Big-bottle rocket build w/pics...& mp3 clip

[rbryerton]

Inspired by RJ's and others KT66/big-bottle rocket projects, I built a KT88 rocket a few weeks ago. A few things I changed/added to the typical rocket circuit:

1. Fixed bias, diode rectifier, stiffer power amp filtering

2. Ultra-linear output transformer/output stage (Magnetic Components Dynaco A-431 clone)

3. Parallel 1st stage in preamp; added an ef86 in parallel w/the 1st stage 12ax7, each with their own gain pot so they can be mixed independently ( sort of like a permanently jumpered Plexi)

4. High voltage power amp section: 560v on the KT88 plates & 43% screen taps. I used a Hammond 278CX 400-0-400 465ma PT; those '88s eat some current!

5. Mid tone control

6. 12AT7 in PI & tweaked PI values for a big voltage swing to drive the 88's in UL mode.

7. Parallel octal P/P output; I use only 2 tubes with KT88's, but it can take 2 88's/6550's/KT66's/EL34's/6L6's or 4 6L6GC's. I need to add a separate heater transformer if I want to run a quad of EL34 or KT66, because the 278CX only has one 6A heater tap

8. Pre-PI Master volume

I'm finalizing voicing now, and still have to clean up the wiring. It took some doing to tame oscillations, but I got it stable. It is *very* versatile tone wise, and depending on which guitar I use, gets some very high gain (approaching an express). It still retains the beautiful cleans and lower gain break-up of a rocket (and sounds just like one at the appropriate settings, albeit w/a bigger & somewhat tighter bottom), and cleans up amazingly well w/guitar volume knob, even when the amp/guitar combo are set to raging overdrive/distortion. The clean to mild break up with a Strat or Tele are just to die for!

Another thing I added was a switch to switch in and out a screen (bypass?) cap on the ef86 for a switchable boost. I changed the typical values you usually see in an ef86; brought the gain way down so it doesn't overwhelm the parallel 12ax7, and to deal w/microphonics a bit. I got these ideas from Merlin's book (which is excellent, BTW).

Oh yeah, did I mention it's LOUD too! Flat out, it pushes 95W from the pair of KT88's.

EDIT: Added sound clip. It's just me hacking around, but you can get an idea of the sound of this beast and how well it cleans up.

http://www.bryerton.com/misc/kt88rocket ... cket_1.mp3


Here's some pics and the schematic:


[img988]http://www.bryerton.com/misc/kt88rocket/v03schem.gif[/img]

[img1000]http://www.bryerton.com/misc/kt88rocket/IMG_0017.JPG[/img]

[img1000]http://www.bryerton.com/misc/kt88rocket/IMG_0018.JPG[/img]

[img1000]http://www.bryerton.com/misc/kt88rocket/IMG_0019.JPG[/img]

[img1333]http://www.bryerton.com/misc/kt88rocket/IMG_0020.JPG[/img]

[img1333]http://www.bryerton.com/misc/kt88rocket/IMG_0021.JPG[/img]

[img1000]http://www.bryerton.com/misc/kt88rocket/IMG_0023.JPG[/img]

[img1000]http://www.bryerton.com/misc/kt88rocket/IMG_0025.JPG[/img]

[img1000]http://www.bryerton.com/misc/kt88rocket/IMG_0026.JPG[/img]

[img1333]http://www.bryerton.com/misc/kt88rocket/IMG_0027.JPG[/img]

[img1333]http://www.bryerton.com/misc/kt88rocket/IMG_0028.JPG[/img]

[img1333]http://www.bryerton.com/misc/kt88rocket/IMG_0029.JPG[/img]

[img1000]http://www.bryerton.com/misc/kt88rocket/IMG_0030.JPG[/img]

[img1000]http://www.bryerton.com/misc/kt88rocket/IMG_0032.JPG[/img]

[img1000]http://www.bryerton.com/misc/kt88rocket/IMG_0033.JPG[/img]

[img1000]http://www.bryerton.com/misc/kt88rocket/IMG_0034.JPG[/img]

[img1000]http://www.bryerton.com/misc/kt88rocket/IMG_0035.JPG[/img]

[img1333]http://www.bryerton.com/misc/kt88rocket/IMG_0037.JPG[/img]

[Colossal]

Wow Rob, that's really cool, Well done! What a beast! You need a floorjack for all that iron! Looks great. Very cool concept of dual input stages and I like what you did with the EF86 voicing. Very interesting hybrid amp. How are the relative clean vs. distortion volumes via the guitar volume knob? Do you get a nice transition with similar apparent volume? How 'bout some clips when you get a chance?

[Structo]

Sweet!
I'll bet it sounds great!

[KellyBass]

Wow! very impressive concept/build! I REALLY think we need to hear some clips of this one!

[rbryerton]

Thanks for the kind words! I've been tinkering with the schematic/design for a couple of months now, and it came together very nicely.

It does retain the ability to clean up w/the guitar knob very well; I was actually surprised at how well that works with this build. I haven't put the scope on this amp yet, but I'm guessing most of the overdrive is coming from the second gain stage being hit w/the 1st stage 12ax7/ef86. I doubt the KT88's are distorting at all, as they take a huge voltage swing to drive them in UL mode; about twice the swing as they take wired as pentode.

Originally I was going to include a switchable Pentode/UL mode, but as I was finishing up, I really started to run out of room for such a switch and it's wiring. It's a rat's nest as it is and I had a hard enough time getting the impedance switch installed due to space constraints/clearance issue with the output sockets. You'll notice I had to install the switch in one of the speaker output holes as it just would not fit in the hole drilled for the Z selector without touching the tags on one of the output tube sockets.

I think I'll wire it up as pentode output in a few more weeks after I've had a chance to really play it a bit. It sounds really, really good IMO in UL, but I'm curious to hear those 88's wired in the traditional pentode output fashion.

I'll get some clips posted tonight for those of you who are interested.

[Structo]

Did you have the face plates made?

Kind of makes you stuck with using what holes you have there.

Interesting on your phase inverter.

I've been experimenting with mine on a 6V6 Rocket.

Man I wish I had your real estate! My chassis is only 14 long...

In fact yesterday I raised the cathode resistor R5 to 2K2 to cut the gain a bit. I wanted a bit more clean head room.

I was going to experiment with different bias and tail resistors on the PI today.

Is the reason the Rocket PI has both input coupling caps at the same value because the two plate load resistors are the same value?
Typically you see the second cap about 10X what the first ones value is.
But I notice on Vox and the Rocket both plate resistors are 100K and both caps are .047uF.


How is your bass response on this amp?

Even though I have .022uF couplers on the PI and 6V6 grids the bass was such that I couldn't really raise the bass pot past 9:00.
Last night I doubled the bass cap in value, we'll see today if that tamed it well enough.

I'm thinking I may add a bypass cap (10uF?) on V1a and have it footswitchable.

[M Fowler]

Rob, nice build.

I haven't had a chance to go over your circuit yet, maybe tonight but I wanted to let you know that the pictures speaks for itself.

Mark

[rbryerton]

Thanks Mark! If you have any suggestions on how to get a bigger swing to the output tubes, I'd like to hear any.

Right now I think (I have to re-measure) I'm getting about 70v PtoP which is a bit low for UL at ~560v plate voltage; the spec sheet quotes 60v swing for tetrode connection and ~120v for UL (my bias voltage is around -60 to -70)

Tom, to answer a few of you questions:

>>Did you have the face plates made?

Actually the face & rear plates and chassis are from Mojo; they're for a 100w Marshall head

>>Man I wish I had your real estate! My chassis is only 14 long...

I knew I'd be using 4 OP tubes and 4 9 pin preamp tubes, plus 2 big chunks of iron, so the 100w Marshall style chassis was perfect...

>>I was going to experiment with different bias and tail resistors on the PI today.

Check this link out, it has a cool LTPI calculator:
http://www.pentodepress.com/home/amplif ... iled-pair/

>>Is the reason the Rocket PI has both input coupling caps at the same value because the two plate load resistors are the same value?

No too sure about that; maybe someone else could answer that question. I tried to stay as close to the Rocket as possible.


>>How is your bass response on this amp?

It's very good; has a strong, firm and deep bottom; not muddy at all, but very authoritative.


>>I'm thinking I may add a bypass cap (10uF?) on V1a and have it footswitchable.

That's a good idea; a bypass cap really kicks up the gain in a big way. A 10uf on a 1500R cathode resistor will give you a -3db point of around 10Hz. I like to use a 1uf which puts the roll-off at 100Hz, but that's just personal preference. Seeing as the low E string's fundamental pitch is ~80 Hz, there's really no point IMO of amplifying anything below that point...

[Structo]

Thanks for that Pentode Press link.
I think I visited that a few years back, but I bookmarked it this time.
Some nice calculators there.

I'm sure you are familiar with Aiken's site.

He has a great article on LTPI.
I believe you want to lower the cathode and tail to increase the gain right?

http://www.aikenamps.com/LongTailPair.htm

[Cliff Schecht]

>>I'm thinking I may add a bypass cap (10uF?) on V1a and have it footswitchable.

That's a good idea; a bypass cap really kicks up the gain in a big way. A 10uf on a 1500R cathode resistor will give you a -3db point of around 10Hz. I like to use a 1uf which puts the roll-off at 100Hz, but that's just personal preference. Seeing as the low E string's fundamental pitch is ~80 Hz, there's really no point IMO of amplifying anything below that point...

Typically you want to set the cathode capacitor to have a rolloff at one decade below the lowest frequency of interest. 10 Hz is about right. Of course that is a matter of personal taste so to each their own .

[rbryerton]

Typically you want to set the cathode capacitor to have a rolloff at one decade below the lowest frequency of interest. 10 Hz is about right. Of course that is a matter of personal taste so to each their own .

Why is that, Cliff? Is it just a rule of thumb, a technical reason, or ??

I'm curious....

[Cliff Schecht]

Well it gives you a boost in gain at lower frequencies. I think comes from the hifi world though and is just so that you don't get a huge boost in your bass frequencies and throw off the mix. The smaller the cap, the larger the boost you get at a higher frequency so if you make it too small, you'll get a fat boost that could be good or bad depending on what you're looking for.

[Structo]

I read somewhere that when you include those lower frequencies it helps with the harmonic overtones.

Or undertones, if you will.

That's why a lot of amps have a large bypass cap on the first stage.
Such as 22uF and higher.
This allows gain of frequencies below 50Hz or the whole spectrum of a guitar.

Then as the signal moves through the various stages that frequency response can be tightened up a bit to take the mud away.

[rbryerton]

So I recorded a clip of me stumbling around on my EC-1000; this is an LTD (ESP) LP type w/an EMG81 in the bridge & an EMG60 in the neck. The bridge pickup is all the way up, and the neck is turned down for a clean sound, and I switch between/combine them for clean, medium dirt, and full on distortion.

The amp is dimed except for: Bass, 12ax7 gain & ef86 gain @ 3 o'clock, ef86 gain boost on; going through a Hotplate into a 2x12 w/WGS Greenback type speakers & miked w/an SM57.

It's just me hacking around, but you can get an idea of the sound of this beast and how well it cleans up.

http://www.bryerton.com/misc/kt88rocket ... cket_1.mp3

[Structo]

Man, that sounds like it has some ballz for sure.

I was surprised at the bass response I got out of a pair of 6V6's.

I may try some JJ 6V6. The tubes I have in it are from the early 60's (silver top Sylvania's) so not sure how strong they are.

Was that with the UL hooked up?