Rocket scope traces and gain structure....pics!

[rbryerton]

So after having a discussion with a couple of members (who will chime in if they choose) about taming Rocket overdrive using modern high output pickups, I decided to put a tone generator on my Rocket clone and scope out the waveform at various stages and see what makes this wonderful circuit 'tick'.

Where does it clip? Where does the majority of the overdrive come from? How can I clean it up a bit at max gain using high output humbuckers? These were some of the questions I (we) were asking ourselves...

Typically, I'll swap out V1 with a 12ay7 or a 12au7 to clean it up, as my guitars all have medium to high output pickups, and I like to retain full travel on the gain pot to tweak with. With a 12ax7 in V1, any gain setting past 12 o'clock for me is useless, as this is the max overdrive I want from my Rocket clone.

Sure, I could turn down my guitar volume pot, but then I'll be in the same position, never being able to use it's full travel.

I came to the conclusion that the best mod *for me* was simple: a micro-switch to switch in and out a 100k resistor to ground, tapped from after the 220k resistor (R10) after the tone stack. This way I could have my cake and eat it too: use the standard tube compliment and have the amp switchable between stock and gain-attenuated modes. This way I can switch back to stock circuit values easily when using my Tele or my single coil Strats if I wanted to.

I've included some voltages for my Rocket that show the gain progression through the circuit, as well as many scope traces showing what the circuit does at various points.

Long story short (is it too late?), all of the clipping and overdrive in a Rocket (or at least mine) comes from the Phase Inverter and the power tubes. The pre-amp seems to only amplify and compress the signal, and add tonal flavor. At least that's what I get from looking at it on the scope.

I tested with a 200Hz, 0.100 v sine wave injected at the input jack to simulate a hot single-coil, and unless noted, all controls were maxed. The amp was output from the 16 ohm tap into a 16 ohm resistive load.

Here's some pics, and I look forward to hearing any feedback.

"test rig"
[IMG1000]http://www.bryerton.com/misc/rocket_tes ... st_rig.JPG[/img]

Voltages:
[IMG:646:417]http://www.bryerton.com/misc/rocket_tes ... ltages.JPG[/img]

Pre-amp compression; note how the top swing of the bottom trace is compressed
[IMG1000]http://www.bryerton.com/misc/rocket_tes ... stage1.JPG[/img]

More Pre-amp compression; note how all of the bottom trace is now compressed
[IMG1000]http://www.bryerton.com/misc/rocket_tes ... stage2.JPG[/img]


[IMG1000]http://www.bryerton.com/misc/rocket_tes ... ter_CF.JPG[/img]

Tone stack effects:
[IMG1000]http://www.bryerton.com/misc/rocket_tes ... treble.JPG[/img]


[IMG]http1000]http://www.bryerton.com/misc/rocket_tes ... treble.JPG[/img]


[IMG1000]http://www.bryerton.com/misc/rocket_tes ... treble.JPG[/img]


[IMG1000]http://www.bryerton.com/misc/rocket_tes ... treble.JPG[/img]


These next 3 images show how the phase inverter clips the signal at various gain levels.
[IMG1000]http://www.bryerton.com/misc/rocket_tes ... t_gain.JPG[/img]


[IMG1000]http://www.bryerton.com/misc/rocket_tes ... t_gain.JPG[/img]


[IMG1000]http://www.bryerton.com/misc/rocket_tes ... x_gain.JPG[/img]

These next 4 images show a phase inverter trace at the top and the output signal at the output jack at various gain levels.
[IMG1000]http://www.bryerton.com/misc/rocket_tes ... t_gain.JPG[/img]


[IMG1000]http://www.bryerton.com/misc/rocket_tes ... t_gain.JPG[/img]


[IMG1000]http://www.bryerton.com/misc/rocket_tes ... t_gain.JPG[/img]


[IMG1000]http://www.bryerton.com/misc/rocket_tes ... x_gain.JPG[/img]

Switch in the 100k resistor at max gain; note how the signal is cleaned up with a minimal loss in volume.
[IMG1000]http://www.bryerton.com/misc/rocket_tes ... pre_PI.JPG[/img]

So what do you think?

edit: added/fixed link

[VacuumVoodoo]

Good work on the scope pics.
Your PI shows ugly overdriven waveform and is losing symmetry due to capacitive loading by the Cut filter.
It's what's causing kinks in the output waveform looking like crossover distortion.
I'd probably try halving R14 value or perhaps having a lower gain tube as PI, EL84s don't need all the signal swing that 12AX7 can deliver to get into serious overdrive.
I think you do not need delayed sweep activated on your scope for these measurements. That's why part of the trace is much brighter.
12AY7 in V1 position. Absolutely.

[rooster]

Wow, fantastic, rbryerton! Er, did you reverse the pics 5 and 6 and the descriptions? These two photos don't make any sense. It seems that #6 would be with Bass on Max in other words...

