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

[JimiB]

you need to fix the ground conection on the bias resistors. The 220K bias splitters and the other end of the cathode bias resistor and cap need to be going to ground. As it is drawn now the power section will do nothing.

[RJ Guitars]

you need to fix the ground conection on the bias resistors. The 220K bias splitters and the other end of the cathode bias resistor and cap need to be going to ground. As it is drawn now the power section will do nothing.

Thanks Jimi... I fixed that and re-posted the schematic.

rj

[RB]

The cap in the HI CUT circuit (C11) is marked as .022uf. The Rocket and VOX schematics indicate .0047uf. If you have changed the value, how has it workrd out?

Thanks
Randy

[RJ Guitars]

The cap in the HI CUT circuit (C11) is marked as .022uf. The Rocket and VOX schematics indicate .0047uf. If you have changed the value, how has it workrd out?

Thanks
Randy

Randy,

Actually that is a mistake. I hope everybody takes the "Unverified Draft" label seriously. I didn't change that cap on the actual amp so I dunno what would happen there.

I actually built the amp using the Rocket layout in an Express chassis and did the power section on the fly... I must be better at building them than drawing them because the amp actually worked first shot out of the barrel.

One cool thing about Trainwreck amps is the common chassis size, tube layout, circuit board size, and such. One driver to build this particular amp is knowing that Ken Fischer was still designing amps in his head when he got sick and wondering what would he have done next using what he had learned on the other builds.

Th 66 Rockster amp now looks amazingly similar to a 59' Bassman except for the cathode biasing... might be that the AC-30 actually borrowed a bit from the 59' Bassman in the same way as those early Marshalls... and of course Fender took their info from the RCA tube Handbook... 50 years later it still makes a great amp!

After I finished the Rockster build I created the schematic document, but obviously with an initial collection of errors from cutting and pasting pieces from other schematics. I had actually never drawn the Rocket schematic because the Matt Taylor document was well done, accepted and tested.

Thank you Randy. I'll update the schematic and if it holds up I'll upgrade it to final draft... might be getting close to the "Prime Time" offering soon.

rj

[soma_hero]

What taps on the output transformer are you using RJ? Probably going to get a transformer from you for my KT66 build, but not sure which one for max flexibility.

Want to use KT66's and probably 5881, EL34, 6L6 at some point. I am correct that you're using the 5.3k tap and that all these tubes are alright with the load impedance. Are you changing speaker loads each time you change tubes?

[RJ Guitars]

What taps on the output transformer are you using RJ? Probably going to get a transformer from you for my KT66 build, but not sure which one for max flexibility.

Want to use KT66's and probably 5881, EL34, 6L6 at some point. I am correct that you're using the 5.3k tap and that all these tubes are alright with the load impedance. Are you changing speaker loads each time you change tubes?

Soma,

I have a Stancor in this amp and it's one that I can't find any pedigree on. (I'll guess 5.2K input impedance and have good intentions of verifying that with a 2nd copy I have on the shelf). Lately I only play through a 4 ohm cabinet to test things and I don't make any impedance adjustments when I swap tube sets.

The transformer that Tweedeluxe is using in his KT66 Rocket is an extremely good choice and well documented. It's an Edcor with dual taps - 5.2K and 6.6K which covers most of the options in Octal tubes. I also have the same basic transformer in 4.3K and 5.2K if you prefer a slightly lower input impedance.

Gerald Weber gives a formula that goes something like "it's safe to match up output transformer impedance on everything within 100%"... Translated than means if you've got a 5.2K / 8 Ohm transformer, it is safe to run it with tubes that have either a 4K or 6.6K load impedance with an 8 ohm speaker... I believe there will be subtle differences in how it sounds depending if you are over, under, or right on the recommended values.

I have not changed the transformer match (partly because I only have one set of taps) when I am swapping tubes since everything of interest is well within that 100% margin. As I tweak the amp and begin to take a more critical look at everything, maybe even use a scope, that impedance match will probably be something I work at a little more carefully.

rj

[gsb]

Hi !
I have read about your amp and I think to build one.Im from Slovenija-Europa
and have a OPT for a JMT 45 from Welter electronic-its a clone OPT for
JMT45 .Primar impendances are 6.6k and 8 k for 4,8,16 ohm secondary
So for KT 66 -6,6 will be ok ? ,and 8k for 6v6 ? I have read also
people run KT66 on 8k in some JMT45 ? What do you think about ?
Thanks for answer and sorry for my English
gsb-darko

[RJ Guitars]

Hi !
I have read about your amp and I think to build one.Im from Slovenija-Europa
and have a OPT for a JMT 45 from Welter electronic-its a clone OPT for
JMT45 .Primar impendances are 6.6k and 8 k for 4,8,16 ohm secondary
So for KT 66 -6,6 will be ok ? ,and 8k for 6v6 ? I have read also
people run KT66 on 8k in some JMT45 ? What do you think about ?
Thanks for answer and sorry for my English
gsb-darko

GSB, That is pretty cool that we can reach all the way around the world and talk about amps... There are a few iterations of this amp starting to pop up here on the AGF. Yours will be a welcome addition.

The JTM45 transformer will work very well for 6V6GT, KT66, EL34, and a hand full of other options. I believe you can use the 6.6K tap for the 6V6's and the KT66's. It might even turn out that you don't have to do anything to the cathode bias resistance between the different tubes... just make sure you check it before you believe that!

