Build thread- 3-channel 50W head, SLO + AB763 clean

[Badside]

Just finally started building today after 2 weeks of planning (measure twenty time, drill once... hopefully)

I don't typically do commission builds for a bunch of reason, but this friend insisted. Not sure what I have gotten myself into.
After playing through a bunch of my builds he settled on my little SLOey practice amp for the lead, and my "Vibroverb without the verb" combo for the clean.
I figured it needed something in between, so I'm adding an extra mode to the SLO side (with its own Drive and Volume) which will bypass 2 stages (which is what the clean mode is on my SLOey practice amp). Gives a Plexi kind of clean that gets all gooey when you crank the gain.

The amp will include an FX loop and a post loop solo boost.
So... 3 footswitchable options in total (switch between the Fender and the SLO side, and between the "Rock" and the "Blues" variants on the SLO side)
5 relays needed.

The whole thing needs to fit on his cab which is 20x10 on the top (slanted vertical 2x12), so starting with a 18.5x8 chassis.

Printed my plan on big sheets... unfortunately I apparently move the board component on the full layout plan before exporting, but it's alright I know where things go.

Cut the drilling plan to size

And went to town

Some interesting things:
- The Relay supply uses the 5VAC rectifier tap off the 290DX power transformer since I don't use a tube rectifier. Using a Delon voltage doubler configuration I can then regulate down to 5VDC and it's super stable. Will be my 4th time using this config. Also using this clean 5VDC to power LED indicator.
- The channel LED indicator will go Green for Fender Clean, Orange for the "blues" mode and Red for the full lead mode. Basically, the relay for the channels lights up the Green side of a dual color LED when in clean, and the Red side when in SLO mode. Then the relay for the sub-mode lights up the Green side only if the Blues mode is activated. So if the amp is set to clean, it doesn't matter if the SLO side is in lead or blues mode, the light will be green. But once the SLO channel is engaged, going to blues will make it go orange by way of lighting both the red and the green! Or at least that's the plan.

 

[Lowerleftcoast]

The Swiss Army SLOeyVerbPlex, or SVP.

Remember to leave a scabbard for the toothpick.

 

[Sam VanLaningham]

Love it!

 

[Badside]

The Swiss Army SLOeyVerbPlex, or SVP.

Remember to leave a scabbard for the toothpick.

Well I've been known to build kitchen sink amps in the past, but this one is actually a lot simpler than it sounds. It's a 50W SLO, I'm just using the extra triode to run a proper AB763 clean channel with its own tonestack... and allowing bypassing two stages in SLO mode for an extra sound.

It is definitely more relays than I've ever used though

 

[dunner84]

Great amp. I built this exact amp a few years ago. I couldn't make out your power supply from the layout, but I would recommend the SLO power supply. Solid state, stiff filtering etc... I first built mine with the ab763 power supply, and the slo just didn't have the immediacy it needed. The ab763 still sounds great with the stiffer PS.

I eventually removed the ab763, and replaced it with a full ODS preamp. I ended up using strips of Arduino relays off amazon to clean things up, and built a little programable foot switch for the channels/modes and some parallel blend options...

Excited to see/hear how yours turns out!

 

[Badside]

Great amp. I built this exact amp a few years ago. I couldn't make out your power supply from the layout, but I would recommend the SLO power supply. Solid state, stiff filtering etc... I first built mine with the ab763 power supply, and the slo just didn't have the immediacy it needed. The ab763 still sounds great with the stiffer PS.

I eventually removed the ab763, and replaced it with a full ODS preamp. I ended up using strips of Arduino relays off amazon to clean things up, and built a little programable foot switch for the channels/modes and some parallel blend options...

Excited to see/hear how yours turns out!

Yeah I'm going stiff on the PSU. Marshall 50W style, 100uF on the plate, 50uF on the screens, diode recto, around 460VDC.

He played through my Fenderey combo and really liked the clean, and that was 64uF on the plate and 32uF on the screens also diode recto, and a pair of 5881. He only wished for more headroom, which this will provide (6L6GC at 460VDC instead of 5881 at 370VDC, and a big fat 50W Bassman OT instead of the 40W OT)

 

[Badside]

Dayum that's a lot of pots. Sure it's just 2 EQ, 2 gains and 3 volume, actually pretty simple and intuitive... hopefully

There will not be an inch to spare in there once I'm done (main board does not include the relays and the FX Loop)

 

[Badside]

After 15 years doing this hobby, it happened... I did the thing

First dry run of the amp 2 days ago (by dry I mean no tubes in). Checking the bias voltage (cause I don't want to melt brand new power tubes), it won't go higher than about 28-30VDC and it keeps going down. Low 20s after a few minutes. This is no bueno.

I check and recheck everything. B+ is fine and stable, but bias voltage just won't stabilize.

Then I realize I have the red probe on ground and black probe on the bias supply and it's still showing negative voltage.

Yeah, I did the thing, I put the diode backwards.

Aka the bias caps were leaking DC all over the place, explaining the low bias voltage (and why it kept changing). Kids, this is why you check voltages BEFORE you put the tubes in.

15 years and not a single misplaced diode so far, and it finally happened.

So anyway, I replaced the caps after installing a new diode the right way (pointing towards the PT not the other way around!) and the amp works! Ok I only have the SLO channel working at this point (I like to build in steps so it's easier to identify what's not working). But it's super quiet, no hum, no hiss, no squeals at high gain.

I don't like the stock 39k NFB resistor though (off the 8 ohm tap), although 100k is a bit too intense and aggressive. Will probably settle with like a 68k or 82k. Want to hear what the clean channel sounds like first (82k would be closest to Fender specs given the 4.7k tail).

It's got more gain than I anticipated, a LOT of gain. Anything past halfway is just too much. But that's why there's a control for that!

Need to add the switching now so I can try the other modes.

 

[Badside]

So the "Blues" mode, which bypasses the 2nd and 3rd stage of the SLO channel and is fed pre-treble peaking circuit... Is way too affected by how the SLO gain control is set (both drive control are fed off the same tube stage).

I knew this would happen, but didn't expect it to be this bad.

But I have an available relay switch right by the drive controls, so I'm gonna interrupt the feed to the SLO Drive control in Blues mode, grounding it instead. This should nullify the interactivity.

So far though:
- This thing is really quiet noise wise! I was apprehensive because of the unproven layout, but it's a success.
- It really has way too much gain. Gonna try swapping one of the two 220k plate resistors to a 100k.

 

[Lorenz1973]

Great job to keep all without unwanted noises. When it's the final revision, I'd be curious to see the schematic.

 

[NSB_Chris]

very cool! Would you post your schematic?

 

[telemnemonics]

Interesting, and that IS a lotta knobs!
Any way to add reverb to the Fender channel?
Maybe I missed that, saw relays so figured one input and channel switching.
Wondering what the customer expects, Fender amp with no reverb is less Fendery while Soldano or Marshall WITH reverb never worked for me. Maybe a tiny bit or preferably delay.