66 Rocker - AKA 66 Rocket ...Eventually the 66 Rockster

[RJ Guitars]

Did you use a 500K volume pot?

[sliberty]

Yes, 500KA.

[sliberty]

Additional info:

I scoped the amp with a 1K signal going into it. All is fine at the plates of V1a and V1b. As V2a is a cathode follower, I soped at the cathode instead, and found that the problem can be seen on the scope at the point in the circuit.

The image below shows traces from V2a's cathode with the volume set at 6 or 7, 8-ish, and then above. The last trace coincides with the point at which the noise floor disappears and the distortion gets ugly (I know, duh!)

[img:355:816]http://misc.sliberty.com/photos/RoctalScope.jpg[/img]

This is where my love / hate relationship with scopes comes into play. Now that I know where, I still have no idea what

I tried a different tube in V2, but it had no impact. I really didn;'t think it would.

So what could cause this mess?

Steve

[RJ Guitars]

hmmmm got me scratchin' my head too. I am just learning about similar issues on my Express and that all seems to be about overloading the PI... you apparently are not getting that far.

Oscillation is a good catch all explanation for these events, but it is also a real possibility. Consider using a snubber... although it will take some thought to figure out on a cathode follower if the typical plate to cathode snubber can be employed... I simply don't know. Anyway, people will tell you they rob tone, but if they identify and stop a problem, then you can always go back and sort out lead dress or component failure etc.. after you know more about the problem.

rj

[klingo]

Merlin recommend the use of grid stopper (even a 100R) between the anode of preceeding gainstage (V1b)and the grid of the cathode follower(V2a).
Since you use two different triodes for that purpose i will try 1k here.
HTH.

[sliberty]

Well, I came home from my weekly blues jam tonight, and decided to try scoping the amp without an input signal, and yes, it still showed the anomoly. So I started chopsticking around, and discovered that the lead dress on V1 was causing the oscillation. I rerouted the wires and now the scope is clean - no more oscillations.

Tomorrow when I get home from work, I'll play through the amp and see how it sounds. Keeping my fingers crossed.

Steve

[RJ Guitars]

that's a great report... looking forward to the sonic test results.

[Zippy]

Well, I came home from my weekly blues jam tonight, and decided to try scoping the amp without an input signal, and yes, it still showed the anomoly. So I started chopsticking around, and discovered that the lead dress on V1 was causing the oscillation. I rerouted the wires and now the scope is clean - no more oscillations.

That's great news, Steve. How severe a reroute did you perform? Got before/after pics of the affected area?

This platform is "known" for friendly assembly and robust stability so I am interested to learn what was going on to cause it to be such a bad actor.

[sliberty]

Re-route is probably too strong a word. Let me describe.

The wires for V1a and V1b were crossing partially. The wires on V1b were just long enough to lift them into an upward arch (without replacing them), and once they were lifted off of V1a's wires, the oscillation disappeared from the scope.

I have a Ceriatone Rocket that I am tweaking for a friend, and the layout of the V1 wires on that one is different. Probably due to tube socket / board relative placement. In any case, the wires all lay down to the chassis nicely without crossing each other. No oscillation there of course.

So, it was really a minor fix once I knew what to address. Isn't it always?

You know the old joke - when you are looking for your keys, they are always in the last place you like. Duh! Once you find them, why would you keep looking

[Reeltarded]

lol I just call a locksmith

Good deal on your amp! I need a scope, huh? A visual representation of the worst noise I ever heard seems like it would really help.

[Zippy]

Re-route is probably too strong a word. Let me describe.

The wires for V1a and V1b were crossing partially. The wires on V1b were just long enough to lift them into an upward arch (without replacing them), and once they were lifted off of V1a's wires, the oscillation disappeared from the scope.

Thanks for sharing that.

[sliberty]

RJ,

I went over your voltage chart from earlier in this thread, and was wondering - did you reverse the triodes in V1? The schematic I have for a Rocket shows the first stage as V1B (pins 6, 7, 8 ), and the second stage as V1a (pins 1,2,3). I ask becaue our V1 voltages are coming up reversed - my higher plate voltage is on V1a, and your higher plate voltage was on V1b.

Thanks,
Steve

[RJ Guitars]

RJ,

I went over your voltage chart from earlier in this thread, and was wondering - did you reverse the triodes in V1? The schematic I have for a Rocket shows the first stage as V1B (pins 6, 7, 8 ), and the second stage as V1a (pins 1,2,3). I ask becaue our V1 voltages are coming up reversed - my higher plate voltage is on V1a, and your higher plate voltage was on V1b.

Thanks,
Steve

I recall going back and forth on that issue and I can't recall the logic that was in place at the time but it seems like one way is more friendly than the other... I would go with the one that holds closest to the original Rocket layout.

[rawnster]

Revive this old thread.

I had a bunch of flashes and sparks coming from the back of an old octal rocket (rockster) yesterday. Blew the fuse, replaced the fuse and tried again and saw the fireworks coming from the KT66s. Didn't have any extra KT66s laying around, so I popped in a pair in Svetlana EL34s. NICE!

I forgot how nice and thick that rocket circuit sounds with EL34s driving it all.

[RJ Guitars]

Rawnster, it's good just to see your post... brings back some good memories from the golden years when everybody was building something. El34's are great tubes and that is the beauty of running octals - you get lots of options and many of them are great sounding tubes.