One watt amp

[HotBluePlates]

I thought I read somewhere that humbuckers will put out 500mV with some peaking at 1V.  I was trying to figure out how you could get any clean tone at all.

Hook a meter to your guitar cable and bang out some chords. YOU tell ME what voltage you get (you may not know peaks unless your meter has a peak-hold function or you have an o'scope).

I put my money where my mouth is, and with my Tele (stock-ish Fralin pickups) and *banging* the strings with both pickups on, I got 99mV RMS max. This means over the measurement period, my meter recorded a maximum signal of 99mV RMS. When recording and playing normally, the maximum signal level recorded was 38.4mV RMS.

Sure, humbuckers will be hotter and some will be a lot hotter. But you also know from experience most amps will distort somewhere between 1/2 & 3/4 volume.

In absence of a tone stack there will be somewhere between 49.5v RMS and 67v RMS hitting V3.  Based on that, I can assume similar performance from V3.  That is, the limits will be hit (HARD) and there will be between 49.5v RMS and 67v RMS output with even more distortion.

In reality, the JTM45 uses a cathode follower then a tone stack so gain will be very different but I wanted to be sure I was understanding the simple case first.

In reality, the JTM45 did not use the supply voltage you assumed, and had a cathode resistor that may have resulted in a different bias voltage than your example. V2 also had no bypass cap, which causes local negative feedback and cuts gain by around half. V3 is a cathode follower, and its output is less than its input, and it then feeds a tone circuit which will introduce some loss.

You also do not calculate gains from input jack to output, but work from the output stage, determine needed drive signals and preceding stage gains, and figure the sensitivity at the input jack. You might also assume that you want your target sensitivity (the signal level smack full output power) to happen at half-volume, so you allow for turn-down for hot pickups and turn-up for weak pickups, so all can allow your amp to deliver all its output power.

You gotta design from speaker backwards to the input jack, then after all done perform your analysis on each stage and determine if there are bottlenecks (really, you should be fixing bottlenecks along the way). Maybe you'll decide that you want some stages to always run clean. Maybe you want every stage in the amp (including the output tubes) to break into distortion at approximately the same moment...

[dbishopbliss]

I know my examples were not representative of the JTM45.  I was just trying to understand how the input voltages are amplified and what happens when you present a gain stage with a very large input voltage.  To be honest, I'm still not entirely sure my assumption is correct... that is, pretty much after the second gain stage you are going to get the same output voltage over and over again assuming all stages have the same operating points and there are no volume or tone controls in the way. 

This probably wouldn't sound good (to me anyway) and would just be layers upon layers of distortion.  But, I wanted to be sure I am understanding what would happen.

[HotBluePlates]

You wouldn't hear it that way... because well before your volume (input signal) got to that point, the output stage would already be distorting, then the phase inverter, then...

You probably wouldn't have kept cranking it up to get to the point your example shows, and if you did you cranked up because you were enjoying the sound.

[dbishopbliss]

I think the next thing for me to figure out is what James Marchant meant when he said,

The 60s (JTM1) has an input stage that has a cathode r/rc network designed to imitate the frequency response of a jumpered input with a little more bright than normal volume level, then gain then anode follower then the tone stack from a JTM45 - 56k slope and 220p treble. Only the treble control is fitted as a tone control. Then there is another anode follower to the PI. There is about 6dB of NFB into the grid of the PI.

.

I know the JTM1 has only 3 tubes - two 12AX7's and one 12AU7.  I know that it uses a cathodyne PI so that leaves 3 triodes for the gain stages.  This is where the description diverges from the actual JTM45 (I think).  The first stage is probably just a typical gain stage like the JTM45 with a volume pot between V1 and V2.  However, the JTM45 has a second gain stage coupled to a cathode follower, then the tone stack feeding the phase inverter.  So, even though the amp sounds like a mini-JTM, the topology of the preamp is different.

Either that or this is where I diverge from the JTM1.  There are a number of interesting input networks documented in the books I have been reading.  One of them might be fun to try... it has two inputs high gain and low gain.  Instead of using a voltage divider for the low gain input, it adds a gain stage for the high gain input using some nifty input wiring.  This would take care of one 12AX7.

