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Hi, I've been lurking around for a few weeks,and I though you might enjoy reading about my 'trial by fire' intro to amp mods. This is a long one guys, skip it if you don't have a few minutes to read!
I've had my Killer Ant for about a year now, and it's a great little amp to keep in my basement for testing out guitars and getting overdrive tones at low volume. For those unfamiliar, the KA is a 1W tube amp that uses two 12AX7s, one for the preamp and one for the power amp. The 12AX7 is the ubiquitous preamp tube, but it is odd to see it as a power valve. And the only control on the amp is a volume knob. I use the matching 1X10 Eminence cab with it.
It comes stock with the common Chinese 12AX7Bs, the supposed 'B' for 'better' Chinese tube. Out of the box, it sounds really distorted at all but the lowest volumes, and not a very pleasing distorted. So I did some tube swapping with some NOS tubes I have kicking around to see what effect that had. I had a Rogers (re-branded Phillips) 12AX7, a Rogers 12AU7, a GE 5 Star 6072A (super-duper 12AY7) and a TEN (no idea?) 12AT7. I had read that they are all "close enough" and could be swapped at will.
The Rogers 12AX7 sounded much better than the Chinese tube as the preamp tube, it cut a lot of the fizz and gave a much warmer tone. I tried the 12AY7 and it really sounded sterile. It's a lower gain tube, but it wasn't that, it just sounded kinda dead. I guess maybe the super-low noise Mil-Spec'd'ness zaps the tone out of it, a regular 12AY7 might be better.
I had read on the BitMo site that the 12AU7 and 12AY7 in the power amp would clean the amp a bit and give more clean head room, because they are lower gain tubes. The 12AU7 really cleaned it up and made it a good bit louder, with a nice bluesy overdrive when dimed. The 12AY7 sounded really off again, and with much less volume.
The 12AT7 was the surprising one, it was louder like the 12AU7, but had more crunch to it. Not fuzzy like the 12AX7, but a good ol' classic rock kinda tone. So I went with the Rogers 12AX7 in the pre-amp and the 12AT7 in the power amp for the last year.
So fast forward to this spring and I found a schematic of the KA circuit.
Link to Schematic in PDF
Which then lead to much reading about tube electronics and how everything actually works! Dangerous for me, since I'm a born tinkerer.
So I learned about gain stages, cathode followers, tone stacks, load lines etc etc, and then decided to sort out what exactly is going on in the Killer Ant.
Looking at the schematic, the first 2 triode stages (the preamp tube, V1A and V1B) are pretty typical. 100K anode resistors, 1.5K cathode resistors bypassed with 1uf caps, with a Marshall 'esq volume/gain dump/tone shaping circuit between them. Following the gain stages is a cathode follower (V2A), which my reading told me was a stage that had no gain of its own, and is used only to match impedance with a heavy load that follows it, usually a tone stack. That didn't make much sense to me, because there is no tone control at all, let alone one that had a high enough insertion loss to warrant the waste of a 1 of only 4 triodes on a no gain cathode follower!
But looking after the cathode follower stage, marshall fans will see something familiar. It's a full on treble/middle/bass tone stack, but instead of pots, it just has fixed resistors with values chosen to replicate a marshall tone stack with everything at 10! So at least the cathode follower makes sense now, even though the whole idea of wasting a triode to drive a non-adjustable tone stack seems pretty stupid.
After the tone stack comes the power amp section, which is a single ended class A using the other triode in the cathode follower tube. So really it's not one preamp tube and one power amp tube, it's 3 triode preamp and 1 triode power amp. That makes it a little tricky swapping out V2 with something other than a 12AX7, because the cathode follower was designed to use a 12AX7, and using a lower gain 12AU7/Y7/T7 might allow the tone stack to haul down the signal a lot.
The power amp section took me a while to figure out what was going on, especially the network of resistors before the grid, R13, 14 and 15. I figured out R15 was a grid stopper, used to block radio transmissions, but the 100K value seems very high. I though R14 might be a grid leak, but then it dawned on my while I was think how dumb a non-adjustable tone stack was. R13 and 14 form a voltage divider, which is basically a master volume control, but again, non-adjustable with fixed resistors. Not only that, but the values chosen (470 Ohm each) replicate a 1M audio taper pot tuned up to about 7. So they're robbing us of the last juicy bit of overdrive, presumably because the stock tubes just sound like azz driven that hard. Bummer because the 12AU7 or 12AT7 would probably sound great with it "at 10".
