Ham-fisted attempt at DIY combo valve amp

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.

Attachment 30207 Attachment 30208

Here's the head version:
Attachment 30209

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.

Attachment 30205
(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:
Attachment 30206
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?

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!

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.

Quote:

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Originally Posted by 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!

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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???)

Quote:

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Originally Posted by 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...

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So heat won't be an issue?

Quote:

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Originally Posted by Andy123

So heat won't be an issue?

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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:

Attachment 30210


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.

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.

@ 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).

Just a few more points to consider:

Attachment 30209

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.

Attachment 30208

^^^ 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.

Attachment 30206

^^^ 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.

Quote:

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Originally Posted by DrNomis_44

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

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To be honest I hadn't even thought about the amp itself overheating (something new to worry about :D). 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.

Quote:

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Originally Posted by DrNomis_44

...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.

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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.

Simon, you're nothing if not generous and thorough. Thanks for all the time and thought you've given this. I should clarify a few things though:

The "kit" amp is a printed circuit board and a handful of components that plug into it. That's it.The chassis comes separate, if at all. The pots and inputs etc for the "front" and the "back" are perforated and separate out from the main PCB but remain connected by wires. In theory you could set this up in any format you wanted to. I included the pic of the head version to give you an idea of what some folks have done with the kit.

I imagine when I remove the amp section from the Kustom combo, the back panel will be coming with it and possibly not getting put back on. I'm not sure at this point. The dimensions of the chassis and possible back panel are still entirely hypothetical.

Quote:

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Originally Posted by Simon Barden

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?

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That's EXACTLY what I want!! :D:D:D

I wanted the combination of tone/wattage/features this kit offers and can't seem to find it anywhere else. I like the idea of smuggling an amp like that in a small, neat, unassuming combo. Most importantly I want to own and play through this thing, and get there on the cheap. This means some assembly is unfortunately required. If I had to learn electronics from scratch and build a cab from scratch I don't think I'd bother.

Quote:

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Originally Posted by Andy123

To be honest I hadn't even thought about the amp itself overheating (something new to worry about :D). 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.


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.

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Unless one or both of the valves have glowing-red plates, which shouldn't happen if the amp is operating normally, the likelyhood of the speaker getting damaged, or glue melting and the leather peeling is pretty remote, I haven't had any of that happen to my Marshall amp or my Fender Super Twin amp at all to be honest.

Looks interesting but there's no pricing on the website that I could find

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Originally Posted by dave.king1

Looks interesting but there's no pricing on the website that I could find

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They're sorely in need of a retailer. I've only found them for sale through their ebay store.

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Originally Posted by Andy123

They're sorely in need of a retailer. I've only found them for sale through their ebay store.

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I didn't even find that, must look harder :D

I stumbled across this about a week ago whilst tumbling down the YouTube rabbit hole:

https://www.youtube.com/watch?v=bF20Ed2RvXY

So, having never heard of that company, I went to check out their website. Sure enough, they sell a US$219 15-watt tube amp. (they also have a small 5-watter as well)

https://www.monoprice.com/product?p_id=611815

If you're dead-set on building one yourself, then completely disregard this. If you just want an inexpensive tube amp, though, this might be worth a look.

I saw that...however it's for the U.S.
So factor in the exchange rate $219 us = $310 aus and then the crippling postage.

cheers, Mark.

Not sure how long these links will remain active, it is ebay after all:

Kit $142USD
https://www.ebay.com/itm/KLD-Pilot15...frcectupt=true

chassis $50USD
https://www.ebay.com/itm/KLD-Pilot-1...frcectupt=true

Quote:

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Originally Posted by Andy123

Not sure how long these links will remain active, it is ebay after all:

Kit $142USD
https://www.ebay.com/itm/KLD-Pilot15...frcectupt=true

chassis $50USD
https://www.ebay.com/itm/KLD-Pilot-1...frcectupt=true

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Nice, I'll talk to my son in law about getting it here ( major freight forwarding company with offices all over the world )

If only we knew someone who was already set up as a business selling kits to a broad customer base who were really into diy musical equipment. Perhaps someone with experience dealing with a Chinese supplier. Do you folks know anyone like that around here? ;)

Quote:

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Originally Posted by Andy123

If only we knew someone who was already set up as a business selling kits to a broad customer base who were really into diy musical equipment. Perhaps someone with experience dealing with a Chinese supplier. Do you folks know anyone like that around here? ;)

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ADAM!!!! Do you know anyone?

cheers, Mark.

