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Ashdown CTM 100 - EQ Fix

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Hey all,

I wanted to share my experience I had with my CTM 100, just in case anyone out there had been pondering this. 

A few months ago I bought a second hand CTM 100. Went and tested it at the guys house, thought it was lovely, and ended up buying it. Turns out I was a bit dumb, and I didn't really test it as properly as I should have, because when I got it home and played with it a bit more, I found that the EQ controls were not as useful as I'd originally thought. 

Mainly that the bass control did practically nothing. The sweep on the knob seemed to show it coming on, but nothing beyond that. It was almost like it could be an on/off switch. I couldn't dial out any bass in my tone on my next gig, so I sounded way too subby, and there wasn't a whole lot I felt I could do about dialling in a useful sound. At least not very easily. 

So I took it to a local amp tech, who agreed the EQ was pretty useless. He called Ashdown and asked them if there was a fault with the amp. They replied saying that's just how they are. I thought if that's the case, then this amp clearly isn't for me, so I did put it for sale on Basschat for a short time. 

But when I was failing to sell it on, I still couldn't really understand why the amp was like this. So I decided to contact Ashdown myself, with a little video showing me using the EQ of my amp, making sure that this was how they're meant to be. 

I got an email back from Dave Green at Ashdown, saying yes that is how they are, and that the bass control only has a boost of 3db, next to nothing. But he also said that there is an alternative solution that he thought I might prefer. 

Dave send me over (free of charge) a new set of replacement EQ pots to put in the amp.

The new values of which were - (Bass - A220K) (Mids - A22K) (Treble - A220K).

A change from what was in mine originally - (Bass - B470K) (Mids - B100K) (Treble - A1M) 

The point of this probably unnecessarily long post, is that I now love my CTM! I had these new pots fitted, and now it's infinitely more useful and versatile. Sounds great with everything set at noon, but for those awkward awful sounding rooms we end up gigging in from time to time, I now have confidence that I can easily make the EQ adjustments to compensate. Even the Mids and Treble control have a wider sweep now so have become much more useful too. The EQ to me has become like a normal usable EQ on any other amp I've used.

Do no other CTM users have an issue with the EQ on theirs? Because I was baffled that I couldn't find anyone with the same issue when I researched it. I'm also a bit confused as to why Ashdown don't make them like this as standard, but I'm no amp expert. All I know is my Ashdown suits me a lot better now, and I now rumble trousers with big fat valve tone for days. Thanks Dave Green! 

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Kudos to you for not settling whith what you had and also to Ashdown for presenting you with a working solution for your issue, even though there was nothing wrong with the amp, as there may be some people that aim for the sound this amp has from factory. This may be a helpfull topic for others in your position.

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Dave Green is a great guy, and has an amazing amount of knowledge over the range of Ashdown products. He helped me out big-time on tightening up the sound from my ABM410 - as above there was nothing wrong with it, it worked as per stock settings but I just wanted that tight/sealed cab sound, so that`s what he helped me out on. This thread is another plus point for Ashdowns customer service, which imo will reach legenderay status pretty soon.

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I wonder if similar applies to the CTM15? I don't really care for the "subtle" and interactive nature of these vintage tone stacks. I'm sure these are used so the marketing department can roll out the "all Valve" line, rather than being useful in real-world playing situations.

The fix for your CTM100 sounds worthwhile though - well done for your persistence and Ashdown's solution.

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The CTM15 has a bigger problem in that the further you turn up the gain control, the more low end you shave off. It's a hangover from it being a common design between it and the Hayden MiniMoFo 15 guitar amp, which is built on the same PCB... I've been meaning to experiment with a few mods to my CTM15 but haven't got around to it yet. I'll maybe slot the EQ values into the Duncan Tone Stack calculator tomorrow and see what it spits out. 

