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

[Zippy]

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.

You respond well to the cattle prod, RJ. Nice work.

[RJ Guitars]

Well I went a couple steps lower in the cathode resistor value and took some more measurements with the KT-66's.


264 ohms gives 109mA / 2 = ~55 mA per tube
235 ohms gives 115mA / 2 = ~57mA per tube


Looks like a 200 Ohm resistor will give me about 60mA per tube... how much is too much of a good thing?

I started a thread over on the technical discussion to see if I could draw in some of the generic tube gurus to help me learn about tube dissipation and cathode biasing... although I think some of you Trainwreck X-Perts will know a lot about this as well so I'll run a bit of a parallel track, at least for this part of my KT-66 Rocket discussion.

How low can I go on the cathode resistor? In the Gerald Weber's book he talks about adjusting the bias on a fixed bias amp by having your buddy wang away on the guitar with the amp cranked and run the bias up until it red plates then back it off until it doesn't.... eventually finding some sweet spot in the non red plate territory. On a cathode bias amp, can you theoretically do the same thing - lowering the cathode resistance until you hit the sweet spot? (stopping of course just before melt down since I'm using Zippy's tubes)?

rj

https://tubeamparchive.com/viewtopic.php?p=76312

[stevek66]

Hey RJ,

This one works pretty good as well.
http://www.webervst.com/tubes1/calcbias.htm

[Zippy]

264 ohms gives 109mA / 2 = ~55 mA per tube
235 ohms gives 115mA / 2 = ~57mA per tube


Looks like a 200 Ohm resistor will give me about 60mA per tube... how much is too much of a good thing?

Are you hearing any difference? Those changes are pretty subtle.

...stopping of course just before melt down since I'm using Zippy's tubes...

Yes, please.

[RJ Guitars]

Hey RJ,

This one works pretty good as well.
http://www.webervst.com/tubes1/calcbias.htm

Excellent info!!


Well,

I went to the 200 Ohm cathode resistor... and all is well! The change is not sonically stunning, but adding something to the mix!

200 Ohms - 26.46 Volts (across the cathode resistor) - 132 mA total - 66mA per tube

I haven't run all the tube options with it yet, but the KT-66's are sounding quite sweet.

I did turn it up all the way and wang away for 15 minutes - no red plate but lots of sound!!!

As with my previous Rocket amp, this one breaks up early and has lots of crunch tone. However, maxed out it still never mushes out. One thing I might do is run either a "PIG" control or another trick or two and see if I can make it run clean at high volumes. It might be just as it is with the EL-84 Rocket, that the Dynaco 470 tranny cleans it up. However, that is not an option I can easily put on a switch.

Right now it's brighter than my EL-84 Rocket but has a lot of that same harmonic character. The Stancor output tranny makes a direct comparison invalid, so I guess I need to build a couple more iterations before it's fully understood and characterized.

I've still got the complete parts array to build an identical one if anybody wants to give it a shot... I'll do these for a low margin if any AmpGarage guys have an interest.

Pictures and clips to come....

rj

[RJ Guitars]

When are we ever finished? Could this be it?

[IMG:320:193]http://www.rjguitars.net/Digital_Images ... _small.jpg[/img]

Since I cranked that bias up I decided to let that rest. EL-34's run at 59mA each if I use a 200 Ohm cathode resistor. They do sound quite nice, I'll let you know how long they last. KT-66's run at 66mA each. I'm a little scared to try the others, but I'll do it eventually. I am really running the plate voltage pretty low for most of these tubes.

I did a couple other tricks as well. I changed the first gain stage bypass cap to 50uF. This gave me a little bit more low end. This particular build wanted a little more - probably the nature of that odd Stancor output tranny. All my other Rocket builds have plenty of bass balz. Oddly, this tweak made the amp more clean?? Anybody understand that?

I also added a 220uF switchable bypass cap onto the 2nd gain stage which give This gives me a pretty good boost as well as even more low end. I added a switch in the bright switch hole for this option... works great but makes a pop when I go to boost. Any thoughts on how to fix this? Maybe a 10 meg bleed resistor?

I moved the wires over on the cut control so that it actually works as a cut rather than backwards like the Rocket builds I did before.

Now it is really starting to settle in and sounds really good!

I also added a direct line out, then put the bottom cover on it and loaded into the cabinet...

It's pretty loud and wants to feedback liberally at 1/4 volume (ES-339).

So since my photo I've added a switch and had it apart three times... is it really ever done?

rj

[xk49w]

... works great but makes a pop when I go to boost. Any thoughts on how to fix this? Maybe a 10 meg bleed resistor?

I guess you are talking about the cathode resistor (R5?) bypass cap? Try to precharge the bypass cap with a series resistor to the cathode. I guess that's what you mean by bleed resistor. Your switch will short it out. Anything over 150k would be more than ten times the value of the cathode resistance, getting close to negligible effect.

