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Phase inverter question


 
3/27/2006 9:58 AM
Randy Phase inverter question
OK...keeping in mind that this stuff is still relatively new to me, so this is a newbie question...  
 
One trick some harp players use to fight amp feedback is to put a 12DW7 tube in the phase inverter position. I've never thought too much of this because I didn't like the idea of driving a push-pull circuit with two signals of different strength...I could see how that could fatten up/break up the tone of an amp, but it didn't strike me as such a hot idea from a circuit wear/tube wear/problems-down-the-road standpoint.  
 
But this particular tube swap works really well in my 5e3 clone. From reading about the phase inverter circuit in a 5e3, it looks like the signal is run through both sides of the phase inverter tube and *then* gets split, so the potential "unbalanced" problems of using a 12Dw7 would not exist.  
 
All I know is that the 12DW7 seems to hit just the right sweet spot for me...provides enough grind so that the amp goes grindin' the way I want, and helps fight feedback too.  
 
Anyway, the question I am asking is: Am I understanding the 5e3 phase inverter tube properly or am I doing something really stupid here with a 12DW7?
 
3/27/2006 9:54 AM
Bruce / Mission Amps
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The reason it works in the 5E3 is because the triode that the volume controls are connected to, (and which drives the phase inverter), would normally be a 12AX7 triode. That gain stage has a mu gain of about 100 and in the 12DW7, that triode is actually like a 12AU7, which has a mu gain of around 17.  
The phase inverter triode has no gain so it doesn't matter too much if it's a 12AX7 triode or 12AU7 triode in this application.  
So, you have reduced the gain into the phase inverter by a factor of less the 1/5 what it used to be.  
Except for that triode having a different impedance and current demands, I can't see how it would hurt anything if you are using at least 1/2 watt resistors.  
 
Bruce
 
3/28/2006 12:24 AM
Greg Simon
Bruce, I was wondering if you might be able to help me with this?  
 
On a similar subject, if I was going to use the 5E5 style phase inverter (cathodyne using 12ax7) but used a 12DW7 instead, and switched the pinout accordingly so that the 12ax7 half was the gain stage and the 12au7 was the phase splitter stage, would the 12au7's ability for higher current help me here? Or would I want to tweak the values on the 12au7 half to better suit that tube type? It will be driving a pair of 7591's or 6L6GC's depending on which ones I like better.  
 
Preamp gain will be similar to a Deluxe Reverb on one channel, whereas the other one will use an EF86 and some other stuff and be approaching JCM800 territory as far as gain is concerned. I've already tried the preamp ideas in a Silvertone 1484 I finished for my brother recently (finally) and I REALLY like it. This also has the Fender reverb circuit, but on both channels, and the one I will be doing will also have the same. I kept the stock 1484 splitter using the 6CG7, and I'm also wondering where that one fits in with things as compared to the 5E5 one?  
 
Thanks for any help you can provide!  
 
greg
 
3/28/2006 10:05 AM
Bruce / Mission Amps
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OK for normal use and respect to the cathodyne driver arrangement, flipping the three leads of each respective triode is correct.  
So, yes, you'd want to try the 12AU7 to be the driver triode section and the 12AX7 section to be the preamp.  
 
I have messed with this a bit and found that if the cathode a plate resistors of the "12AU7" section are lower, around 27K-33K, it works very well... but it works OK at 56K too, just not so well when you use the 12AU7 section loaded and biased with typical 12AX7 plate and cathode loads of 100K to 220K in the driver.  
What also matters is the actual plate voltage across the cathodyne driver triode if you are driving power tubes that require a larger voltage swing, because of their relative, deeper negative bias voltage, etc.  
 
Now if a player was going to blow harp through this, I wouldn't worry about the PI driver and 12AU7 section too much and instead mess with the 12DW7 as the first preamp tube.  
The 12AU7 section is usually good enough with a hot mic to kick the next stage and PI driver right in the pants.  
 
Bruce
 
3/28/2006 11:48 AM
Greg Simon
Thanks for the info Bruce. This will be a guitar amp, and is intended for some dirty blues type stuff. Knowing which way to go to tweak is much appreciated!  
 
Greg
 
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