Another idea is to have one input that would bridge both halves of the first 12AX7 (like a jumpered plexi).  Each triode would have its own volume control that feed into a common second gain stage, cathode follower, tone stack then phase inverter.

Of course I'm just throwing out stuff that sounds like I know what I'm talking about.  Perhaps I should examine why the JTM1 would have a final anode follower before the PI first.

[HotBluePlates]

I think the next thing for me to figure out is what James Marchant meant when he said ...

He didn't build a JTM45 with a 12AU7 output stage. He built a 12AU7-output amp with a JTM45 flavor.

He says, " an input stage ... then gain then anode follower then the tone stack ... Then there is another anode follower to the PI."

I think you should read that as "common-cathode gain stage" (the kind you already know about), Volume control (cause it's the only knob aside from the single Tone control), then common-cathode gain stage, the tone circuit, then common-cathode gain stage, then split-load phase inverter. We already know from previous discussion that he used the split-load inverter.

WTF is an "anode follower"? Well, in this case, he almost certainly means a gain stage with an output at the plate. There is a circuit in tube lore called an "anode follower" or see-saw circuit, but it has nothing to do with what is going on in this amp (and usually implies the use of 2 triodes).

"The 60s (JTM1) has an input stage that has a cathode r/rc network designed to imitate the frequency response of a jumpered input with a little more bright than normal volume level ...

This is easier than you think. The normal input (with the 330uF cap) is just insane mud. I know from having an early 70's 50w Marshall. The Brilliant channel is too ice-picky for normal use. Jumpered input allows you to use the volumes to get a balanced sound.

So I'd interpret his comment as having less bass than the 330uF cap, but more bass than the 0.68uF/2.7kΩ setup. Subjectively tilted a bit closer to brighter than darker.

I'd read the tone circuit as being a full-up Marshall tone circuit, but with fixed resistors in place of the Bass and Midrange pots. That way you control the overall sound with the single (Treble) Tone pot.

I think the next thing for me to figure out is what James Marchant meant when he said,

... There is about 6dB of NFB into the grid of the PI.

.

If you want an exact clone of the amp, you'll need some feedback from speaker output to split-load grid. 6dB worth.

...  There are a number of interesting input networks documented in the books I have been reading.  One of them might be fun to try... it has two inputs high gain and low gain. ...

If it were my amp, I'd just stick with the basic setup.

The folks I've known who had a JCM800 (the input setup that adds a gain stage to the High jack) never use the low gain setting. Of course, they also got the amp because it was master volume and they wanted distortion.

[JB]

This is easier than you think. The normal input (with the 330uF cap) is just insane mud. I know from having an early 70's 50w Marshall. The Brilliant channel is too ice-picky for normal use. Jumpered input allows you to use the volumes to get a balanced sound.

So I'd interpret his comment as having less bass than the 330uF cap, but more bass than the 0.68uF/2.7kΩ setup. Subjectively tilted a bit closer to brighter than darker.

That's true of an early 70's Marshall, plus the two input stages used different coupling caps.  But in the JTM 45 the only difference between the channels is a treble bleed cap on the "High Treble" channels mix resistor, and a bright cap on its volume control.  So the bass response is the same for the two channels, but some treble lift on one of them.

However I agree - he's using the cathode network of the first stage to shape the response a little, so it's brighter than "Normal" but not as much as the "High Treble".  Maybe something a little more complex to give a stepped response, rather than just a smaller bypass cap ?

Edit - The designer says "... a cathode r/rc network...".  So I think that's it - series resistor under the bypass cap to give a bit of hf shelving, to emulate the bleed/bypass caps.  Don't know how you'd calculate that, personally I'd temporarily tack in a 10-20K trimmer, tune by ear then remove and measure!

[HotBluePlates]

That's true of an early 70's Marshall, plus the two input stages used different coupling caps.  But in the JTM 45 the only difference between the channels is a treble bleed cap on the "High Treble" channels mix resistor, and a bright cap on its volume control. ...

Good point. I mainly took the cue from the designer that it was accomplished with cathode cap/resistor tone shaping.

... I think that's it - series resistor under the bypass cap to give a bit of hf shelving, to emulate the bleed/bypass caps. 