We know BPG won't do amp kits because of the electrocution potential. And the pre-built 5-watt is no longer available, probably because it didn't sell particularly well.

Curiosity got the better of me. However I go about doing this, nothing can be decided until the amp is removed from the Kustom and I have a look around.

First of all, with the black rear timber panel removed exposing the innards of the amp:

Attachment 30254

The amp was attached to the cab via one screw in each of the rear plastic corner guards on top. Also, it appears there's a kind of wall behind the speaker. There are four screws going through the back of the chassis into that wall.

Here's what it looks like with the amp removed.

Attachment 30255

Whether I

1. install the new amp in the same orientation with valves pointing in, or
2. lay it flat across the top with valves pointing down

...that wall has to go. I'm thinking I'll go with option 2 to avoid heat issues. I have one concern with that approach though. There's something next to the speaker near the top of the cab which could potentially get in the way. Any idea what the Dickens this is?

Attachment 30256

It looked like a HF tweeter and a check shows that it is an acoustic combo with a 10" speaker and a tweeter. Looks like the tweeter is fed from the main speaker and has an in-line resistor, which almost certainly means that its a piezo tweeter.

So the speaker won't sound that good as an electric guitar speaker because it will have a very flat response. Also, a speaker for a 30W acoustic solid state amp will probably struggle with the full output from a 20W valve amp.

So it looks like you'll need a new speaker and you'll have to remove the tweeter and block over the hole that's left. Costs are starting to mount though. You could fit another 10" speaker, though if you can remove the front baffle, I'd enlarge the hole and fit a 12" speaker.

You've obviously got the Kustom, so you can measure it and see if the kit amp will actually fit in flat (with the controls pointing backwards -which means they will be upside down?). But pointing upwards would still be my choice providing you can fit valve retention devices (necessary for inverted operation anyway) and provide vents in the top.

Thanks Simon. I don't suppose simply disconnecting the tweeter and pretending it's not there would be viable? I think I might be able to work around it.

If I could manage to keep the whole project below $500 I did have intentions of replacing the speaker. I had my eye on one of these bad boys:
https://thespeakerfactory.com.au/col...g10c-s-75-watt
Attachment 30259

I understand that in theory a 12" is supposed to be better, but some decent 10" speakers will out perform cheapo 12" ones in a guitar amp. Hopefully that one will go alright.

It's going to mean some mucking around with a custom chassis, but I'll have the amp horizontal with the valves hanging down towards the back of the amp (heat, easy access etc). The controls will be on top and hopefully I can line up the back panel stuff to actually be on the back panel. It'll be a tight fit and I'll have to line everything up "just so" but I'm optimistic.

Yes, you can certainly leave the tweeter in place and just cut the connections. You just don't want to leave a hole in the speaker baffle.

Those Warehouse speakers (formerly WGS) are really nice. I had one that came as standard in my Two Rock combo and that sounded lovely. I upgraded it to a Celestion Creamback (the Two Rock suggested upgrade) but it didn't really make any difference to my ears.

You obviously loose a bit of bass end with a 10" speaker compared to a 12". It's an open back cab, so you aren't going to be able to extend the bass response like you can in a ported bass cab. Unfortunately WGS don't show any frequency response graphs or give any frequency limits, which makes it hard to compare the difference between the 10" and its 12" equivalent. The 12" is about 1.5dB more sensitive/louder which is going to be almost inaudible. You've really got a choice of 8 ohm or 16 ohm versions of the speaker (it's a valve amp so the impedance won't make any difference to the volume). I'd avoid 4 ohms as it stops you using it with an extension cabinet.

