trinityamps.com

I finished my Trinity Plexi build this evening (Plexi brown layout with no cascade). Was stoked that voltages etc. were in spec and it booted up with no major issues first try. Normal channel sounds great

But alas, it appears I have an issue with the TMB channel ... volume is quite low, sounds OK (good tone but very little gain). Tone pots seem to work OK. Completely dimed it is still clean, barely clipping and not nearly as loud as normal channel. Boost switch works but boosts the clean to louder semi-clean and still not nearly as loud as it should be. Its like I am missing a gain stage

Any suggestions what I should be looking at? - I've spent a couple hours now trying to figure it out but can't seem to figure it out. I've gone over the layout a few times, and have re-soldered a lot of connections - I could use some suggestions or fresh eyes - a few pics attached

Perhaps one of the more experienced builders can help walk me through likely issues or how they would diagnose something like this.

thanks!

I had a similar problem.. double check your wiring on the gain pot.. if you like you can remove resistor R8 and try it. My TMB channel is still not as loud as the normal channel, so mine isn't 100% yet. You should also triple check all your soldering... also try swapping the 12aX7 tubes around, and the power tubes... and I also understand that voltages are critical, so measure them all.

Is there not a way to measure voltages along the signal chain to determine where there is an issue vs blindly re-soldering or replacing every component??

Do you have any tubes to swap in. Check that all the resistors are the correct values too. People have been known to mix 470 ohm with 470K !!

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

so after a ton of checking values, rechecking connections, re-soldering etc I found that by removing the 470k resistor across the 1 + 2 lugs on the gain pot that I all of the sudden I had some decent gain

Is there supposed to be BOTH the 470k resistor on the V2 tube grid pin AND the 470k on the pot as per the Plexi Brown layout? The regular plexi layout only has a 100k on the pot which got me thinking maybe there should not be 2 resistors, so I removed one pin of the resistor on the pot to test if that would help my gain issue and it sure does.

any comments would be appreciated

There should be two resistors but after looking again, the layout is wrong. The way the layout has the 470K on the pot before the signal goes to pin 2 is creating a voltage divider with the 470k resistor on pin 2 which will reduce the signal considerably. The Schematic is correct where the second 470k should come after the inline 470k with the cap across it. It should go from pin 2 to ground.

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Les

http://www.soundclick.com/unimind

I got lucky there...I'm building a single input, switched cascade version & am using on-hand trannys & misc bits. Since I had to do my own schematic & layout I never actually looked at the erroneous layout. I started with the regular TMB layout & modified it based on my modified Brown/Cascade schematic.

FWIW, here's how I'm planning to deal with that:

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b0b0, R24 in your layout is still creating a voltage divider. It needs to connect from pin 2 to ground.

Like this:

This is the area we are looking at (green circle):

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Les

http://www.soundclick.com/unimind

Sure enough! I had it on the wrong side of the RC. Thanks!!!

- Edit -
Removed the layout pic.....Stephen's revised layout is much cleaner. You da man, Stephen!

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Updated the layout to fix this error.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I had been running my amp without the 2nd 470k from pin 2 to ground and today decided to put the 470k in to see what it sounds like vs without it and I still get the voltage divider effect.

Either I have something wired incorrectly or this 2nd resistor is incorrect or unnecessary- without it I can dime the gain and get preamp distortion, with it, if I dime the gain it is totally clean and I have to dime the master to get any distortion and even then it is 1/4 the volume of the normal channel. I wired a metal film 1/2w resistor right from pin 2 to the tube mounting bolt and clamped it with a nut since I do not have a terminal strip at v2

I have checked and rechecked 100 times and I cannot find a wiring error on my amp so I thought I would re-post this for the gurus to chew on

pic shows amp after I pulled the 2nd 470k resistor to confirm the gain issue - once I remove it I seem to have the expected gain. What is the purpose of the additional 470k to ground supposed to be?

I may have found my own answer - if you look at an actual plexi layout there are two 470k resistors but one goes to the bright volume and one to the normal volume - I think this may be an oversight on the layout?

Ceria-tone Brown plexi 100 shown

No, not an ovrsight. The purpose in the plexi is to mix the two hcannels at V2. In the 28, the two channels re mixedt the PI sfter the tonestacks. The pleix channels just share the same tonestack.

The idea of the 2-470K in the 18 plexi is a voltage divider to reduce the gain, but it looks like in yours, it works better without it.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Coco's correct. I was wrong earlier in this thread when I said it was a voltage divider when the one 470k was on the volume pot.

It's weird that on my amp I have had to add another 100K resistor from pin 2 to ground after the mix resistors to reduce the signal entering v2 where as on Lindz' amp that resistor is cutting way too much signal.

Lindz , I would like to compare our amps more closely. If you like you can PM me and/or we can start a new thread to compare our amps.

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Les

http://www.soundclick.com/unimind

call me a dope but I can't figure out how to PM you

Hmmmmm - if I am plugged into the TMB jack with the master volume off but I have the gain on the normal channel dialed way up and I play I can hear sound .. its not very loud but essentially sounds like the TMB channel would with the master barely on. If I dial the normal volume down it attenuates the sound

Is this normal? - I posted way back when I first built this amp that I noticed that hiss from volume controls from each of the channels would interact on the other and was told this is normal. I assume that a small amount of actual guitar signal getting through is too, or do I need to adjust resistor values or something to kill it?

Or perhaps I do have some wiring or components off spec a bit that are causing both this and the gain issue on the TMB channel when I add the 2nd 470k?

Yeah it looks like they have it turned off for this forum. You can send me a message through my site and then I can reply. http://www.unimind.us/contact.html

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Les

http://www.soundclick.com/unimind

You have some crosstalk between preamp channels. In other words some signal is being radiated between the two channels. this in itself is perfectly harmless. But consider what you get if a preamp channel picks up radiated signal from the power stage. You get feedback which may manifest itself as whistles, excessive hiss or blocking distortion.

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Great tones come in small glass jars!

Both my Ceria-tone and the Trinity exhibit this cross talk - do other peoples amps not do this at all?? It does not really bug me in itself as both amps sound great, but if it is a potential source of problems I should probably chase it down

I would have thought there would be a difference between the 2 as they are laid out differently (combo vs head, one steel chassis with no sides, one aluminum chassis, as well as minor wiring layout differences) but they are almost exactly the same in terms of this crosstalk.

if this is not normal where are the basic places in the circuit where it would most likely be introduced so I can try and chase it down?

would wire moving and chopsticking be the approach to try and fix it?

PM back on BTW.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps