New Rocket Launch

[gktamps]

For some reason, I keep building variants of Rockets, each with their own features, tone and character, though they each sound great. In fact, while the basic character of the Rocket is always present, each of the mods add to the unique character of each amp.

Here's my latest, finished just today. I used a Heyboer HTS-9144 PT, and RJ's Edcor 4.3k/5.2k OT, which I wired to an impedance switch. I used F&T filter caps on a board, with the power tube cathode resistor and bypass cap (wire/sand) on the board ala Liverpool. Also added an HT fuse, and bleeder resistors. Heater elevation is intended but not yet implemented (Thanks, Colossal Dave for the advice off-list).

Grounds are arranged differently from stock, and I added diode protection on the rectifier tube socket, and Merlin's CF protection on that tube socket. I diverged from the classic selection of caps, using polystyrene instead of mica, Russian PIOs for the .022uF, second stage switchable bypass caps (none-1uF-10uF) hooked to a back panel jack, Mallorys for the .047uF, PIO for the cut cap, and used a switched treble pot to select one 47pF stock, or paralleled with a 500pF for some boost.

There are a few unused turrets where I moved the PI coupler cap, and replaced the 220k resistor with a 250k pot - a squish control. Also, like I've done with another build, a treble cap switch was added up front.

Voltages looked good, so I played it for an hour and a half to an encore call from the wife, who gave this amp two thumbs up. The amp is quiet, no bad manners, and the tone shaping is amazing, with the 47pf/500pF switch having a remarkable effect, but even more so, the 4.3k vs. 5.2k output impedance really changes the character of the amp. In the 4.3k setting, the sound is more what I am used to with a Rocket, with great cleans that get nice and dirty with the guitar up full, and even dirtier with the various tone shapers in action. But with the amp running at 5.2k, the sound is more compressed, with lots more grit and grind - very ballsy. I bought the Edcor and a Pacific from RJ, intending two build, each with their own "thing", and this Edcor OT build certainly has its own thing going!

Output tubes are biased at 10.8 watts using a 62 Ohm bypass resistor, and I used 470 Ohm screen resistors, but still need to take screen voltage readings. Just using JJ power tubes for now, but swapped around 8 to get them balanced in the amp - even though they were all supposed to be matched. Preamp tubes are Svetlanas. Also need to adjust the chassis mounting holes a bit, though the front panel also needs to be shaved on he bottom at one end - not square.

Pics to follow.

Cheers,

Greg

[gktamps]

Photos

[gktamps]

More pics.
Extra hole in front panel is for another mod. Need a small body pot, so nothing in there yet.
I didn't build the cab.

[Mark]

Output tubes are biased at 10.8 watts using a 62 Ohm bypass resistor, and I used 470 Ohm screen resistors, but still need to take screen voltage readings. Just using JJ power tubes for now,

I hear you loud and clear on this one, those output tubes NEED to have 290VDC on them to run anywhere near where you have them. Mine runs between 306VDC and 310VDC depending on the wall voltage.

I think what you did makes more sense, and who is to say the original Rockets didn't run at 306VDC to 310VDC?

[gktamps]

Thanks, Mark. I'm away from the amp for a week, now, but will post a full voltage chart when I return.
Cheers,
Greg

[gktamps]

After giving this build a couple of months to settle in with me, I decided to pull the Edcor OT (RJ-EM7020) 4.3k/5.2k and replace it with the Pacific OT I bought from RJ at the same time (RJ-040112).

When I built this with the Edcor I installed a switch for the primaries, and found that the 5.2k setting was really just too grindy and harsh for my tastes. Voltages are right in line throughout the amp, and tubes are biased for about 10.8 watts. The 4.3k setting was more to my liking, but still pretty aggressive compared to my other Rockets, including one I built with an original Dynaco A-470.

Taking a break from my current Express build:
https://tubeamparchive.com/viewtopic.php?t=25086

... I spent a couple of hours swapping the Pacific OT in, making no other changes, I have found that there is a noticeable difference in the tone and response of the amp. I prefer the Pacific, which gives the amp a darker feel and "bluesier", less aggressive distortion. Midrange seems a bit more pronounced, but the bass seems a bit cleaner. Turned up, the amp has plenty of dirt and grind, but is more musical to my ears. It passed the "wife" test, too - she volunteered that she preferred the change.

This puzzles me a little, because RJ spec'd 5k for this take on the Dynaco A-470, which makes it closer to the higher primary setting on the Edcor that I didn't enjoy. Also, compared to my Dynaco OT build, I prefer the tones I am getting out of this amp with the Pacific OT, although there are some other circuit changes and refinements in this one.

