Modded Princeton Reverb/5E3 circuit critiques

[Ibanezgrind]

Greetings!

For those of you that remember, I am the same guy who recently tried to get a presence control to work on my home built 6L6WGB princeton reverb and gave that idea up. This is what I came up with and think it's much better. Go easy on me please, it's my first schematic and those little connection dots between symbols gave me trouble so if you see some missing or in the wrong spots - I'll get better.

My idea is combining a mod I saw on Rob Robinette's 5E3 mod page that cascades one gain stage into the other. The channels are then separated by 2 mixing 220K resistors.

What I am not sure of:

Am I correct to place the mixing resistors in there? If so, are they in the correct spots?

I currently have a Master Volume (represented in the schematic) that works great in my Princeton circuit . Now this cascade mod included a Master Volume for it's channel which is really the bright volume knob on a 5E3. The normal volume knob is now the gain level knob. I kept everything the same in regards to the schematic on his page but have no idea if having 2 master volumes, one for each channel is ok. Maybe one would be more simple but have no idea how that would look in my schematic. What would you guys suggest?

I know too much overall amp gain can be a factor but I think what I have is ok since this proven mod is for a 5E3. However, my concern is for the V5A 12AX7 half in my schematic. I have the cathode tied to a 10K resistor for cold clipping. Not sure about that one either but thew it in. Maybe too much? Put far less resistance here? Maybe bypassed with a cap? I don't know..

I really can't think of anything else in my limited knowledge. If you see anything at all aside from my questions, please let me know. Feel free to make edits on the attachment if that is easier and apprecaite everyones knowledge.

Miguel

[shooter]

here's my example, the "master vol" I just call gain, since they are "left" of the mix

[tubenit]

Some of the coupling caps off the plates of the triodes aren't drawn correctly.  I made the same mistake early on in drawing schematics several times.   

Overall, it's a well drawn schematic, IMO. 

With respect, Tubenit

[Ibanezgrind]

Great catch Tubenit, thank you! Fixing now.

[Ibanezgrind]

I finished this at 2am and then doublechecked my work this morning. Arrhg! I thought I got everything 100%. Thanks for editing the schematic and point out the others!

[Ibanezgrind]

I found a few others at the bottom with the same issue. Here is the updated schematic.

[2deaf]

I see that you are catching some noise over there on TDPRI.  Just as a head's up:  Bendyha is the guy you want to pay attention to over there.  Rob just made a mistake when he didn't show the 5E3 input grounded.  All you will get as drawn will be hum and noise and a potential shock hazard.

The 470K grid stopper on the cathodyne doesn't affect the audio frequencies.  The essentially unity gain makes the input capacitance about twice the grid-to-plate capacitance, or 3.4pF.  This puts the cut-off frequency (corner frequency) at 99.6KHz. 

The input impedance of the cathodyne grid is infinity for all practical purposes, so the 470K grid stopper is having no effect on your 220K/220K mixing resistors.  The input impedance of the whole cathodyne is over 16M, so the effect on the 220K/220K mixing resistors is nil.

The 470K isn't there to prevent blocking distortion, but rather to prevent that ugly sound you get when you overdrive a cathodyne without a grid-stopper.  To prevent blocking distortion, you might consider large grid-stoppers on the power tubes.  These grid-stoppers are part of the grid circuit for the power tubes and there are limits on how big the resistance can be.  But a cathodyne with balanced loads can drive some pretty low resistance grid leaks, so reduce the grid leaks and increase the grid stoppers. 

[Ibanezgrind]

Thanks for the reply 2deaf. He is very knowledgable. Speaking of Bendyha, I read his thread on TDPRI about "Fender Cathodyne Phase Inverters" I'm sure you have read it and the thread does mention what you suggested to prevent blocking distortion.

https://www.tdpri.com/threads/fender-cathodyne-phase-inverters.755580/

He suggests the grid leaks to be reduced to 180K and increase the grid stoppers to pins 5 of the power tubes to 33K. He reported some nice results with those values. Although I am using 6L6WGB, I would assume that makes no difference, correct?

In that same thread, on post #6, he mentions:

So... what are the modifications? Simply add a grid-stopper resistors to the cathodyne, and to the output tubes.

1. Add a 1M Ohm grid-stopper resistor to pin 7 of the 12AX7. This will prevent any frequency doubling. The easiest way would be just to solder it straight onto the tube socket. Although this is placed between the tube and the 1M grid-leak resistor, it will make no difference to the grid-leak resistor function, or to the amps previous sound. Although data sheets vary in the limit given for maximum grid circuit resistance, with Q at 1mA and 2M Rg, a 12AX7 should be alright. If, alternatively, the grid-leak resistor is between the grid-stopper and the tube, we then form a 1:1 voltage divider, and input strength will be effected.

This 1M grid leak resistor he mentions, is it the same 470K resistor I used in my schematic on pin 7 of the phase inverter? Would this be a better value over the 470K?

[2deaf]

I only made it to post #4 at which point I disagreed and quit reading. 

The IM grid stopper is at the same location as your 470K.  I used a 470K and it seemed to be sufficient.