New rocket build

[pablogt]

I just finished a rocket build along the lines of the "hybrid" rockets build in the group build initiated by rj some time ago. Many of the pieces come from rj, including the gorgeous cherry cabinet, custom faceplate and transformers.

There are not many "innovations" on my build:

- I incorporated Power Scaling (SB-84) in the design from the start. I only scale the power section, so I installed PPIMV to balance preamp and amp signals. Initially I was going to use a bootstrapped MV as in my previous build but decided on PPIMV because that keeps the gain structure before PI intact. I put Master before Cut on the faceplate because the bootstrapped MV lives around the PI so it made more sense from a lead dress perspective. For PPIMV it would have been slightly better to invert their positions

- I installed initially a switch for parallel input, with a separate 1k5/2.2uf cathode and paralleling another 220k anode resistor but, after trying it the first time, I removed the switch and hardwired it in.

- I send each electrolytic cap's ground along with the B+ (you can see it in the picture of the the boards' back side). The bus you see just connects the 5 local star grounds. I'm not sure if it was worth the effort, but the amp is certainly very quiet.

- There is a voicing switch heavily "inspired" on the old Rocket Tweaks thread. It parallels a 500p cap with C2, a 470p cap with C5, a 5.5k resistor with R14 and a 220k resistor with R11. This combines the mid boost "mod" and brings the PI to Liverpool specs. I sounds really sweet, I leave it on most of the time.

- Last, I used the switch that was initially intended for parallel input for a boost switch. Instead of bypassing R5 with a cap, I bypass it with a variable resistor (a trimpot) that is adjusted so that the plate voltage of v2a is exactly half of B+4. According to this post http://www.ax84.com/bbs/dm.php?thread=430662 from the AX84 forum there is some magic going on with harmonics when that voltage relation is set. I can tell you that I like how it sounds, I would describe it as "crispy". In my case, the sweet spot is at a combined cathode resistor value of 440R.

I'm using NOS Jan 5751 at V1 and V3, TAD 7025 highgrade at V2, NOS GZ34 "La Radiotechnique" and NOS Saratovs for the power section. The amp sounds really sweet, has a lot more gain than I expected (that's part of the reason for using 5751s) and it is the first time I'm out of ideas for modifying a new build, I just don't want to screw it!

Thanks everyone for sharing so much info. I had a great time building this amp, and the results exceed my expectations. Special thanks to rj, both for sharing his experiences and for caring so much when he sent me the parts I needed.

Pablo

[Gibsonman63]

Clean. I'm digging the twisted wire. Nice build.

[ampmike]

Wow Pablo,Awsome build,really neat too.Just wandering how you made the front faceplate? Very pro looking build,Have fun.I bet it sounds sweet!!
,rj is great to work with also. Mikey

[pablogt]

Faceplate is rj's merit. As a mater of fact, he had one version printed in one side, he thought the power control was too close to the standby switch and took it back to the print house to have it re-printed on the other side. What a guy!

[Gibsonman63]

I just noticed the offset handle. I did that on my last build. It is amazing how much lighter the amp feels when the handle is over the center of gravity.

[DocJames]

Great build, makes me think of rebuilding my rocket and putting more bells and whistles into it . I just noticed the dual ganged PEC pot, how much did that send you back, and where did you get it?

[pablogt]

I live in Madrid... Got it on a German on-line store for the ridiculous quantity of 46€. I just could not stand seeing a tandem Alpha among all those nice PECs http://www.banzaimusic.com/PEC-MIL-STE-LOG-250k.html

[M Fowler]

Very nice build and love the cabinet, which I did know rj offered that wood.

[davesi]

Very nice!

[RJ Guitars]

Hey Pablo, very cool build. I was just wondering yesterday how this had turned out for you. It looks awesome! If you get a chance for more photos and some clips I'd be jazzed to learn a little more. Thanks for the kind words!! rj

[pablogt]

Ok, here go some clips. They were recorded with a Zoom H2 (sorry, I should have increased the mic gain), all amp controls at noon except for power and master witch were reduced to get about 5watts output (the neighbors are at home!).

The guitar is a HSS strat with vintage type singles and a hot Bareknukles humbucker. The speakers were a EV12L in a closed cabinet and a Celestion G12H30 in an open combo cabinet. The three clips contain a clean section, with neck pickup at about half volume, then a crunch section with neck pickup dimed, a chunky section with bridge humbucker at full volume and a lead part with humbucker full volume and a Menatone Blue Collar overdrive with all controls at noon.

The first clip is with the amp "plain", the second with the voicing switch up and the third with voicing up and the boost on.

Please excuse my lousy technique

[pablogt]

There are some details that some people might find interesting. On the photo below you can see:

- The white trimpot on left, which is a mids control pot. The mark indicates the 10k position, rocket's standard value for the resistor the pot replaces

- the switch just at the bottom edge alternates between 260 and 300 volt PT secondaries. It is not very accessible, but I did not want to cross all the OT leads with 300VAC cables. I think I prefer 260v, although I just noticed that I recorded the clips on the 300v position

- Every filter cap is bypassed by a 0.47uf Solen fast cap. Since I'm not quite comfortable with the idea of having all the filters in one side, I wanted to be sure to have clean B+´s

- The standby switch is bypassed by a 100k 2w resistor and a 47nf film cap. The resistor allows a slow charge of the filter caps (except for the first one that is before the standby) and the cap removes the pop when switching to the play position. With the standby switch bypassed, it would have been better to have the first filter cap behind standby, although the tube rectifier should allow a slow charge of that one too.

- The rectifier tube is paralleled by 2 diodes. These would let the amp work even if the tube failed (although at a higher B+) but, most importantly, would protect the OT in case the tube shorted. There is also a 0.01uf 3kv cap across the pt secondaries, to prevent possible ringing noise by the diodes.

Many of these additions come from Merlin's books. The more you read, the more stuff ends up crammed in your amps!

Pablo

[Reeltarded]

You are a machine! That amp is a machine!

Nice work!

[dcribbs1412]

pablogt
congrats on a outstanding looking build
nice mix of the original design along the "innovations" you've added
love the grape resistors...they have to add a nice flavor...
nicely done

Darin

[pablogt]

Thanks all for your nice comments. On the CC resistors, I like to use them on the plate resistors. I only had one left, and I used it for V2a. I'll change the PI and V1 ones when I get more.

Pablo