TW Express combo?

[gcenker]

Against my better judgment, I built a TW combo for a friend and put it in a Marshall 18-Watt style cabinet with a Celestion Vintage 30 (8 ohm) speaker. I reinforced the chassis for vibration and put tube retainers all around. Now an interesting problem arises...

With the amp fully mounted on an aluminum plate & bolted to the wooden back of the amp (but still resting on my bench). It works without a hitch. Pure TW tone! But put the amp together and with all the controls set to “off” I’m able to turn up the presence control and get a really annoying high pitched squeal. After much cursing & trouble shooting, I found an interesting phenomenon. With the amp fully assembled, if I take the speaker out and use an external cabinet, the problem goes away. Further more if I take the speaker out and run it through a power attenuator and back to the combo, the problem goes away.

My thought is it must have something to do with the NFB circuit being tied to the 8 ohm tap. Would moving the wire going from the presence control to say the 4 or 16 ohm tap fix the problem?

Anyone experience this? I was considering buying one of these to see if it makes the problem go away:

http://www.4electronicwarehouse.com/pro ... comparison

Somehow I’m not entirely convinced it’s the cable opposed simply to the proximity of the speaker magnet with the output cable. Or am I missing something?

Thoughts?

[Aharon]

I had an amp that had an annoying buzz,nothing loud but noticeable.
The speaker cord was 3 ft long just in case I wanted to use the cab for another amp........one day I decided to use a 12" cord and the buzz was gone.I'd say on some amps it makes a difference especially "on the edge" amps.
Aharon

[Bob-I]

I'd say the problem has to do with the routing of the speaker wire. Try moving the wire around and see if the squeel changes. Also try twisting the speaker wires together. Make sure the speaker jack is correctly grounded.

[gcenker]

I'd say the problem has to do with the routing of the speaker wire. Try moving the wire around and see if the squeel changes. Also try twisting the speaker wires together. Make sure the speaker jack is correctly grounded.

I actually used a twisted pair speaker cable, crimped a connector to the ends and got it out of the way. Problem still there. That's why I was suggesting the monster 90 degree stuff.

As mentioned, the problem isn't there if I use the exact same cable but use an external speaker or if I use an attenuator... I'll try just about anything at this point.

-Greg

[LeftyStrat]

Now you know why Ken never wanted to do an Express combo (I think there is only one Liverpool combo, but could be wrong).

I'd try using some rubber grommets between the chassis and mounting board of the combo. Every Trainwreck I have ever built, the entire chassis was "live" and microphonic. So you need to isolate the vibration of the speaker from the chassis.

[gcenker]

Now you know why Ken never wanted to do an Express combo (I think there is only one Liverpool combo, but could be wrong).

I'd try using some rubber grommets between the chassis and mounting board of the combo. Every Trainwreck I have ever built, the entire chassis was "live" and microphonic. So you need to isolate the vibration of the speaker from the chassis.

Thanks Lefty.You're absolutely right, the entire chassis of a TW is microphonic - especially the wire coming off the volume pot going to pin 2 of V1(I use shielded coax there). I did use dampening but the problem seems to be associated with the cable proximity with respect to the speaker magnet. The guy who uses it plays with an attenuator so it's not a big issue to him but I wanted to solve the problem. That's the first and LAST TW combo I'll ever build! Too many rattles, squeals and headaches! The TW on it's own is a virtually "out of control" beast let alone stuffing it into a combo!

-Greg

['67_Plexi]

Yup, this is why there are no combos.

The magnet on the speaker causes some bad noise issues. To stand half a chance, you need to have a decent thickness of wood (the denser, the better) and as much air space as possible between the chassis and the speaker housing. Then mount the chassis on some kind of suspension mechanism like what you would find in an old Ampeg combo.

Presence pot squealing is usually nothing to do with the presence circuit directly, it just sniffs out instabilities in the first two stages. Once you have those stable, you should be able to move the prescence wire anywhere you want without any change in noise floor or tone.

I dare anyone to place their finger near the bright switch with all controls on 10 !!!

Alan.

[gcenker]

Presence pot squealing is usually nothing to do with the presence circuit directly, it just sniffs out instabilities in the first two stages. Once you have those stable, you should be able to move the prescence wire anywhere you want without any change in noise floor or tone.

You're correct. The presence pot only "sniffs" out instabilities. Fortunately the guy who uses it was aware of the combo issue before and is cool using an attenuator. FWIW, there must be only 1/16" clearance between the chassis & magnet

I suggested just using an extension cabinet (2 x 12) but he specifically wanted a TW combo .... I'll still try to fix the problem.

I appreciate everyones input!

[rfgordon]

I'd be interested to see what happens if you swapped out a neo mag speaker--the magnet is so small (inside the finned heat sink) that it would be farther away from the chassis.

Also, I never finish an amp with less than 8 feet of speaker wire. I have a way of twisting it all together. Looks like a boogered up mess, but it does make a tone difference.

[gcenker]

I'd be interested to see what happens if you swapped out a neo mag speaker--the magnet is so small (inside the finned heat sink) that it would be farther away from the chassis.

Also, I never finish an amp with less than 8 feet of speaker wire. I have a way of twisting it all together. Looks like a boogered up mess, but it does make a tone difference.

Neo-mag? I've never heard of it. Guess I'll have to check it out. What I'm going to do (if the Moster Cable doesn't work) is rebuild the cabinet to look like this:

FWIW, I actually tried about 6' of speaker cable with the same results - squealing. Just for the hell of it I could run my 20' cable to see if there is indeed any difference. Thanks for the input.

-Greg

[LeftyStrat]

I wonder if a Vox AC30 style cabinet might help. It would provide a little more acoustic isolation from the speaker. I have plans for this style cab if anyone is interested.

Here are some pics of one done in mahogany:

http://www.vintone.com/rc_ac30_o

[gcenker]

Lefty,
that would work except you'd have to make the chassis shelf wider to accomodate the TW width. Looks killer though bro!

Are you going to try that experiment?

[LeftyStrat]

I've been toying with the idea of building something AC30-ish, perhaps a Rocket and putting it in a cabinet like that.

I showed my wife those pics and she said it was pretty enough for the living room, so now of course I *need* to build one for the living room.

But if it becomes my living room amp, it might be a waste to put a thirty watt power amp in it. It would be used mostly at low volumes, either giving my daughter guitar lessons or just me fooling around at night.

Might be an excuse to try power scaling though.

[gcenker]

I've been toying with the idea of building something AC30-ish, perhaps a Rocket and putting it in a cabinet like that.

I showed my wife those pics and she said it was pretty enough for the living room, so now of course I *need* to build one for the living room.

But if it becomes my living room amp, it might be a waste to put a thirty watt power amp in it. It would be used mostly at low volumes, either giving my daughter guitar lessons or just me fooling around at night.

Might be an excuse to try power scaling though.

A valid excuse is a valid excuse. An old USMC quote was "if it's a dumb idea and it works, it ain't a dumb idea". I agree with you, looks killer even though it would be for the living room ... that what power brakes are for!

My wife gave up ... everytime I say I *need* to build _____, she lets me be but gets even by buying "things" at Victoria Secret in which case it's a win-win situation!

[LeftyStrat]

I should have just posted the link for those AC30 cab dimensions:

http://music-electronics-forum.com/show ... php?t=4258

There are two pdf files, one has drawings and the other the actual dimensions.