trinityamps.com

Hi all,

I've just completed my first build of a TMB 18, and am having fun reading the forums, and exploring gain mods. I've encountered one problem that I've been unable to diagnose, and am hoping that someone on the forum can help me.

The problem is on the high gain input only on the TMB channel. When a guitar is plugged in, and the gain is turned up above 7, the amp makes a high pitch (e.g. much higher than 60 Hz) squeal. Here's a few more details on the symptoms:
a) It only effects the TMB high gain input
b) The squeal kicks in at a Gain of about 7. Between Gain of 7 to 8, the squeal can be eliminated by turning down the "Middle" tone. Above Gain 8, Middle tone doesn't impact it.
c) At a gain of about 6), there is a "squeally" tone when you do play the guitar.
d) Effects pedal/guitar volume impacts the squeal. For example, when plugged directly into the guitar, the amp squels. Turn down the guitar volume, squeal goes away. Plug into an effects pedal first, the squeal goes away.
e) If you just put an open patch cord in the high gain input, the amp squeals. With a 470 ohm resistor across the patch cord, the squeal stops. With a 4.7K resistor, it stops. With a 470K resistor, the squeal changes frequency, but remains.

I've read the instructions, and the forum threads on noise, but nothing has helped so far. In particular, I've:
1) I've used chopsticks to move wires with no significant effects
2) I've rechecked wiring and grounding of the high gain input (and all others), and they appear fine
3) I've checked ground on the input jack, and it's fine
4) All inputs from the jacks to V1 are sheilded and run above the board. Moving them has no effect.
5) I've removed R8 - (both for more gain, and to try to get rid of the squeal)
6) I've even tried reversing the blue & brown leads to V4/V5 (as suggested in one thread) with no effect

Any pointers people could provide would be much appreciated.

Thanks,
Brian

I have heard that with some really high gain amps it can squeal like that when it is not in a shielded cabinet. If you do not have a cabinet you try to just cover the open part of the chasis with a piece of Aluminum sheet metal or flat piece of cardboard with Aluminum tape on it. But it might also be some kind of parasitic oscillation. Need the experts to chime in on that. Have you done any other gain mods other than remove R8?

_________________
Les

http://www.soundclick.com/unimind

Are you using shielded wire for the connections to the Gain and Volume pots? Are other connection wires inside the amp short and tidy?

You may also find it useful to twist the OT primary wires together. Also if your OT is the Trinity 18W OT, then the unused 5k connections need to be capped off and coiled *above* the chassis. And make sure the OT secondary grounding is good and solid.

_________________
Great tones come in small glass jars!

Thanks very much for the ideas above.

Thank you for the quick response!

In response to the points above:
1) I've put the chassis in a cabinet. Also tried a couple of locations in the house, so i don't think it's outside signal.
2) I've used shielded wire for the input and Gain pot (per instructions). I also added the cascaded channel mod, and have used shielded cables for the two lines from the Normal Volume to the DPDT and back to the Normal channel C7 capacitor. (note that I've also changed R4 and R7 to 220K per mod instructions, but this didn't impact the noise).
3) All wiring is tidy - not "point-to-point" shortest, but "right angled" shortest to keep tidy looking.
4) Trinity OT blue&green twisted up to V4 and V5. The unused connections are coiled outside the chassis (but I haven't cut them off)
5) I don't understand the point about secondary OT grounding. There are no additional wires coming out of the OT other than any grounding through the mounting (sorry if this comment shows my ignorance).

Therefore, unless it's something in point 5) I'm missing, or I need to cut off the unused wires in point 4), I don't think these are the problems.

One final note about high gain. In cascaded channel mod (Normal then TMB), I don't get the squeal. However, at the very highest gain mod (all volumes & gains at full other than the master), there is a hint of high pitch noise in the background.

Thanks again for the tips, and I'll gladly accept any other hints people may have.

Cheers,
Brian

I am not sure but you might just have too much gain and it is self oscillating. Did you have the squeal before removing R8 and changing R4 and R7 to 220k? Both those mods are to increase gain. Maybe try to decrease gain a little.

The only time I got the squeal like that was when I first hooked up my NFB and presence control. The problem for me was that the output jacks were not grounded so I still had an open loop. Once I grounded the output jacks the squeal went away. Don't know if that helps, maybe retrace your entire circuit and check for proper solid ground connections.

_________________
Les

http://www.soundclick.com/unimind

All,

A quick update - problem solved!

I pulled out the two TMB input jacks, and found with them pulled out, the squeal went away. After a bit of fiddling, it turned out that the problem was the routing of the wires from the TMB Middle, Treble, and Gain pots to the board. While I was very tidy about the layout (right angle corners, carefully laid out), I went up from the board, and then across the chassis to the pots. The secret was to keep these wires low (at the level of the board) across the front of the chassis, and only bring them up to the height of the pot right below the pots. Keeping these wires as far as possible from the TMB Hi Gain Input Jack (tough given the close quarters) solved the issue.

Hopefully this will be of help to anyone else who has similar problems.

Thanks again for all the timely help and pointers! Now it's time to explore some of the recent Plexi mods....

Cheers,
Brian