New PI tube - kick me! should have done this long ago

[bluesfendermanblues]

My #124 50 clone, improved sound wise after changing V1 and V2 to JJ ECC83s, but I have struggled with a much too bright and thin sound overall.

V3 (PI) was a Sovtek 12ax7wa, which I thought was 'ok' and not that significant in this amp. I was wrong. Changed it today and the sound is now much rounder and nicer. No cap on master.

Setup is Strat 57RI => #124 => EMV12L in thielecab.

Just wanted to share.

Moral: Get those tubes right, before modding too much. I guess most of you knew this already

[Structo]

Sweet!

Glad you got good results from that.

I agree, the PI tube often overlooked as a tone modifier.

I have another amp ( 6V6 cathode biased) where I couldn't believe the difference between PI tubes, it was dramatic!

[doveman]

I changed my PI tube in my old Boogie MK1 ... every time I changed it back. That PI was (still is) the original from '78 and never could be improved at least so far.

[David Root]

I have seen it said that long plates in the PI work best for Dumbles, but I can't remember why. Most important is good balance, both AC and DC.

[ChrisM]

Sovtek LPS for me! The WA does not work well in the PI. You will see this echoed across the board on other topics.

[bluesfendermanblues]

Sovtek LPS for me! The WA does not work well in the PI. You will see this echoed across the board on other topics.

I a have a couple of LPS' lying around. I'll compare them to JJ's in my next build. In the #124 the jj is a keeper for PI.

[johan]

Can I change the PI without rebiasing?

Are both V1 and V2 as important? Will a Dumble amp benefit from NOS tubes just like a BF Fender.

[bluesfendermanblues]

Can I change the PI without rebiasing?

Yes, but you probably have to adjust the PI trim setting

Are both V1 and V2 as important?

IMHO they are

Will a Dumble amp benefit from NOS tubes just like a BF Fender.

Its a matter of taste. The JJ seem to be the most popular for V1 and V2,
but some people on the forum report that NOS (like 803) are the way to go.

No matter what you use, a D amp is very sensitive to the sound of V1 and V2 and different tubes will result in different voltages due to variations in current consumption.

[talbany]

No matter what you use, a D amp is very sensitive to the sound of V1 and V2 and different tubes will result in different voltages due to variations in current consumption.

+1....When your dealing with any tube that is cathode biased( where a resistor is placed in the cathode). Current draw through this resistor makes the grid potential negative with respect to the cathode. And, changes in tube conduction are automatically compensated by changes in bias ..Since each tube draws different amounts of current they each bias up slightly differently (Just like power tubes)...Tone changes... unless you replace that tube with the same type from the same manufacturer and both tubes draw the exact amount oF current they will sound different and put you in a slightly different part of the curve also effecting headroom and quality of breakup...The variances can go from virtually inaudible to very noticeable.. These amps seem to be more sensitive to tube types than most...... This isn't to say that each tube type (803/LPS/83/AX/WA/etc) doesn't carry with it it's own personality just like power tubes just be aware just because a particular tube doesn't sound so good in V2 of your Dumble does'nt mean it won't sound good in v1 of your Fender with more voltage applied..I don't mean to lecture and for the old timers here is common Knowledge..Just elaborating on Blues point..

Tony