What a nice thing to share here, thanks! As to losing the 12AX7 in V1? Um, no, not for me.

[rbryerton]

...
Your PI shows ugly overdriven waveform and is losing symmetry due to capacitive loading by the Cut filter.
It's what's causing kinks in the output waveform looking like crossover distortion.

Ugly....overdriven.... I thought that was mojo...
It sounds much better than it looks... look at the last shot (after I dumped ~65% of the signal before the PI)...it still has a notch, but the symmetry is much better and not as clipped. It sounds *great*.

I'd probably try halving R14 value or perhaps having a lower gain tube as PI, EL84s don't need all the signal swing that 12AX7 can deliver to get into serious overdrive.
I think you do not need delayed sweep activated on your scope for these measurements. That's why part of the trace is much brighter.
12AY7 in V1 position. Absolutely.

Thanks for the scope hint. I really didn't want to get too far from the stock circuit for silly reasons... it seems I reached my objective here with just 1 added resistor which is cool, and I can save the 12ay7 for another amp... lots of ax, only have a couple ay's.

Plus, I'm thinking keeping the ax7's installed will help with sustain, because they basically seem to act as a compressor in the pre amp.

...
Wow, fantastic, rbryerton! Er, did you reverse the pics 5 and 6 and the descriptions? These two photos don't make any sense. It seems that #6 would be with Bass on Max in other words...

What a nice thing to share here, thanks! As to losing the 12AX7 in V1? Um, no, not for me.

No, I'm almost positive they're labeled correctly, but I could've screwed something up.... ignore the amplitude of the waveforms in the preamp pics, I pretty much normalized them to fill the screen.

I didn't mess w/the amplitude in the output section shots though, as I wanted to show how the signal progressively distorts as the amplitude rises in the output section.

I was really surprised to see that there is basically no overdrive/distortion generated in the preamp compared with the power amp section.... it seems that as long as you get a volt or two to the PI input, it will drive those output tubes like crazy... I knew el84's were sensitive, but daaayyuuum...

I should mention that I built this amp several months ago, and it has always sounded great, it just had too much overdrive for my tastes... my max gain setting was like 9 or 10'o clock on the gain pot.... now I have use of the full travel.... sweet!

[RJ Guitars]

Rob,

It's always been my experience with these amps that if you smack them too hard with a high output pickup they will really break up early. They have a sweet crunch to them but folks have often mentioned they would like more headroom before the crunch thing started. I get a notable difference in the breakup point between my Strat and a Les Paul, although mine does not stay clean with either guitar if you run the volume up on the guitar and set the amp past 10 o'clock... which is really loud!

This is a really well done look at things. There have been a lot of us speculate on what is happening at each stage but nothing really tells the story like a set of scope pictures. If you ever get the free time I'd be curious to see the same scope shots if you start out with that 12AY7 or 12AU7.

Thanks for sharing with us. It looks like a real nice job on the amp build as well.

rj

[KellyBass]

Thanks very much for sharing this, Rob! I'll admit...I was scratching my head a bit after my first rocket build. The amp wasn't quite what I expected. Why does this thing sound somewhat "fizzy" at all settings? Other 4xel84 amps I've experienced, at least have the ability to, sound so much more "open". After seeing this, I'm definitely going to install a voltage divider pre PI. Now...whether to go with a switch and a 100k or with a pot and the PIG configuration...

Or how about losing the cut control and going with a matchless-style MV?

Uh oh...just kidding...

In any case, Thanks again for posting this!

[Bear]

Makes me take back up my old thought of a "Lite" Rocket with a cathodyne PI from the unused 2nd triode stage of V2. I'm not looking for a lot of amp distortion out of it, so that might not work for everyone.

[rooster]

Bear et all - Where rbryerton uses a 100K value here, it was 2Tone who chimed in with a different value directly from Ken Fischer for 2Tone's Ken built Rocket. Ken suggested a switched 220K value, to duplicate the load of the 'Normal' channel in an AC30. ...And Ken was kinda into this mod, FWIW, something that he came up with after he sold and built the Rocket for 2Tone. (All of this, BTW, is somewhere in the forum posts)

Also, maybe worth noting, 2Tone did the mod and much prefered the stock Rocket setting. Just saying.

Again rbryerton, thanks for the post.

[Noel Grassy]

Great stuff Rob! It's about time someone put up pics of their O'scope exploits
and empirical amp data. The typical adjectives can only inform us as an opinion. IMHO!

Another opinion, you may do very well to heed Vacuum Voodoo's suggestions
as he may be regarded as an authority at the very least. I mean who else
has built an amp with Mad Professor (Bjorn Juhl) and has their name on it?
Of course I respect your desire to remain faithful to the Rocket's original
design too!
I think your O'scope is the same one I have. Tektronix 485?

Noel Grassy.

[PlinytheWelder]

I'm going to try this with my express when it's all set up. I should have all the gear to do it.

Even triangle waves....