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

[fishy]

Your right about the 66's, might have to get some of those

[Zippy]

Just got back from playing and listening to RJ's amp. Schweeeet!

We started with some 6V6s, then plugged in some Mullard EL34s, and then went to the set of KT66s. By the time we had played his Strat, my G&L ASAT Jr (w/ P-90 style pickups), and his ES-339... I could do nothing but smile. There was not a bad tone to be found. Clean to crunch - it's all there.

We'll be doing some tweaking to be sure - mostly 'cause the amp has so much promise, so many great sounds as-is just off the bench.

This really takes the Rocket platform to the next level.

Hey RJ, how 'bout calling it the "Space Station"?

[xk49w]

RJ, this is basically a tweaked Rocket pre and an octal power section? What about NFB - any?

[RJ Guitars]

RJ, this is basically a tweaked Rocket pre and an octal power section? What about NFB - any?

Yep, that's pretty much it. If you like comparing schematics, this will also look a whole lot like a 59' Bassman (Thank you Zippy for the early call on that).

The Rocket preamp is really pretty simple, but there are several meaningful tweaks you can throw at it. It almost always sounds good and works with a variety of guitars. I expected to have to do more to bring this into the tone zone. I might still do a little more but it didn't start off in a bad place at all.

The output section is similar to a lot of 2 tube Octal outputs except for the cathode biasing. This makes it pretty friendly to a variety of tubes. So far I have tried:

KT-66 - Excellent
5881 - Excellent
6K6GT - Excellent
EL-34 - Very Good (Didn't have any vintage British tubes)
6V6GT - Good
6L6GC - Good

I don't really understand the 6L6GC performance. I have several sets high quality tubes but they just didn't get it for this amp. Might be simple to tweak it into shape with them, I haven't tried.

Nope, no fixed negative feedback. Might be something to consider if you could figure out how you want to do it? It does have a cut control although I don't find it as responsive in this amp as it is in the EL-84 iteration.

Thanks for inquiry.

rj

[Blind Lemon]

I hope you don't mind if I butt in, you could copy a Fender on the NFB, but I really think that it would neuter this thing. Its got balls. But you could figure the riz size by ear.....slave in a 100k pot and find the sweet spot. Measure input to wiper and that's what you like.

My 2 cents.

BL

[Zippy]

The NFB is one of the first things I get after when warming up a not-so-hot tweed Fender. Dial up a lil' gain, adjust the tone stack, tweak the cathode follower and do a lil' more judicious tube swappin' - voila! - you've got a Rocket with KT-66s.

Anyhow, whatever path you take to get there, this KT-66 "Rocket" is a beautiful thing. Try it...

... and I was trying SO hard to get into the whole EL-84 thing...

[M Fowler]

RJ

Nice build. Is that PT the same one I have? I fired up my Rocket and it sounds great with your PT.

About the KT66 I have been meaning to build one but from what I have seen the voltage is about 375 for this type of circut such as Rt 66 and JTM 45. I want more power so I think I would go with the Heyboer 300 volt tranny and Rocket preamp as I really don't see having a mid pot and can adjust the mid I want in the circuit. Never found any use for the mid pot.

I am going to stick it in this chassis and cab, attached.

Are you going to use this with the 6v6 or KT66s?

Mark

[xk49w]

Anyhow, whatever path you take to get there, this KT-66 "Rocket" is a beautiful thing. Try it...

I would like to try if they will fit. I'm cramming them into an AO-35 chassis with the first noval hole step drilled to fit an octal socket. The octals are very close together, 1.525 center-to-center if I measured right. It will work with straight envelope types like 6V6 or EL34. KT66 looks like a coke bottle envelope. Anyone KT66 owners with a micrometer please measure the external diameter?

-- Bradley

[Zippy]

About the KT66 I have been meaning to build one but from what I have seen the voltage is about 375 for this type of circut such as Rt 66 and JTM 45. I want more power so I think I would go with the Heyboer 300 volt tranny and Rocket preamp as I really don't see having a mid pot and can adjust the mid I want in the circuit. Never found any use for the mid pot.

Hey Mark,

I'm into big tubes (EL34, KT66) at low(er) voltages - that's part of what was driving this particular substitution. If you do go with higher voltages, I'd recommend adjusting your dropping string to keep the preamp voltages in the Rocket ballpark (my other compulsion is tweed-range voltages on V1).

