New Rocket build

[Ron Worley]

Strato, you live 20 miles away, you're more than welcome to come hear it, I'm around this weekend....

Ron

[Fischerman]

daveg,
Is your PI clipping before your power tubes? On mine the power tubes clip first. I'm using a 9k1-9k1-9k1 string and a 260-0-260 PT...but my wall voltage is always high.

[Stanz]

A quad set of EL-84 from the sarge are very competitive with new tubes.

[Ron Worley]

I got the matched quad from the Tube Depot for ~$31.00....

What is the Sarge charging??

Ron

[skyboltone]

When calculating a dropping string, one usually assumes a current of about 1 mA for a 12AX7 - so somewhere around 1V is appropriate to read on a cathode (for a first order approximation).

Depending on the cathode resistor value. Ron's first stage has .679v across a 1k5 resistor for a cathode current of only .42mA. That was why I used 9k1 instead of 18k in the dropping string...it just seemed the voltages and currents were so low (even though I was aware that's where they are supposed to be in a Rocket).

Time I think for a quick look at table 9 of the resistance coupled amplifier section of your local RCA Vacuum Tube Manual. The gain thru a triode of a resistance coupled amplifier is dependent on four things. #1 plate voltage, #2 plate resistor, #3 Cathode resistor, and #4 the input resistance of the following stage. Table 9 puts alot of the math in solved form for a variety of situations; none of which include the initial stage of a Rocket. But, you an interpolate what yer gonna get from the values given. This is a much lower gain amp than the Express as many have noted. The voltages at stage one and two are low enough that I'm real interested in the outcome of those projects that have included the VVR. At some point, (50V?) 12AX7s crap out.

If you want more hair on it you can fool with the dropping string, or the cathode resistor/plate resistor pairings. This amp doesn't follow a 66:1 ratio of plate to cathode resistor values.

[Zippy]

The voltages at stage one and two are low enough that I'm real interested in the outcome of those projects that have included the VVR. At some point, (50V?) 12AX7s crap out.

I've heard a Rocket build with VVR that, at low settings - certainly below 1/4, the sound just turns "fizzy". How others claim that one can go to near zero without degradation of sound just flies in the face of logic, physics... (insert your favorite religion).

[Noel Grassy]

Really nice example Ron! That looks alot like the one Allyn built for a benchmark. If you run that and the Express I bet you'll get the gout or
something.

In the words of Emeril the Chef, "Aw yeah Babe!".

[tweedeluxe]

Ron, great looking build! Hope mine turns out half as nice.

I can't quite make out the ground wiring from the photos - are you grounding the 40uF caps at the rectifier socket and grounding the 20uF caps on the same lug as 220k grid resistors?

[M Fowler]

Nice build Ron and I like having your pictures and information you and others are giving which helps with getting started on my Rocket build.

Mark

[Fischerman]

Time I think for a quick look at table 9 of the resistance coupled amplifier section of your local RCA Vacuum Tube Manual. The gain thru a triode of a resistance coupled amplifier is dependent on four things. #1 plate voltage, #2 plate resistor, #3 Cathode resistor, and #4 the input resistance of the following stage. Table 9 puts alot of the math in solved form for a variety of situations; none of which include the initial stage of a Rocket. But, you an interpolate what yer gonna get from the values given. This is a much lower gain amp than the Express as many have noted. The voltages at stage one and two are low enough that I'm real interested in the outcome of those projects that have included the VVR. At some point, (50V?) 12AX7s crap out.

If you want more hair on it you can fool with the dropping string, or the cathode resistor/plate resistor pairings. This amp doesn't follow a 66:1 ratio of plate to cathode resistor values.

I think you meant 'supply' voltage where I bolded but whatever. Good question on the VVR...now I'm interested in that too.

In the Rocket; the one less gain stage (and the Rocket's second stage cathode isn't bypassed either), much lower voltages, and a much less gainy PI circuit all contribute to much, much less gain. But when I added a bypass cap to the second stage cathode it came right back out...sounded much better without it imo. I have been meaning to try tweeking that second stage cathode resistor value (might try that this weekend)...it should affect gain quite noticably since it is not bypassed.

There's plenty of extra gain to be had but consider this; it only takes about ~250mVac at the PI grid for the amp to reach full clean power. Guitars with regular ole passive pickups can put out around that on hard strums. Point being that we only need a tiny, tiny bit of preamp gain to reach full clean power...and that's with our somewhat 'choked' PI values.

