Trace Elliot AH350SMX Question

[Addison]

I have a Trace Elliot AH350SMX... fantastic head. Has that classic Trace sound, and is LOUD for what it's rated... it's also pretty light.

I am the original owner... I bought it in the early 90's. If I remember right, it was a Musician's Friend "Special Edition" of some kind... it was basically an AH350SMX amp, inside an AH300SMX suitcase style green-carpeted rack. So there's no blacklight or anything.

This is not my head, but it looks a lot like this:



ANYWAY...

Here's my question:

I recently dug it out of storage and I'm in the process of restoring it. I've taken it completely apart and replaced the input jack... totally cleaned it out, threw in a new preamp tube, and put it all back together. It sounds fantastic, but there is one kind of weird behavior and I was hoping someone here might be able to help me with.

For some reason, even when the input is set to "passive", I need to turn the input gain up all the way to get it to go into the "red" using a passive bass. To get it to "yellow" it has to stay on at least 9. With an active bass, or with some boost style preamp or effect pedals, I can get it to go into the red at like 9 or so... it will flutter into the yellow at about 8.5 or so.

I tried 3 different tubes... same thing.

It seems like I used to be able to get it to clip at a much lower setting than that... and I was wondering if maybe the pot was bad, or if there's anything else anyone can think of that might have gone bad that might be causing this. If it's something simple, I could probably just order the part from British Audio Repair and replace it myself...

It still sounds great... and, other than that, it works perfectly... it just seems odd that I'd have to crank the input gain that high to get a decent signal.

Anyway... if anyone knows about these things on this level, I'd sure appreciate any input you have.

Thanks!

[BassmanPaul]

By taking it apart - what exactly did you do????

[Addison]

The problem existed before I took it apart... if that's what you're getting at. The input jack was cutting out... and, at first, I thought that the bad input jack might also be why the input gain was so low.

I don't claim to be an electronics expert... but I've done my own wiring and simple repair projects in the past. I know what to be careful of and when to stop, etc.

But... if you want to know... I took off the chassis, the knobs, the front face-plate... then detached the graphic EQ section... and then removed the entire preamp section of the board in order to replace the input jack.

Blew it all out, cleaned the fan, replaced the preamp tube, and reassembled... lots of steps and parts, but it was pretty easy.

After reassembly, the input gain was still really low... however, the jack replacement fixed the "cutting out" problem.

[Jim C]

FWIW, I had a Trace AH 350, British made, black light, no tube (I thought SMX model)
It also had a really low input gain with a passive bass and the input gain maxed
I had to use a pre-amp to light the red light with some passive basses
Crazy amounts of output power even when just the yellow light blinked though

[Addison]

Ok, this is getting VERY interesting...

I just stumbled upon this post where TB'er "Linuxius" is complaining about the EXACT same problem I am...

Trace Elliot RAH600SMX low output level problem

Then, in this thread:

Team Trace Elliot!

He says, "I finally fixed my RAH600SMX... Man that thing has power. The problem was the one FET used for the muting circuit broke and pulled up the gate voltage of the other FETs as well. That made the amp mute to about 10% of max. capacity."

Ok, now we're getting somewhere.

Unfortunately he hasn't posted since May, 2009... but I sent him a PM anyway to see if he could help me.

[Addison]

I wanted to post an update, in case someone else experiences this.

First of all, I DO NOT have an AH350SMX... it's a '96 AH300SMX, pre-Gibson, British made.

Next... I worked for several weeks with Shane from British Audio Service, who also happens to be one of the most awesome people alive.

I explained that I was capable of doing some minor soldering myself, so he was willing to give me a couple of ideas.

The 1st thing he said I should try was to replace the 3 JFETS on the preamp board. I replaced the JFETs located at TR1, TR22, and TR23... no change.

2nd, he told me to try to replace the only TL072 opamp on the preamp board... no change.

He then directed me to check voltage levels for each pin on the preamp tube, and this is when we found the problem. Pin 6 was reading about HALF normal voltage... it was 73.8vdc and should be at about 150-180vdc.

Shane was then nice enough to begin to describe how to go about trouble shooting, and it started to get over my head... so I gave up and sent it to him.

Here's what he noted on the repair order:

Repair Action:
1.) Disassemble unit
2.) Remove preamp board and hardwire AC input connection - it was intermittent at one leg
3.) Factory update mute circuit
4.) Reassemble and test

I talked to him briefly on the phone and, if I remember right, I believe he said he was able to make the fixes "permanent"... so I'm thinking the mute switch pop and the AC input connection problem might have both been "known issues" at one point.

Anyway... hopefully that info helps someone else down the line.

I haven't actually tried it out yet... but, I can't wait to get it home and try it out tonight!

Thanks for listening...

[Sneakypete]

Hi. I'm currently working on an old but lovely AH350X - this is like the AH250 but also has a separate 150W output stage high-passed at 250Hz plus. I had the same problem except it's just a red led no yellow. Although I only have a passive 4-string I could always drive the clip indicator on with all previous amps. Also, it has a front facing XLR input but the clip led still wouldn't light even when using my Sansamp bass driver pedal.No preamp tube/valve in this one but three 8-pin op-amps in push-in sockets. I carefully removed each one (using a static discharge mat - I'm in the electronics trade) and gently scraped the oxidization off the pins. Carefully re-seated them and the clip led now works as expected - anything past 7.5 and it's on with every note. I've also cleaned each pot and slider with cleaner and a small fibre brush and used compressed air to blow out the dust from every nook and cranny. Now it's sounding absolutely great. One last job to do now - I don't like XLR speaker outs but the full range B channel has one although the high-pass A channel uses a 1/4" jack. I'll fit a panel-mount Speakon socket, probably next week as I'm doing this in the odd lunch-hour at the day job. The trouble is I keep noodling on the bass instead of getting on with the amp! Tracebassplayer has advised on uploading photo's (amongst other good advice - thank you) so hopefully I'll be able to do that soon.
Here's a thought; I don't know if you boys over that side of the pond have heard of Landrovers? 4 wheel drive vehicles beginning in 1948 and once the preserve of farmers but now seen at trendy bars everywhere. I can't remember the percentage but a very high proportion of them are still on the road. It's quite common in rural areas to see ones that are thirty or forty years old and still working hard every day. Could TE stuff go the same way? My Ah350X is over 20 years old and, I reckon, good for another twenty. In the future, when everyone else is using 3kW heads the size of an iPhone - TE's old soldiers will still be staggering into gigs with those weird, green old amps.

[Downunderwonder]

My brand new '96 AH250SMX went back for one of those "permanent known fixes" for going all quiet, when she was just a day old. Man that t'eed me off but all was forgiven very shortly and we're still in love.