So next thing I wanted to do was calculate the load lines for the tubes I've been using in V2, to see what was really going on when I was swapping them all willy nilly. I realized later that the "any 12XXX will work" idea is really only for preamps, and it's a good bit more complex thinking about how it works in the power amp.
I found some other pertinent info online like the output transformer has a 50K primary, and B+ is 300V. The 50K OT was interesting, because it is much much higher than for typical power tubes (between 2-8K), but makes sense when you figure the typical plate resistor in a preamp 12AX7 is 100K.
So I printed off the Plate Voltage vs Plate Current graphs for the tubes I'd used and put pencil to paper and finger to calculator. Out of the box with the 12AX7, the 1.5K cathode resistor sets the bias voltage at ~2.5v, which makes it biased quite cold at 300v, and the load line falls far below the max dissipation line (where you'd get the most output). The output was calculated to be a whopping 0.112 W! I guess to be fair, Blackheart does bill the KA as a 'sub' 1W amp. I know it's trite, but it is amazing how loud 0.112W is. I don't know how anyone with a 5W amp could stand it in a small room at full volume. I also calculated the % of 2nd harmonic distortion, which was 9.75%. I don't really have anything to compare that value with, other than I didn't like the sound of it. Although the fizziness is probably odd order harmonics, which I didn't calculate.
Subbing in the 12AU7 gave an output of 0.24W at 7.5% 2nd HD. So exactly as I had heard, louder and less distortion. The load line with the 1.5K bias resistor landed very close to the max dissipation line, and the 13V bias at 300V puts it just a smidge cold biased, but very close to perfect class A. It is really a much better fit at those conditions than the 12AX7, and makes me wonder if it had not been originally designed to use a 12AU7, and economy forced the 12AX7 substitute.
The 12AT7 gave 0.24W of output at 6.82% 2nd HD, which sort of fits with my observations. The load line falls well short of the max dissipation, and the 5v bias voltage is quite cold. The cold bias might be what gives it the crunchiness that made me like the 12AT7 over the 12AU7.
Looking at the graph for the 12AY7 made it apparent why it sucked, the max plate voltage is much lower than the other valves, and the bias was forced very very cold. The grid lines were too bunched in the area to even try to calculate the output and % 2nd HD. This is were the "any 12XXX will do" really falls apart for use in the power amp, because it's 100V higher than a typical preamp and it's just too much for the 12AY7.
It's worth mentioning that the 12AX7 has more voltage gain than current gain when compared to the 12AU7 or 12AT7. Typically power tubes are more current gain devises, so that may be why I preferred the sound of those tubes over the 12AX7. So maybe they actually work better as power tubes, or maybe my ears just recognize the tone better, who knows.
So the next idea I had to do some non-invasive mods was to play with the output transformer loading with the 12AT7 to try to get closer to the max dissipation. The KA has jacks for 4/8/16 Ohm speaker loads. So that ment I could change the plate load of the power amp from 50K to a high or lower value by plugging in different speaker loads in the 'wrong' jacks. For example, the 8 Ohm tap has a ratio of 50 000 / 8 = 6250. So if I plug a 16 Ohm speaker in the 8 Ohm jack, the OT now loads the plate of the power tube with 6250 x 16 = 100K.
Going back to the graphs, I found that in general, increasing the plate load reduces the output and increases the %2nd HD. Plotting the load line revealed that I was a bit closer to the max dissipation with the 12AT7, but I really couldn't tell much difference when I actually tried it. I tried the 12AU7 before plotting the load line, and the tube got super hot in a few minutes. When I did go back and plot the line, at 100K I was over the max dissipation. So be very careful if you try the 'wrong' jack approach.
I also played with plugging a 4 Ohm speaker (the only other one I had around) into the 8 and 16 Ohm jack to reduce the plate load, which should increase output and reduce %2nd HD. But looking at the load lines, this forced the operation into the very non-linear part at the bottom of the grid curves, and in practice didn't sound good at all.
