Ham-fisted attempt at DIY combo valve amp

[Andy123]

So the mad (and highly unqualified) scientist in me has conjured up another way to waste money unnecessarily on musical equipment.

I'd like a valve amp:

• quiet enough for bedroom volumes
• just loud enough to play with a quiet drummer
• with at least gain, bass/mid/treble, and volume knobs
• a physically small, light, portable combo

I struck upon the idea of getting a cheap, crappy, second-hand solid state combo, ripping the amp out and replacing it with a kit valve amp (more on that in a sec). A few minor cosmetic changes to the cab, and whamo there's the project.

Of course its way cheaper to just buy a small valve amp off the shelf, ready to go. My issue was that the smaller, cheaper ones tended not to have the three band eq. To get that you had to go high wattage and high price.

...until I discovered a Chinese mob selling kit amps. They mostly do clones of classic amps, but they also have one which:

• is based on vintage Fender tone
• has a 3 band eq
• is 15 watts
• has reverb and a few other bells and whistles
• is pretty cheap for what it is

No knowledge of electronics required, just plug the transformer and valves into the circuit board and you're off.

Here's the head version:

I haven't been brave enough to pull the trigger on the kit yet, but I spotted a Kustom Sienna 30 for sale at a practically throw away price. The size and weight seemed perfect and the tan leather covering looks pretty nice. I couldn't say no, so it's now sitting in my lounge room awaiting a good butchering.

(pic not mine, pinched off the net)

Here's where the trouble starts. Most combos have the amp running along the top of the cab front to back, face plate is either along the front or along the top. I didn't realise until I got it home that Kustom amps do something a bit weird. The amp sits 90 degrees from where you'd expect it to, forming the rear panel of the cab:

Its a great space saving solution and allows for a smaller, neater unit with easy access to the amp itself. All well and good while the amp is solid state.

If I were to swap this for a valve amp, the valves would have just enough clearance to sit inside the cab, running along the top of it and sitting just above the speaker cone. I suspect I may run into heat issues if I do this.

What do you lot think?

Will I have to ditch my nice pretty box in favour of something a little more conventional?
Will the heat not really be that much of a problem?
Should I continue the mad scientist approach and explore impractical and unwieldly solutions to the heat problem?

[Dedman]

Well, personally, I'd build a whole new cabinet, because a pet hate of mine is combo's and heads with the the controls on the top at the back. WHY do they do that!!!!!?? On top at the front I could understand, but prefer them on the front where I can see them from pretty much any angle. In fact I'd build it as a head, then you can build a few diff cabs with different speakers and be even more mad scientist!

[DrNomis_44]

Fender used to make their early Tweed covered amps, such as the 1950's Tweed Fender Bassman, that way with the chassis mounted on the back panel, so that's not exactly new, building the amp chassis into a head cabinet means more flexibility with regards to speakers so you could easily plug it into a 4 X 12 quad box, or a 1 X 12, or 2 X 12 speaker cab.

Vox amplifiers used to be built with the chassis positioned at the rear of the cabinet as well, back in the early days when powerful PA systems didn't exist, most guitar amps were positioned at the front of the stage so that the audience could hear the band better, so rear-mounted control panels made it easier for the musicians to adjust the controls, since powerful PA systems are more common nowadays, amps are generally placed at the back of the stage either side of the drum riser in a backline arrangement, this meant that the amp control panel had to be positioned at the front of the amp for easy access to the controls.

[Andy123]

Well, personally, I'd build a whole new cabinet, because a pet hate of mine is combo's and heads with the the controls on the top at the back. WHY do they do that!!!!!?? On top at the front I could understand, but prefer them on the front where I can see them from pretty much any angle. In fact I'd build it as a head, then you can build a few diff cabs with different speakers and be even more mad scientist!

The control placement doesn't bother me too much. The idea of building the cabinet from scratch would put me off doing this altogether. Making it as a head so I could build many cabs doesn't bare thinking about!!

I'm doing this because I'm a tight-arse and I want something that can't be bought off the shelf. The having to do it myself bit is a mild inconvenience that I hope to minimise.

End goal is a small, lightweight combo that will surprise other unsuspecting jam session attendees with its tone and volume (who knew that thing was full of valves???)

[Andy123]

Fender used to make their early Tweed covered amps, such as the 1950's Tweed Fender Bassman, that way with the chassis mounted on the back panel, so that's not exactly new...

So heat won't be an issue?

[DrNomis_44]

So heat won't be an issue?

As long as there's reasonably good ventilation, heat is not really much of an issue unless the output valves start conducting too much current causing them to overheat, most valve amps usually have their output valves biased so that their power dissipation (in the form of heat) is at reasonable levels, the six 6L6 power valves in my Fender Super Twin amp do get pretty warm (it has two retrofitted cooling fans), but not hot enough to start a fire, same with the four EL34 power valves in my Marshall MA100C amp.


Most power valves have what's called a Maximum Plate Dissipation specification for a certain amount of watts of heat before there's a risk of damage to, or shortening of life of the power valve, and the power valve is typically biased so that it operates at the point where it is dissipating about 70% of the Maximum Plate Dissipation, Marcel could probably explain it a bit better, but essentially the power valve is biased so that it is operating within safe limits so it's not overheating too much.


