taming harsh distortion

[M Fowler]

The PT and OT choice is good I think, nothing wrong with using those.

Increase dropping string resistors to lower to preamp tubes but can't really do much to drop some screen voltage if you don't do zener on CT. Using 100R screen resistors is stock perhaps increasing those to 1k or so. Biasing of the EL84's is crucial 50R would be too hot.

Hard to tell from this end.

[JoshBernstein]

Alright. Well i will try the bypass caps on the plate load resistors and will do the feedback stuff. If that doesn't help ill try the zener trick. I did adjust the bias resistor for the el84's to compensate for the higher voltages. This would definitely be much easier with a scope!
Anyway, thank you for the help!

[Theashe]

You could also try increasing the grid stopper resistors on the power tubes to around 15k or even higher to see if the cause of the harsh sound is blocking distortion.

The local feedback suggestion I made before can be done with a couple of alligator clips, so its easy to test if that cleans up the distortion to your liking.

A scope is a great investment in your hobby. I bought a Hantek DSO5072 two years ago and I'm thrilled with it, but by no means do you have to spend over $100 on a scope for guitar amps.

[RJ Guitars]

AC30 OT trannies work well in a Rocket, I've done that before and it makes a sweet sounding amp. If you're a bit high on a voltage that has also been done by a few guys here on TAG - I think Mark Fowler actually prefers it that way. So I wouldn't say that is a tone killer.

1) I am suspect that the tone circuit may not be doing everything correctly.

2) I am wondering about some oscillations or cross talk somewhere in that circuit. Especially watch (and move around) the location of your wires going into pins 2 and 7 on the preamp tubes.

3) Then maybe speakers - try an alternative cabinet.

What happens when you run the dial up on your Cut control? That should really knock the top end off and although it kinda kills tone it also might tell you something if it kills the harshness. I would also suggest you play with Duncan's tonestack calculator. It really reveals a lot about the AC-30 interactive behavior on the tonestack.

Also, do some tube rolling and include the PI tube.

Thanks for posting, this is good stuff for us all to sort through.

[JoshBernstein]

Well its looking like this issue has been resolved as well. I put the bypass caps across the preamp plate load resistors, and the improvement is unbelievable. The distortion now phenomenal; bright and defined, yet warm and smooth. Thank you all for the help, and also to RJ for the chassis and faceplate for this particular build.
Now its over to the speaker section for some speaker ghost note issues!
Anyway, thanks again!

[Colossal]

Just for everyone's edification as well as future reference, if this thread is searched by someone with a similar problem, what value cap(s) did you use and where did you put them?

Thanks.

[JoshBernstein]

I used a 130pf polystyrene across R4, and 100pf silver micas across R15 and R16 (based on the schematic found in the rocket files section). These are the plate load resistors for the input triode, and for both triodes of the phase inverter. Also meant to mention in the previous post that i put a 270pf polystyrene across the pi plates. As a side note, i know polystyrenes are becoming more difficult to get as they are out of production, but i order mine from watts tube audio. Just another source in case anybody is interested.

[Colossal]

Excellent, thanks for that. The details are very helpful. Glad you got it figured out to your liking.

[Theashe]

I used a 130pf polystyrene across R4, and 100pf silver micas across R15 and R16 (based on the schematic found in the rocket files section). These are the plate load resistors for the input triode, and for both triodes of the phase inverter. Also meant to mention in the previous post that i put a 270pf polystyrene across the pi plates.

I'm still relatively new to amp building, so I've never heard of bypass caps being put across plate load resistors to tame harsh distortion before this. I'm trying to improve the distortion in my own home-made amp. I'll give this a shot for that project, and maybe it'll do the trick! Will report back when I do.

[JoshBernstein]

I am relatively new to amp building as well, this is my 4th. The concept behind the bypass caps is they let some high frequencies around the plate load resistor, giving them an easy path to ground- just to help you judge if the caps will help you or not.

[RJ Guitars]

Congrats, glad you solved the problem. I've done this on a few high gain amps before but not the Rocket. You might find that with different preamp tubes the need might or might not go away. also, thanks for the kind words.

I'd like to see a photo of the amp in the cabinet to complete the big picture once you have it ready for prime time.

[JoshBernstein]

Rj,
Heres a couple of pics i have. This was my first time finishing wood, and it was much more difficult than anticipated. Theres a few glitches, but i dont think they're too noticeable unless your'e looking for them.

[JoshBernstein]

Here's the back...
As you can see i decided to go with ss rectification after experimenting with the gz34. I guess im just crazy, but i have prefered ss rectifiers in all 4 of my builds so far.

[RJ Guitars]

It looks great... Should be a welcome addition to your collection.

If your first woodworking project came out perfect and you said it was easy most people wouldn't like you much anyway... It's supposed to be one of those crafts that you grow to love and develop over years.

I also like SS rectifiers although I do think they behave differently than tubes. This could be one of the contributors to some of those differences you reported on your build.

Now looking forward to your gig report and any sound clips you make.

[Colossal]

Interesting to note about SS vs. tube rectifiers. I had a high gain test build once with a quad of EL84s. The power supply was similar to a Trainwreck and had a dual secondary PT. I put SS diodes on the lower voltage tap and a GZ34 on the higher voltage tap. The equivalent rectified voltages were pretty much the same. With the GZ34, the tone was beautiful, full, and had amazing depth...like you could reach into the sound. Switching to SS diodes just killed the whole thing. The amp became way too immediate and the distortion became buzzy, flat, and one dimensional.