I should have been following this post better, after all I started it.
I've been doing my builds with mostly carbon film resistors. I understand that CC's have more shot noise than the other types CF and MF, but I didn't realize that this shot noise could actually add more even order harmonics. I'm going to try using CC's for the plate resistors of the PI tubes in my next build and see if I notice a difference. And BTW, no more balanced PI tubes for me!
Or maybe take Alexo's suggestion, and only have one of the resistors carbon comp, and keep the other one carbon film. That should help maximise the sweetening effect getting through.
BTW it's not the shot noise that's increasing the even harmonics. That shot noise is nothing but a nuisance. Rather, it's the asymmetric non-linear behaviour of CCs under high voltage conditions which provides the tone benefit.
You want to keep the carbon films, metal films, whatever everywhere with one exception for using carbon comps: In an 18 watt, but the carbon comps in the PI plate load positions. (generally, in any amp, you want the CC to be in the driver's plate - only). There was an article somewhere that talked about they where and why - maybe someone has a link - but that was the long and short of it.
Also, I find these balance/unbalance conversations to get real confused because often people are not really talking about the same thing.
There are two kinds of balance in the PI: In the plate loads, and in the dual triode itself.
Sometimes people are actually asking about whether to $pring for a balanced 12AX7 for use as the PI (I thought that was what the OP was asking here at first, but then...) The general consensus seems to be don't waste your money because even if that tube is balanced, there are so many other things that need to be balanced to get the benefit that you'll just be bopping ferrets all day, as Gil says.
The other kind of balance, plate load balance, is further complicated by the fact that it is actually opposite initial assumptions. Lots of people confuse the idea of having 100K/100K plate loads in the PI with having the PI balanced. IN FACT, for technical reasons beyond immediate import, this is the imbalanced PI load. The balanced PI load will be someting like 82-92K/100K (YMMV).
I know this confused me at first until I figured out the decoder key. (and if I'm still wrong, someone correct me)
So, if you want an imbalanced PI for what some prefer as the best sounding amp, then use the 100K/100K CC's in the PI plate for an imbalanced PI. If you want as much balance as possible, you can either guess at 82K/100K, which is likely somewhat balanced, or tweek around and find the exact combination that suits you.
That's what I understand and I have tried a few combinations of balance, imbalanced, CF, and CC and found that I much prefer 100K/100K CC in that position. I use CF resistors everywhere else (excepting the power supply, obviously).
I tried various resistors and found that it's not a matter of balance being better than imbalance or visa versa, it's a matter of whether it works well for you and the amp. To my ear, more imbalance and the tone gets softer/squishier. So in an amp thats rather hard or harsh sounding, imbalance seems like it would be a great idea, tho adjusting the plate R seems a easier way then trying a ton of tubes. A amp thats already smooth may not benefit from imbalance, it may actually make it sound/feel worse because too much squish/smoothness can lead to a inarticulate tone. I was using several things in the amp to get a really smooth squishy sound, and providing imbalance was just too much after a while. So i think it's just a matter of whether the amps tone needs it or not. Like treble for example....you wouldn't want to add treble to an amp thats already bright or visa versa.
Seems to me you aren't going to whip out your resistor collection every time you change a PI tube? so TRYING to balance the PI is an excersise in futility.
Stew
stoo wrote:Seems to me you aren't going to whip out your resistor collection every time you change a PI tube? so TRYING to balance the PI is an excersise in futility.
Stew
On the other hand you could use a pot as outlined here by BooBird and just adjust it by ear.
I guess that depends on the person. For me it's not an issue because preamp tubes last me years. I have some Chinese from the 80's i still use and sound like new. So i can't envision needing to do it once every 8 or 10 years. But even then, when i imbalanced my PI, the imbalance was such that it was more than any tube makes. When i first built my current project i tried a number of tubes in the PI and none made a difference like imbalancing the PI via the resistors. In fact, never in any amp have i noticed much of a change in the tone when switching PI tubes unless i was going to a different brand that just inherently sounds different. Adding even 4.7k in series to the 100k resistor made a difference no swapping would ever do. at least thats what i find. So once you change the balance via the resistors the tonal difference will probably be there no matter what tube you put in it.
I just read the artical on the Aikens sight re; the LTP PI. He says ..if you're using the second i/p on the PI as a mixer for the other channel then the plate resistors should be the same so as to not compound the issue for the other channel. Also that a high value tail resistor mitigates the inbalance to an extent and I'd say 56K is fairely high.
Stew
merlinb, who has the valve wizard web site, did some tests with PIs and found that after about 15k there was little improvement in balance as you increased the tail resistor value.
stoo wrote:Seems to me you aren't going to whip out your resistor collection every time you change a PI tube? so TRYING to balance the PI is an excerise in futility.
Well provided you stay with the same generic type of tube - eg 12AX7 - then the PI will stay approximately (un)balanced to the same extent. Although no two 12AX7s (or 12AT7, 6SL7s, etc) are the same, there isn't enough variation between them to make a significant difference to PI balance in a guitar amp. In a finely knife-edge balanced hi-fi amp it may be different,
I have a couple of questions about this, if anyone can help. The trimpot idea sounds great, but I'm having trouble visualizing how it lays out. I was thinking the logical place for the pot would be across the HT ends of the two load resistors, with the wiper connected to Vc, but that doesn't really fit with the description in this thread. Any chance of a partial schematic?
Also, is 0.5W OK for the carbon comp resistors? By my inexpert estimate, they could be dissipating around 0.45W when one triode is off - does that sound about right?
I've never tried the trimpot thing myself. 0.45W sounds rather high to me. As I recall, you usually you get less than 0.25W in that position. Don't forget that there's a significant voltage drop across the PI's tail resistor.
Resistor failures are a matter of taste - when I was a lad, metal film were desirable because they were the most exact value types, combined with low noise.
As time went by, people avoided the MF for a while, because they "went high" if they failed, and usually took out some associated transistors in the process.
Parts you buy these days are a long way forward - they are all "low noise" compared to the old days, the precision is amazingly high, you need to look at long term life to decide.
The objective here is to build amps based on old designs. Remember, when those designs were conceived, the valves and inductances were the precision parts. Resistors came +/- 20% as standard, and capacitances were as bad as 50% off the nominal value - and people succeeded in producing working designs.
That's why some old amps sound better or different to others with the same designations - you get variations.
90% of the specified work is probably OK for any resistor - but don't ever try to exceed the values quoted.
Sometimes capacitances will work at higher voltages than specified, especially PIO and silver/mica types - but don't try pushing it with other components.