[v00d00blues79]

I started on my Rocket-66 build last night. I should have it done by this weekend.

RJ,

Do you have any clips of yours yet?

Thanks,

Andy

[andrew]

What are the milliamp requirments on KT-66s? Thanks.

[RJ Guitars]

What are the milliamp requirments on KT-66s? Thanks.

Andrew - here is where I go for that kind of info - http://tdsl.duncanamps.com/show.php?des=KT66

I use the tube specs to get things in the ball park then move to more of a try it and listen technique. I did some reading from Ken Fisher's Trainwreck pages and Gerald Weber's first tube amp book, particularly on the topic of biasing. There are a couple of interesting things in those books about biasing an amp hotter in the cathode biased mode as opposed to a fixed bias amp. I did push that specification up pretty high on my build in the end.

Also, there is some interesting reading in those KT66 spec sheets including the requirement for separate cathode resistors when using a pair of KT66s in cathode biased mode... I didn't do that but it's probably worth investigating why they ask for it and again listening to it both ways. Does anybody know why they have that in the spec sheets?

I'm really looking forward to a couple more guys putting these together and tweaking them into the tone zone. It's a pretty easy build to create by using an Express chassis and adding in the Rectifier tube. I've got my as built drawings and schematic almost done and I'll post those up in the near future.

rj

[v00d00blues79]

Finished my build up this afternoon. I'm really digging it so far. I have a bit of a hum problem when I get close to the input. I ran a regular lead, and haven't tried shielded yet, but it grooves pretty good! I have a set of old Mullard XF2's in the output right now, but I'm getting ready to order some KT66's. I also need to get some new pre's to try in it. All in all I'm really happy with this build. Any recommendations on what tubes to feed this beast?

Thanks!

Andy

[RJ Guitars]

Finished my build up this afternoon. I'm really digging it so far. I have a bit of a hum problem when I get close to the input. I ran a regular lead, and haven't tried shielded yet, but it grooves pretty good! I have a set of old Mullard XF2's in the output right now, but I'm getting ready to order some KT66's. I also need to get some new pre's to try in it. All in all I'm really happy with this build. Any recommendations on what tubes to feed this beast?

Thanks!

Andy

Congrats Andy... post up a photo when you get something available, I'd love to see it.

Mine is pretty quiet and since it's not a gain monster like the Express, it should be easy enough to tame.

Guitar player did a review of KT-66's here a while back and the bottom line was that there really aren't any bad ones in new or old production.
I am running the Shugang-Valve Art tubes right now and they are very good sounding. I'll look for that GP article and review it for more specific info....

just Googled this up - http://thetubestore.com/kt66reviews.html

I have mine biased at 66mA and there is a comment on thetubestore site that says these need to be biased hot... I agree. http://thetubestore.com/shuguangkt66.html

For a high quality tube $50 for a matched pair doesn't seem outrageous.

As far as preamp tubes, I have a lot of old RCA and GE stock and a pristine set of old Mullards plus a couple old Bugle Boys. The bottom line is I have not bought any preamp tubes lately so I don't know what to suggest.

Do I recall correctly that people are pretty happy with the Mullard reissues?

Let us know how the tweaking goes. I think I have schematic ready to post up... I'll get that done and provide a baseline to tweak from.

rj

[M Fowler]

I got a matched set of Groove Tubes KT66 #6 rating for medium breakup range. Tried them in my Express combo build and they were nice. This amp is so loud, quiet (no hiss/hum) and clean that I had decided to use the 6V6 and the NFB resistor is 68k instead of 100k.

Bolted the chassis into a Line6 212 xl flextone amp cab that I never use any more and they don't fetch any money so why not use the eminence speakers and cab. The Line6 amp will go into a rack mount for recording.

Mark

[v00d00blues79]

I added a trim-pot in place of the mid resistor so I could fine tune the mids on this one, but I pretty much nailed it with the setting I had on it when I put it in place, so I haven't measured it. I also plan on putting a VVR in this one, so I plan on using the presence pot hole for that. I'll post some pics when I have it open next time.

Thanks!

Andy

[RJ Guitars]

EDIT - 2/16/09 - Replaced the schematic with Rev. 1d (Thanks Jimi)... then 1e (Thanks Randy).

I have what I believe is a correct schematic of the 66 Rockster. The preamp is basically a Rocket preamp with a variable mid control. Some folks are reporting success just tweaking the mid resistor and not bothering with the pot.

I think I started with an Express power section then converted it to a cathode biased unit. I have not done anything to optimize these values but the result is at the least a good starting place.

As usual, I made a board for the power supply. The chassis and preamp board are both Trainwreck specification so if someone wants to try a "Stack O' Caps, I think it would work just dandy.

I'll be polishing up a layout drawing next. If anyone sees anything that is absolutely not gonna work or is a major mystery with this schematic... lemme know.

thanks,

rj