Regarding the mid pot: I agree that you don't need one if you're open to tweaking to taste. It might be useful to put a 25k mid trimmer in the circuit so one can more easily adjust the two-knob tone stack for a particular user. 10k to 25k seems to be the sweet spot in this particular application - maybe a bit less but certainly not too much more.

[RJ Guitars]

RJ

Nice build. Is that PT the same one I have? I fired up my Rocket and it sounds great with your PT.

About the KT66 I have been meaning to build one but from what I have seen the voltage is about 375 for this type of circut such as Rt 66 and JTM 45. I want more power so I think I would go with the Heyboer 300 volt tranny and Rocket preamp...

Are you going to use this with the 6v6 or KT66s?

Mark

Mark,

Thanks for the compliment.

This is the same power tranny that you have 245-0-245 & 250mA. I have not looked at the tube data sheet for the KT-66, but I planned to starve it a bit and i'm sure this is not much juice for that tube. They do great even over 400 volts. You might even think about a Fender Tranny with a 325-0-325. Those are pretty common and not too expensive.

I didn't care much for the tone of the 6V6's in this amp, so I'm not driven to use them at this point. Maybe after I get some tweak time I'll like that idea better.

rj

[andrew]

RJ, I have a dual tap PT for 340v or 275v with 150ma. Would this drive the KT66s? Thanks.

[moonrock]

RJ nice work. Have you checked how much power you are dissipating from the different tubes? I’ve done cathode biased before and found a resistor value may make the KT66 and EL34s sound sweet, but running 6V6 and 6L6s requires a different value to get them to sound good.

How is New Mexico? I just moved to Dallas after living in Albuquerque for 20+ years. I miss the mountains.
Bruce

[moonrock]

RJ nice work. Have you checked how much power you are dissipating from the different tubes? I’ve done cathode biased before and found a resistor value may make the KT66 and EL34s sound sweet, but running 6V6 and 6L6s requires a different value to get them to sound good.

How is New Mexico? I just moved to Dallas after living in Albuquerque for 20+ years. I miss the mountains.
Bruce

[RJ Guitars]

RJ nice work. Have you checked how much power you are dissipating from the different tubes? I’ve done cathode biased before and found a resistor value may make the KT66 and EL34s sound sweet, but running 6V6 and 6L6s requires a different value to get them to sound good.

How is New Mexico? I just moved to Dallas after living in Albuquerque for 20+ years. I miss the mountains.
Bruce

Bruce,

You've caught me in some fundamental neglect... Zippy has been telling me to get some voltages for a few weeks now before I worry about any further tweaks on this amp. I need to do that but have restricted myself from messing with the this amp until I get two others off of my workbench and into a cabinet.

I hope you are right about the tweak on the bias resistor for the 6V6's and the 6L6's... I'd love to hear the background and which way you had to go to tweak things into order for those tubes.

I'll make that a goal for the weekend to get the voltages and figure out where I am at as far as power dissipation goes. I know that I am probably starving these dudes a lot since I think I've only got about 300V on the plates...

New Mexico is amazing this time of year. Albuquerque has been clear and bright, in the 60's for a couple weeks straight. Up north here it's not quite as warm but they are having a great Ski Season.

thanks for the note and good to hear from you!

rj

[RJ Guitars]

RJ nice work. Have you checked how much power you are dissipating from the different tubes...

Yes, now I have done that. I'll attach a chart that tells the whole story. I looked into the dissipation on the 12AX7's and decided that it is negligible in sorting out the total load on the power transformer and it made my chart too big to do a screen shot, so those aren't on the graphic.

...I’ve done cathode biased before and found a resistor value may make the KT66 and EL34s sound sweet, but running 6V6 and 6L6s requires a different value to get them to sound good...

Any suggestion as to what direction I should go with that cathode resistor to get the 6V6's and 6L6's to get in line?

At a glance it looks like the 6V6's should be happy at an average dissipation of 33mA so I dunno what to say about that. I'm a little concerned about the 6K6's running that hot, but I need to look at the spec sheet and see where they like life. I think the KT-66's could take a little more heat so I plan to play with the cathode resistor and I'll get back to you on that story.

I also will put in all the options on the rectifier tube and see what happens to the voltages then.

As a side note -- It seems that you could build one of these amps with a 150mA tranny and still be in the safe zone. If the manufacturers have a built in 2X safety rating then everything should be very cool... so to speak.

Finally I'm curious what the gurus think about voltage starving the KT-66's (and the other big power tubes)? What should a person expect in performance from a voltage profile like I have for these tubes?

rj