6V6 Rocket

[sliberty]

I have been enjoying my 6v6 Rocket a lot. It is a bit of a different animal from my "normal" Rocket, although there are certainly some aspects of its tone that are similar. It does break up more easily, and a little more agressively than it's EL84 sister. (I've added a pot that dumps some signal to ground right in front of the PI to help clean things up a bit when I want to).

One thing that has been bothering me though was that it sounded a little scooped compared to my regular Rocket. Not really sure why, but today, I replaced the 10K mid resistor (R9) with a 47K. According to the Duncan tone stack sim, this would reduce the scoop from 25db to 15db - not an insignificant change. And it helped a lot.

Was wondering if anyone else has had a similar experience with a 6V6 Rocket, and how you dealt with it.

[Zippy]

I have been enjoying my 6v6 Rocket a lot. It is a bit of a different animal from my "normal" Rocket, although there are certainly some aspects of its tone that are similar. It does break up more easily, and a little more agressively than it's EL84 sister. (I've added a pot that dumps some signal to ground right in front of the PI to help clean things up a bit when I want to).

One thing that has been bothering me though was that it sounded a little scooped compared to my regular Rocket. Not really sure why, but today, I replaced the 10K mid resistor (R9) with a 47K. According to the Duncan tone stack sim, this would reduce the scoop from 25db to 15db - not an insignificant change.

47k is too much - try a 25k trim pot there or bring the mids to a front panel tone control.

You may also want to try changing the output impedence for its effect on the breakup point.

RJ and I played around with a variable leg on the PI - we call it the PIG control (Phase Inverter Gain).

[Ian444]

In my humble experience with stereo amps, the 6V6 has a lot more bass than other small tubes, and a nice top end. The extra bass might be making it sound "too scooped" in the mids, and also causing early breakup, and the more "aggressive" nature of breakup. Maybe try a filter that pulls some bass out. Not sure where, maybe try half the value of C8 feeding the phase splitter. Or even 1/10 the original value. Pull the bass out and then keep going up a size until it sounds just right.

[sliberty]

47k is too much - try a 25k trim pot there or bring the mids to a front panel tone control.

I tried alligator clipping in various values and 47K was what I settled on to my taste. Why would you say it is too much? Do you mean too much for your taste, or is there something else you had in mind?

RJ and I played around with a variable leg on the PI - we call it the PIG control (Phase Inverter Gain).

Do you have a schematic fragment that would show this feature?

What I did was run a pot from the juntion of R10/C8 to ground to dump signal before pushing the PI. The impact is that there is less push on the power tubes as well of course. I have to use it judisiously though as dumping too much mkaes the tone wimpy.

[sliberty]

...the 6V6 has a lot more bass than other small tubes, and ...The extra bass might be making it sound "too scooped" in the mids, and also causing early breakup...maybe try half the value of C8 feeding the phase splitter...

Sounds like a worthwhile experiment, and easy enough to do. Thanks for the suggestion. I'll report back. I'll also try a smaller cathode bypass cap on the 6V6's - 220uF is hefty, and maybe a 25uF or 50uF might smooth the response a bit.

[Zippy]

47k starts to swamp out the tone stack but if you like it...

I'll hope that RJ can scratch out the PIG control in a drawing. I'm only up for a few minutes at a time lately - back surgery tomorrow for a herniated disc.

G'luck with the lil' octal Rocket. I'll check back in a couple days to see how you tweak it into bliss.

[sliberty]

Good luck with the back man - hope you are upright and soldering again real soon.

[sliberty]

Reducing the pwoer tube cathode cap from 200uF to 47uF helped. If I decide to also try the .047uF cap (C8) that leads into the PI, should I also change C9?

[jelle]

Hee Steve, nice hanging out with you guys today. I have done a 2x 6v6 rocket once and I put a 500pF treble cap and a .0022 mid cap in there. very significant change....

Jelle

[sliberty]

Jelle, I enjoyed it too.

So I get the 500pF, but when you mentioned the mid cap, honestly, i am not sure which you mean. is it the one in series with the 10k resistor? And what did these two cap mods "sound like"?

Of course I'll probably try it before you get a chance to answer.

[jelle]

The mid cap...yes, the one in series with the 10K. The sound....less mid scoop by increasing more highs (via lowering the knee) and adding low mids as well to compensate for the added high end content. This balances the tone and fills the scooped EQ nicely.

jelle

[RJ Guitars]

...Do you have a schematic fragment that would show this feature?