So any further experimentation would mean taking the next step, and formulating some ideas for modifying components, values and topography. Here are some of my thoughts on possible avenues of improvement:
1. The anode resistors, cathode resistors and cathode bypass capacitors are basically all the same throughout the preamp, presumably for ease of construction and cost. Swapping the values, perhaps starting with well known similar preamp values, may really open it up and/or change the tone. Modeling the preamp after the plexi or ac30 would be very easy, as both are 2 gain stage, 1 cathode follower designs. I think just replacing the bypass cap on V1A with a larger value, say 22uf, would make for an improvement by adding some gain and letting more treble through.
2. Ditch the fake tone stack and see if the result is pleasing. Some Mesa Boogies have a switch that lifts the ground connection of the middle pot, bypassing the tone stack and results in a gain boost. It'd be as easy as de-soldering one leg of R12 to try it.
3. Remove the fixed resistors in the tone stack and replace with proper pots. If you're going to have the cathode follower, you might as well get some real use out of the T/M/B tone stack! Be tricky to fit all those pots on there though.
4. Remove the fake master volume by jumping R13 and lifting the ground of R14. That'd be pretty easy. Or put a proper pot in there.
5. Reduce the value of R15, the grid stopper. Most designs seems to use around 5K, and I've read that large grid stoppers can dump treble.
6. Replace R16, the power stage cathode resistor with a value calculated using the load line of the tube you like the best. I figure that 470 Ohm should work well with the 12AT7 and 12AX7. But NOT the 12AU7, it'll run very hot at that bias.
7. If you've removed the fake tone stack, add a low insertion loss tone control (say the tweed control) to the preamp. This would also mean that you wouldn't technically need the cathode follower anymore, so now you have a free triode to play with, and added a tone control!
8. With the free triode from implementing #7, add another gain stage to the preamp! More crunchy goodness... I've worked this out, and it could be done with adding the necessary components and only cutting a few traces on the circuit board.
9. With the free triode from implementing #7, add a parallel triode to the power stage. More clean headroom and more volume! Again, I've worked it out and it could be done.
10. Add a whole 'nother tube!
I've had my Killer Ant for about a year now, and it's a great little amp to keep in my basement for testing out guitars and getting overdrive tones at low volume. For those unfamiliar, the KA is a 1W tube amp that uses two 12AX7s, one for the preamp and one for the power amp. The 12AX7 is the ubiquitous preamp tube, but it is odd to see it as a power valve. And the only control on the amp is a volume knob. I use the matching 1X10 Eminence cab with it.
It comes stock with the common Chinese 12AX7Bs, the supposed 'B' for 'better' Chinese tube. Out of the box, it sounds really distorted at all but the lowest volumes, and not a very pleasing distorted. So I did some tube swapping with some NOS tubes I have kicking around to see what effect that had. I had a Rogers (re-branded Phillips) 12AX7, a Rogers 12AU7, a GE 5 Star 6072A (super-duper 12AY7) and a TEN (no idea?) 12AT7. I had read that they are all "close enough" and could be swapped at will.
The Rogers 12AX7 sounded much better than the Chinese tube as the preamp tube, it cut a lot of the fizz and gave a much warmer tone. I tried the 12AY7 and it really sounded sterile. It's a lower gain tube, but it wasn't that, it just sounded kinda dead. I guess maybe the super-low noise Mil-Spec'd'ness zaps the tone out of it, a regular 12AY7 might be better.
I had read on the BitMo site that the 12AU7 and 12AY7 in the power amp would clean the amp a bit and give more clean head room, because they are lower gain tubes. The 12AU7 really cleaned it up and made it a good bit louder, with a nice bluesy overdrive when dimed. The 12AY7 sounded really off again, and with much less volume.
The 12AT7 was the surprising one, it was louder like the 12AU7, but had more crunch to it. Not fuzzy like the 12AX7, but a good ol' classic rock kinda tone. So I went with the Rogers 12AX7 in the pre-amp and the 12AT7 in the power amp for the last year.
So fast forward to this spring and I found a schematic of the KA circuit.