Good call. I'd tend to believe that, and agree that you'd have to tune by ear just like the designer did. You calculate to pick good starting values for the bypass cap, then tune the shelving resistance by ear.

[PRR]

> Assuming the gain of the first valve is 60, does this mean that if the volume control was all the way up and the signal coming from the guitar was 0.1V that the voltage going into the second valve would be 6.0V

Yes. And the 2nd stage would distort and sound like %$#@! If this were a Hi-Fi, you would TURN IT DOWN. (Guitar tone-creation sometimes uses %$#@! for flavor.)

> if the second valve has also has a gain of 60 (it probably wouldn't because its not bypassed)

Pencil-check ordinary stages un-bypassed as "half".

The Fender 12AX7 100K 1.5K stage is more like Gv=50; unbypassed maybe 20-30.

> and it has an input voltage of 4.0V, does that mean that the output voltage will be 240V?

It will try. It can't (with normal 270V-350V B+ supply). Like a 7-foot truck in a 6-foot garage, it's gonna have damage. Actually for your *assumed* values, like a 6-foot garage swallowing a _60_foot truck. It's gonna be nearly all damage.

>   That doesn't seem to make sense

With all knobs full-up, 20mV (5mV-50mV) at the Inst jack should just-touch 50 Watts (1W, whatever) at the speaker jack.

AND for nearly all "normal" plucking, for any nearly-clean sound, you will Turn Down one or more gain-knobs.

You should be able to put-in 500mV, turn-down, and get your Full Power out cleanly.

Or not, and take it dirty.

[dbishopbliss]

I've been working on a schematic as a starting point. While I will have the schematic in hand, I plan on building this "bread board" style with lots of room so I can clip in different bypass caps and 10K pots for cathode resistors (should work fine for these tubes).

I will post it soon but work has been very busy. 

[JB]

One curious point about this amp from the designers description is:  "... There is about 6dB of NFB into the grid of the PI. "

I've never seen negative feedback taken to a cathodyne grid - it's usually (in Fenders at least) to the cathode of the previous gain stage.

So would he be forming a voltage divider from the OPT and then a mix resistor into the grid, along a mix resistor coming from the pervious stage?

Why do that, rather than go the Fender feedback route?

[HotBluePlates]

One curious point about this amp from the designers description is:  "... There is about 6dB of NFB into the grid of the PI. "

I've never seen negative feedback taken to a cathodyne grid - it's usually (in Fenders at least) to the cathode of the previous gain stage.

Yep, that's what I've seen, too.

FWIW, I don't know exactly how this amp is set up. So everything from here on is a guess...

So would he be forming a voltage divider from the OPT and then a mix resistor into the grid, along a mix resistor coming from the pervious stage?

You're always gonna have a series resistor coming from the OT secondary for any feedback loop. If the grid in question has a resistor from grid to ground (or from grid to the top of a cathode load resistor), then that existing resistor can be the 2nd half of a voltage divider network. So the only trick involved would be sizing the series resistor from the OT to a value that provides 6dB feedback when connected.

Or maybe he looks at this phase inverter the way I do, in that the gain stage ahead of the split-load is always needed, and whose grid is basically the input port to the power amp. And since the split-load has no gain of its own, any feedback applied would have to go to either this pre-gain stage, or would steal for the output tube's ability to amplify.

That said, 6dB of voltage feedback is a halving of gain. The 12AU7 might have a gain of 12-16 or so, depending on the exact operating point used. Maybe this was deemed a bit hot for the amount of preamp gain stages used, as typical power tubes might have gains more like 8-10. If the feedback was applied to the split-load input, it couldn't reduce the split-load's gain any further (which is already less than 1) so it's really allowing the 12AU7 to accept a bigger input signal.

So I guess there are arguments for either connection.

[JB]

I guess if negative feedback is going to the PI grid then it will reduce (halve, by his account) the signal into the PI.  There's probably still enough signal from the pre-PI stage to fully drive the 12AU7.  It's the kind of amp that will be using a certain amount of output stage distortion so he must have plenty of level there.  Alternatively you could be right in that he's thinking of the pre-PI stage as an integral part of the PI and the feedback is returning there.  Even so it's odd to take it to the grid.

