So...in my quest for a tube that "gives it up" in the PI slot, and partially so I'd be prepared for the Portland Dumblefest that never was, I decided to try to document things using a handful of PI tubes. I had installed an external 10-turn trimmer, so I could adjust from the outside.
With the amp open, I would plug in a tube, adjust with a signal generator for AC balance between the halves, and then write a couple of parameters on some heat shrink tubing that I'd then put on the tube. What I measured and wrote was
DC voltage difference between the feedback side and the non-feedback side of the PI
How many turns from full counterclockwise the trimmer had to be set
Make and type of tube
For example:
+8.5 VDC
12.5 turns
Mullard 12AT7
This is proving to be a reliable way to swap PI tubes and still be able to adjust the trimmer appropriately. What is interesting is the DC readings. Most of the readings I get are + a few volts to the feedback side of the tube. But I've seen readings of -2V there, as well as +25V. I'm wondering what, if anything, the DC voltage reading tells a person about the tube and whether it is appropriate/balanced for the PI? Any thoughts? I haven't noticed an audible change that I can attribute to the DC voltage...for example a Sovtek LPS that measures +8V sounds about the same as one that measures +1V. But I wonder about the couple of tubes I measured that seemed WAY out...+25V.
Out of all the tubes you have tried did any stick out, as sounding better than the others?
Was there much of an audible difference between the different types at all?
Which tubes have you tried so far? NOS? New? both?
Which sounds best to you so far?
Sorry for all the questions! I have set the PI on mine using a matched triode mullard by ear looking for what sounded the best. Before that it was unmatched JJEcc83 and the PI was just set at 12:00 on the 10k piher pot per recommendation. I do not hear a difference at all! Could be my ears though.
There is a thread from the past few months about adjusting the PI trimmer that you should read through if you have not. It goes into adjusting for AC balance, and why that is a good starting point.
The tubes sound very different. I've tried Sovtek LPS, RFT, JJ803, Mullard AT7 short plate, and a bunch of others. The ones that seem to stand out are the Mullard (less volume, but tons of harmonics!), the Sovtek, and the RFT. This position is less sensitive tone-wise than the other two, but it does make a difference. You want to try to find one that yields feedback easily, and in my limited experience, this is often a long plate...but not always.
And for some reason, on my amp the "sweet spot" seems to be right at AC balance. Others have said this is a starting point, and you want to adjust off to one side of that to really find it. I haven't found a spot I liked better on any of the tubes.
I think I went through the thread you mentioned right before I adjusted my PI a few weeks back. I ended up just tweaking by ear and with a mullard RI matched triode. While adjusting the PI the change in sound was so minimal to my ears it made me wonder if the tube type/brand is more important than the setting itself
I like that you put the PI pot on the chassis though thats the main thing that keeps me from messin with it all the time. It is a PITA to take out of cab and put back all the time.
Thanks for the tube info, I might have to try the Mullard you mentioned next
I test for both MU and TC matching. Measuring DC volts has little relevance unless you are using a perfectly matched and balanced tube.
It does matter. Maybe more evident on the nicer tubes. Mullard RI is a very congested and thuddy beast and nothing you do can make it as nice as an RCA Blackplate, Bugle Boy, or real Mullard, for example......
Hi,
I put on the PI a fine tune 10k Trimmer. So I found MY sweet point at 2,5k //7,5k on the trimmer and a difference from around 6VDC between the two sides. You can hear now every single touch on the strings and a high compression (I guess it is compression). Very nice tone evan from any guitar I put on my 124 ODS clone (low plate).
dogears wrote:I test for both MU and TC matching. Measuring DC volts has little relevance unless you are using a perfectly matched and balanced tube.
I have a couple that are supposedly matched and balanced...those are the ones that come in at +3-7V higher on the feedback side when AC balance is achieved.
But is that DC voltage tightly correlated with anything really? If I have a tube that is at +24V or -3V with AC balance, does that mean that the tube is imbalanced to the point where a static AC balance measure is really not worth anything?
greiswig wrote:I have a couple that are supposedly matched and balanced...those are the ones that come in at +3-7V higher on the feedback side when AC balance is achieved.
But is that DC voltage tightly correlated with anything really? If I have a tube that is at +24V or -3V with AC balance, does that mean that the tube is imbalanced to the point where a static AC balance measure is really not worth anything?
