Balanced phase inverter tube?

[Rex_Rocker]

Re: Balanced phase inverter tube?

Yeah, I'm confused as well. The LPS is hugely popular and often recommended as a PI by a lot of reputable sellers. I knew about most Russian tubes not being able to handle cathode follower positions because their filaments are encased within the plate structure (I think that's why?), but had never heard about Russian tubes having problems in the PI position. I've run the Sovtek 12AX7LPS for ages in my Krank Rev Jr. and Peavey XXX with no problems so far.

Not saying you guys that are saying the LPS is bad on the PI position are wrong, I'm just confused.

 

[Gone_Shootin]

Re: Balanced phase inverter tube?

Yeah, I'm confused as well. The LPS is hugely popular and often recommended as a PI by a lot of reputable sellers. I knew about most Russian tubes not being able to handle cathode follower positions because their filaments are encased within the plate structure (I think that's why?), but had never heard about Russian tubes having problems in the PI position. I've run the Sovtek 12AX7LPS for ages in my Krank Rev Jr. and Peavey XXX with no problems so far.

Not saying you guys that are saying the LPS is bad on the PI position are wrong, I'm just confused.

I'm not saying that they're bad for that position, I'm just saying that I don't like them in that position. They're too clean and sterile sounding to my ears. I like JJs simply because I like the way they sound in that position. They have a little more gain, so I get a little more growl in my distortion. In other words, I like a dirty PI hole. :burnout:

 

[JeffB]

Re: Balanced phase inverter tube?

I'm glad none of this is complicated.

Don't let yourself get overwhelmed. Buy some quality tubes from a reputable dealer and use your ears. Learn the amp.

It's only complicated if you make it complicated.

As mentioned, the TungSol gets eaten up in the CF. Thats about the only "no no".

 

[Little Pigbacon]

Re: Balanced phase inverter tube?

Don't let yourself get overwhelmed. Buy some quality tubes from a reputable dealer and use your ears. Learn the amp.

It's only complicated if you make it complicated.

As mentioned, the TungSol gets eaten up in the CF. Thats about the only "no no".

It has TungSols now. No idea how old they are. I ordered from Tube Depot and got:

- a matched quartet of JJ EL34, 24-hour burn-in
- five 12AX7-EH
- a balanced 12AX7-EH
- a low-noise/low-microphonics 12AX7-EH

 

[JeffB]

Re: Balanced phase inverter tube?

And just go with what you bought. Get the amp biased properly and play the hell out of it.

After awhile experiment with the old tung sols if you want (and they are still good). Or just buy a variety of others.

Or not.

 

[Little Pigbacon]

Re: Balanced phase inverter tube?

Speaking of biasing, I've never biased an amp before, and there's a lot of information out there that seems contradictory. For example, Tom Mitchell says in his book that the tube manufacturer determines how much bias their tube needs, rather than the amp manufacturer determining how much bias the tubes in their amp need. But most of the other information seems to indicate there is one right place to set the bias for an amp, without regard to which tubes are in it. And of course everyone has a different number. And the tubes from Tube Depot have numbers on them, and I have only a hazy idea of what they mean. (Three are 34, and one is 33.)

I've been able to narrow it down to where I think I'm looking for numbers at the test points somewhere between 60 and 90 mv.

 

[JeffB]

Re: Balanced phase inverter tube?

I had never done so either. I found a great load of info on the Marshall forum (a 330+ page thread on the dsl40c alone) as well as a excellent how to video on youtube for my specific amp. The amp has easy to use bias test points (once the chassis is out).

Generally plate voltage is measured, a calculation is made, and the bias is set. Or a general guideline is provided by people who know the amp in question and go with that.

http://www.tedweber.com/webervst/tubes1/calcbias.htm

That said, I know NOTHING about the specifics of your amp, and it can kill you if you are not very careful inside the amp while it is on. So do the research or let a qualified tech take care of it.

My understanding of the tube ratings by tube depot and others is so that you can simply re-order the same rated tubes when it's time to experiment or replace. For example all the Tube Depot el34s (various brands) and Kt77s I bought with similar ratings so I could just swap and not have to rebias. However my amp seems to have a hefty drift every six months or so, so it's not quite that easy, but once biased properly I can back to back different sets to check tonality within a couple minutes (as long as amp is out of chassis).

 

[Little Pigbacon]

Re: Balanced phase inverter tube?

I'm hoping I can get enough information to avoid having to take measurements anywhere other than the millivolt-level test points.

 

[zzmoore]

Re: Balanced phase inverter tube?

Is this a modern day amp.?
Without measuring plate voltage, the mili "Volt" number is meaningless.
To make things "easier" for consumers.....modern day amps suggest a certain mV because they know (within reason) what plate voltage their amps run at.
That said. you have no idea what percentage of bias any given manufacture is recommending when they tell you to turn the knob until it reads XYZ.
There is biasing, and then there is biasing.
Sticking a meter in the ports on the back of an amp is designed to let you safely install new power tubes and keep on playing. It is a calculated convenience but is not exactly "setting the bias" of an amp.
good luck

 

[Little Pigbacon]

Re: Balanced phase inverter tube?

Maybe now I'm getting somewhere.

 

[GilmourD]

Re: Balanced phase inverter tube?

If you look at the PI section of almost any amp with a long-tail PI you'll notice that one half of the PI has an 82K plate resistor and the other half has a 100K plate resistor, and this includes your JVM. The circuit itself isn't balanced so a balanced tube won't add any benefit.

Later silverface Fenders, though, did switch to a matched 47K plate resistor setup on both halves, so a balanced tube might help there, but... Still... Not really.