...Which you may never, ever use, but if you do, you really want to have the extension cab the same rating as the internal speaker. 1x 12" extension cabs tend to come in 8 ohm versions if designed to pair with solid state amps (to give 4 ohms combined impedance - the lowest most SS amps go to but at which they produce more power), but 16 ohms if designed to pair with valve amps. 4x12" cabs are almost always 16 ohms (some switchable to 4 ohms for use with SS amp heads), so I'd lean towards 16 ohms as the most overly compatible value - and if you want a 15W amp to sound significantly louder than with its internal speaker, then a 4x12" is the only way to go. You don't want to mismatch impedances as the speaker with the higher impedance will take a lot less power than the other one, so won't be as loud, whilst you are stopping some of the amp power going to the lower impedance speaker, making that quieter. So then you are normally best either sticking with the internal speaker or (if the external speaker has a much higher efficiency) disconnecting the internal speaker and just using the external one.

But never use a valve amp without a speaker (or correct impedance attenuator/load box) fitted, and always make sure that the correct impedance output tappings on the amp are used.

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Originally Posted by Simon Barden

Unfortunately WGS don't show any frequency response graphs or give any frequency limits, which makes it hard to compare the difference between the 10" and its 12" equivalent.

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On my budget it was going to be a semi decent 10" or cheap and nasty 12". Looking at the specs of the equivalent speaker in the 12" version would have only been helpful if I wanted to get bummed out about my budget. ;)

Thinking I might have to start again with a different cab. In the meantime, here's a video of some one else having a go at the exact same kit I was looking at (this guy is doing it as a head instead of a combo).

https://www.youtube.com/watch?v=UXS6SdKYBqk

Buggar.... China does it again....

Cost comparison.
This kit amp landed at your door is what? AU$315 + freight...
Parts alone for my AC-15W scratch build head (AC15 circuit with Marshall 15W transformers) set me back AU$950 at trade discount prices. Mains transformet alone cost AU$185
Damn, a new fully assembled Fender Blues Junior IV cost upwards of AU$850 from the right vendor.

This is a cheap Chinese 90% assembled kit. Price makes this a good buy, the proof will be in the playing...

Just had a look at their ebay store, two channel 25W 5F6A kit $221 with free delivery

https://youtu.be/kGCw9TwtYaE

Don't know if it's of any revelance, but I saw this KLD kit on ebay.
It infers Australia requires a different power transformer

https://www.ebay.com.au/itm/KLD-Pilo...YAAOSwYMldVe8d

It's also only 15 watts.

cheers, Mark.

Yep, that one comes with the correct transformer.

Valve Heaven sell Australian designed and made valve amp kits:
https://www.valveheaven.com/diy-amp-kits/

However they are not super cheap, and do not come with a cabinet of any form. Most don't have the EQ options that the OP was looking for either. Still, I think they are interesting...

So I've obtained a different combo with controls along the front.
Attachment 36677

This looks like a straight forward case of flipping everything upside down, except for one thing:

All the pots and sockets for the front of the amp are mounted to a perforated section of the circuit board, which is joined to the main board by a bunch of really short leads.
Attachment 36678
Attachment 36679

They designed this section to be broken off the main board, but installed close to it in a similar relative position. This would mean the valves are hard up against the rear of the front control panel, possibly coming close to (if not touching) the speaker.

To pull this off I'm going to have to use much longer connecting wires and move that front panel to the opposite side. It was inevitable I guess, but it looks like I'm going to have make use of a soldering iron after all. I've never done that before, anyone go any pointers on how to unsolder those wires and replace them without destroying the whole kit?

Also as Simon pointed out, with this amp getting installed upside down, the valves could shake loose and will require some kind of retainers. From what I can see, retainers usually screw into the base of the socket, but these sockets are soldered in place with nowhere to screw in a retainer. Is there a work around for this? Would I likely be able to just screw them into the chassis?

Attachment 36680