Link to Schematic in PDF
Which then lead to much reading about tube electronics and how everything actually works! Dangerous for me, since I'm a born tinkerer.
So I learned about gain stages, cathode followers, tone stacks, load lines etc etc, and then decided to sort out what exactly is going on in the Killer Ant.
Looking at the schematic, the first 2 triode stages (the preamp tube, V1A and V1B) are pretty typical. 100K anode resistors, 1.5K cathode resistors bypassed with 1uf caps, with a Marshall 'esq volume/gain dump/tone shaping circuit between them. Following the gain stages is a cathode follower (V2A), which my reading told me was a stage that had no gain of its own, and is used only to match impedance with a heavy load that follows it, usually a tone stack. That didn't make much sense to me, because there is no tone control at all, let alone one that had a high enough insertion loss to warrant the waste of a 1 of only 4 triodes on a no gain cathode follower!
But looking after the cathode follower stage, marshall fans will see something familiar. It's a full on treble/middle/bass tone stack, but instead of pots, it just has fixed resistors with values chosen to replicate a marshall tone stack with everything at 10! So at least the cathode follower makes sense now, even though the whole idea of wasting a triode to drive a non-adjustable tone stack seems pretty stupid.
After the tone stack comes the power amp section, which is a single ended class A using the other triode in the cathode follower tube. So really it's not one preamp tube and one power amp tube, it's 3 triode preamp and 1 triode power amp. That makes it a little tricky swapping out V2 with something other than a 12AX7, because the cathode follower was designed to use a 12AX7, and using a lower gain 12AU7/Y7/T7 might allow the tone stack to haul down the signal a lot.
The power amp section took me a while to figure out what was going on, especially the network of resistors before the grid, R13, 14 and 15. I figured out R15 was a grid stopper, used to block radio transmissions, but the 100K value seems very high. I though R14 might be a grid leak, but then it dawned on my while I was think how dumb a non-adjustable tone stack was. R13 and 14 form a voltage divider, which is basically a master volume control, but again, non-adjustable with fixed resistors. Not only that, but the values chosen (470 Ohm each) replicate a 1M audio taper pot tuned up to about 7. So they're robbing us of the last juicy bit of overdrive, presumably because the stock tubes just sound like azz driven that hard. Bummer because the 12AU7 or 12AT7 would probably sound great with it "at 10".
So next thing I wanted to do was calculate the load lines for the tubes I've been using in V2, to see what was really going on when I was swapping them all willy nilly. I realized later that the "any 12XXX will work" idea is really only for preamps, and it's a good bit more complex thinking about how it works in the power amp.
I found some other pertinent info online like the output transformer has a 50K primary, and B+ is 300V. The 50K OT was interesting, because it is much much higher than for typical power tubes (between 2-8K), but makes sense when you figure the typical plate resistor in a preamp 12AX7 is 100K.
So I printed off the Plate Voltage vs Plate Current graphs for the tubes I'd used and put pencil to paper and finger to calculator. Out of the box with the 12AX7, the 1.5K cathode resistor sets the bias voltage at ~2.5v, which makes it biased quite cold at 300v, and the load line falls far below the max dissipation line (where you'd get the most output). The output was calculated to be a whopping 0.112 W! I guess to be fair, Blackheart does bill the KA as a 'sub' 1W amp. I know it's trite, but it is amazing how loud 0.112W is. I don't know how anyone with a 5W amp could stand it in a small room at full volume. I also calculated the % of 2nd harmonic distortion, which was 9.75%. I don't really have anything to compare that value with, other than I didn't like the sound of it. Although the fizziness is probably odd order harmonics, which I didn't calculate.
Subbing in the 12AU7 gave an output of 0.24W at 7.5% 2nd HD. So exactly as I had heard, louder and less distortion. The load line with the 1.5K bias resistor landed very close to the max dissipation line, and the 13V bias at 300V puts it just a smidge cold biased, but very close to perfect class A. It is really a much better fit at those conditions than the 12AX7, and makes me wonder if it had not been originally designed to use a 12AU7, and economy forced the 12AX7 substitute.
