Question about Phase Inverter

Richman1 · 10-31-2009, 06:09 AM

What is the real deal about this 12AX7 in V4 being balanced. Eurotubes claims that it must be a special hand picked balanced tube for V4. Some people say thats bullcrap and you won't hear any difference if you stick any new 12AX7 in there????

PaoloJM · 10-31-2009, 10:39 AM

Quote:

Originally Posted by Richman1

What is the real deal about this 12AX7 in V4 being balanced. Eurotubes claims that it must be a special hand picked balanced tube for V4. Some people say thats bullcrap and you won't hear any difference if you stick any new 12AX7 in there????

A 12AX7, being a dual triode, has two gain stages enclosed. The gain is not exact on both and is within a tolerance. This means that the gain on one part might 94 and the other might be 104 for example and so the phase inverter will be unbalanced.

That, however, assumes that the circuit itself and the OT is in perfect balanced which it never is in any amplifier.
Therefore an unbalanced 12AX7 may actually balance the stage, and vise versa a balanced one might throw it off.

That brings us to what sounds good. 99% of your audience will not notice any tonal difference.
Most guitarists with well tuned ears will notice a difference but not be able to state whether the difference is actually "better" or not.
In general a perfectly balanced PI stage sounds a little more sterile and HiFi, whereas a bit of imbalance brings out some warmth and sweet harmonic content.

As mentioned earlier a balanced PI valve may bring the stage more in balance or make it even further out and so wont give consistant results in different amplifiers and therefore, IMO, is a waste of money, especially when you consider that your target definitley will not notice.

MajorNut1967 · 10-31-2009, 11:54 AM

Quote:

Originally Posted by PaoloJM

A 12AX7, being a dual triode, has two gain stages enclosed. The gain is not exact on both and is within a tolerance. This means that the gain on one part might 94 and the other might be 104 for example and so the phase inverter will be unbalanced.

That, however, assumes that the circuit itself and the OT is in perfect balanced which it never is in any amplifier.
Therefore an unbalanced 12AX7 may actually balance the stage, and vise versa a balanced one might throw it off.

That brings us to what sounds good. 99% of your audience will not notice any tonal difference.
Most guitarists with well tuned ears will notice a difference but not be able to state whether the difference is actually "better" or not.
In general a perfectly balanced PI stage sounds a little more sterile and HiFi, whereas a bit of imbalance brings out some warmth and sweet harmonic content.

As mentioned earlier a balanced PI valve may bring the stage more in balance or make it even further out and so wont give consistant results in different amplifiers and therefore, IMO, is a waste of money, especially when you consider that your target definitley will not notice.

Yes exactly +2!

Hey Paolo have got your site up yet and open your shop yet?

PaoloJM · 10-31-2009, 12:40 PM

Quote:

Originally Posted by MajorNut1967

Yes exactly +2!

Hey Paolo have got your site up yet and open your shop yet?

Hey, just spotted your 800 build, sweet.

My GFs been doing exams so I've pretty much been working (my day job) and minding my 2 yo for the last couple of weeks. Not done any real work except a few amp repair jobs for local guys. Did buy a MacBook Pro though so I'll starting into the wonderful world of iWeb soon...

MajorNut1967 · 10-31-2009, 03:50 PM

Quote:

Originally Posted by PaoloJM

Hey, just spotted your 800 build, sweet.

My GFs been doing exams so I've pretty much been working (my day job) and minding my 2 yo for the last couple of weeks. Not done any real work except a few amp repair jobs for local guys. Did buy a MacBook Pro though so I'll starting into the wonderful world of iWeb soon...

Choice Mate, all the best to you! Ya I got a 7 month old in my house now. Please do your best make a shop and a Web your approach and knowledge are priceless

MartyStrat54 · 10-31-2009, 05:10 PM

I used to be caught up in the balanced PI tube bit, until I continued my research on it. The bottom line is your Marshall amp is not "balanced." It has multiple components that are + or - 20 per cent being used in the circuits. The gain stages vary from each amp.

In our experiments, we have found that you can use a regular gain tube or a high gainer in the PI slot and it can be lop-sided or even and we couldn't really tell much difference. I run a super high gain in my PI, just as a way to use an NOS tube that I cannot use in the V1 or V2 positions. If you ran a scope on the amp, the wave form would be asymmetrical, but I doubt if your ears could tell it. Even if you put a balance PI in, the waveform of your power tubes would still be asymmetrical due to imbalances in the power circuits.

Now the technical paper I read on this matter goes on to say that some guitarists and many harmonica players will actually use a 12DW7 in the PI. This creates a heavy asymmetrical waveform, but it is rich in even order harmonics. BTW, a 12DW7 has a gain of 100 on triode A and a gain of 17 on triode B. So one side is like a 12AX7 and the other is like a 12AU7.

Save money. You don't need a perfectly balanced tube. Just get one that is close if you feel the need, but don't spend extra cash on "perfectly" balanced tubes that probably aren't perfectly balanced.

HOT TUBES 70 · 10-31-2009, 09:28 PM

Question : ? Is the PI tube always in the V4 location on all marshalls etc ?
And what is its job exactly ?

