phase inverter/bias question

[jmv]

A thought just struck me.... for my birthday, i was gonna ask for a new tube for the phase inverter position for my amp. Right now, its a 12ax7, but i wanted to get a NOS 12AT7 (which is the type specified on the tube chart, NOT a 12AX7, like i have in there now....). But i was thinking, the phase inverter is the 'driver' for the powertubes, right?? Well, if i change it to a lowergain tube, would that mean i'd need to rebias the powertubes in the amp?? Because my amp has an odd biasing setup which i don't think i could do myself... you have to bias one tube by changing out resistors, and then match the other tube to that by using a bias pot.

 

[Pandemonium]

Re: phase inverter/bias question

No affect on bias. I'd get a balanced tube for the phase inverter.

 

[Curly]

Re: phase inverter/bias question

jmv,
I'd recommend getting the "bias balance" changed to a regular bias, if I understand your explanation correctly. It's a fairly straightforward mod that a good tech will have no problem with. Again, maybe ask Jon from Austone at tonecraft , or email him - muse@texas.net

til then, you could just get the AT7 and give it a try and see how you like it, but I have a feeling you may not care for a lower gain tube than that. As far as the issue of "balanced phase inverters", I personally don't think it's an issue with the NOS tubes, but might be worth the couple extra bucks on a newer production tube.

 

[jmv]

Re: phase inverter/bias question

The bias mod is tempting... but at the same time, i'm really not sure about any mods, because the thing is completely original (which, judging from ebay, is more and more rare). I'll have to think on that.

And you're right curly, i might not like a lowergain tube in there... but i'd like to see what it sounds like, and it'd also be nice if i'm playing a larger or outdoor venue to be able to swap that and get some more clean headroom.

And the tube is only $22, and its only $5 extra for the 'balanced' thing, so i figure why not.

But thanks for the info, guys!!

 

[Pandemonium]

Re: phase inverter/bias question

I agree with Curly about balance sides not being an issue with NOS tubes, for most of my life I had never even heard the term and my amps always sounded great. I only use NOS tubes and I never bother with that detail myself, but according to people I trust, it's worth the extra money with the new tubes to specify a balanced tube for the PI, because the quality ain't what it used to be.

 

[Scott_F]

Re: phase inverter/bias question

First of all, the first time some tech changed resistor values, it no longer qualifies as "original".

Now, to the heart of the matter, think about what the P.I. does. In a "push-pull" amp, the phase inverter provides the exact 180 degree opposite of the original signal to one of the two power tubes. The other power tube gets the "proper" signal. I'm no expert, but having a matched PI tube provides both power tubes the proper signal.

I'll have to go back and read my Gerald Weber book for the technical explanation, but not all NOS tubes are going to be matched. That's ridiculous to think just because they were produced way back when, they'll be matched. Their quality control (because of military spec tubes, when themilitary was still using tubes) was definitely better then.

Think of a preamp tube as having two separate tubes in one housing. You have two halves of a 12A...7. One halfdriving one power tube, the other half driving the other. If one side is stronger than the other, it makes sense that you are not going to get the ultimate in sound. Every other "pulse" is going to be a bit stronger than the other. That imbalance will not sound so great.

Think of the sine wave. As one power tube is peaking, the other one is at it's lowest. You want each side's peak to be the same and both side's valleys to be equal.

I'll type up a better explanation later.

 

[Curly]

Re: phase inverter/bias question

The bias mod is tempting... but at the same time, i'm really not sure about any mods, because the thing is completely original (which, judging from ebay, is more and more rare). I'll have to think on that.

And you're right curly, i might not like a lowergain tube in there... but i'd like to see what it sounds like, and it'd also be nice if i'm playing a larger or outdoor venue to be able to swap that and get some more clean headroom.

And the tube is only $22, and its only $5 extra for the 'balanced' thing, so i figure why not.

But thanks for the info, guys!!

jmv,
the bias mod is basically just moving a couple of existing wires, and it improves a not-so-great idea, if what you have is really "bias balance" rather than "bias adjust". that's why I said to ask a tech about it - they know more than me, but my BF was pretty much dead stock, and I had no qualms about doing it.

and let me be clear, I think you'll like an AT7, but not something lower gain than that.

if you're talking about a new production tube, $22 sounds high to me, and $27 is just too much for a newer one, especially when Bob at Eurotubes sells matched AT7's (his EC 81's) for about $10.

as far as the balanced phase inverter, you guys can read the info over on guitar amp blueprinting if you want, but I'm pretty sure even Myles says it's to help rid an amp of certain problems, and he also talks quite a bit about the fact that modern tubes vary wildly in gain compared to good NOS tubes.

 

[jmv]

Re: phase inverter/bias question

curly, let me clarify, that price of $27 is for a NOS mullard 12AT7 thats been balanced. From all i've heard, its supposed to be a real super tube, and that doesn't seem like a bad price for NOS, just from what i've seen.

I'm still not sure about the bias mod... like i said, i'll have to think on that one.