trinityamps.com

I have the Plexi version -

Been looking at the schematics and ended up with a really nasty noise in the amp - I did this once before with a different build and it was nice - I liked the sound of it so would like to try it on Stephen's circuit. Has anyone done this here with this type of amp and if so can you help a guy out?

THanks - Steve

What's a nasty sound?

Squealing? Whining? Oscillation?

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Squeal. I used a 18 Watt lead schematic that was on this site and had a presence control.
Being that it's a different circuit I just did the PI changes and control.

It squeals and buzzes with vOlume off.

Try swapping the primaries of the OPT. Sounds like you have PFB instead of NFB.
Stew

I got the squeal as well when I first installed mine. I had to ground my output jacks to fix the squealing.

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Les

http://www.soundclick.com/unimind

I grounded the jacks and also flipped the tranny leads and it works. Now I'm faced with either putting in a toggle to activate either channel or some other method of separating the channels that I don't know about. It also didn't really get rid of the fizz especially when I kick in the humbucker. I think I'm faced with finding the sweet spot and using pedals for the lead sounds. I may toggle between using it as an 18 watt stock and switching in the presence when I'd like to fool around more.

By Fizz I mean the distortion when really dimed has a fizzy sound to it - however I am using attenuators and that may be the sound of too much compression as when I crank open the atten I get less fizz and more tone.

While I'm playing with this - is the output transformer on this amp strong enough to handle 2 - 6V6GTs? I'm toying with adding them if the OT can handle it - in this case it would be switched with either EL84s or 6V6s not both same time - I built a Twreck which could sub out either EL34s or 6V6s and it was fun to play with.

So many ideas too little time

A couple of things that helped with fizz in my amp, I changed the power tube coupling caps C8 and C9 to .01uf and the PI tail from 470ohm to 820ohm like on the sIII or TMB. These are standard 18 watt values.
There is a layout for that in the resources section here viewtopic.php?f=12&t=763

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Les

http://www.soundclick.com/unimind

+1


That can certainly affect the tone.


Totally. The 18W OT is actually quite a bit beefier than you will find in many F*nder 6V6 amps. 6V6s will also tend to have less (or even no) fizz.

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I did the deed and have the EL84s switchable with the 6V6s now - no presence added at this time - however the EL84s are Louder than the 6v6's - is that normal or could it just be the tubes?

That's normal. The difference is minimal though.

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Interestingly enough the amp sounded good with todays production tubes - JJs and EIs. I put in my Telefunkens and some Made in Holland EL84s organ tubes, GEs JAN NOS for the 6V6s - I bought and they sound really good too - the thing was, I heard the damned fizz again - subdued but still there.

Its also very dark compared to how it was with the resistor and cap changes in the 6V6 / 18 Watt schematic. The Plexi side sounds pretty brash. I think I'm going to tweak it some more. THe Presence might actually do something with the dark sound - don't get me wrong I like the bass response but I'd like more variety.

New Question but still in this thread. I'm really thinking about the presence control - actually the Presonance control like on Ampgarage, but with either I need to separate the two channels - ie my plexi side and the tmb side - is there a way to allow both into the phase inverter at the same time without them becoming part of each other like they do if you simply wire them in? Or will I be forced to use a switch?

Without the presence control this isn't a problem but the PC takes up an entry point into the inverter.

I really don't like the idea of mods that involve adding negative feedback to an 18W amp. The lack of NFB is one of the key reasons the 18W tone is so rich. I'm not sure why you want a Presence control so much, but if it really matters to you, I would recommend doing a Mesa DR style one, which taps the presence control straight off the tone stack, rather than putting it into an NFB loop
.

EL84s have quite a lot more gain than 6V6s, so you need to modify the PI a little for 6V6s. Mainly the 56k tail resistor in the PI needs to be reduced to around 20k. If you've built the Trinity "Plexi" version, then the PI cathode resistor should already be 470 ohms, which in other regular 18W versions would be 820 ohms. Of course, with these mods the PI becomes a little too hot for EL84s. So if you want to be able to run either, you have to create a little ladder network of resistors at the power tube grids, so that the EL84s then get driven with an attenuated signal from the hotted-up PI.

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Great tones come in small glass jars!

