Monkeymatic S'express 15-watt

[xtian]

Witness! The inception of my small Express, thus, "S'express".

Thanks to RJ for the inspiration, and to others for the spare parts, including the iron, which is a 1980s tranny set for Rivera-era Fenders. I have taken many liberties with the layout, stealing ideas from Hoffman, Huss, HAD, etc., and completely throwing caution to the wind.

Chassis is a blank from Watts, 12.5" x 8" x 2.5". [updated layout, see next page]

[dcribbs1412]

Thanks xtian
Is this a build in the works?
I have a 18watt PT and a DR OT
on the shelf I might give this a shot.

Darin

[xtian]

I have 95% of the parts, but haven't started building yet. This will probably get built over the next few weeks. I'll keep posting.

[JoeCon]

Xtian

Is this gonna be a 6V6 version? MV, really??

[xtian]

No, those are noval sockets for EL84. Yes, MV.

[JoeCon]

Xtian

Nice addition to your amp choices....

[dcribbs1412]

Will be looking forward to updates
What is your iron specs?
I might see 350V@120ma on the plates using the 290v-0-290v PT
hoping its close enough for a 6V6 xpress.

Darin

[xtian]

Mine is going to be very low voltage. I'm using a Fender A019118 "Super Champ" transformer, with 310vac unloaded HV. That will probably give me 260vDC on the plates. Might have to throw up my hands and put in a different PT when I'm done...but we'll see.

EDIT: Well, I'm less concerned about the voltage after reviewing the EL84 datasheets, which have a spec for 250 on the anodes of a P-P pair in class AB.

[dcribbs1412]

Mine is going to be very low voltage. I'm using a Fender A019118 "Super Champ" transformer, with 310vac unloaded HV. That will probably give me 260vDC on the plates. Might have to throw up my hands and put in a different PT when I'm done...but we'll see.

What math are you using....Not sure if this is right but I have seen others use 1.2 and 1.4 as a multipliers
1.2x310V=372V loaded? or 1.4x310v=434 unloaded?

After looking into the load calcs a little further (1.2x290V=348V)
I may only see around 350V at the plates.

Darin

[xtian]

Check out Hammond's Design Guide for Rectifier Use:

http://www.hammondmfg.com/pdf/5c007.pdf

Mine is a full-wave bridge, cap input. V(peak) = 1.4 x secondary HV, and V(average) = 0.9 x secondary HV.

So, I'm multiplying my 310vac unloaded by 0.9, and then AGAIN by 0.9 as fudge factor for value under load.

[dcribbs1412]

Thanks xtian
good info
looking forward to your build

Darin

[JoeCon]

Xtian

But an Express preamp with EL84s? Liverpool??

[cbass]

Xtian

But an Express preamp with EL84s? Liverpool??

it looks like its fixed bias though

[JoeCon]

Ok so a cross between the Liverpool and Express! I like it...
I've got a chassis punched for EL84s maybe I should build one too...

Thanks.

[xtian]

I'm following RJ's lead on this one. Express preamp, EL84 power section, fixed bias. Yup.