Building a Rocket - Layout Check for group build

[DocJames]

Hi nickt,

please correct me if I'm wrong cause I'm no epxert, but the schematics by Matt Taylor show the V2a being used for the 2nd gain stage. Also, looking at those initial build pictures from Dana, I think on Page 1, it appears he's using both triods on the V2 also. It does however look like the second triod is used for the first gain stage.

I think the better question is, will either of these changes make any difference in the final tone of the build?

[Fischerman]

I think the better question is, will either of these changes make any difference in the final tone of the build?

The way you just described seems like it would yield preamp voltages closer to the AC30 (which apparently has a much higher plate voltage for the first stage than the Rocket). It will make a difference in the preamp voltages how you do it. And if we wire the second stage to V1...does it still use the same PS node as the first stage or does it use the node for V2? These are all things I intend to try...and they are very easy to try...but which preamp tube gets the second stage is a bit more involved. To be honest...I like the Marshall/Vox way (i.e. cathode follower and stage driving it on same socket) as far as looks and layout and it obv works fine (tonally) in those amps. Then I can try paralleling V1 for the first stage. So many options!

EDIT: I just checked that pic on page 1 and Dana used V1 as the input but put the second stage on V2. Remember...the 33k grid stopper for the input triode is on the board at the very end (another thing I'm not crazy about...grid stoppers go on the socket).

[txbluesboy]

UPS just delivered my chassis and transformer order on the rocket group build. I have to say its not exactly what I expected--- I ordered a chassis, transformers, choke and faceplate. What I got instead was a half built amp!!! The tube sockets are already installed, the transformers and choke are already mounted, the circuit boards are mounted, the aluminum chassis is a work of art! GREAT JOB BOB, Thanks , I cant wait to get started.

[nickt]

Hi nickt,

please correct me if I'm wrong cause I'm no epxert, but the schematics by Matt Taylor show the V2a being used for the 2nd gain stage. Also, looking at those initial build pictures from Dana, I think on Page 1, it appears he's using both triods on the V2 also. It does however look like the second triod is used for the first gain stage.

I think the better question is, will either of these changes make any difference in the final tone of the build?

I think you're probably right - I was quoting others not looking at Matt's drawing.

WRT the tone thing: my understanding is that the second triode in 12AX7's is designed to be used as the input stage. How? why? I don't know - again I'm quoting. Does this change the tone? Don't know either.

[Fischerman]

nickt,
I have read that in a 12AX7, the second triode (i.e. pins 6/7/8 ) is ever so slightly more quiet than the first triode. I can't remember why (IIRC it has something to do with the center-tapped filaments) but that's what I've read. I usually try to wire the input to the second triode and when I wire up a gain stage/cathode follower pair I usually try to use the second triode as the gain stage and first triode as the cathode follower (now you can use a 12DW7 in that socket and the cathode follower will use the 12AU7 half...no loss in gain). But I can't say I've ever heard any difference either way.

UPS says Monday for me.

[Fischerman]

Another thing: I'm not sure what the originals did but I was planning on connecting the heater/filament center tap to the power tube cathodes (or actually to the cathode resistor/cap on the board). I'm not sure what our cathode voltage will be but I'm guessing in the 10vdc neighborhood. According to this article by Paul Ruby, this is the best place to connect it (you have to have a big bypass cap though...but we do).

[Sven]

Greetings,

Well, I have done it so far, but it would be nice if someone perfected the layout, which could be done if someone volunteers to recive my AutoCAD file of the present layout, and if possible make interventions in the AutoCAD format, and than it can be transfered into .gif format here.

In other words, the basis for the layouts published here so far, is still available in the most updated form in AutoCAD format in these quarters, so people are welcome to ask for that CAD file and perfect it, as it needs it.

Just post request on THIS THREAD, and I shall send the AutoCAD file via PM to the person that posted request. Rerhaps the person who started this project would be the best person to do it.
Sven

[Blindog]

Hey Rocket builders,

Since chassis kits are being received and or on the way, has anyone decided on a particular layout? I thought it would be good to know what we are building off of, in case anyone (like me!) runs into trouble we would have a partner in crime to bounce ideas, etc . Plus, I really need a complete layout to work off of. Don’t get me wrong, I can read a schematic and such, I just like a good roadmap in front of me until I’ve built a few amps. It looks like Sven is really trying to get a definitive one together, but on what variation I’m not too sure. On the power supply, I’m leaning to the no stack o’ caps like Fischerman. I’m gonna use a Hammond 158L choke and a OT from a Vox AC30 re-issue.

Anyway, got my care package yesterday and I’m quite impressed! Should be a lot of fun.

Regards to all,

Mark

[RJ Guitars]

Hello Rocket Builders and Amp Gurus,

After a few busy weeks a dozen Rocket build parts are are on their way all over the world... some have in fact landed. I have more chassis and circuit board sets and I am contemplating a 2nd round of the group build project if I sense enough interest... lemme know.

Finally, back to discussing and ultimately building a Rocket amp...

The Matt Taylor schematic calls for a 250 Ohm cathode resistor on the output tubes. VOX used a 50 ohm for the AC-30 and I see most other Rocket schematics are calling for a 50 ohm... I'm inclined to go with the 50 ohm, at least to start out... but why?

Here is a great "teachable moment" for one (or more) of the real amp gurus to tell us about cathode resistors. We of course want to use "whatever KF used", but only to the extent that we want it to accomplish the same end result he wanted and it in fact does that. Rather than my own drivel on this, I'd like someone else's technical input here to help us understand the association of cathode resistor value and how it affects things like "Headroom - Early Breakup - total output power - etc..

