50W Valve Head build

[JapanAxe]

Following the success of my B15-ish build I have decided to go for a giggable 50W head. I was originally looking at something based on a 60s Fender Bassman but then I came across a MojoTone 50W bass amp kit. Although it's discontinued you can still access the schematic and the layout. It's based around the iron from a Marshall JTM45 and runs two KT66s in the power amp, two 12AX7s in the pre, and a solid state rectifier.

 

I'll be housing mine in a Hammond steel enclosure with a mesh cover as I want it to sit neatly on either of my BF cabs - overhang makes my teeth itch!

 

Here's a video from a happy chap who built one of the kits:

 

EDIT: Links removed as they now point to a Bassman kit build.

Edited August 6, 2022 by JapanAxe

[Jimothey]

Following with interest!  👍🏻

[JapanAxe]

The enclosure arrived yesterday but I won't start any drilling or cutting yet because I have only just started work on the layout.

 

I draw the layout at 100% scale to make sure no component fouls any other. You can see I have made a full size 'footprint' of the PT. That board will get chopped around a bit. The input grid stoppers will go as I am only having one and will mount it straight to the valve socket. I will also be adding the filter caps next to their respective stages, rather than using cap cans. I may have to move the bias circuit to a separate small board.

 

I already have a few bits in my stash, the rest is on order. Things like power switches soon add up in cost so I use what I've got. With resistors I tend to order extra so that I always have a stock for mods. Bottom right you can see my home brew bias pot. Fender ones retail at £6 but I found an old 10k pot, carefully undid the clips, took out the shaft, and cut enough off so that it no longer projects from the threaded portion. There is a screwdriver adjustment slot on the back of the pot.

[JapanAxe]

First draft of my board layout. I will revisit this tomorrow and check that it makes sense! That may not be its final position within the chassis, that will depend on where the transformer bolts and cable holes end up.

[JapanAxe]

Some more bits have arrived today, including a pair of KT66 valves. Those things are monsters, they dwarf the 12AX7s  from my stash - I'll have to space them a bit further apart on my layout!

[GrowlyBassDude]

Nice stuff. I've used 50-100w valve amps in the past for bass and they can record well, but didn't have enough grunt for live use in a loud rock band.

 

Is the output stage pentode or ultra linear? You could get more clean headroom with UL, but you lose the Pentode 'growl'. 

 

 

[JapanAxe]

1 hour ago, GrowlyBassDude said:

Nice stuff. I've used 50-100w valve amps in the past for bass and they can record well, but didn't have enough grunt for live use in a loud rock band.

 

Is the output stage pentode or ultra linear? You could get more clean headroom with UL, but you lose the Pentode 'growl'. 

 

 

 It's pentode. I've gigged a 50W valve head (Ampeg PF-50T) in a 3-piece rock/pop covers outfit and it was plenty loud enough.

[JapanAxe]

The iron has landed! Michael at Modulus Amplification wound me a JTM45 OT with a 6.6k primary. The PT is sold to fit a JCM800 but has the right voltages for my build and by default is in stand-up format. All the rest of the bits arrived with the same order. Woot woot!

 

[Jimothey]

Just out of interest what’s the price difference between buying an amp head and building one?👍🏻

[JapanAxe]

2 hours ago, Jimothey said:

Just out of interest what’s the price difference between buying an amp head and building one?👍🏻

Depends!

 

My build will come out at about £450. That might buy you a used Ampeg PF-50T - I had one and liked it a lot. I don’t think there’s much else around in the way of 50-100W valve heads in a package that will sit neatly on top of a Barefaced One 10, except the Handbox WB-100 at about £1,000.

 

 

[JapanAxe]

I used the 'headphone trick' to work out the best position of the OT in relation to the PT: I connected the 240V PT primary to a mains cable while all other leads were safely parked in a chocolate block connector. Then I connected a set of headphones to the 16ohm secondary of the OT and listened for hum  while I positioned the OT. No surprise that the hum was least when the transformers were at right angles at opposite ends of the chassis.

 

I marked the transformer positions onto a piece of paper. I also tweaked the eyelet board layout with the actual components to hand for their sizes, and drew in some under-board HT runners (red dotted lines). The pink circles are where the mounting stand-offs will go.

Edited May 31, 2022 by JapanAxe

[JapanAxe]

There's been a bit of a hiatus in this build for a couple of reasons.

 

First off I had a run of 4 gigs and 3 rehearsals, followed by a 2-day trip up to Liverpool to play the first ever David Bowie World Fan Convention!