MartyStrat54 · 10-31-2009, 09:46 PM

Only if your Marshall has four tubes. On 800's with three preamp tubes, it's V3. Same with older Marshall's using three preamp tubes.

Of all the circuits in a tube amplifier, the Phase Inverter, also known as the Phase Splitter, is the most difficult to understand by even some experienced techs. Here's what it does. It takes a signal input, and create two outputs, one that is identical (e.g. in-phase) to the original, and another that is a mirror-image (phase-inverted or flipped phase). Each signal then feeds the power tubes that are connected to each side of the Output Transformer’s primary winding in the typical Push-Pull (Class AB) configuration. Single-ended power amps, like those found in the Fender Champ, which have only one power tube, do not require a phase inverter. They only need a driver tube before the power tube to boost the preamp signal to a level usable by the lone power tube.

10-31-2009, 06:09 AM	#1 (permalink)
Richman1 Member Join Date: Sep 2009 Posts: 62	Question about Phase Inverter What is the real deal about this 12AX7 in V4 being balanced. Eurotubes claims that it must be a special hand picked balanced tube for V4. Some people say thats bullcrap and you won't hear any difference if you stick any new 12AX7 in there???? __________________ DSL-50 DSL-50 1960AC 1960BV 1960A 1984 Jackson RR Custom 2006 Les Paul Classic

10-31-2009, 05:10 PM	#6 (permalink)
MartyStrat54 Senior Member Join Date: Feb 2009 Location: Formerly Phoenix, now north of KC Posts: 2,583	Re: Question about Phase Inverter I used to be caught up in the balanced PI tube bit, until I continued my research on it. The bottom line is your Marshall amp is not "balanced." It has multiple components that are + or - 20 per cent being used in the circuits. The gain stages vary from each amp. In our experiments, we have found that you can use a regular gain tube or a high gainer in the PI slot and it can be lop-sided or even and we couldn't really tell much difference. I run a super high gain in my PI, just as a way to use an NOS tube that I cannot use in the V1 or V2 positions. If you ran a scope on the amp, the wave form would be asymmetrical, but I doubt if your ears could tell it. Even if you put a balance PI in, the waveform of your power tubes would still be asymmetrical due to imbalances in the power circuits. Now the technical paper I read on this matter goes on to say that some guitarists and many harmonica players will actually use a 12DW7 in the PI. This creates a heavy asymmetrical waveform, but it is rich in even order harmonics. BTW, a 12DW7 has a gain of 100 on triode A and a gain of 17 on triode B. So one side is like a 12AX7 and the other is like a 12AU7. Save money. You don't need a perfectly balanced tube. Just get one that is close if you feel the need, but don't spend extra cash on "perfectly" balanced tubes that probably aren't perfectly balanced. __________________ MARTY 2009 Gibson Explorer-Hell Raising Machine 1989 Fender Strat Plus USA 2008 Fender Tele USA 2004 Schecter Elite w/DiMarzio's TSL100 with custom 4x12 cab. TSL122 with Man 'O War Speakers. TSL602 #1 with JBL D123 Speakers and 602 #2 with Altec 417 Speakers. "Oh so sweet my friends." DSL401 #1 with Red Fang Speaker and 401 #2 with JBL D123 Speaker. Tubes: Let's just say they are NOS that kick ass.

10-31-2009, 09:28 PM	#7 (permalink)
HOT TUBES 70 Senior Member Join Date: Oct 2008 Location: SASKATOON Posts: 1,675	Re: Question about Phase Inverter Question : ? Is the PI tube always in the V4 location on all marshalls etc ? And what is its job exactly ? __________________ MARSHALL 6100LM MARSHALL 1960A W/-G12H 30's CUSTOM BUILT 2X12 W/-V30's "MARSHALL" - only the best will do !

10-31-2009, 09:46 PM	#8 (permalink)
MartyStrat54 Senior Member Join Date: Feb 2009 Location: Formerly Phoenix, now north of KC Posts: 2,583	Re: Question about Phase Inverter Only if your Marshall has four tubes. On 800's with three preamp tubes, it's V3. Same with older Marshall's using three preamp tubes. Of all the circuits in a tube amplifier, the Phase Inverter, also known as the Phase Splitter, is the most difficult to understand by even some experienced techs. Here's what it does. It takes a signal input, and create two outputs, one that is identical (e.g. in-phase) to the original, and another that is a mirror-image (phase-inverted or flipped phase). Each signal then feeds the power tubes that are connected to each side of the Output Transformer’s primary winding in the typical Push-Pull (Class AB) configuration. Single-ended power amps, like those found in the Fender Champ, which have only one power tube, do not require a phase inverter. They only need a driver tube before the power tube to boost the preamp signal to a level usable by the lone power tube. __________________ MARTY 2009 Gibson Explorer-Hell Raising Machine 1989 Fender Strat Plus USA 2008 Fender Tele USA 2004 Schecter Elite w/DiMarzio's TSL100 with custom 4x12 cab. TSL122 with Man 'O War Speakers. TSL602 #1 with JBL D123 Speakers and 602 #2 with Altec 417 Speakers. "Oh so sweet my friends." DSL401 #1 with Red Fang Speaker and 401 #2 with JBL D123 Speaker. Tubes: Let's just say they are NOS that kick ass.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode