More Rockette Tweaks

Finally got around to putting the bright cap on a switch. Also added a 500pF cap option. Eh, you got some center-off DPDT switches lying around, you stay using 'em.

It's okay, nothing to write home about.

I'm also planning on making the bridged channels selectable. Using both sides of the LTP wins up costing you some gain, and why not get that back?

Also, I might as well add a sag resistor, put that on a switch too. I'm pretty proud of how rock-sold the power supply is, and the modest plate voltage, but

So a total of three switches that are pretty superfluous.

I don't know; what the hell. It's my amp.

MPS Iron

I got a Musical Power Supplies OT40PP for the Rocket build. I have one of MPS's earlier offshore versions of this OT in Bodie and it works well in that amp.

This new model is made in the US with M6 steel, 4/8/16 ohm taps on the secondary, and 4k2 and 3k3 primaries.

It's a great chunk of iron at a great price. No idea who's doing the winding, but this guy knows how to spec his iron. I'm super happy, especially compared to the stock Crate iron.

It's kind of odd to think how similar the Fender 5F6A, Marshall JTM45 & Plexi, Vox AC30 (Trainwreck Rocket), and Sunn Model T are. Okay, the first two really aren't much of a surprise, and there are countless other examples... I guess it's just a really popular topography.

I can kind of see it, but it's just not doing much for me. Maybe with a different tone stack or some NFB. Just a little more midrange and a little less bass would go a long way.

Final-ish Rockette Schematic

The previous .sch files appear to be lost, so I drew up a new one showing the current schematic of the Rockette.

Some of the parts values might be off, but dig that normal channel. Super useful.

Rocket in the pocket

So, I got the Rocket finished a while ago. I decided to add a Vox "normal channel" by using the spare gain stage through a gain pot into the other side of the phase inverter. It took a little tweaking, namely an additional grid leak, stopper, and coupling cap, but it's worth it.
Drive is reduced slightly on the TB channel, which is fine, and the single gain stage normal channel can drive the amp into overdrive just fine. The "cut" control still works for both channels, of course, since it's after the phase inverter.
I used the other side of the PI (the second triode's grid, which is usually grounded with a large cap) because the two channels are out of phase. I kept the two channels permanently bridged because I didn't feel like adding three input jacks and the ability to blend the two channels is great. You can dial in some mids to get a thicker sound than a typical Vox, or use either channel independently.
It's not a revolutionary idea, I mean, the AC30 uses the PI to mix the normal and overdrive channels with the reverb/trem signals on the second grid. Typically the normal and tb channels can't be bridged, so this is novel, at least, while bringing the pi closer to the original.
I'm tempted to add channel switching by using an on/off/on SPDT to shunt either grid to ground with a large cap. Heck, why not do it with a relay and footswitch? Ah, but how to power a relay?
Unfortunately, the rocket's going to be out of commission for a while due to a shorted output tube, but if anyone is looking for ideas about the Trainwreck "unused triode" I just have two words: normal channel.
Oh yeah, and here's a nasty gut shot as I still haven't drawn an updated schematic!
I also moved the hv fuse to the front panel because it just made way more sense there from a wiring perspective.

PI

So! Finished the power tube wiring. Used one bias resistor for the inner pair and one for the outer, so one set can be pulled if need be. The tubes are biasing at 13W and 13.5W, which should be fine.
I also got the PI wired - I promise it's in there somewhere. Pretty much as far away from the power tubes as possible. Next time, I'm totally drilling the chassis; I'm lucky it's quiet.
Well, that's not entirely true. I did my best with the grounding and layout that I could. And the preamp isn't wired in yet, but compared to the "are you sure it's on" Dynaco gold standard for quiet power amps, this guy's not quite there. Compared to my Twin with the master on 0, though, the Rockette is an order of magnitude quieter. And it's not even connected to the chassis yet!
Okay, enough. I thought that the massive leads to the output tube grids would be a death sentence, and I'm glad they're not. Also, those chicklets are totally 630v film caps. I vaguely remember Mouser being out of the particular caps I wanted and apparently I never followed through on ordering them. Same reason there are some different 470k metal films in there.
Oh well. I'm also running out of teflon wire, so the color scheme is probably going to get even wackier in the preamp.
I really need to give this thing a different name.

I have some cascode wackiness planned for the unused triode in V1 so I can get both the AC30 top boost channel and something like the AC30/4 normal channel at the same time. Bridged, even. I'll decide if that's worth it after I get the stock schematic working.
Or I could still try the "jumpered Plexi" thing. I should at least try the amp as Ken Fisher might've designed it first.
Maybe that's some variant of "blind urge to mod" syndrome. "Blind urge to make a design more 'original'" syndrome. Eh. It's my basement; I make the rules!

The five percent nation of filter caps

Finally got a little more work done on the Rockette. I need to think of a better name.
Anyway.
Got the power amp finished, except for the grid leaks. Maybe I will put that PPIMV in after all. The PI tube is all the way across the chassis so I'll probably need shielded cable anyway.
Also finished wiring in the filter caps. I don't like turrets; tag strips have ruined me forever.
It's crazy how time-consuming this is when you're not painting by the numbers. If I had a board of the preamp components that I could just plop in, I'd be done by now.
Hopefully I can avoid noise problems, because fixing them is going to be a bear. The "distributed star" grounding is going to help, and I should be able to attach the components to the preamp sockets in a "bicycle spoke" pattern also, which is pretty much ideal.
Time will tell, but I'm optimistic.

Incremental progress

Not much time spent today, but I got the grid stoppers in place and the screens connected.
Really wish I had a chassis-mount resistor for biasing the power tubes, but I think I'm just going to plow ahead with the cement blocks I have on hand.
I did spend a little line on the load line for this 3k OT. Probably the only thing I ever thought "when am I ever going to use this" about in college was graphical analysis, rulers and pencils and such. "We've got computers now!"
Har, har.
Little did I know I'd be trying to extrapolate anode curves for a 6V6 at 350V, for fun.
...I gotta learn Spice.

I've got the power (supply) (almost)

So, pretty wild that I've had another productive day. Didn't have a ton of time, but still managed to get the power supply laid out and the power tubes halfway wired.
Oh, and I finally measured the Crate OT: 3k plate-to-plate (to 8, 16 ohms) which probably seems low for a guitar amp, but I'm thinking it'll be pretty close to ideal. I'll have to plot some loadlines, just to be sure.

Bridge to B+

Ugly, sure. But it works. For the first test fire, I forgot the bleeder. Yay, 375V! Um... Where's that cap drainer?!
I did leave the B+ fuse on the back panel. That whole corner of the chassis is completely off limits for any signal wiring. AC radiating everywhere.
Next donor chassis... I've gotta drill it. This is just silly.

Rockette - more wiring

So, I got the PT primary wired up. The safety is wired to its own chassis connection, and the purple is just a static shield for the PT.
No rectifier yet, but we're getting 273V AC, both unloaded and with the tubes in (heater load). Forgot to check what the heater sags to, but unloaded it's at 6.8V AC, which is fine.
Admirable performance from a PT sold as 250V. Added bonus: it's rated for 100VA at 50 Hz. Long story short, it should be able to supply much more current than the amp can draw. Which is great, because how guitar amp PTs have been sized in the past irks me.
To that end, I don't think I'm going to install a sag resistor. At least not initially. I might abandon cathode bias later on too. We'll see how punchy it is, and if it's not enough I'll go from there.
I think I'm going to mount the B+ fuse internally. As much as I like the pretty fuse holder on the back, it's just going to be inconvenient for wiring. Same reason I installed a Carling instead of using the front panel power switch it came with.
Slow and steady...

Heaters

...I hate wiring heaters.
I'm hoping having the heaters run perpendicular to most of the signal wiring + the correct approach to the sockets + an elevated humdinger should add up to no hum, but I guess we'll find out.
I probably should make a genuine elevation node for the heaters instead of just referencing the power tubes' cathodes, what with the DC cathode follower raising Vhk on v2b into dangerous territory.
Kind of moot until I can figure out where to put the bridge rectifier and the reservoir & screen supply caps.
The rest just falls into place though.

More Progress on the Voxy Build

Just a quick update; making some tweaks to the Rockette. The Trainwreck Rocket completely ignores one triode of V1, which seems wasteful, and the AC30 dumps that output straight to the phase inverter, which seems silly. So, I got a little Plexi goin', and ran the "normal channel" into the next gain stage. Basically permanently jumpered channels that you can blend with the two volumes between bright and regular. Maybe I'll split those V1 cathodes if there isn't enough difference between the two volumes...

Moved some other things around, nothing too serious. Might add zeners on the cathodes? Maybe just on one pair? Maybe not, come to think of it.

Oh yes, and the parts are ordered, so the build will start shortly!

New Crate Plans

Alright, so, I've decided to take the Crate rebuild in a different direction - Vox land. I was wondering if anyone would be interested in a single channel AC30 with no reverb or tremolo and it turns out yes, they call it a Trainwreck Rocket!

Now, of course, I'm going to have to deal with heat. I suppose I would have anyway (seeing how I don't work for Crate and thus am not comfortable hanging tubes below a completely sealed chassis) but cathode bias ups the heat load I'm going to have to dissipate. Also I'm going to put in (switchable) sag resistors to emulate tube rectification. Needs moar recs!

Here's what I've come up with so far. Still a lot of tweaking to be done. It's based on what people assume is in a Trainwreck Rocket, as well as the original AC30 top boost channel. I've added a handful of tweaks throughout; we'll see how they work.

• Parallel first triode. Why the hell leave it unused. Same gain, lower noise. 
• Arc protection on the cathode follower to help the tube survive switch-on.
• Grid stopper on the cathode follower. Just something I feel like trying. Figured I'd split the difference between 10k and 1M for starters. 
• Reduced grid leaks on the LTP to reduce noise. Reduced cap across the second input to help recovery from blocking distortion.
• Lar/Mar PPIMV
• Sizeable grid stoppers on the power tubes for better distortion.
• Power tubes biased in pairs, partially to spread the heat around, partially so one pair can be pulled. I might do individuals, we'll see.
• Zener-assisted bias for the power tubes. Cathode-biased 6V6s seem to distort nicely anyway, but this might be worth looking into.
• Switchable sag resistor for a "tube rectified" feel. Might make these bigger.
• Heaters referenced to power tube cathodes for free elevation. Humdinger to get the last of the noise out.
• 1k screen grid stoppers. 
• Grid stoppers everywhere. 10k for starters. 

Layout is going to be a big pain. All the tubes are in the center of the chassis, so I'm going to have to get clever. I'm tempted to start drawing it up in DIY Layout Creator or sort of CAD program. I may just use paper. It's going to have to be all point-to-point, but since this isn't going to be as much of a ground-up as Bodie was I'll be able to plan better.

Welp, I've been spending even less time in front of the computer lately, but I'll post again when I've got more to go on.

The next one!

So, still in the process of homebuying, yeah? Brainmeats don't want to stop thinking about amps. This one popped into my head today - okay, I've been thinking about Plexis a lot and it doesn't look like building a Plexi from a kit makes a whole lot of economic sense w.r.t. resale possibilities. And I've got this Crate GT50 Stealth combo that's been begging to be de-Lee Jackson'ed into some proper Marshall thing.

So why not a Plexi? Sounds like a plan, right? Well, my stupid brainmeats got around to thinking about a couple building blocks that it really likes, a couple it really wants to try out, and somehow I convinced myself to build a new "original" design. Well, original in this case means "stolen from Merlin" but... wait, why am I telling you this?! I am a God of Mojo! I invented the pentode!


*cough*

Sorry; I'm a little delirious. I just spent three hours or so typing up this:

Okay, admittedly, the power supply is a little lacking. The rest though... ho ho ho.

So, starting out: the first two stages can be either run in parallel (think: jumpered channels) or cascaded (think: one-wire mod) at the flip of a switch. Then those slam into a small-signal pentode (think: EF86). The sound gets smoothed out by a directly coupled cathode follower and the rest is pretty much "every amp ever made" though I'm going to try to design a lower output impedance long-tailed pair phase inverter so I can ixnay blocking distortion in the FOUR 6V6 OUTPUT SECTION. 

I do have a working OT this time but I've got to pick a PT which means the power supply world is my oyster. I've already started 'funny' for a guitar amp by using a full-wave rectified bias supply.

Okay.

Well, to summarize: this amp is going to be pretty high gain but extraordinarily versatile. Cleans are going to take a back seat to this glorious preamp distortion and who knows what the final result is going to look like considering how many times I changed Bodie around, but man... this'll be interesting.

On the bench

It looks like it's going to take a while for me to post details of everything I'm working on, but here's a quick list:

• Stromberg-Carlson AU-36B (100W beast)
• Bogen CHB-35A - 30 W of glory, if I can ever settle on a schematic. Planning on making this my main gigging amp.
• Crate GT50 Stealth - Combo with 4x6V6 output section and a fried PT. Might try to avoid point-to-point for this one. Probably going to be a classic Marshall, like a 4010 or 4001.This one I'd likely be willing to sell.

I've also been looking at modding a Silverface Deluxe Reverb for a customer. Been pretty interesting so far pushing the schematic around; really excited to start tweaking the bugger. 

And if $600 falls in my lap from the heavens I'm thinking about throwing together a 5E3 just for fun, maybe flip it for a tiny profit that I'd roll into future amp parts.