Tweaking cathodyne PI in Magnatone 260 for smoother drive

[dinkotom]

Hi, can anything simple be done to this PI to make the overdrive sound smoother? I've been reading about cathodyne PI a lot lately, but the more I read, the more I'm confused (eternal noob :BangHead: ). I already put a 1M grid stopper between PI grid and plate of the previous stage, not quite sure if I can hear a difference. Should I try to lower the B+ voltage for PI (don't remember where I read that it helps it make smoother) or something else? There are no voltages shown in this schematic, my readings are:

B+ for PI (B node in schematic):320V

PI:
Plate:209V
Grid (with 1M stopper):99V
Cathode:110V

Previous stage:
Plate:109V
Grid:0V
Cathode:0.9V

Schematic:
http://4.bp.blogspot.com/-r_TK-MCP5Xs/U ... +260+2.JPG

Any thoughts welcome.
Thanks, Dinko.

[tubeswell]

Reducing coupling cap(s) and cathode-bypass cap(s) earlier in signal path to remove bass flab?

[jazbo8]

Saw your post over at Hoffman as well, since this is not a common complain about the Magnatone's PI, what do you mean by smoother overdrive? What is it doing now?

[trobbins]

The grid stopper on PI grid would likely only influence heavy overdrive, if the PI was the stage being mainly over-driven.

The output valves have no grid stoppers, so they would be similarly an option.

If you are maxing the VOL pot, then that too may be overdriving the subsequent 12AX7 grid, so a grid stopper may be appropriate there as well.

[martin manning]

The operating point is not very well centered. Consider converting to the 260a AC coupled CF, and include the change to the previous stage (replace 220k from supply to cathode with 120k). You could also add grid stoppers to the power tubes.

[trobbins]

Isn't the 220k bypass for the 470 ohm cathode pushing the bias voltage to that pre-PI up too much (and hence pushing the anode voltage up to 110V - rather than aiming for say 80-90V), and hence that 220k should be increased?

[martin manning]

Hi Tim, the voltages are listed above. The supply is at 320 and the PI grid is at the previous stage's anode voltage, 109. It's on the hot side now, but that may not be a problem since the 260a schematic voltages are similar. Driving into cut-off would be much worse, I think.

[trobbins]

I was just quickly estimating 30V min across 12AX7 in PI, and 30V min swing of driver 12AX7. So 320-60=260V is available swing, which would then target the idle PI cathode voltage to be towards 30 + 260/4=95V.

My initial guess of 80-90 was a bit far me thinks

Just thinking of a tweak as first step to test, rather than a change in circuit.

[dinkotom]

Guys thanks for suggestions, I put 47K output grid stoppers, that helped a lot, it's almost there! Also, I have a switch for the first coupling cap to choose from stock .010 to .002 - that takes out most of the flub when driven.
I will also try a grid stopper after the volume pot.
But back to the PI - I still can't pretend I understand most of what Martin and Tim are saying, so I'll try any tweak you suggest and hear if it helps. Should I try a bigger resistor than 220k bypass for the 470 ohm cathode then as a first step?

[sluckey]

I consider the Magnatone amps as having a clean sound. I don't think they were intended to have an overdriven sound.

Your 260 has a direct coupled cathodyne. A later and more popular cathodyne is the bootstrapped cathodyne. The attached schematic is from a M10A but it's also found in many other popular amps (Princeton Reverb, 5E3, etc.). It would be an easy circuit to try in your 260 (that is if the wiring under the hood is not too messy). I circled the only three components you would need to add. It would also be easy to reverse.

I don't know if this will give the results you want but it should be worth the effort to try.

[dinkotom]

I consider the Magnatone amps as having a clean sound. I don't think they were intended to have an overdriven sound.

Hi Steve, yes I know that, but this one overdrives for some reason, so I just wanted to see if tweaking the PI can make it smoother. With 47k stoppers it's pretty close, I won't complain if it stays as is now. I also have a 260A model with newer PI and that one stays clean almost all the way.

[tictac]

Here's Merlin's comments

If you are using a cathodyne always give it a nice big grid-stopper. The tonal reward is startling!!! Yes I know Leo Fender didn't use any, but he wasn't designing amps to be overdriven, and this is the 21st century.

[dinkotom]

see my first post

I already put a 1M grid stopper between PI grid and plate of the previous stage, not quite sure if I can hear a difference.

[tictac]

yes I saw your first post but you didnt show a schematic of your current amp so I wasn't sure if you actually knew what a grid stopper was....

Merlins design also has a few other do-dad's which you can read about on his ValveWizard website if you want to know what they are for....

TT

[trobbins]

But back to the PI - I still can't pretend I understand most of what Martin and Tim are saying, so I'll try any tweak you suggest and hear if it helps. Should I try a bigger resistor than 220k bypass for the 470 ohm cathode then as a first step?

Sorry for our chit-chat discussion about the centring of the PI idle point. The aim is to allow the PI output voltage to swing so that clipping occurs at top and bottom peaks evenly. That is best done by looking at an oscilloscope, as the datasheets don't always indicate your valves curves. My guestimate was that raising the 220k a bit may give a titch more clean output voltage swing from the PI - but a test is the only way to really check, and the difference is likely to be negligible anyway. Hence the recommendations to change the circuit design.