Here's a pic of the back panel of my Marshall amp showing the ventilation cut outs in it to give you some idea of how much ventilation you can get away with, the four EL34 power valves do get pretty toasty:




So I personally think that there should be relatively few issues with regards to heat with the Kustom Amp cabinet you're planning on using, have a look at some pics of Vox AC30 amps, not only do they have less ventilation than my Marshall amp, they do tend to run pretty hot, and the amp chassis is enclosed inside the cabinet too.

[Simon Barden]

I've seen that kit before. Basically a Chinese copy of a Fender Blues Jr with a couple of basic modifications.

But that head version really doesn't lend itself to mounting in that style of combo cabinet. You'd normally have the amp chassis held on by a couple of bolts from the top and a couple at the lower end supporting the chassis from the sides. There are no means of doing that with this amp head design.

The best you could do is screw the chassis to a backplate which then screws to the back of the amp. But that means all the valves and transformers will be pointing into the amp, so any valve removal or checking will be impossible without taking the chassis out completely. You may also run into problems with the amp components then actually touching the speaker chassis so it may simply not fit! The chassis itself is 36cm long, which I'm guessing is probably a good 6 cm shorter than the length of the amp chassis in the Custom, so you'll also have gaps at the ends that will need plugging.

And I'm afraid Doc hasn't really thought this one through with regards ventilation as unless you create some grilles in the top of the amp, the valves and transformers will be sitting in an enclosed section with no through-ventilation at all. You can't make ventilation slots in any backplate a) because of the high voltages present and b) because you've still got the metal base of the chassis in the way.

The one benefit of the (probably) shorter chassis than the Kustom is that it would allow you to create vents at either end of the chassis.

Note that you'd want/need to create some panels for the open ends of the chassis, ideally of metal, that are grounded and help cut down noise pickup by, and emissions from, the amp. The wooden back panel would also benefit from a thin metal sheet or shielding tape, on the inside, for the same reason.

[DrNomis_44]

@ Simon Perhaps I mis-interpreted Andy123's question, but yes you're quite right, although there is quite a large opening on the back of the amp cabinet that he intends to use, which I think may provide ample ventilation, that's assuming that the amp chassis is mounted on the back panel with the controls facing up through the opening on the top of the amp cabinet(my interpretation).

[Simon Barden]

Just a few more points to consider:



1) Note how the valves are near the front of the amp? That means that they'd be right under the top of the cab when the chassis is tilted so heat build-up would be quite severe without some top-mounted vents in the cab. On the plus side that probably means they wouldn't foul the speaker.

2) The valves would be mounted sideways, not vertically as they normally would. As the kit is for an upright head, the kit doesn't seem to include any valve retaining devices, so adding some of your own would be pretty vital otherwise they will vibrate loose over time.

3) The reverb tank is attached to the top of the chassis. If left like this, it would then be hanging sideways and reverb tanks are not meant to hang like this and will sound terrible. So it will need to be re-located to the floor of the cab. Easy enough to do, just need a longer dual RCA lead. Don't be tempted to re-use any reverb tank left on the Kustom, as it will invariable have the wrong input and output impedances and not work.



^^^ This is the side of the amp chassis that will be facing outwards (towards the back). So any vents in a back panel (see below) won't really do much for the valve heating problem. There's not a lot of heat-producing components within the chassis, so it doesn't really require much cooling. The big hot items (transformers and valves) are all on the other side of the metal, away from the back panel. You will need top vents, but as said before, it looks like this amp chassis is smaller than the existing one, so some mesh grilles at either end of the control panel cut-out will help a lot.



^^^ So you'd end up with something like this -(you may even be able to re-use the same panel depending on the chassis size. Note that the current panel is bolted to the chassis only, so you'd probably need to add battens to the sides of the cab to screw the back to. Looks like there are 8 small screw holes for fixing the chassis to the base of the head, but fixing to wood, you might be better utilising the four larger square cut-outs with cage nuts and bolts (like 19" rack fixing use) to get larger-sized bolt heads. It's also easier to undo 4 large screws than 8 small ones!

However,

I'd also suggest that this isn't a 'kit' amp in the true sense of the word. You get complete PCBs, so you've only really got to fit some leads together, wire in the transformers, screw the PCBs to the chassis fit the valves, fit the knobs and you're there. You won't learn anything worthwhile about valve amps along the way. Not like getting a kit where you have to wire components to tag or turret boards, do all the wiring yourself and then (carefully) test it yourself. But maybe that's just what you want?

You'd really learn just as much by getting say a Blues Jr (or a Chinese copy), taking it apart and then putting it back together again.

[Andy123]

...but essentially the power valve is biased so that it is operating within safe limits so it's not overheating too much.

To be honest I hadn't even thought about the amp itself overheating (something new to worry about ). I was concerned about the heat generated by the amp damaging the speaker (valves practically sitting right next to it) or causing glue to melt and the leather (or faux leather) to separate off.

...have a look at some pics of Vox AC30 amps, not only do they have less ventilation than my Marshall amp, they do tend to run pretty hot, and the amp chassis is enclosed inside the cabinet too.

Perhaps its less ventilation than your Marshall but it does have a vent on top, directly over the valves where the Kustom currently doesn't. I think I may have to take a saw to the top of my cab.