12AT7 PI vs 12AX7

[MonkHouse93]

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.

DaveH Removing the bypass cap brought the gain down, but I also lost a lot of treble response, and I didn't like how the volume changed as I turned the knob; to explain that fully.

I've tried a 500k pot for a volume pot, since it parallels with the load resistor of the first stage but that didn't really do much anyway. . . I don't remember what half way was on my 1MEG pots, but the 500k mark was just a few notches down from maxed. I am using Alpha pots.

And I think the easiest solution might be to bias the power tubes a little colder, since i put a trim pot in there. I've thought about it but havent actually done it. When I get a chance to get at this thing I'll try some of these suggestions out and report back, as of now there's a trace elliot consuming all my desk space D:

Thanks all

[MonkHouse93]

For my next make, I'm thinking Long tail pair? I'm also thinking of using fender's Silverface style tone stack to get some bass response out of it, and If i leave the 2nd stage cathode unbypassed the amp won't distort at all, so I think that's something I'll have to work with too

[Chuck H]

I think the first thing to determine is that it is indeed the power tubes that are clipping first. Almost certain. In this light I would back the suggestions of replacing the tube recto with a SS, reducing the coupling cap value to the PI and biasing the amp a little cooler. If it's more clean volume you want nothing beats a more efficient speaker! As to the aesthetic of where the amp begins to break up, I think you should let that go for two reasons. One is that it's a silly compulsion. If it's the power tubes that are clipping then the amp will be producing full power, all it can make, before that volume knob setting. Having a higher volume knob setting before that point won't make the amp any louder so it doesn't matter in any relevant way with respect to circuit function. The other reason is that you may wish to use another guitar with the amp some day. If that guitar has lower output pickups, and the amp is modified to only reach full output with higher output pickups then you will be stuck with less than full output available whenever you use that guitar. Likewise, suppose you find a tone on your current guitar that involves trimming the volume control. If you want that tone at full volume, but the amp is modified to only make full volume with the guitar volume turned up, you'll also be stuck with no way to get the tone you want at full volume. Amplifier sensitivity isn't the same thing as headroom. Good amplifiers are designed to accept most possible levels of input signal and amplify it to full power. That criteria must include that the amp will over amplify (clip) higher input signals. It's part of how amps are supposed to work. If the amp is clipping, turn down the volume control and try not to be bothered by it. Just because it's not at it's maximum setting doesn't at all mean the amp isn't producing maximum power.

[Dave H]

Removing the bypass cap brought the gain down, but I also lost a lot of treble response, and I didn't like how the volume changed as I turned the knob;

I hate interactive volume/tone controls. That is the problem. It is now operating with the vol pot near maximum which means the tone control has no treble boost only treble cut and the vol pot is working over a different range making it interact differently with the tone pot. I don't think it matters how the gain of the amp is reduced, if the volume has to be at near maximum the treble loss will be the same.

[MonkHouse93]

Sooooo What I'm getting from all this is leave the 5F10 as is except for the smaller cap to the PI and biasing colder, otherwise it's as good as its gonna get as far as tubes and component values?

I'm just not understanding why the power tubes clip before the PI does. . .

My guitar puts out 50mV for example (yes this is designing for a specific instrument, but just bear with me) the first stage has a gain of 60, 60*.05=3V, this tone stack I simulated on PSPICE and in the mid gutiar frequencies drops about half a volt, which (for those frequencies at least) puts 2.5V at the input of the 2nd stage, which can take upwards of 4Vpp, which in terms of the rms my meter gives me is like 2.8V. I don't remember where the bias point is but if this doesn't clip here, then the output across the 2nd stage's 100k should be30*2.8=84V (not the same numbers I used before, but close enough for the point). So the PI grid is pretty much taking that 84V. . . it'll clip that since it exceeds the input swing, I think, and then it'll send that clipped signal to the power tubes, which may further clip the signal. . . I suppose I'm neglecting the feedback loop with these calcultions, I'm not quite sure how to factor that in honestly with a feedback resistor to the cathode.

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.

But if the output PI clipped signal is less than input maximum swings of the poweramp then the poweramp wont be clipping, otherwise it would be?
Am I doing this wrong?

[Chuck H]

You surmise correctly. The PI clips ahead of the power tubes to insure that full power can be reached. And if the PI clipped ahead of the power tubes then the power tubes wouldn't. Could this be idealized better? Oh yeah. An amp could be made so that the PI driving stage, the PI and the power tubes all jive at about the same time. This is actually where the discussion of headroom starts. The PI should be able to stay clean well behind the power tubes. This keeps distortion for previous gain stages low. The preamp and PI stages and their distortion component are critical and a good deal of headroom is more ideal in that regard. The amp performs better in general, and as a guitar amp, with ample headroom for the PI driver and the PI capable of more output than necessary. That doesn't mean you couldn't, or shouldn't experiment with your idea of a whole system with balanced headroom. The distortion component would be high. Not clipping distortion, but other sorts of compressions, phase errors, harmonics and waveform anomalies other than clipping. This, for better or worse. In a hi fi amp this approach would probably cause instability in the various feedback loops as well as global feedback. In a guitar amp it would just simply be very different from what we're use to. Again, for better or worse, depending on a great deal of personal objectivity.

[Dave H]

I'm just not understanding why the power tubes clip before the PI does. . .

The power tubes clip when the peak input voltage to their grids is greater than the bias voltage of 27V peak. If the PI can provide more than 27V peak without clipping then the power tubes clip first. If the PI clips at less than 27V peak then the power tubes don't clip and the amp won't produce full power.

If the PI biases to the voltage on the schematic I think the power tubes will just about clip first but the PI supply is only 250V. To be sure I'd change the 22k resistor supplying it to 10k for more output voltage.

[g1]

You can pull the power tubes and see where the PI clips. Like Dave H mentioned, you need at least 54Vp-p plus a bit of leeway to account for voltage rise of B+ without power tubes.

[MonkHouse93]

Sorry I kind of abandoned thread; I ordered a solid state rectifier and then I never actually found the time to plug the thing in. . . First of all I reverted the amp back to stock 5F10, dumped the 12AT7 and put the AX7 back in. It sounded much more robust even when clean than it did with the 12AT7, first of all. Then I threw in some jumper cables so I could mess around with center biasing the preamps vs stock. The PI's cathode value between 1500 and 3000 didn't really change much. . . the second stage made the most difference between those two values. definitely cleans the amp up a lot, but again with 1500 it sounds much more robust than 3000, even well before being overdriven. By robust I mean louder, but not like turning the volume louder necessarily, but just overall thicker sounding, as opposed to thin. The first stage has that affect as well, but it's very subtle.

That's with the 5Y3. Then I stuck in the solid state rectifier, my B+ jumped up from 330 to 370, and everything else kinda went up the same percentage. It definitely yielded much more headroom. I didn't really notice sag vs lack of sag, but I guess this amp is too low powered to really notice? However with the SS rectifier it did go into overdrive much more harshly, as in it kinda hit a brick wall. The 5Y3 took it into overdrive a little more gradually. Maybe that's a slight sag effect happening there.