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Author Topic: What kind of phase inverter tubes are you using?  (Read 11954 times)
blfmmm
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« on: January 27, 2009, 08:00:02 PM »
I use NOS Mullard AT7.  The best AT7 I've ever heard.  It opens up the high end in a sweet almost glassy way.  Like a Fender Twin but without any shrillness.  Anyone concur?  What others have you all tried and to what avail?  Thanks.
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Jimmyd
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« Reply #1 on: January 27, 2009, 10:20:33 PM »
I have Nik's  JJ 12AX7A in mine.  Some purists believe you need a matched PI in the ODS style amp.

jim
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blfmmm
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« Reply #2 on: January 27, 2009, 11:31:58 PM »
Quote from: Jimmyd on January 27, 2009, 10:20:33 PM
I have Nik's  JJ 12AX7A in mine.  Some purists believe you need a matched PI in the ODS style amp.

jim
I use matched PI's.  they push the power section more evenly.  AT7s give a more of a glassier feel, thats why I like them.  They "sparkle" more.
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JohnE
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« Reply #3 on: January 28, 2009, 03:11:01 AM »
Do you mean balanced (phase balanced)? If so, that would be correct since the PI tube  splits the signal to your output tubes 180 degrees out of phase and having them balanced will ensure max performance of the output tubes. Do you notice any drop off in volume with the 12AT7 vs AX7?  The drop off in my Super Reverb is very significant when the pi is a 12AT7.

Bob at Eurotubes has very good JJ balanced 12AX7s and AT7s
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erwin_ve
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« Reply #4 on: January 28, 2009, 10:26:49 AM »
I use a Tung-sol 12ax7 balanced gold-pin. A absolute top tube.
The OTS really opens up  if you use a bright PI tube.
« Last Edit: January 28, 2009, 12:35:25 PM by erwin_ve » Logged
Pickmaster
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« Reply #5 on: January 28, 2009, 11:51:32 AM »
AT7 Significantly!!! Reduces power output so you are loosing at least 10-15 watts and loads of warmth. That’s why your amp feels “glassy” because low frequencies gone.
It’s OK if you like it this way as far as it compliments your style.
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blfmmm
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« Reply #6 on: January 31, 2009, 01:55:17 AM »
Quote from: Pickmaster on January 28, 2009, 11:51:32 AM
AT7 Significantly!!! Reduces power output so you are loosing at least 10-15 watts and loads of warmth. That’s why your amp feels “glassy” because low frequencies gone.
It’s OK if you like it this way as far as it compliments your style.

My OT is 100 watts so the power drop is no problem.  I would not agree that any warmth is lost, you just have to dial your eq accordingly.  I also would not agree that the bottom end is gone, I have plenty, more than I need at my disposal.  AT7 was standard inverter in classic fenders.  Derek Trucks uses a Mullard AT7 in his Super Reverb and he has one of the warmest tones you will ever hear.  Thankyou for taking an interest in my posts though.  I enjoy the differing perspectives.
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blfmmm
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« Reply #7 on: January 31, 2009, 02:01:35 AM »
Quote from: JohnE on January 28, 2009, 03:11:01 AM
Do you mean balanced (phase balanced)? If so, that would be correct since the PI tube  splits the signal to your output tubes 180 degrees out of phase and having them balanced will ensure max performance of the output tubes. Do you notice any drop off in volume with the 12AT7 vs AX7?  The drop off in my Super Reverb is very significant when the pi is a 12AT7.

Bob at Eurotubes has very good JJ balanced 12AX7s and AT7s

I have not noticed any significant drop in volume, none that is a problem for me at 50 watts or 100 watts.  My OT has 4 6l6s and a 1/2 power switch.  If you like the tone of Derek Trucks, I know that he uses Mullard AT7 in his PI.  You can get them at KCANOSTubes.com.  It is a real great NOS online tube store.  Lots of choices and great info.  Reasonably priced too.  Thanks for the response.
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Franc
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« Reply #8 on: February 07, 2009, 06:59:28 PM »
From what I understand you can create a balance with the internal pot?
How did you guys set the pot? What;s the voltage difference between both plates?
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erwin_ve
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« Reply #9 on: February 07, 2009, 07:36:02 PM »
Hi Franc, the dutch connection again Grin

As you know, swapping PI tubes also alters the voltages between the triodes. So you have to pick a balanced one with not too much voltage difference between the plates.
You can balance it by ear. Lows are affected a lot and harmonics as well when turning the PI trimmer.
For me it is ending up somewhere between 5-12 volts between the plate voltages.(depending on the tube)
« Last Edit: February 07, 2009, 07:58:44 PM by erwin_ve » Logged
pine
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« Reply #10 on: April 14, 2009, 12:18:59 PM »
Quote from: erwin_ve on February 07, 2009, 07:36:02 PM
Hi Franc, the dutch connection again Grin

As you know, swapping PI tubes also alters the voltages between the triodes. So you have to pick a balanced one with not too much voltage difference between the plates.
You can balance it by ear. Lows are affected a lot and harmonics as well when turning the PI trimmer.
For me it is ending up somewhere between 5-12 volts between the plate voltages.(depending on the tube)


What would the balancing process be if I wanted to do this electronically? Say I got a new PI to replace the OEM one, and wanted to adjust the trimpot to set it up correctly. Where would I take readings from with a DMM? From plates to ground, i.e, pins 1 and 6 to ground?
« Last Edit: April 14, 2009, 12:47:41 PM by pine » Logged
Franc
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« Reply #11 on: April 14, 2009, 01:37:16 PM »
Just make the difference between de platevoltages as small as possible.
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Steven_nl
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« Reply #12 on: April 15, 2009, 01:36:20 PM »
I use a Sovtek 12ax7LPS. I did notice the bias on my power tubes (matched Sovtek 5881) is  a bit higher and further apart then what it used to be. If I remember correctly bias was around 0.34mA (one slichtly higher) and now the Bias is 36 mA and 38 mA I think.
Is this a problem?
Is the difference caused by the fact that this Sovtek LPS isn't balanced?
« Last Edit: April 15, 2009, 01:57:20 PM by Steven_nl » Logged
JohnE
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« Reply #13 on: April 15, 2009, 04:27:35 PM »
36 & 38 are acceptable. If it sounds OK, there is no need to re-bias. You may want to bring it down some  (to say... 32mA) to extend your tube life, but the effect will be minimal.
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nickm57
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« Reply #14 on: April 16, 2009, 12:33:34 AM »
Is the difference caused by the fact that this Sovtek LPS isn't balanced?

No this is because the power tubes are not perfectly matched. They rarely are. You may find they drift around as they age.
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