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Yes, that is an irritation on the gain control. If you do come up with a decent resolution to this, please let me know. I'm a bit surprised they let this carry over from a guitar amp design. If anything, I would have thought you'd want to lift the bass content a bit to keep up with the increased harmonic content you get as the distortion kicks in.

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21 hours ago, MoonBassAlpha said:

Yes, that is an irritation on the gain control. If you do come up with a decent resolution to this, please let me know. I'm a bit surprised they let this carry over from a guitar amp design. If anything, I would have thought you'd want to lift the bass content a bit to keep up with the increased harmonic content you get as the distortion kicks in.

@MoonBassAlpha Short out C12 on the pcb, or short the junction between R8 and C12 to ground, whichever is handiest. Haven't done it myself yet, but I've modelled it in LTSpice and C12 is the culprit. When I get around to it I'll be playing with the MOSFET clipping in the circuit too...

There are differences in the overall circuit between the CTM15 and the MiniMOFO, but not much until after the tone stack. The minimum gain available is very slightly lower on the CTM15 (different value for R9). Now maybe shorting C12 will highlight a deficiency somewhere else in the circuit, but to get a quarter turn of travel on the gain control without it turning to crap I'm willing to give it a rattle!

Some plots to show what simulation says. I've only simulated up to just after the tone stack, and have bass and treble cut, mid boosted to get the flattest response (fender-style tone stack). Should have measured before the tone stack, but you get the idea... Obviously there is some high and low passing going on elsewhere in the circuit, but I'm not planning on playing with that... yet...

First, minimum gain, original circuit:

mingainoriginal.thumb.JPG.42d8ec6a75ed231ff0a1fc82fd920146.JPG

Now minimum gain with C12 shorted out:

mingainc12short.thumb.JPG.3e654d510b3b4dacf5a4ae29c2ccce4b.JPG

Now back to original circuit but gain about half way up (assuming 100k ohms on 1meg ohm log taper pot):

halfgainoriginal.thumb.JPG.352164823320c917fe01a365c3615845.JPG

Now half way gain with C12 shorted out:

halfgainc12short.thumb.JPG.5bc859303d6673ed3577a5c43d278f39.JPG

Edited by Bigwan
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Just got my CTM100 back from the tech with it's new pots fitted & wow what a difference,a massive improvement.

I love my CTM even more now 😁

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Hmm...it sounds to me like there probably was something a little awry with the bass pot. The amp has a very bass-heavy tone, but I've never found the bass control to be that ineffective. (The bass-shift control was in the 'off' position, I assume?!)

That said, I have come to like the old-fashioned tone stack. I appreciate that it's not everybody's cup of tea; you certainly can't "sculpt" a tone like you can with a lot of modern amps, but I already like the "baked-in" tone so it works for me.

The biggest revelation for me - and you may have already tried this - was pushing the mid-range shift. Made the mids much clearer and more aggressive - more to my taste, anyway!

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I always have the mid shift pressed too 😁

The new pots make a massive difference & are definitely worth changing.

I run mine through a fender bassman neo 410 & found in some rooms I had far too much bottom end with no means of reducing it.

Now I have plenty of adjustment on tap to remove the boomyness,so I'm much happier with my amplifier.

 

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As I said I contacted Ashdown about the bass pot, and they said there was nothing awry about it. As well I had it looked at by a tech who also said there was nothing technically wrong with it. It's just designed to do only 2db cut, which wasn't useful enough for me. Maybe yours is different to ours, EliasMooseblaster. 

That being said, most of the time when I'm gigging the amp now, I still usually have the settings at noon. Just being able to alter the tone enough to work in different rooms is what I struggled with. And it's not because it's a passive EQ. I used to have an Orange Ad200b that I never thought had a useless EQ.

But yeah that mid shift thing is cool for added gnarlyness!

 

Glad to hear you're loving the change with yours though, Artisan!

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Indeed I am 😁😁

A big thank you for your original post as I was considering selling mine & getting something else.

Now my amp is perfect.

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