taming harsh distortion

[JoshBernstein]

With the help of TAG, i recently finished a rocket build. After some ghost note debugging, im left with some harsh distortion. The amp has a really nice clean tone, exactly what i was aiming for. But when i turn it up so that it starts to clip its just kind of harsh and ice-picky. Ive tried with several different speakers, including a 2x10 celestion gold open back cab, 1x12 closed back with some miscellaneous 100 watt speaker, and my open back 212 with a 25 watt hellatone ceramic and hellatone ceramic. Ive tried some tube rolling, and the best combination ive had so far is and amparex bugle boy for v1, chinese groove tube for v2, jan phillips 12at7 for v3, and a quad of jj's for the outputs. Still harsh though. My next experiment will be to add 100pf polystyrenes accross the preamp plate load resistors, and maybe a cap accross the pi plates. If that doesnt do it, i dont know what else to try. Anybody had this problem before, or any suggestions? Thanks
-josh

[Colossal]

Try paralleling another 1k2 alongside the phase inverter cathode resistor to lower the overall value to 600R or just reduce the value to 750-820R. I understand Ken Fischer used two 1k5 in parallel for 750R to warm up the bias of the phase inverter.

[Theashe]

When I finished my first amp build, I wasn't happy with the distortion tone - it was fuzzy and harsh. One of the things I did was use a scope to find the first tube in the amp that was getting badly overdriven (really ugly wave) and drop in enough local feedback that it rounded out the wave a bit and cleaned up some of the higher frequencies.

[Littlewyan]

I think the JJ EL84s are harsh sounding valves. I don't have much experience with them but I think others have found this.

[JoshBernstein]

Will definitely change the pi cathode resistor and see how that does. For local feedback, im guessing that its going to be a resistor from plate to grid. Do i need to have a cap in series with the resistor? If so what values should i use?
As for the jj output tubes, ive tried a couple of different brands and the jj's were the least harsh to my ears.
Thank you!

[Theashe]

Here's a great resource for planning local feedback:
http://www.valvewizard.co.uk/localfeedback.html

In my case, instead of placing the feedback resistor after the coupling capacitor, I just soldered a resistor and 2.2nF capacitor in series point to point from the plate to the grid, since it was easier given my layout. You can use a higher capacitance to let more frequencies into the feedback loop, but I was trying to restrict the feedback to just high frequencies to eliminate them. My recommendation would be to connect the feedback loop using alligator clips at first, and try out different values to see what suits your ear best. When you find a sound you like, solder it in.

[JoshBernstein]

Thats a good read, thank you for that, ill give it a shot. I dont have a scope, so ill try it first on the cathode follower. I cant imagine that the input tube is clipping so ill skip thag for now, unless someone thinks i should try that on too.

[Theashe]

A cathode follower already uses 100% negative feedback - that's why its gain is slightly less than one. The cathode output signal is in phase with the grid input in a cathode follower, so shunting the two won't provide any benefit (I'm not even sure what the effect would be - maybe positive feedback/oscillation).

I just familiarized myself with the Rocket schematic for the first time via:
https://tubeamparchive.com/files/taylor ... 08_758.pdf

I would put the negative feedback on V1A in that schematic. And since its DC coupled to the cathode follower, definitely use a 600V coupling cap.

Another option would be to put global negative feedback from the output to the phase inverter. A slight combination of both may even yield a nice sound.

I also use a 1nF capacitor shunted across the plates of my phase inverter, with no potentiometer. It definitely lowers the brightness of the whole amp.

[JoshBernstein]

Im not sure why i didnt think about that in regards to the cathode follower, but that does make sense. Will give the feedback options a shot and report back with my results. Thans for the help!

[leadfootdriver]

Maybe let the amp burn in for a day. New amps sound harsh I think.

Also, an AT7 in place of an AX7 sounds thin and ratty IMO. Good luck.

[M Fowler]

To me something else has to be going on here, I've never heard a harsh sounding four EL84 Rocket when using transformers spec'd for a Rocket build.

I would suspect incorrect value component some where in the circuit.

Rockets are smooth sounding amps with just enough good distortion when turning up the volume pot to get to the sweet spot. Even better with PPIMV.

Mark

[JoshBernstein]

Is there any particular area that would be suspect? If it helps, i did use a larmar ppimv, and the distortion is just as harsh/fuzzy while using that at any level.

[RJ Guitars]

To me something else has to be going on here, I've never heard a harsh sounding four EL84 Rocket when using transformers spec'd for a Rocket build.

I would suspect incorrect value component some where in the circuit.

Rockets are smooth sounding amps with just enough good distortion when turning up the volume pot to get to the sweet spot. Even better with PPIMV.

Mark

That has also been my experience. I have had them get muddy and some folks have to take a little bottom end off them, but I can't recall a harsh one.

[JoshBernstein]

I went over all of the components, checking their connections and values. The resistors were easy, as they are the brown dale ones with their values written directly on them. Checked all the cap codes as well. Everything matches up with the schematic. My voltages are a bit higher than normal, as my pt is a hammond which i believe wound for the lower canadian mains voltages. Could that be my cause? If so is there any way to lower them, maybe with big resistors in the pt primary or secondary? Id rather not take the zener approach, and also would rather avoid vvr. I still havent gotten around to trying feedback, but will still give it a shot.

[JoshBernstein]

Also meant to say that i dont think the ot is suspect, as it is a large ac30 type tranny