100p cap across PI plates on a Rocket?

[johndandry]

Can I put a 100p cap across a Rocket's PI plate resistors to knock down some high end or does the Cut control pretty much do the same thing?

[redshark]

No need for that, the treble, bass and cut control work in a very eficient way.

[Markusv]

I may be mistaken, but I used one on my 15 watt rocket and I felt it removed fizziness when driven hard

YMMV
IMHO
FWIW

Markus V

[sliberty]

I may be mistaken, but I used one on my 15 watt rocket and I felt it removed fizziness when driven hard

YMMV
IMHO
FWIW

Markus V

I am always "just about to do this", and never do. Not so much on my Rocket, but on my Express. fizziness can drive me a little nuts. Did you use 100pF? Did you try any other values as well? Did you do it on the PI< or elsewhere in the amp? How effective was it? Did it just remove fizz without dulling the chime?

[Markusv]

Sliberty

Yes that's what I did
I used the 100pf to take some fizz off. Used it on the PI
I think it still retained the chime

On the Rocket I changed the 50 Pf treble cap to a 100 pf too
Not an issue with the express - having a 500pf there, but I feel the Liverpool benefits from the same treatment

Markus

[sliberty]

Oh, wait, it just sunk in. you guys are talking about a 100pF from one plate of the PI to the other.

the trick I have been considering is to put a small cap in parallel with the plate resistor on a tube stage. For example, parallel with the 100k plate resistor on V2. I had heard that this will kill fizzies too. Anyone with experience? How do you select which tube stage to do this to?

[Reeltarded]

47 or 100 on plates of the PI is better than on the plates of a preamp stage, from experience with Marshalls here.

Less is more. You will like it dull, I am warning you.

Try it, but before you decide to leave it (haha like we ever decide?) try a bit of added suppression at the V1 grids, and balance it on both ends. It sounds better to take up the slack in those two places than in hammering it in either one.

[Markusv]

I find 47 too little
Always have gone with 100 or 120 on the plates

But I have never added the cap parallel to the plate resistor in addition
May try that and reduce the PI cap a bit?
Thought I'd also mention that I have lately steered away from Silver Mica PF caps and I use Polystyrene almost exclusively. Like them very much especially in the tone stack.

Markus V .

[johndandry]

I'll try it and start with a 100pf.

[RJ Guitars]

I want to follow this thread... would be curious to learn more ways to slow down the abrupt takeoff on the express amp I just built. It slams the gain hard starting at 1 and just keeps going... forget fizz this thing boils over madly.

[Markusv]

RJ

Slow/fast switch? ala Komet

Markus

[RJ Guitars]

I am getting ready to try a little bit of filtering on my Express. I just received the parts today to this... http://www.guitarplayer.com/printprevie ... cleid=5505 . has anyone done this on an Express before? It sounds like it almost perfectly describes the behavior I am seeing on a build that I recently finished.

I never build clones for myself but in this case I finished up an express clone for a guy that is having severe health issues. It just seems a little too unruly and goes from cool sounding to extreme distortion really fast. Several people have told me this is how it is expected to behave but it just strikes me that the transition should be more smooth.

Let me know if this is not relevant but it strikes me that this problem could be messing with the PI and they do mention it is intended to take the fizz out of an amp.

[Markusv]

Never tried it but I've seen people do the Cap/resistor in series thing between plates or between secondaries

M

[Tillydog]

RJ,

I have played with similar R-C networks on the secondary of the OT in an Express style circuit (a bastardised 4-4-0 with EL84 power section).

I was successful in getting rid of a nasty fizz, but in this case, the problem was clearly visible on the 'scope as a nasty spike on the output waveform. Note that it only showed up when driving a speaker. When driving a resistive dummy load, there was nothing to see.

Before:

[IMG1326]http://i255.photobucket.com/albums/hh14 ... C_9841.jpg[/img]

I tuned this out with a switched R/C network, and as the spike reduced, the tone got better. I ended up with 56R & 2u2 in series across the 16 ohm tap of the Hammond 125E OT (I had the OT wired for 10K:8 ratio). I found the values quite critical, though, so I would be wary of any 'cookie cutter' type solutions.

After:
[IMG1322]http://i255.photobucket.com/albums/hh14 ... C_9844.jpg[/img]

I figured this was only needed because of the mediocre quality of the OT.

My network has a cross-over frequency of ~1.2kHz with an equivalent impedance of ~70k ohm in parallel with the OT primary (if my maths is correct). The one in your link has a cross-over frequency of ~2.6 kHz with an impedance of ~6k in parallel with the OT primary, which seems quite a heavy load.

I don't know if it will help you, though.

I've also played a lot with the balance between the PI and the power section - the transition from clean to distorted can be really abrupt if the PI gain / output swing is high.

IMHO, the distortion flips on in an Express when the NFB breaks down due to grid conduction in the power section. The extent to which the power valves get over-driven then immediately depends on the available PI headroom when this happens. Again IMHO, I think 'tuning' involves trying to get this to happen just as the PI is starting to compress, too.

You might try swapping the PI for a lower gain valve (not necessarily a different type), or maybe getting the PI B+ down a little, or find some 'harder to drive' power valves.

I'm not an expert by any stretch, so take the above with a pinch of salt.

Andy