trinityamps.com

I moved my noise chasing to this new thread because I think I was getting off topic on the plexi brown mods thread

To recap: After listening to Uniminds "brown" sound clips and following that thread with great interest I went through my TBM amp and changed all the various component values to the "brown" values that he, Coco, Zaphod and the other gurus came up with

It sounds pretty good so far, not quite as good as his though I am still tweaking it. However, I am having 2 issues; the TBM channel is VERY hissy, and the amp seems to be quite feedback prone (microphonic squeal when I dial the master past 4 or 5 with the gain and tone controls dimed) - possible it is just pickups but it does it on all my guitars (a variety of Gibsons, Fenders and others) and Unimind does not seem to have the same issue

Any suggestions on components or something on the amp that may contribute to squealy feedback? It seems to be volume related (squeals when I am not playing even with strings clamped, my back to the amp way off to the side of the speakers - does not seem like bad tube microphonics where it rings when I play). I suppose it could be that ALL my guitars have squaly pickups but this amp seems to squeal more than other high gain amps I have

As for hiss - the Normal channel is much, much quieter than the TMB, leading me to think I have introduced some noise somewhere along the line in the TMB circuit or perhaps my original amp was hissy and I just never noticed because I did not play it at "11" before these brown mods

It is hiss, not 60 hz. There is a tiny amount of 60hz buzz when it is dimed but nothing to write home about. The hiss is VERY loud

Any suggestions where to start? Most of what I read about hiss on the net seems to suggest it likely being in or around v1A. I suppose I can just start swapping components, re-soldering, swapping tubes etc. but thought the gurus might have a logical approach based on experience.

I did try changing to some other 12ax7 tubes I have with no real difference - but I was not particularly methodical - just swapped a few and all were equally hissy. I have ordered some metal film resistors, some carbon film, some 1 and 2 watt carbon comp (assuming they may be quieter than 1/2 watt), and some other caps etc assuming it may be interesting to try some swaps sound wise and noise wise - but it would be great if anyone can chime in with any other suggestions on the most logical places to start chasing the hiss

Also, I noticed that the Trinity Plexi layout uses shielded wire and a 470k resistor/330pF cap soldered right close to V2 pin 2, running across the board to the gain pot - the Ceria-tone layout on my amp uses non-shielded wire going from the tube pin to the board where there is a 470k resistor running across the lugs then to the pot again with non shielded wire - I simply soldered a 330pf cap across the lugs to match the brown layout. Do you think this non shielded wire and component placement difference would be adding hiss?

Anyway I would really appreciate anyone chiming in ...

thanks!

Lindsay

You had said that some of the components are different types than what I had in my amp. Can you post what and where they are (i.e. V1a plate - 100k carbon comp 1/2 watt)? Could you also post some gut shots of the board? I don't know if guys are reluctant to help because it is not a Trinity amp but I can try to help if I can since you have converted it the 'Brown' layout.

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Les

http://www.soundclick.com/unimind

Pics below - I did mention in my first post in the Brown Plexi thread that I was not trying to freeload on the forum and had ordered an 18 watt head kit from Trinity a couple weeks back - I have not got it yet as they were waiting on some Transformers, but none the less I thought that should earn me the right to ask a few questions here without breaking any forum etiquette.

As a first time builder that has not really messed much with electronics, I thought an 18 watt would be a good first project and Trinity's kits looked suitable for a rookie with the forum, manual etc. I actually bought the Ceria-tone chassis off Ebay so I would have a working 18 watt in front of me during my first build. The layout is a little different than Trinity's but I thought it would be good to have something right in front of me that I could really study.

After following your thread I got a little impatient and started messing with it in the mean time - Its been good experience .

3 pics below - board view, pots view and tube socket view - since it is a combo chassis its a little harder to see components from just the top view -

Also the Ceria-tone layout swaps the position of V2 and V3 on the Chassis so it will look a little confusing vs Trinity

I have already tried swapping a few resistors so you will see some metal film and metal oxide in a few places where there was carbon comp. Still hissy as hell - I'm curious if you think the non shielded wire from V2 I mention above would be an issue - does shielding quiet hiss and white noise or just hum (or both)?

I see a few things that we can try.

First is the grid leak to V2a. This is the 470k resistor on the TMB ch gain pot in the picture below. This will reduce the gain a little going into V2. This is in addition to the mix resistor that you have on the board already.


Next is the V2b cathode resistor marked 100k in the next pic. This will also reduce gain a bit. I am not sure what you have in there now. It looks like you have gray,blue,blue which would be 86Mohm which can't be right. In any case you want it to be 100k (brown,black,yellow).

Also in that last pic I circled 2 resistors. Are those gray,black,red 8k? Or are they brown,black,red 1k? They should be 820R gray,red,brown. If you have 1k there it should be ok but I am not sure what 8k would do.

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Les

http://www.soundclick.com/unimind

the 2 resistors that should be 820r are 1k - I was just trying the 1k resistors I happened to have at the house in a few places to see if they would quiet it down vs the 1/2 watt carbon comp. I have ordered a variety of other types at the correct values that I will try in a few days when they arrive. My thought is I might try and quiet it down then get back to the tone quest

The 100k you marked is 100k - double checked it - perhaps the colors look off on the picture

Ironically I had just pulled the 470k grid leak resistor to see if it might be a noise culprit

I'm kind of shooting in the dark as I do not know much about the most likely places where hiss would be introduced and was hoping some of the gurus might chime in - The sound is OK but its quite hissy

Sometimes what we hear as hiss is actually very high frequency oscialltion. When that is the case, shielded wire can be a huge help. Also BTW shielded wire has almost no benefit against hum. It mainly deals with higher frequency issues.

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