mods.

First I must say I appreciate what a great job the designer has done to produce a very good sounding unit at such a reasonable price. OK, I've changed it and I know I've improved it but that does not imply any disrespect for the designer - quite the opposite!

I didn't build it as standard for some good reasons. It's not that easy to change components on a pcb (I know, I've done it several times) and there is the danger of damaging the pcb. Also I have confidence in my changes. While it would be nice to hear each change one by one, it would be a real pain to do this taking many months (allowing for running in every time). You'll have to trust me ... I know what I'm doing with valve circuits. I did intend to try varying two things but results are so good I have not yet tried this; when I get a unit this good I don't mess about with it.

The main mod is to change the operating point of the 1st stage valve. As standard, it runs at only 0.5mA which is low for any valve, but especially low for the 5670 which looks to run well at about 5mA. This low a current will struggle to drive the 2nd stage well; 'slew rate' will be limited. Transients, leading edges and dynamics may be compromised. Also, the low current will likely result in a degree of noise.
The voltage of about 180V does not allow much change to the op point, but I increased the current to 1mA by changing R5 to 1800 ohms and R11 to 100kohm (in fact I got 0.9mA but that's OK; the 1800 could be reduced by one value to get closer to 1mA).

The volume control pot was changed to Tocos 10kohm stereo (yes, 10k) in conjunction with a 47kohm series resistor. This is a good sounding pot, and the way I use it improves it further and reduces the gain usefully giving a reasonable volume control setting.
To do this: omit the pcb with volume control. The Tocos pot fits the front panel OK. Identify the earth and wiper terminals on the pot (the 3rd terminal is not used). Fix the 47k resistor to the wiper. Connect a wire from Vol-in G to both channel earth terminals on the pot, and from there to the Vol-out G.
Connect a wire from Vol-in L to the free end of the 47k R; connect a wire from the wiper to Vol-out L. Do similar for R channel.
The result is that the attached source will see a load of about 50kohm, and only the lower half of the pot track will be in circuit. I use this method a lot and with a good pot, I find it is about as good as a stepped attenuator; yes, that good (in fact I prefer it!). As it fixes the issue of the over sensitive volume control, this mod is most highly recommended.

Coupling caps: omit C1, replace it with a link; also remove R1 as it's not needed now.
C5 must be changed to something good. Victor's suggestion of a Teflon cap is good, but I have my favourites (after trying *many*, including Teflon); Mundorf silver/gold/oil, and Jupiter copper/beeswax. Both are superb, very transparent, fine tonality and natural sounding; I use Jupiter here, perhaps it's a touch more natural.

Rectifiers: standard, two ready made bridges are used. A useful improvement can be had by replacing these with schottky diodes. I have not found suitable ready made units so I made up a bridge, on the pcb, using in each case 4 individual diodes. For the heater (the big unit) I suggest 4 off, 2amp 40V schottky diodes (try ebay). For the main circuit (the small one), I use 4 off 1 amp 150V schottkys. In theory 100V ones will do but go a bit over for safety.
Look up on the web how to wire these up.
This mod will give a bit more 'purity' and tonality to the sound and for the small cost involved, it is definitely recommended.

More to follow .....

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