Ham-fisted attempt at DIY combo valve amp

[king casey]

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?

ADAM!!!! Do you know anyone?

cheers, Mark.

[Simon Barden]

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.

[Andy123]

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:



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.



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?

[Simon Barden]

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.

[Andy123]

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


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.

[Simon Barden]

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.

[Andy123]

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.

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.

[Andy123]

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).

[Marcel]

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...

[dave.king1]

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