 

Secondly, the PT secondary hits 360V AC unloaded, which would translate to about 509V DC. The standby arrangement leaves power applied to the first filter cap, which is rated at 500V, so I have obtained some smaller 450V caps to use in series (with balancing resistors) like Fender did in amps such as the Twin. The original JTM45 design employs 32uF filter caps at the first two nodes, whereas this design has 100uF at each! Two 100uF caps in series will give a total of 50uF at the first node, so I bought 4 in the hope of doubling that. I can only fit 2 on the board but if the smoothing turns out to be inadequate (unlikely!) I can possibly piggy-back them on the 2 that I'm installing (left of the circuit board).

 

Today I finished drawing the layout. I'll check it again tomorrow, then start marking the eyelet board.

 

[JapanAxe]

Made some progress on the board:

 

(1) Covered the board with sticky paper recycled from an eBay label and marked out all the positions of eyelets, mounting holes, and bias pot.

 

(2) Drilled all the holes.

 

(3) Set the eyelets. I ended up moving one because I thought it was a bit close to its neighbour, given the 720V between them!

 

(4) Fitted the stand-offs and the bias pot, straightened some copper wire and soldered the earth bus in place.

[Geek99]

This is total Greek to me, but it’s interesting 

[JapanAxe]

Last night and this morning have been all about metalwork. Never my favourite subject at school but it just has to be done. The big challenge here was cutting a rectangular hole in the rear panel (0.91mm steel - about 28 gauge for our US friends) to fit the IEC mains inlet. I started with a 6mm hole inside each corner, then took out the bulk of the material with a 20mm panel hole cutter. I managed to remove some more with a hand nibbler (yes it's a thing!) before defining the edges with a cutoff disc and a grinder on my Dremel-a-like tool. I'm pretty pleased with the result.

 

That's all the front and rear panel holes done, plus the mounting points for the eyelet board.

 

[JapanAxe]

Valve socket holes cut and mounting holes drilled.

 

[JapanAxe]

Marked, punched, drilled and de-burred all the holes in the top surface of the chassis. That's a lot of holes! Also fitted the grommets for the transformer lead holes. Oh yeah, fitted the feet to the bottom plate.

 

[JapanAxe]

Installed some of the hardware. I hate wrestling with bear-trap retainers but not as much as I would hate the sound of a broken KT66 rattling around inside the enclosure. I haven't fitted screening cans for the preamp valves as this is a fairly low-gain design and the enclosure is one big Faraday cage.

 

Also wired up the circuit board. Some of the eyelets are as yet unsoldered as they will take leads from the transformer.

 

[bertbass]

I'M LIKING THIS.

[JapanAxe]

Wired up the input jack and pots, plus some runs of screened cable.

 

[tauzero]

On 17/07/2022 at 17:41, JapanAxe said:

Installed some of the hardware. I hate wrestling with bear-trap retainers but not as much as I would hate the sound of a broken KT66 rattling around inside the enclosure.

 

I'm sure I remember some valveholders with clips that went over the valve - I've never encountered a KT66 in the flesh but have met up with a few ECC series and 12AX7s. Would it be preferable to use those, so your 12AX7s don't become KT66-seeking projectiles, or are the valveholder bases sufficiently strong to resist earthquake-type forces?

[JapanAxe]

7 hours ago, tauzero said:

 

I'm sure I remember some valveholders with clips that went over the valve - I've never encountered a KT66 in the flesh but have met up with a few ECC series and 12AX7s. Would it be preferable to use those, so your 12AX7s don't become KT66-seeking projectiles, or are the valveholder bases sufficiently strong to resist earthquake-type forces?

I’ve used twist-on shielding cans on the 12A_7 valves in all my previous builds but they were all the sort where the valves hang below the chassis. I’ve just re-biased a friend’s amp where the line 12AX7 hangs like this, and despite there being no retainer it has stayed there for several years since the last time I did the job! I’ll be the only person transporting the head around and I’ll make sure it stays right-side-up in transit. The valve bases are good quality Beltons.

[JapanAxe]

I reached the point where I had to fit the transformers and the eyelet board, so I did. There are now loads of wires everywhere, but from here on in things get tidier.

 

[3below]

Inspiring work , well built valve amps are a thing of true beauty.  I also value that they can be easily repaired in the event of component failure - smt technology is not friendly to those of us with ageing eyesight.  Any reason for choosing eyelets over turrets other than personal preference? 

[JapanAxe]

12 minutes ago, 3below said:

Inspiring work , well built valve amps are a thing of true beauty.  I also value that they can be easily repaired in the event of component failure - smt technology is not friendly to those of us with ageing eyesight.  Any reason for choosing eyelets over turrets other than personal preference? 

Personal preference. Turrets are easier when it comes to connecting more than about 3 components or leads, but they take a lot of heat to solder properly.

 

And thanks!

Edited July 24, 2022 by JapanAxe