Thanks - the presence is simply an experiment to see if I can get a more "plexi" sound out of the amp. I had redone the PI cathode to 820 which fit the schematic I was working from - I think it was 470 prior. I'd have to check. But that explains why the plexi channel doesn't sound good anymore when it used to sound great. I have a switch box for resistors so I'll try playing with the 56K - 20K and see what is a good sub - I really don't mind the 6V6s being quieter - its a more mellow sound. I'm just playing around with this.

Not sure what you mean about a ladder of resistors on the power tubes - I assume this is the EL84s but could you describe this or draw a schematic?

I do have another question - some amps use a resister across the output jacks incase you forget to plug in the speakers - any recommendations on what this should be.

Thanks for your help.

I doubt that will really help much. The reason Plexi amps need a "Presence" circuit is to give back some of the harmonic complexity removed by the negative feedback. A regular Marshall 18W has no NFB and hence no loss of harmonic complexity.

Look at a Plexi PI, it has 15k in the tail (including the 5k presence pot). Octal power tubes, like 6V6s and EL34s, have way less gain than EL84s. So to drive them fully, you need to have a higher gain PI. 470R/20k drives 6V6s pretty well, whereas EL84s only need 820R/56k. You won't be able to fully drive 6V6s or overdrive them with EL84 PI values.

From a previous post...

100 ohms 5W should be fine

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Great tones come in small glass jars!

I tried the 470R back in with the 56K and it sounded fizzy - the 6V6s sounded better but the EL84s were fizzy. Am I to understand with the ladder arrangement - you'd have the proper bias for the 6v6 running it hotter but not too much and the EL84s backed down a bit so they wouldn't be as fizzy? That might work.

I ask this because with the 820R/56K its not a bad sound at all - just the 6v6 is weaker which isn't that big a deal. Now if I can dial in the tone just a tad on the plexi channel it would be better - thats why I was looking at the presence to add that extra tightness on the bass and a wicked glassy high end.

I did this years ago with Graydon's design and it sounded really good although I had to balance the caps a bit so it wasn't so bright. I got a lot of compliments on those amps.

I also didn't have a 100R 5 watt but I did have a 150R so I put that in for now across the speaker jack. BTW I'm not a purist on the 18 watt sound - I'm looking for more of a hybrid between it and the 50 watt plexi. I love the plexi sound - just don't like the weight of carrying one around!

You got it.

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Great tones come in small glass jars!

I cant upload my drawing layout for some reason - it tells me the attached file max has been reached for the board.

So I'll attempt to describe this ladder modification that you suggest. Using the 6V6 Layout in Resources

I take the pink wire between the 8.2K and pin 2 of the EL-84s and connect with a 250K resistor bridged to the other pink wire from the other end of the 8.2K, going to pin 6 of the 6V6 - do one for each EL84/6V6 set.

Then put a 220K resistor on pin 2 of the EL84 to ground on each tube.

After I change the 820R/56K to 470R/20K.

Is this it?

You have to put the file in a photo sharing site, such as photobucket or flickr, and then just post the link.


I'm afraid I'm partially colourblind - and all this "pink wire" stuff confuses the heck out of me... Just think of it as a voltage divider on each PI output. The 6V6 grid gets fed from the top of the divider, so it gets the full drive level, and the EL84 grid gets fed from the junction at the middle of the divider.


Yes, that's correct.

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Great tones come in small glass jars!

Zaphod - that fix worked perfectly. Thank you very much. Except I didn't have the 250K resistors so used 270Ks and now the two OTube sets are close to the same level of volume. I also tried that Mesa presence mod - didn't like it. THe amp is more responsive without it. There are the two 8.2K resistors in the output section - I had originally installed one 8.2K and a .082K as they looked really close band wise and I didn't catch the error - I had run it this way for a year - when I did notice it I put the proper 8.2k in I then noticed slightly more fizz - I put it back it sounds better. I also switched the cap from a .01 to a .02 for slightly more treble and mid. The result is a little more brightness and some interesting tone.

I have to play it some more today and listen but this is very interesting and a success. Again thanks for your help during this little experimentation.

Addendum - I did add a neg feedback loop and presence - the 18 watt sound as it was just wasn't smooth enough - bass flabby and it had a sort of harsh sound - adding the presence fixed all of that - now its just tweak time. It tends to sound more like a plexi now - but thats ok with me.