What power transformer do you have? I'll assume a 230V primary and I am curious about which secondary. If you don't have one, I'd suggest a VOX AC-30 power transformer. They are a heavy duty unit and the secondary voltage (280V) will lend itself to this project. One other good option is a transformer from Toneslut here in the USA. I think he carries the European voltage units and his has a 260V secondary that would be perfect for these amps.

Thanks for checking in with us, Good luck, and let us know if we can help,

rj

[gsb]

Hi !
Thanks for answer.For my tube builds I always use custom
toroid PT from local shop here .I have one with 275 V ,450ma , 6,3v-6A
15v -500ma ,5v -2A.Its 230V model.I know i must change primary fuse . I think it will be fine for project it has plenty of power and noo humm .
thanks gsb

[RJ Guitars]

Hello AGF,

I added several options to allow me to tweak this amp. Some of these I think I have already mentioned...

I have begun the process using my ears. Eventually I'll scope this, but some of them are so obvious you won't need a scope to sort out what's happening.

I added a few things:
1) Variable Output cathode bias resistor
2) Variable Phase Inverter Tail Resistor
3) Variable Cathode Follower tail Resistor
4) Balance pot on the Phase Inverter
5) Switched cathode bypass on the 2nd gain stage
6) Added 6.2K Screen resistors
7) Added a 470K resistor across the volume pot

For the output cathode bias resistor I also added a couple measurement nodes so I can tell what resistance/voltage I am working at without having to pull the amp out of the cabinet. Very handy for switching tubes in and out!

Of all the tweaks you see in my photo, the only one that has a huge audible impact is the bypass cap on the 2nd gain stage. Everything else is more subtle.

You can hear the difference in the cathode bias resistor, but it doesn't jump out at you.

The other three pots will require a scope or a better set of ears to really know what is happening. I kinda thought I could hear things happening, but nothing so obvious I want to claim it until I see it with a scope.

The resistor across the volume pot really took away that huge jump in volume between go and 9:00... I like it this way but I am curious if it has changed other things that I haven't noticed yet.

The amp still sounds absolutely great and I've been doing a lot of tweaking with a set of 5881's in it. They sound really good although it's clear they do not pack as much horsepower as the KT66's!

Curious if others have done similar tweaks and their results?

rj

[Structo]

That's a pretty cool test bed.

I know what you mean.
On my last build which was the Hammond conversion, I swapped different values around from different amps just to see what they offered.

I was kind of surprised when sometimes a value change of up to 300-400% didn't make an audible difference to me.

But, like you said, without a scope to tell the story it may be a good change or a bad change that is happening.

Ain't this stuff fun!

[Greg]

Lets hear it

[gsb]

Hi !
I,m little confused about screnns resistors.Tweeddeluxe used 220 ohm
RJ 1k .I have look on original JMT schematic there are 470 ohm and then
one 1k .What should be used ? Are there any sound diferences betwen
one cathode resistor for KT66 (200 ohm) or separate resistors -each 400 ohm ? And one question on Tweedeluxe -how do you made paralel mod
on V1 -schematic of your build will be fine .
Thanks for your answers
gsb

[RJ Guitars]

Hi !
I,m little confused about screnns resistors.Tweeddeluxe used 220 ohm
RJ 1k .I have look on original JMT schematic there are 470 ohm and then
one 1k .What should be used ?

I chose my values because I was converting an Express output section from fixed bias to cathode bias. The 1K is a little better match for the EL34's you would commonly find in an Express... I liked how mine sounded so I haven't changed that value yet, it's a good experiment to try in the future though... I wonder if I could put a pair of rheostats on there?

I'll let Tweedeluxe answer for himself but it is is is a bit of "season to your own taste" in the formula. Fender tends to use the 470 ohm resistors. You can find a lot of info on this topic in the Dumble discussion group. As we get a few of these out there I'll expect that we find an understanding how they affect the tone and establish some common standards.

Are there any sound diferences betwen
one cathode resistor for KT66 (200 ohm) or separate resistors -each 400 ohm ?

The KT66 spec sheet asks for two cathode resistors rather than a common single... in hindsight I wish I would have done that just to hear it. I was adding the output layout idea from the Rocket in this case, which uses a single cathode bias resistor. I don't know why they asked for it but I'd like to try it both ways. Somebody who has the two resistors can add a parallel link and tell us the difference sonically...

And one question on Tweedeluxe -how do you made paralel mod on V1 -schematic of your build will be fine ...

If TD doesn't get back to you, I think Blind Lemon gave us pictures of the parallel V1 on his first Rocket build. He also gave us the info for matching the cathode and plate resistors for the new values.

thanks

rj

[tweedeluxe]

gsb - I think this is how I'm going to wire the normal/parallel inputs on my upcoming EL84 Rocket clone:

[img:640:480]http://img113.imageshack.us/img113/7097/img3704.jpg[/img]

Ra is 100k from stock 220k (twice the current, half the resistance). Rks stay the same because the cathodes are still separated.

The scheme is the same on the KT66 Rocket, just with a toggle switch. I don't know if this is the best way to do it though.

Check out the 5E8a schematic for a slightly different setup: http://www.freeinfosociety.com/electron ... php?id=797

I used 220R on the screens because the datasheet recommended that value. I used 5k6 grid stoppers because I had a bunch of them already in 1W and they seem to work just fine in a JTM45.