12AT7 PI vs 12AX7

[MonkHouse93]

In a 5f10 harvard ( http://www.thevintagesound.com/ffg/s...5f10_schem.gif ) Im attempting to make the amp cleaner sounding by swapping out the 12AX7 PI tube with a 12AT7, so that the cathodyne PI and the 2nd gain stage just before the PI are both halves of a 12AT7. I'm planning on using a 51k load resistor with a 3000 ohm cathode resistor (attempting to center bias) for the 2nd stage, and the cathodyne will have the load and tail resistors add up to ~50k in series with a 3000 ohm cathode resistor.

OR, I use 27k with 1500 and the equivalent series resistance on the cathodyne.

I've read a 12AT7 has a higher transconductance than a 12AX7. I know what transconductance is but I'm not too sure what that implies relative to these two tubes. From what I can see on the I-V curves the 12AT7 can take an input of about 8V max vs the 4 or 5 of the 12AX7, while also having much less voltage gain than a 12AX7 too. So if I create a low gain 2nd stage, and feed its output to a grid that can take larger voltage swings than a 12AX7, I should yield less distortions, in terms of clipping or compression.

To get to the point and the question I'm trying to ask, using the 51k vs 27k results in gains of 13 vs 8, while still maintaining about the same idle current with the correct cathode ressitance, just the output current will vary more with a 27k load, and I can drive the power tubes with more current. . . I guess I'm trying to ask what benefit does one load resistance provide over the other in terms of driving the output tubes, minimizing overdrive. What benefit does a larger variation in plate current on the 12AT7 provide?

[pdf64]

My guess is that your proposed changes don't offer much scope for improving how clean the amp is, compared to just turning the volume control down.
The limiting factor for power output tends to be power tube grid clipping (ie class AB1) and a cathodyne is not suitable for pulling Vg1-k>0, as required for AB2.
And both sections of V2 are within a global feedback loop, which will act to negate lower gain in the 2nd gain stage.
Also lowering the open loop gain should act to reduce the NFB ratio, thereby potentially increasing the distortion.

So I doubt that the mods will offer any measurable improvement, but different tube types sound different and you may prefer the 12AT7.

The 5Y3 should be able to accommodate a greater value of reservoir capacitence than the schematic's 16uF, which should help to sustain a higher average VB+ under high signal levels and so increase the (continuous) power output a fraction.

[jazbo8]

Here is a great response taken from a similar thread, which is just a search away...

[Jazz P Bass]

Changing to a 12AT7 will be the same as turning the volume control down a tad.

10:1 it's the power tubes that are clipping.

Welcome to low powered amps.

[eschertron]

Changing to a 12AT7 will be the same as turning the volume control down a tad.

10:1 it's the power tubes that are clipping.

Welcome to low powered amps.

... and turning the volume control down a tad will allow the tone control to be a tad more effective.

If the distortion you're hearing is 'farty' distortion, then reduce the value of the coupling cap between the gain stage and the PI.

edit: have you plugged the AT7 into that spot to hear what it sounds like with no other mods?

[MonkHouse93]

eschertron, I did do this mod already, It kinda works as just turning the volume down. I was trying to get it so that amp just started to break up around the 10/12 or 11/12 (ideally not at all) position as opposed to around 6 or 7/12 (numbered to 12 rather than 10 on the dials). The distortion isn't farty really.

Jazbo8 I read that and I realize that is what's happening here. If I wanted more headroom, I'd need more power is what I'm getting from that, so I should try a different set of ouput tubes? I didn't really expect to get more power out of it this way, I was just shooting for a wider span on the volume knob before it clips. So like in the paragraph above, my clean tone was pushed to halfway and below on the volume, and now that range is expanded at the expense of being able to distort less, albeit I'm getting the same clean power, just at a different number on the dial. . . Sort of. . .

What is 10:1 Jazz P Bass? the NFB ratio? in terms of A/(AB+1) ?

And if the cathodyne is taking voltage swings geater than its input can take why isn't it distorting too? Or am I neglecting something there?

[jazbo8]

I was just shooting for a wider span on the volume knob before it clips. So like in the paragraph above, my clean tone was pushed to halfway and below on the volume, and now that range is expanded at the expense of being able to distort less, albeit I'm getting the same clean power, just at a different number on the dial. . . Sort of. . .

You can also try biasing the output tubes colder which give you a bit more headroom before they are clipped, or try a volume pot with a more aggressive taper if it is the "dial setting" that you are worrying about.

[pdf64]

I was just shooting for a wider span on the volume knob before it clips.

What taper is your volume control, eg set at 50% of its rotation, what is the resistance from the wiper the either track end?
If it's already ~10%, ie 100k to grounded track end, 900k to hot track end, then a good way to reduce gain is the divided load, as the 2nd stage of the 6G3 http://www.thevintagesound.com/ffg/s..._6g3_schem.gif

[eschertron]

eschertron, I did do this mod already, It kinda works as just turning the volume down. I was trying to get it so that amp just started to break up around the 10/12 or 11/12 (ideally not at all) position as opposed to around 6 or 7/12 (numbered to 12 rather than 10 on the dials). The distortion isn't farty really.

Is it a volume issue? Do you want the amp to be louder? Or is it a tonal thing, the tone knob only does what you want when the vol is all the way up? These two different things will call for different solutions...

[Dave H]

I was trying to get it so that amp just started to break up around the 10/12 or 11/12 (ideally not at all) position as opposed to around 6 or 7/12 (numbered to 12 rather than 10 on the dials).

Removing the cathode bypass capacitor from the first stage would do that.

[MonkHouse93]

It's a tonal thing really, although more volume would be cool, I'm really just trying to have a less overdriven tone.

The volume is audio taper regardless of what the schematic says; the tone knob has more functionality at lower volumes, but I'm not too concerned about it.

Removing the bypass cap on the first stage actually makes the volume respond weirdly I didn't like it so much, but I did put a 3k there.

The divided load is the 15k and 100k where the two channels combine on the 6g3? Unfortunately I didn't make the board I made won't really allow that :/

[pdf64]

Audio taper can mean anything really, all we can take from that is that it's probably less than 50% / linear; eg Weber is ~30%, Alpha ~15%.
If you want to know then need to measure it.
The CTS standard seems to be ~10%, so that may work best for you.

[Dave H]

Removing the bypass cap on the first stage actually makes the volume respond weirdly I didn't like it so much, but I did put a 3k there.

That's strange. I removed it on my similar 2 x 6V6 amp and it reduced the gain by a factor of two making the onset of distortion at a higher volume setting (as required) but volume pot still responds normally.

[nickb]

How about you replace the tube rectifier with a solid state one? That will give you more headroom. You might need to reduce the bias current a tad to suit the new B+.

[Gingertube]

Changing to a 12AT7 will be the same as turning the volume control down a tad.

10:1 it's the power tubes that are clipping.

Welcome to low powered amps.

Replace the 1500 Ohm grid stops on the 6V6 outputs with something way higher, I suggest 39K or even 47K.
Cathodyne splitters suffer when the anode and cathode loads are not equal, this happens when the output tubes start to show grid rectification current. High Grid stops minimize the changes in the anode and cathode loads on the cathodyne splitter and thus minimise its "bad behaviour".

Cheers,
Ian