 

[aleclee]

Re: Balanced phase inverter tube?

Phase inverter gets a lot of stress. I've read it's best ot replace it well b4 the rest of your tube compliment.

I'm hearing from a lot of people that it should be changed along with the power tubes.

That makes sense. From what I understand with modern amps with the tube sockets on the circuitboard, with modern production tubes, it's a best bet to replace them one a year , assuming you play the amp regularly. If one of them Power tubes gives up the ghost , you have a real mess on your hands with one of those amps.

I don't think it's that easy to wear out a 12AX7, regardless of position. If you think about it, the gain stages in any high gain amp are gonna get slammed harder than the PI 99% of the time. That might be the opposite of a Plexi-style amp where what is often referred to as "power tube distortion" is actually the PI clipping. In many of those designs, it's just about impossible to clip the power tubes without overdriving the PI.

In summation:

• If you're playing a modern (preamp-driven) amp, the PI isn't pushed hard because such amps aren't driving the power tubes that hard.
• If you're playing a plexi-style amp, the PI is more likely being pushed but probably still not as hard as in a modern high gain amp. Even if you're driving those amps as hard as AC/DC, the amp probably doesn't have tube sockets mounted on the circuit board (assuming it has a PCB at all).

 

[Gone_Shootin]

Re: Balanced phase inverter tube?

Speaking of biasing, I've never biased an amp before, and there's a lot of information out there that seems contradictory. For example, Tom Mitchell says in his book that the tube manufacturer determines how much bias their tube needs, rather than the amp manufacturer determining how much bias the tubes in their amp need. But most of the other information seems to indicate there is one right place to set the bias for an amp, without regard to which tubes are in it. And of course everyone has a different number. And the tubes from Tube Depot have numbers on them, and I have only a hazy idea of what they mean. (Three are 34, and one is 33.)

I've been able to narrow it down to where I think I'm looking for numbers at the test points somewhere between 60 and 90 mv.

Like I said the other day, go to JVMforum.com . There's a thread over there on this very subject. With pictures, settings, and everything.

http://jvmforum.com/phpBB3/viewtopic.php?t=237

 

[Little Pigbacon]

Re: Balanced phase inverter tube?

I'd already read the first thread and found it pretty helpful. I was looking for corroboration and a way to understand it in terms of the Tube Depot numbers. The secondary thread had potential but is missing the photos.

 

[GilmourD]

Re: Balanced phase inverter tube?

I don't think it's that easy to wear out a 12AX7, regardless of position. If you think about it, the gain stages in any high gain amp are gonna get slammed harder than the PI 99% of the time. That might be the opposite of a Plexi-style amp where what is often referred to as "power tube distortion" is actually the PI clipping. In many of those designs, it's just about impossible to clip the power tubes without overdriving the PI.

In summation:

• If you're playing a modern (preamp-driven) amp, the PI isn't pushed hard because such amps aren't driving the power tubes that hard.
• If you're playing a plexi-style amp, the PI is more likely being pushed but probably still not as hard as in a modern high gain amp. Even if you're driving those amps as hard as AC/DC, the amp probably doesn't have tube sockets mounted on the circuit board (assuming it has a PCB at all).

The only time you have to worry is when you have a tube, like the Tung-Sol 12AX7, which wasn't built to proper spec and can't take the voltage difference between the heater and cathode that a cathode follower circuit puts on the tube. Tung-Sols like to die there. They're OK in any other position but not the CF.

 

[Ascension]

Re: Balanced phase inverter tube?

I always replace the PI when replacing power tubes it's just cheap insurance. I also use a balanced PI because i understand the theory behind a push pull and want the efficiency. It's kind of like a preference with balanced or unbalanced pickup coils and depends on the amp design and power tube used how much difference it makes.

 

[Gone_Shootin]

Re: Balanced phase inverter tube?

I'd already read the first thread and found it pretty helpful. I was looking for corroboration and a way to understand it in terms of the Tube Depot numbers. The secondary thread had potential but is missing the photos.

Oh, ok, I misunderstood.

I guess I don't pay too much attention, as long as they're matched up well. And if I'm ordering power tubes for an amp with a non-adjustable fixed bias or a cathode bias, I call and tell them what they're going into and if I want tubes on the warmer side, cooler side, or somewhere in between. This is what I did when I bought those Svetlana EL-34s for that Mesa Recto I used to have, since it is a cathode biased amp.

So anyway, in the case of my JVM, Splawn, or Fender Supersonic (which all have an adjustable bias), I double check the plate voltages and scroll down to the corresponding chart on this page of the Ted Weber site to be certian where I want the bias (I really should print it out one of these days).

http://www.tedweber.com/webervst/tubes1/calcbias.htm

 

[Little Pigbacon]

Re: Balanced phase inverter tube?

So that's what the tube-socket bias probes are for? A relatively safe way to measure plate voltage? I see the difference it makes when you index it against bias current.

 

[Gone_Shootin]

Re: Balanced phase inverter tube?

So that's what the tube-socket bias probes are for? A relatively safe way to measure plate voltage? I see the difference it makes when you index it against bias current.

I don't know, I don't own any of those. I just test the pins on the bottom of the tube sockets when I have the chassis flipped upside down (the headshell doubles as a stand so you can have the chassis upside down with the tubes in).

 

[crusty philtrum]

Re: Balanced phase inverter tube?

Got a balanced output transformer ?

Probably not, in a guitar amp.

So all this is waffle, just try various PI valves in YOUR amp and go with what gives the sound you like. What works in one amp may be very different to what works in the next, even the same make and model.