What I did was run a pot from the juntion of R10/C8 to ground to dump signal before pushing the PI. The impact is that there is less push on the power tubes as well of course. I have to use it judisiously though as dumping too much mkaes the tone wimpy.

I made my "Rockster" with cathode biased 6V6GT's, but soon moved on to El34's and eventually KT66's. It was my interpretation of the Octal Rocket that was driven by a conversation with a close associate of Ken Fisher and also Zippy's observation that an AC-30/Rocket was basically a 59' Bassman with a quad EL84 output section. Normster had already made a Rocket with a quad of 6V6's that he raved about so he had an influence in this as well.

Zippy was a partner in the "Rockster" effort from the beginning and once we had the amp done, he kept feeding me tweak ideas for it and the other EL84 Rockets that I had built. There were two master volume ideas we took from the Trainwreck pages. I think it is #2 that has become popular as the PPIMV more recently. The #1 MV is the Phase-Inverter-Gain (PIG) control. The PIG will give you the option to really clean up the amp and I agree that you lose something if you take it too far. At this point I think I would prefer a multi position switch and go from clean to dirty, but never get to "wimpy".

If you ever get curious, try some KT66's in this amp. I found them to have all the good stuff of an EL84 but more of everything.

rj

[sliberty]

Thanks RJ. What were the characteristics of the 6V6 version that caused you to keep searching for "the sound"?

ALso, thanks for the PIG schem. It is a little like the control I added, but not exactly the same. Mine has the wiper of the teble pot going to the wiper of the new control. Then, the two legs of the new control are either the "output" to the 220K reistor, or ground. This probably has a slightly different impact, as mine doesn't place additional resistance in the circuit - it just dumps signal to ground. I believe I tied in after the 220K resistor too. I'll have to try it your way too and see which I prefer.

Currently, I doubt my iron could handle KT66's. I used an OT that was specifically for a 6V6 amp, and my PT can only push around 100mA.

[RJ Guitars]

Thanks RJ. What were the characteristics of the 6V6 version that caused you to keep searching for "the sound"?...

I had used pretty big iron with the thought that I wanted to try various tubes. A friend of Ken Fischers had mentioned that Ken really loved the sound of KT-88 tubes. I was also curious about the EL34 sound in an octal Rocket. The Liverpool kinda evolved into the Express and I was curious if Ken had evolutionary thoughts about the Rocket as well. This was all speculation and daydreaming on my part... nothing solid to work from. I did start with the 6V6's because of Normsters report on his 6V6 Quad Rocket.

With the cathode biasing it was pretty easy to start dropping in other tubes and I found several I liked. One set of EL34's sounded great, they had the most edge and added a real "British" sound to the amp. I also tried a pair of old Tungsol 5881's that just oozed with sweetness. I think they really like cathode biasing.

The KT66's came about because Zippy had read up on them and he was quite keen to try them. In fact so keen that he provided me his own set of them to try in this amp. At that time there were several new offerings showing up in the KT66 design and he had found a good price on the Valve-Art tubes. There is a good KT66 comparison out there -- http://thetubestore.com/kt66reviews.html . What I found was that these really seemed to sound most like the EL84 Rocket but with more of everything - Volume - sparkle - Bass - Mid - Treble. Very cool sounding tubes. Now when I sell a Rockster package I usually suggest the KT66 but really all the tubes I tried sounded good. I did the least amount of work in optimizing the 6V6's, so it's not fair to say they aren't the answer. It was just easy work to get great tone with the KT66's.

I haven't gotten around to those KT88's yet but I have built a bigger chassis with them in mind. The extra breathing room between power tubes is also an important consideration for using the Valve-Art KT66's. I built my original Rockster in an Express chassis and there is really not enough between the power tubes for 66's or 88's.

rj

[sliberty]

Thanks RJ. Your big iron seems to have served you well here. SInce my intent was always a pair of 6V6's, I didn't plan for anything else as you did.

I also love KT66's. I built a JTM45 , initially with 5881's,m and later the VA KT66's, and those KT66's just killed when the amp was cranked (didn't really care for the low volume sounds on that amp regardless of the tubes). I also built (and still own) a Blond Bassman, again, initially with 5881's, and later with VA KT66's -- wow!

Have you ever tried KT66's in an Express? This conversation has me wondering how that would turn out.