New Rocket build

[Ron Worley]

I finished my Rocket "Ryann Leslie" (daughter) last night and had a chance to fire it up today.

Ran all the usual checks without then with tubes and used a bulb limiter- all was OK.

I got rid of the limiter and checked everything out:

Secondaries: 506V
B+1: 298
B+2: 289
B+3: 220
B+4: 197
B+5: 189
Bias voltage: 9.1V

V1b Plate:125V, Cathode:.878V
V1a Plate: 90V, Cathode: .679V
V2a Plate: 194V, Cathode: 125V

V4 & V5 Plate: 289.6V Cathode: 9.07V
V6 & V7 Plate: 290.5V, Cathode: 9.07V

The Cathode voltages on V1 had me really perplexed.... so I re checked the layout on the Preamp board, re checked continuity on all wire and connections- all was OK... I did however catch that the board mounting #6 machine screw by the input jack was touching the turret for the wire from the input jack... thought that might have been it...

Rechecked all voltages again and got the same for the V1 cathodes..... so now I'm thinking that either this is what I should be seeing, or something really stupid has happened in the build (wrong wire, wire wisker shorting to ground, whatever). The amp was pretty quiet, but enough hiss to know it was breathing.

So I figured that I would just go ahead and try it with a guitar.... and it sounds amazing... it's not as hairy as an Express, but dimed, it is every bit its peer.

So I must now conclude that due to my electronic ignorance, it's supposed to have these low cathode voltages, but I'm probably wrong...

So Skyboltone, Allyn, Dana, Jana, and all of you that know a hell of a lot more than I do, what's the deal here- is it right? if it's not, it sure sounds good for a screw up....

Many thanks in advance!

Ron

[Ron Worley]

More pics

[Ron Worley]

More

[Ron Worley]

Yet More

[gahult]

Ron,

Looks nice. Thanks for the pictures. Hope mine turns out as good.

Gary

[fishy]

Ron,
A great looking build.

FWIW, here are my voltages from my build. Several people have seen these and I have had no feedback questioning any to date.

I used 260v taps,
1k screens
56k replaced with 68k
Cathode resistor is 50ohms.

Original EL84s were replaced with Siemens and the cathode voltage came up to 9.4v . I have also used copper top but the voltages are the same as with real glass when I checked.

I tried to post these othe other day but had problems somewhere. Hopefully I can do it now....

[Ron Worley]

Fishy- Many thanks for the voltage chart, it would seem that my V1 cathode values are actually supposed to be that low...

Clearly I need some skoolin on the preamp layout and how it works...

Thanks Gary!

Ron

[DustyBadlands]

Hi Ron, Nice looking build! What brand of tubes are you using for this TW Rocket? I'm planning a rocket build next. How is the sound compared to your express? Thanks. -db

[Fischerman]

Nice build Ron. It just so happens I was piddling with my Rocket this weekend...wait...that didn't come out right.

I measured/recorded voltages since I couldn't find my voltage chart for the amp. This was a build from RJ's group build a while back...apparently my PT is slightly higher voltage (either that or it's my AC wall voltage which is high @ 123.7vac). I always measure/record the incoming AC wall voltage when doing my measurements.

AC wall voltage 123.7vac
B+1 313v
B+2 303v
B+3 258v
B+4 223v
B+5 214v

First stage: 117v/.96v
Second stage: 140/1.2v
Third stage: 223/140.5
PI: 206v/204.5v/51.5v/50.1v (last two are both sides of cathode resistor)
V4-V7: 304v/301v/9.45v

NOTE: I just remembered I have a 9.1K dropping resistor instead of 18K!...my dropping string is just 9.1k-9.1k-9.1k so that's why all my preamp voltages are higher. But I'll leave the voltages just in case anybody was interested. Also, my first stage is paralleled triodes but it's biased pretty close to the same.

I'm using Saratov EL84M from Sargeant Overdrive (aka Tube Tramp, aka Terry Kilgore).

[Zippy]

I finished my Rocket "Ryann Leslie" (daughter) last night and had a chance to fire it up today.

Ran all the usual checks without then with tubes and used a bulb limiter- all was OK.

I got rid of the limiter and checked everything out:

Secondaries: 506V
B+1: 298
B+2: 289
B+3: 220
B+4: 197
B+5: 189
Bias voltage: 9.1V

V1b Plate:125V, Cathode:.878V
V1a Plate: 90V, Cathode: .679V
V2a Plate: 194V, Cathode: 125V

V4 & V5 Plate: 289.6V Cathode: 9.07V
V6 & V7 Plate: 290.5V, Cathode: 9.07V

The Cathode voltages on V1 had me really perplexed.... so I re checked the layout on the Preamp board, re checked continuity...

Congrats on the build, Ron. As always, the proof is in the playin', eh?

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).

Looks like it is time to drive the holders and (successful) clone builders for a gathering of the voltages! Yes, there are a few different choices of iron out there, but a representative collection of data will reveal the ballpark values for a good build.

[Fischerman]

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).

[Zippy]

So he has almost 0.5 mA for one half of the 12AX7? Seems right to me...

[dave g]

Just a thought...

Has anybody tried messing with the dropping tree resistors for B+4/B+3 and B+3/B+2? I have been seriously thinking about it a lot lately as a means of reducing PI clipping (and therefore allowing you a larger swing on the grids of the EL84s, meaning a larger fraction of the total distortion will come from the EL84s). To get around 290v on the PI in my amp while maintaining the preamp voltages, I calculated that the 18.2k needs to go to 10.5k and the 9.1k after B+3 needs to go to 22.3k...so using the two 9.1ks in between B+3 and B+2 instead of between B+4 and B+3 would get me pretty close.

I'm using 275-0-275 iron, though, and run my PI biased hotter than the stock Rocket, so the exact values may differ quite a bit from amp to amp.

Thoughts?

[Ron Worley]

Hi Ron, Nice looking build! What brand of tubes are you using for this TW Rocket? I'm planning a rocket build next. How is the sound compared to your express? Thanks. -db

DB-

I'm using stock new production JJ EL84s and Tung Sol 12AX7s as startup tubes. I don't have hundreds of $$$ for nice NOS tubes (I will have built 4 TWs this year, and at about a grand a pop, it's a tad tight), but will eventually work my way into into some tasty preamp tubes.

I should check out the good Sargent's Saratovs... about how much are they running for a matched quad??

Ron

[strato17]

Hi Ron, Nice looking build! What brand of tubes are you using for this TW Rocket? I'm planning a rocket build next. How is the sound compared to your express? Thanks. -db

DB-

I'm using stock new production JJ EL84s and Tung Sol 12AX7s as startup tubes. I don't have hundreds of $$$ for nice NOS tubes (I will have built 4 TWs this year, and at about a grand a pop, it's a tad tight), but will eventually work my way into into some tasty preamp tubes.

I should check out the good Sargent's Saratovs... about how much are they running for a matched quad??

Ron

Ron... I really need to hear this thing!