The 12AT7 gave 0.24W of output at 6.82% 2nd HD, which sort of fits with my observations. The load line falls well short of the max dissipation, and the 5v bias voltage is quite cold. The cold bias might be what gives it the crunchiness that made me like the 12AT7 over the 12AU7.
Looking at the graph for the 12AY7 made it apparent why it sucked, the max plate voltage is much lower than the other valves, and the bias was forced very very cold. The grid lines were too bunched in the area to even try to calculate the output and % 2nd HD. This is were the "any 12XXX will do" really falls apart for use in the power amp, because it's 100V higher than a typical preamp and it's just too much for the 12AY7.
It's worth mentioning that the 12AX7 has more voltage gain than current gain when compared to the 12AU7 or 12AT7. Typically power tubes are more current gain devises, so that may be why I preferred the sound of those tubes over the 12AX7. So maybe they actually work better as power tubes, or maybe my ears just recognize the tone better, who knows.
So the next idea I had to do some non-invasive mods was to play with the output transformer loading with the 12AT7 to try to get closer to the max dissipation. The KA has jacks for 4/8/16 Ohm speaker loads. So that ment I could change the plate load of the power amp from 50K to a high or lower value by plugging in different speaker loads in the 'wrong' jacks. For example, the 8 Ohm tap has a ratio of 50 000 / 8 = 6250. So if I plug a 16 Ohm speaker in the 8 Ohm jack, the OT now loads the plate of the power tube with 6250 x 16 = 100K.
Going back to the graphs, I found that in general, increasing the plate load reduces the output and increases the %2nd HD. Plotting the load line revealed that I was a bit closer to the max dissipation with the 12AT7, but I really couldn't tell much difference when I actually tried it. I tried the 12AU7 before plotting the load line, and the tube got super hot in a few minutes. When I did go back and plot the line, at 100K I was over the max dissipation. So be very careful if you try the 'wrong' jack approach.
I also played with plugging a 4 Ohm speaker (the only other one I had around) into the 8 and 16 Ohm jack to reduce the plate load, which should increase output and reduce %2nd HD. But looking at the load lines, this forced the operation into the very non-linear part at the bottom of the grid curves, and in practice didn't sound good at all.
So any further experimentation would mean taking the next step, and formulating some ideas for modifying components, values and topography. Here are some of my thoughts on possible avenues of improvement:
1. The anode resistors, cathode resistors and cathode bypass capacitors are basically all the same throughout the preamp, presumably for ease of construction and cost. Swapping the values, perhaps starting with well known similar preamp values, may really open it up and/or change the tone. Modeling the preamp after the plexi or ac30 would be very easy, as both are 2 gain stage, 1 cathode follower designs. I think just replacing the bypass cap on V1A with a larger value, say 22uf, would make for an improvement by adding some gain and letting more treble through.
2. Ditch the fake tone stack and see if the result is pleasing. Some Mesa Boogies have a switch that lifts the ground connection of the middle pot, bypassing the tone stack and results in a gain boost. It'd be as easy as de-soldering one leg of R12 to try it.
3. Remove the fixed resistors in the tone stack and replace with proper pots. If you're going to have the cathode follower, you might as well get some real use out of the T/M/B tone stack! Be tricky to fit all those pots on there though.
4. Remove the fake master volume by jumping R13 and lifting the ground of R14. That'd be pretty easy. Or put a proper pot in there.
5. Reduce the value of R15, the grid stopper. Most designs seems to use around 5K, and I've read that large grid stoppers can dump treble.
6. Replace R16, the power stage cathode resistor with a value calculated using the load line of the tube you like the best. I figure that 470 Ohm should work well with the 12AT7 and 12AX7. But NOT the 12AU7, it'll run very hot at that bias.
7. If you've removed the fake tone stack, add a low insertion loss tone control (say the tweed control) to the preamp. This would also mean that you wouldn't technically need the cathode follower anymore, so now you have a free triode to play with, and added a tone control!
8. With the free triode from implementing #7, add another gain stage to the preamp! More crunchy goodness... I've worked this out, and it could be done with adding the necessary components and only cutting a few traces on the circuit board.
9. With the free triode from implementing #7, add a parallel triode to the power stage. More clean headroom and more volume! Again, I've worked it out and it could be done.
10. Add a whole 'nother tube!
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