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Phase inverter yet again (sorry)

4/7/2003 9:28 AM
Chris W
Phase inverter yet again (sorry)
Sorry this is a question on similar lines to others posted recently.  
As I understand it the original phase inverter is unbalanced and one improvement would be to include a 1K5 in series with the 56K resistor on the plate so that it matches the cathode. This is supposed to provide more power, more headroom and be slightly cleaner. I have not tested this yet.  
On the other hand we then have the Paul C mod which again balances the Plate and Cathode and is supposed to do a similar thing but I donít understand what the 2M2 is doing and I thought this design needed the tap from the grid the Cathode.  
Any ideas on which is the best and how the Paul C mod works would be gratefully received, or if there are any other ideas floating around out there.  
Many thanks,  
4/8/2003 12:59 PM
In his book, Dan Torres calls it 'blueprinting'.  
By better balancing both sides of the split-load driver, you get more power, headroom, etc.  
I didn't use this method with my 1st 5E3, but I will on the one I am building now.  
Then, I'll be able to see what difference it makes.  
The 1st step is to add the 1.5K to the plate, but you also have to match the 2 56K's and the 2 1.5K's.  
In addition, you have to match the 2 .1uF's and the 2 220k's.  
All the components in the driver.  
I never tried the Paul C. mod.  
4/8/2003 4:05 PM
Bruce /Mission Amps

"Dan Torres calls it 'blueprinting'"
I've tried it in the 5E3 and found virtually no tonal difference.  
Try the AMPEG fixed bias PI mod with the 2m2 resistor and no 1k5 resistor.  
You might hear a difference there.  
4/9/2003 1:35 AM
I finished the amp & I don't hear any difference either.  
Then again, it's not exactly the same amp I built the 1st time- different PT, OT, & 6L6's this time.  
But, it sounds great.  
Question- I'm using a Hammond 270FX PT & I'm getting 372 plate & 336 screen with a 2.2K 1st dropping resistor, 250 ohm 10W cathode resistor & a 5AR4.  
If I'm never going to use 6V6's, I could put a 1K resistor in to bring the screen voltage up for a little more clean & a little more power, right?  
4/9/2003 3:35 PM
Bruce /Mission Amps

Yes, you will get a little more clean power with higher screen voltage but the tubes will try and draw way more idle current too, so watch out for that.  
I've used a 2k2 to 2k7 for the screen dropping resistor with 6V6s and 6L6s, but a 1K is OK... keep in mind you'll need about the same plate voltage to the 12AY7 and 12AX7 as before so the next dropping resistor, the 22K, might need to be fudge around a little.  
If you are using 6L6s exclusively with a 1K screen/preamp dropping resistor, do as I do and also install a 1K-2w resistor on each power tube socket... just as the 6L6 BF Fender amps had.  
4/9/2003 3:48 PM
I'm getting 175 on the plate of the preamp stages & 216 on the PI with the 2.2K dropping resistor.  
What would be the highest I would want to get on these?  
Also, you're saying change the 470 ohm screen resistors I have already to 1K if I drop the power resistor from 2.2K to 1K?  
4/9/2003 5:51 PM
Bruce /Mission Amps

No, you're OK with the 470s on the sockets.  
I ment that I use a pair of 1K-2w screen resistors on the power tube sockets of my amps.  
175vdc on the first preamp tube is a bit high for a tweed amp like this.  
I like to keep the actual plate voltage on the 12AY7 at around or less then 130vdc.  
That means changing the 22K to something higher.  

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