FWIW, when I put a 100mV/880Hz signal into the input of my Rocket I get 6.2vac on the first plate...but that's with triodes paralleled, 100K shared Rp, 1K shared Rk, and I just stuck with the 25uF cap (technically I should increase it but didn't).

Ron, I can't remember but IIRC the Saratovs were around $24 per matched pair. More...but not outta hand.

[67plexi]

Ron, Looks great but you need to change the red standby from the 2- 40 uf caps to pin # 8 on the gz34 as marshall jtm 45 not pin two.
Outstanding work AAA+++

Best regards, Steve.

[skyboltone]

Time I think for a quick look at table 9 of the resistance coupled amplifier section of your local RCA Vacuum Tube Manual. The gain thru a triode of a resistance coupled amplifier is dependent on four things. #1 plate voltage, #2 plate resistor, #3 Cathode resistor, and #4 the input resistance of the following stage. Table 9 puts alot of the math in solved form for a variety of situations; none of which include the initial stage of a Rocket. But, you an interpolate what yer gonna get from the values given. This is a much lower gain amp than the Express as many have noted. The voltages at stage one and two are low enough that I'm real interested in the outcome of those projects that have included the VVR. At some point, (50V?) 12AX7s crap out.

If you want more hair on it you can fool with the dropping string, or the cathode resistor/plate resistor pairings. This amp doesn't follow a 66:1 ratio of plate to cathode resistor values.

I think you meant 'supply' voltage where I bolded but whatever. Good question on the VVR...now I'm interested in that too.

In the Rocket; the one less gain stage (and the Rocket's second stage cathode isn't bypassed either), much lower voltages, and a much less gainy PI circuit all contribute to much, much less gain. But when I added a bypass cap to the second stage cathode it came right back out...sounded much better without it imo. I have been meaning to try tweeking that second stage cathode resistor value (might try that this weekend)...it should affect gain quite noticably since it is not bypassed.

There's plenty of extra gain to be had but consider this; it only takes about ~250mVac at the PI grid for the amp to reach full clean power. Guitars with regular ole passive pickups can put out around that on hard strums. Point being that we only need a tiny, tiny bit of preamp gain to reach full clean power...and that's with our somewhat 'choked' PI values.

FWIW, when I put a 100mV/880Hz signal into the input of my Rocket I get 6.2vac on the first plate...but that's with triodes paralleled, 100K shared Rp, 1K shared Rk, and I just stuck with the 25uF cap (technically I should increase it but didn't).

Ron, I can't remember but IIRC the Saratovs were around $24 per matched pair. More...but not outta hand.

You are quite correct, I thought about that after posting and was going back to correct it when I got distracted, maybe by that debug business but whatever. It is indeed plate supply voltage.

[Ron Worley]

Ron, Looks great but you need to change the red standby from the 2- 40 uf caps to pin # 8 on the gz34 as marshall jtm 45 not pin two.
Outstanding work AAA+++

Best regards, Steve.

Steve- I am no genius on Rectifiers, but I put the middle 40uf wire on pin 2 because that's how it's wired in pictures of a real Rocket and in Allyn's replica of a Rocket. They're both heater connections, can you 'splain why you think it should connect to pin 8??

Thanks for the compliment, I try really hard to get it right.... but there are things on this build that aren't perfect- slightly miss aligned preamp board installation and other examples of Anal behavior on my part....

Ron

[stevek66]

Ron,great looking build.I will be building a Rocket myself as soon as parts start showing up.Where did you ground your Filter caps?I noticed it was different than the rocket layout.Did you just ground to one of the choke lugs.
Again great build and thanks for sharing pics........Steve

[Ron Worley]

Ron,great looking build.I will be building a Rocket myself as soon as parts start showing up.Where did you ground your Filter caps?I noticed it was different than the rocket layout.Did you just ground to one of the choke lugs.
Again great build and thanks for sharing pics........Steve

Steve-

The layout is wrong, and we're correcting it. The bottom 40uf caps are all tied together on the ground side and the wire goes from the center cap to the grounding lug on the rectifier's Cinch style socket (which you can get from Mojo Musical Supply for ~$8.00 ).

The 2 20uf caps closest to the preamp board are tied together and a wire goes from the center cap to a ground lug mounted between the preamp board and the front edge of the chassis. The 20uf closest to the power switches has it's own wire that goes to the same ground lug. This lug is also used for the "Bias" connection off the preamp board as well.

Hope that helps!

Ron