I think the Edcor would be good in a Rocket type build modified for less gain, so the OT could exert its influence without compounding too much. Or, maybe it's just the ticket for someone else's ears.

[RJ Guitars]

Greg,

I think I missed this when you first posted it so I didn't follow you through the phases at the time... too bad, it's a great piece of work. I just now read the whole thing through and it's a great piece of documentation on more Rocket research, and another great amp build.

I like this work because you made only one change and it apparently made a big impact. I don't know if it's the 7-layer wind, it's got more iron, or any of the other subtleties of transformer winding technology, but those Pacific style Dynaco's really fill in the tone spaces. I think the 35W Edcor output trannies are useful for an amp where you are looking for a simple gritty sound (limited number of cases where you might want that) and it's becoming more apparent that they Dynaco style remains among the best choices for adding depth to the sound of a Rocket.

I am setting up an amp test platform https://tubeamparchive.com/viewtopic.php?t=25030 to be able to quick swap OT's in an Express style build. Once that is done I'll need to rebuild the test amp to accommodate Rocket OT's so we can compare them as well.

[Colossal]

Nice build Greg! I like the added options for more versatility and tonal variety. You may have already changed it, but is that 22uF bypass cap in the input stage reversed? (printed line towards ground)

[gktamps]

Hi RJ,

Glad you have chimed in, because I am really intrigued by your test rig, and think it's going to be a fantastic tool for making these types of comparisons, and more.

The original Dynaco OT at 4.3k has a slightly lower primary impedance than your 5k, 7-layer Pacific version, so even that would be a useful comparison. As I mentioned earlier, I built a slightly different version previously, using different tone and filter caps and a real A-470, and the sound is definitely different than with this Pacific iron.

Holding the Pacific next to the Edcor, they seem to have identical total lam width, but each lamination in the Pacific appear thinner. Not sure what that accounts for. I didn't compare weights of each. The real A-470 total lam width is of course significantly larger than either the Edcor or Pacific - by more than 1/2" - but that's probably because the screen taps are not included.

My observations on tone with this amp were independently echoed by my wife, who also preferred the Pacific iron.

Although I haven't built any faithful repros of a Rocket, the design makes a fantastic launching pad (pun intended) for variants that each offer their own excellent tone characteristics.

Cheers,
Greg

[gktamps]

Thanks, Dave!

The options are outwardly subtle, but the effects are really remarkable. There is a lot of tone shaping available and plenty of gain on tap for my tastes.

Yes, I caught the reversed bypass cap before firing it up. Then I promptly made the same mistake on my Express build that you've already read about, but Jelle caught it in the photos. Fired that one up last night without tubes to check wiring and voltages, and tonight it gets the full treatment. Just waiting for Jeff to complete my cabinet (no pressure, Jeff, honest).

Cheers,

Greg

Nice build Greg! I like the added options for more versatility and tonal variety. You may have already changed it, but is that 22uF bypass cap in the input stage reversed? (printed line towards ground)

[jeff12]

[quote="gktamps"]Thanks, Dave!

Just waiting for Jeff to complete my cabinet (no pressure, Jeff, honest).

Cheers,

Greg


Ok,Ok time to post something here. Have been busy with lot of projects, I should be posting more I know. Here are progress pictures for you Greg.
Had to set up a new miter sled on my table saw, sorry for the delay but you will like the results. Incra makes great stuff. Look at the miter on that corner.

[gktamps]

Hah!
Knew I'd get you out of the woodwork with that, Jeff.
That is beautiful, man!
Sang your praises in my Express build thread - really appreciate your craftsmanship, Jeff.
Cheers,
Greg

[rp]

You know if the Pacific is a 7 layer M6 job with same mounting as a Dynaco it could be an interesting sub for an A470 in Hifi duty. Just a thought.

Wonder how well it rates on tech details? Anyone deep into Hifi around here?

[M Fowler]

I really like this style of Jeff12 TW cab in cherry wood and the stain is perfect shade to me.

Richie had a cab vendor that made his cabs very close to Jeff's.

[cbass]

Thanks, Dave!

Just waiting for Jeff to complete my cabinet (no pressure, Jeff, honest).

Cheers,

Greg


Ok,Ok time to post something here. Have been busy with lot of projects, I should be posting more I know. Here are progress pictures for you Greg.
Had to set up a new miter sled on my table saw, sorry for the delay but you will like the results. Incra makes great stuff. Look at the miter on that corner.

Showoff