Silver Jub

Davo · Post by **Davo** » Sun Apr 15, 2007 8:25 pm

Hi Guys,

Long time watcher / first time poster!

Firstly Steve - I reckon you have to be one of the most generous and helpful guys on the planet - the work you do here is invaluable to all us "wannabe" amp techs.....

I have been planning a Jube build for a while now and the SDM layouts are obviously central to this - a couple of questions;

1) Has anyone built and validated Steve's extra gain control layout (2550X) - this does seem like a logical mod and a confirmed lack of pop on switching would be nice... otherwise I'll just give it a try!

2) I have PT iron for a 100W but was originally intending to build the 50W with pentode / triode switch. In the past I have successfully switched the outer two power tubes to ground on a 100W build and I figured this coupled to a triode switch on the inner tubes would provide a flexible 100 / 50 / 25 watt amp. The question I have relates to halving the speaker impedance setting when these lower power modes were running: consensus seems to be this is not critical for master volume builds as there is less stress on the output stage compared to the non-MV models running flat out - but would this also hold for dropping down the extra step to 25W? Alternatively would it make any difference if all four tubes were pentode/triode switched to hit 50W with the switch to ground dropping the outside tubes to hit 25W?

Any and all thoughts welcome before I start buying the extra parts....

paulster · Post by **paulster** » Mon Apr 16, 2007 1:11 pm

I've just finished building a 2550 based on Steve's layout and I'm really pleased with it.

I used 50W iron though as I already had it spare, and that was what started me off down the Jube route.

As regards the 100/50/25W switching, if you take a look at the schematics for the 30th Anniversary amp (see www.drtube.com for them) then this has pentode/triode and 100/50W switching ready drawn out for you.

You leave the speaker impedance the same when (electrically) removing a pair of tubes because the tubes are still in circuit, only they're not amplifying a signal anymore. They still have the same output impedance though, so it keeps everything looking the same as far as the tubes/transformer/speaker relationship.

SDM · Post by **SDM** » Mon Apr 16, 2007 11:25 pm

Hey Davo,
Welcome to the board. The 2550X layout was just something I just kinda threw out there as the shared gain control always bugged me on those amps. I did just update that layout with a few changes - .033uf first coupler and 1 meg pots (rather than the 2 megs as they are much easier to come by). All in all the first stage is loaded a tad more with the extra pot in there, but just a bit, really should be something that could be more than made up for with normal variances you'd find with different 12AX7's. So, hopefully it's basically inaudible. Would need to make the switching more complicated to absolutely ensure no subtle differences exist, and since there may well be no real benefit, I just kinda leave that stuff up for people to play around with if they'd like.

All the layouts were meant originally just for ideas, never expected so many to actually build them. Glad many have had success though! Also I do believe someone is about finished with a 2550X version, and if they don't post here. I'll let you know if popping is a problem or not.

Davo · Post by **Davo** » Tue Apr 17, 2007 3:04 am

Hey Guys - thanks for the swift responses!

I'll chase down those 30th anniversary schems to see the specifics and start ordering some parts - should be fun to try a dual gain 100W Jube Steve - I may just use separate switches to allow complete flexibility on the pot values (i.e. would you consider 2Meg to be preferable?) - if you do see any new 2550X feedback please post it and I'll do the same of course....and thanks again for sharing such quality work!

Cheers,

Davo · Post by **Davo** » Sun Apr 29, 2007 9:12 pm

Hey people - are the 220K linear pots on the lead volume and treble critical on a 2555 build or can I get away with 250's?

Thanks

SDM · Post by **SDM** » Mon Apr 30, 2007 12:11 am

Davo wrote: would you consider 2Meg to be preferable?

- Only if you always play with both gain pots dimed, so not really as that kinda defeats the point of two gain controls and Jubs IMO sound best with the lead gain rolled back some. Actually the bright cap should have been half the value shown with the old 2 meg pots layout, but I wouldn't worry about that and just go with the 1 megs as per the current layout.

250K pots will do fine.

Davo · Post by **Davo** » Mon Apr 30, 2007 12:57 am

Thanks Steve - I have parts arriving from all over the place - just need to start the always "interesting" chassis fabrication!

Cheers,

Davo · Post by **Davo** » Thu Jun 28, 2007 2:06 am

Hi "Jub guys" - see: http://forum.metroamp.com/viewtopic.php?t=12354

JDavisApple · Post by **JDavisApple** » Sun Jan 06, 2008 11:34 am

I too would like to see a jub kit out because I deffently would buy one since my favorite guitar player is slash.