I've been playing with low power amps, SE and PP, for some time now.  My current PP low power amp is pretty much the same topology as this little Marshall - same number of gain stages, tone and volume controls in the same place, cathodyne PI. Major difference is I'm running a pair of EL91's for about 4W output, with a VVR circuit to drop the PI and output stage B+ for domestic friendly SPL when required.  To me these little power pentodes sound better than running the triode halves of a 12AU7 as a PP output stage. I'm not using any negative feedback at the moment, might be interesting to try his arrangement - and also the usual Fender feedback method on their cathodyne PI amps.

[HotBluePlates]

I guess if negative feedback is going to the PI grid then it will reduce (halve, by his account) the signal into the PI. 

That's not what it means.

Let's pretend our output stage (with fictitious tube "X51") has a power output of 1w with an 8Ω load. That implies √(1w*8Ω) = ~2.83v RMS at the speaker output jack.

Let's also say our X51 output stage has a bias of -13v, and so a 13v peak signal (9.19v RMS) is needed to drive it to full output power. Our 12AX7 split-load inverter has a gain of 0.97, and the 12AX7 pre-gain stage has a gain of 50.

Normally, the X51 will convert the 9.19v RMS input signal it receives into a much larger voltage swing (amplification to the plate), while the OT convert the big-voltage small-current signal at its primary to a small-voltage big-current signal at its secondary. Without even knowing the gain of the X51 or the step-down ratio of the OT, we can see that the overall gain through this system is 9.19v RMS (X51 input) -> 2.83v RMS (speaker terminals, or a gain of 0.31.

The split load then is more loss, which tells us we need 9.19v RMS/0.97 = 9.47v RMS at the split load input to get full power output. If the feedback is returned to the split-load input grid, then 6dB of negative feedback means you need twice the input signal, or 18.9v RMS at the split load grid for full output.

The end result is the same if the feedback is returned to the pre-gain stage, but now this stage is influenced by the feedback and becomes the new point of reference for required input signal level. Originally, this stage had a gain of 50 and it would take 0.184v RMS to deliver the 9.19v RMS needed by the split-load stage. After closing the feedback loop, it would take 0.368v RMS at the pre-gain stage to deliver the full 2.83v RMS to the speaker terminals.

Shortening the feedback loop (reducing the number of stages inside the loop) reduces the number of reactive components in the loop, and allows more feedback before the loop turn the amp into an oscillator. Anyway, 6dB of feedback is petty modest, and unlikely to cause a problem.

[dbishopbliss]

The normal input (with the 330uF cap) is just insane mud... So I'd interpret his comment as having less bass than the 330uF cap, but more bass than the 0.68uF/2.7kΩ setup. Subjectively tilted a bit closer to brighter than darker.

I'd read the tone circuit as being a full-up Marshall tone circuit, but with fixed resistors in place of the Bass and Midrange pots. That way you control the overall sound with the single (Treble) Tone pot.

I put in some place holder values for the bypass cap on the first gain stage and the fixed resistors that replace the pots in the tone controls.  I looked at the JTM45 tone stack for the capaictor values.  I'm sure these are great places to play with different values on the breadboard.  I can clip in bypass caps but I may actually use pots on the breadboard, then measure values I like for the fixed mid and bass resistors.

If you want an exact clone of the amp, you'll need some feedback from speaker output to split-load grid. 6dB worth.

Just when I think I'm beginning to understand you throw feedback at me.    Time to read the chapter on Feedback.

[JB]

Thanks for the feedback thoughts HBP.  Nicely written. 

I have plenty more gain than necessary so I think some negative feedback wouldn't go amiss.  Might also tighten the sound up a little which would be useful.

[RobBozic]

Just to keep this thread going, Mojotone have a 1 watt amp on the market using a 12BH7.
 
http://www.mojotone.com/Marketplace/Mojotone-Studio-One-Amplifier-Head#.U2MOgp1-9Fo
 
It looks like a tweed front end for the 'clean'channel', and a JCM800 style cascaded style preamp for the 'dirty' channel.
 
I took some screen shots from the video demo.
 
They also sell an OT for the 12BH7.
 
Rob