Updated: 20-Jan-03
Doede's Audio Tweaks...

Tweak of the week:
Changing the gridresistor in the Penthode of the MU-Stage

On the Ultranalog Site from Remco, he describes the effect the gridresistor of the Penthode has in a MU-Stage. I will not go in  to technical details, as you can read this on his site.... Any way, it had something to do, with HF junk being "mixed" in the Penthode and resulting in IM distortion. Mhhhh, this sounds a bit like the problems which were solved by the Mains Cable Tweak. It seems that the less HF is around in the circuit, the cleaner the sound.... OK, make sense, so how to reduce this effect than ?? The higher impedance the grid from the Penthode sees, the worse the effect. But if I reduce the Rg, the Triode will be loaded too much, I hear you ask?? I made the same thinking mistake as most other people I guess. Is that the reason we see at that specific spot ALWAYS "normal" values, like 470k - 1Meg ??? I think so, because that is what we normally take as a gridresistor, don't we? But just think about it, this resistor is as MUCH BOOT-TRAPPED as the the resistor which loads the Triode; the basic working of the MU stage, remember??? So the value the Triode sees is easily 100-200 times higher. This would result in a virtual gridresistor  of 100's of   MegaOhms. That looks indeed a bit overdone  to me and also very high impedance to close the grid circuit....  So I thought, lets do this with a 10k resistor, which looks approximately as a 1Meg one and use again my famous A-B switch   comparison.........................


mutweak.gif (9553 bytes)


How does it SOUND ????
With the A-B switch is was very easy to get the result. There is no need for burning in or so. Again a major step forward ??? To be honest?? No ! This is not a major thing..... NEVERTHELESS, if you go to the 10k position, suddenly there is more "rest" in the whole sound stage..... Actually the same effect as I moved to the special mains ring and power cables (see tweak of the week nr 6). I even would state, that the effect was less clear as the latter. Let's say, a 1 point tweak....

So, am I disappointed it was just 1 little point?? Of course not !!  If the amp sound slightly better, because of the use of a 10k resistor, in stead of a 470k (or whatever) resistor, the choice is VERY, VERY simple...... at  NO cost the amp is better. From now on, I will only use these configuration in MU- stages, you bet !!

Is the story ready now?? Actually not, I decided to make it statistically more relevant and do the same test with another Penthode (A-B Switching...) I happened to just have received two very nice SQ goldpin versions of the 7788 (very cheap from EBAY :-)..... One of the most famous Penthodes for use in a MU-Stage. Result? Switching from A-B and yes, exactly the same result, so it is very likely, that this tweak indeed has some positive effect on the "HF-mixing" thing. 

Ready now?? NOT at ALL !! What I "forgot" to mention, is that this 7788 sounded better than the D3A !! This was the first time, I used this SQ  goldpin  high grade version and I was impressed by its powerful dynamics !! So I gained at a totally different sound aspect (Dynamics...) extra points !!

No wonder, the 7788, as per today is still playing in my Golidav's ........  with 10k gridresistors.........


Happy Listening and Building !!!!

Doede Douma


IMPORTANT: The information provided on this page is intended as guide for DIY activities and therefore free to copy and or publish. If any one wishes to use any of the information from my WEB site, please make sure to refer and footnote to my URL Link as source! Doede Douma