Low volume, possible cathodyne issue, help needed.

[wozt]

I have a 6v6 build which is suffering from low volume. My main concern is the cathodyne PI. I followed valve wizards description (or thought I had). A 100k-1M grid lead is tied to a divider in the cathode resistance and a large grid stopper is utilised to prevent some nasty stuff.
So from the coupling capacitor do I place the grid leak first then the grid stopper?
If I use a 470K grid stopper and a 100K grid leak (for example) won't the current flow through the path of least resistance, ie the grid leak and on through the cathode resistors to ground rather than pass through the grid stopper.?

Thanks for your input.

[Chuck H]

A 100k-1M grid lead is tied to a divider in the cathode resistance and a large grid stopper is utilised to prevent some nasty stuff.

I have no ability to visualize what you've described.

It's impossible "for me" to interpret your text description. Text descriptions of circuits to be analyzed border on useless, even from the most literate. Abort that practice immediately and figure out how to sketch and upload diagrams/schematics of what we need to evaluate.

If I use a 470K grid stopper and a 100K grid leak (for example) won't the current flow through the path of least resistance, ie the grid leak and on through the cathode resistors to ground rather than pass through the grid stopper.?

There's a little misconception here. The current is not important for the power tube grids WRT guitar amps (there are exceptions but I won't confound the issue here). It's the voltage that is important. And the voltage will indeed see the division as well, for better or worse. Since there is (for the most part) no significant current at the power tube grids you should focus on the voltage division created by the circuit.

Pretty please, with sugar on top, figure out how to post actual schematic or diagram references for the circuits you are actually implementing. You can even hand draw and scan them for upload. Even that would be miles ahead of a text description.

[wozt]

Dee-zaster-2.png Photo by paintrdude | Photobucket

Chuck this cathodyne has input coupling cap, then grid leak, and then grid stopper. I have built it like this one
http://i406.photobucket.com/albums/p...psc3df59f0.jpg

Im losing volume somewhere and the cathodyne is my first suspect. Tones nice just too little volume.
Thanks for your help across a few threads, it has been appreciated.

[eschertron]

Dee-zaster-2.png Photo by paintrdude | Photobucket

Chuck this cathodyne has input coupling cap, then grid leak, and then grid stopper. I have built it like this one
http://i406.photobucket.com/albums/p...psc3df59f0.jpg

Im losing volume somewhere and the cathodyne is my first suspect. Tones nice just too little volume.
Thanks for your help across a few threads, it has been appreciated.

The first schem, the "Deezaster" one, has the placement of the grid stopper correct. The second one is not a grid stopper per se, but really acts as a voltage divider as you and Chuck discussed.

So which schematic did you follow? other than the 560k (960k? can't really make it out) resistor between the previous triode's coupling cap and the PI grid, I like the values in the second schem better. More 'standard'. If you want Merlin's grid stopper, put the 470k resistor right on the pin of the tube, and take the other end to the terminal strip/eyelet board. Just like the resistor's not there as far as the other circuit components see. That placement won't interfere with the gain of the stage.

[wozt]

This is how it is currently. I'm using a JJ 12dw7 tube, the cathodyne on pins 1-3, a preceding 12ax7 stage on pins 6,7,8.

[eschertron]

[ATTACH=CONFIG]33369[/ATTACH]

This is how it is currently. I'm using a JJ 12dw7 tube, the cathodyne on pins 1-3, a preceding 12ax7 stage on pins 6,7,8.

It looks to me like you're losing half your signal (~6dB) across R1 and R2. You can test to see how much gain you can get here by jumping out R1. Merlin's great big grid stopper (TM) is there for overdrive conditions. Shouldn't bother you to test without it.

From the DC voltages you have posted, there's a few 10's of volts headroom between cathode and ground, and between plate and HV supply. Yet there's hundreds of volts available between the two operating points. You may not get more gain but you'll get the headroom if you massage the values of R4 and R5. Somewhere on the web there's a spreadsheet that allows you to calculate gain (etc) for a PI circuit. Alternately, plot a loadline to see how it's behaving. Alt-alternately, Merlin suggests thumbrules for picking R4 and R5 based on setting the idle voltages as a proportion of HV. The Valve Wizard

What's the AC voltage at the grid of the PI? For 6V6 output tubes you'll want to be able to get in excess of 45vpp (16vrms or so) to even get to max HR on the output.

edit: Also, taking the non-inverting out put off from the junction of R3 and R4 instead of R3 and the cathode you're giving up a smallish amount of gain, at least on that leg. [gain = R4/(R3+R4) or about -2.3dB]

[Dave H]

As eschertron said try shorting R1 (or move it to between the tube grid and the top of R2) and I agree it is biassed cold. I'm not sure of the saturation voltage of 12AU7s but for a 12AX7 the cathode voltage should be set to between 1/4 and 1/5 of the HV supply voltage for max headroom (tm). Also R4 and R5 should be the same value for balanced outputs or C3 could be moved to the top of R3 for more output voltage but but with a small output imbalance.

[wozt]

So i turned my DMM to ac, clipped the black to chassis and red to the grid of the PI. It shows 3v hitting the guitar hard and amp and volumes on full.

[wozt]

If I'm measuring the AC correctly, V1 (which is a parallel 12ax7 stage) is producing 2v onto the grid of V2a, which is a single 12ax7 stage. This stage is only increasing the 2v to 3v at the PI grid. V2 is a 12dw7, 1/2 12ax7 1/2 12au7.

[eschertron]

If I'm measuring the AC correctly, V1 (which is a parallel 12ax7 stage) is producing 2v onto the grid of V2a, which is a single 12ax7 stage. This stage is only increasing the 2v to 3v at the PI grid. V2 is a 12dw7, 1/2 12ax7 1/2 12au7.

Can you repeat the experiment with R1 jumpered out?

[wozt]

I've jumpered R1 and cathodyne grid is reading 4.6v now at full volume.

[wozt]

I have moved r2 to before the grid stopper to break the voltage divider. The two stages with the 12dw7 are shown below.

[potatofarmer]

Can you post a full schematic? While your cathodyne could/should be biased warmer, it looks like the problem is earlier in the preamp.

[eschertron]

+1

For 6V6s, you'll want more signal to drive them. For a rough first guesstimate, multiply the 6V6 bias voltage by .707 to get rms voltage needed to just reach max headroom. To make it crunchy, 2 or 3 times more than that!

There may be opportunities to design a less lossy tone stack.

[wozt]

OK I'll get on the case for a full schematic.
As the 12ax7 side of the 12dw7 is only providing a 1 volt increase isn't this the problem, what would expect a 12ax7 to amplify 2 volts too?