This discussion then should lead us right into another discussion on the half power option and sorting out the resistor values when we split the cathode sets and drop out one set of output tubes.... but understanding the impact of the resistor values on the cathode has to be figured out first.

thanks for all the great support on this project so far.

rj

[RJ Guitars]

This was actually a double post...

Anyway... I'll just edit it to mention that we also need to look at the purpose of those cathode bypass capacitors and what happens when we change those values...

rj

[Fischerman]

Which cathode bypass caps Bob?

My take on it in general:
With no bypass cap there is some signal on the cathode and that signal is essentially NFB for the gain stage. Signals at the grid are amplified out-of-phase at the plate but signals input at the cathode are amplified in-phase at the plate. And the signal on the grid and cathode are in-phase with each other. So they tend to cancel a bit at the plate...i.e. NFB.

If you put a sufficiently large bypass cap in parallel with the cathode resistor then the cap blocks DC so the bias stays the same but the cap shunts all the AC signal stright to ground (just like a filter cap). So now there's none of that NFB and the gain stage is at max gain (given the chosen bias point, plate resistor, etc.). If the cathode resistor is bypassed with a sufficiently large cap (like the 25uF value often used) then adjusting the cathode resistor value (within limits) won't affect the gain that much...if it's not bypassed then adjusting the cathode resistor value will have a significant affect on the gain. If the cap is not sufficienly large (for example; the Marshall .68uF value)..then the gain stage will still have some of that NFB at low frequencies (because the cap is too small to let them shunt to ground) and the result is that you'll get a brighter tone because the lows are slightly attenuated relative to the highs (the highs are at max gain...and there's a roll-off in there somewhere). The frequency response is a actually a function of both the cap value and the value of the cathode resistor it is in parallel with. If the cathode resistor value goes up then the bypass cap value must go down to have the exact same frequency response.

In the circuit we're using it looks as if all the cathode bypass caps are intended to be sufficiently large to be bandpass...i.e. all audible frequencies are shunted to ground. The value of the cap is also dependant on the value of the cathode resistor. If the cathode resistor value goes up then the bypass cap value must go down to have the exact same frequency response. That's probably why we have that big honkin' 220uF on the power tube cathodes...because the cathode resistor value is so small (only 50ohms).

One more thing...IMLE you get similar amounts of sustain either way...even though you have so much less gain without the bypass cap. That NFB stuff happening in the tube actually yields some sustain that you don't get when you bypass the cathode resistor. But since you have so much more gain with the bypass cap you get your sustain back that way.

Lastly, I'm pretty sure that you can't (or shouldn't) connect the PT filament center tap to the power tube cathodes unless you have a sufficiently large cap there. That large cap ensures that we have nothing but DC voltage there...and no AC signal/voltage.

[jeffkt]

I have seen the inside of a rocket. It did not use a Liverpool power supply board. It used a single aluminum cathode resistor/bypass cap bolted directly to the chassis to disipate the heat. Values of components are Vox AC30.

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[Fischerman]

Bob, For some reason I only read your second post (the dbl post you editted)...so that's why I went on that long rant about cathode bypass caps. The cathode resistor just sets the bias. 250ohms would likely be way too cold at the plate voltage we're at. 50ohms should be a fairly warm/hot bias at our plate voltage.

Remember, when you only run two tubes the cathode resistor value should be double the 4-tube value (because you only have 1/2 the current flowing through it so you only get half the voltage drop). It's the voltage of the grid relative to the cathode that's the important thing...and the grid must be negative (relative to the cathode). In fixed-bias we derive a negative voltage and apply it to the grid and ground the cathode...in cathode-bias the current through the cathode resistor causes a voltage drop across the resistor and the grid is grounded (through the 220K resistor) and is at 0 volts...so either way the grid is negative relative to the cathode.

Whatever sounds best and doesn't overdissipate the tube (too much) is the best choice. I ordered a couple of 56ohm/10W resistors as well as the 50ohm/10W resistors (actually, 5W is fine)...just in case 50ohm is a little too hot.

[jsn]

Hi everyone. I am an IRL friend of RJGuitars' and he has been talking about this project a lot lately so I thought I would stop in and say hello.

There was some talk of which 3 out of the avilable 4 12ax7 triodes to use where. My inclination would be to use half of one tube for the cathode follower, and use both halves of the other tube for the gain stages. This is because it is generally considered a good thing not to have the cathodes of the 2 triodes in a tube at radically different voltages. Since the cathode of the cathode follower will be at 100V or so, it would work out well to have it be it's own tube. I'm not sure how this would affect the layout though.

Thanks!
jsn

[RJ Guitars]

Fisch... that is good stuff! Way more understanding and eloquence than I could have added! I'm going to use and recommend 5W resistors simply because they will fit my board... on the other hand if you are creative or don't intend to use my power supply board, then use what ya got.

Jeffkt - your eyes have seen the insides of the magic amp... way cool! The Rocket has been kinda the lost child of the Trainwreck family. Posts like yours are really helpful in bringing to light more Rocket info. Thanks for helping out! I know AllynMey also mentioned the use of those gold panel mounts for cathode resistors so you have confirmed his input as well. All the AmpGarage forum would be greatly delighted to hear and see anything further you can share about an authentic Rocket Amp.

Also, it's cool to jsn stop in and talk tubes with us. If you ever get the chance to look at http://boozhoundlabs.com/ you will find a gold mine of good technical info... Jason is one of the most helpful guys in the analog audio business. I never stay stuck on a tech problem for very long if he is available to help me sort it out.

rj