New Express, non-traditional layout

[pablogt]

Hi all. I finished this amp just before summer. It's essentially an Express circuit. I have moved the second gain stage to V2 so I could have normal and parallel input jacks. I also adjusted the resistor drop down string so that voltages are close to Francesca's, compensating the extra load of the unused-no-more triode. The second gain stage now draws from B+4 instead of B+5 so there are some differences.

I used a Pacific 25525 OT I got from rj and Hammond's 372jx PT, Sozo Vintage caps, Murata 1000kv ceramic caps for smaller values, Koa carbon film resistors, carbon comp for plates and metal for the drop down chain and grid stoppers. F&T filter caps. You may notice that two 1000pf poly caps are used in series as treble cap.

I also installed Dana's VVR, scaling the whole amp

There are three small RC filters in the PS section (two before rectifiers, one after), to smooth out switching noise or ripple (that really smooths out distortion). That also adds some PS sag, that helps compensating the "stiffer" 372jx. Elevated heaters by 70v

The layout is all new and it works great. I've had no oscillations at all, even with everything dimed and parallel entry. The hiss is very reasonable for that kind of gain.

The sound is great. Very smooth, and balanced when VVR is maxed. When dialing power down, some treble and gain is lost, nothing you cannot compensate with settings. I use the bright switch to compensate for lost treble at lower settings.

The most amazing thing about this circuit is the clean to mean thing. Last week I jigged for the first time in my life without any effects and with a single channel amp. The sound guy just could not believe it.

The voicing was spot on since the very beginning. Most time was spent rolling tubes, mainly looking for the less noisy ones. It was particularly difficult to find a good tube for V2. I also had to try several pairs of El34's till I found the right one. I finally settled for Brimar (V1), Brimar (V2), Telefunken (V3) and RFT (V4 and V5).

I have enclosed the Visio files I used in case they are useful for someone. After building the amp, I did some small corrections to the layout. The biggest one was to move the VVR board and pot towards the main board so that they did not interfere with the PT's secondaries. In the photo you will see the VVR board turned 90 degrees to avoid that interference. I also had to cover the bottom plate with cork board so that it cannot accidentally short the VVR board.

cheers,
Pablo

[pablogt]

The Layout and schematic

I wired the PT for 220v operation, while there are 230v at the wall at home. That little extra voltage was used to compensate for the extra load of the RC filters and the extra pre triode.

Actual /Francesca's
B+1: 393 /395
B+2: 370 /380
B+3: 374 /298
B+4: 250 /282
B+5: 238 /267

You may also notice the dual bias pots and bias test points

[pablogt]

Visio files. They were done with Visio 2007

[Reeltarded]

Always great work! Looks as good as it sounds like it must sound.

[pablogt]

Thanks...

Visio files have different layers that allow to show or hide elements. Here is the layout without VVR.

P.

[overtone]

Impressive work at all levels.
I enjoyed reading through your schematic, thanks.

[BIG Dave]

Very nice work!

[cbass]

Greatjob a work of art I expect nothing less from pablo

[Kregg]

Major amp porn, beautifully done.
I want one.
Audio clips?

[pablogt]

Hi. Thanks all for your kind comments. I'll try to have some clips recorded next week

P.

[Firestorm]

Oh my! That looks sweet!

[RJ Guitars]

As always, a beautiful build. I wish for some clips if you ever have the chance.

[pablogt]

Promise I will... I just cannot play loud at home and haven't been at the rehearsal studios for the last couple of weeks.

P.

[pablogt]

Hi there. This is my friend Marco Ciargo trying out my Express. Settings are gain at 2 PM, the rest at noon, including VVR. He is plugged in the "hot" input (parallel triodes) and using an old Gibson SG. He switches from full on bridge to neck at maybe 4 at 0:50.

Recorded with my shitty Zoom H2 with no effects.

The second (parallel) input triode was voiced to compensate for the treble loss that happens at this VVR setting, which is the one I use most.

Pablo

[katopan]

Great clip - love it!