trinityamps.com

I currently have some MESA EL-84/6BQ5 which I think are Russian but I don't think they are military grade.

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Les

http://www.soundclick.com/unimind

I don't trust anything from Mezza.


Yes, but with cathode bias, you can run the amp close to 100% dissipation at idle. The bias voltage then rises when the amp cranks and compensates itself. Edit: Just got round to listening to the clip. BTW do you get that same sound if you play through just the Normal channel on its own? That's way more than a bit of fizz! I would normally interpret that sound to mean that there's something seriously wrong with the amp, and would instinctively start by swapping out the tubes.


There's a very good reason those two caps are 0.01uF in the Marshall 18W design. To me it looks like your amp needs the lower value caps *plus* the hotter bias.

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Great tones come in small glass jars!

I changed the PI tail resistor R15 from 470R to 820R like on the sIII and TMB. I think that finally brought the fizz/buzz under control. Maybe.

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Les

http://www.soundclick.com/unimind

820 ohms is the original value used in the 18W PI. 470 ohms is indeed a bit hotter, and also gives some crunchy Plexi-style distortion. Perhaps the extra gain with the 470 ohm resistor is a little too much with EL84s, when the preamp is also cascaded for extra gain. It might be interesting to see what would happen if 6V6s or 6CM6s were used in instead of EL84s, staying with the 470 ohm PI cathode resistor.

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Great tones come in small glass jars!

I have actually been thinking about that as well the past couple of weeks. I would just need to drill the chassis for the larger octal tube sockets.

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Les

http://www.soundclick.com/unimind

Or just go out on eBay and snag yourself a few 6BW6s or 6CM6s. These are both 9-pin versions of 6V6s, so no need to change any sockets. Like EL84s they have a max dissipation limit of 12W, rather than the 14W of a regular size 6V6. There's quite a few folks who lswear by these tubes in an 18W.

However, there are three resistor values which will need to change:

- PI tail = 22k/470R - yes, you have to go back to the 470 ohm value to help compensate for the lower gain of the 6V6s
- Power tube cathode resistor = 270 ohms to 300 ohms ball park. You may need to try a few different values.

Being beam tetrode tubes, rather than power pentodes, these mini 6V6s have a higher damping factor than EL84s and are less prone to suffer from fizzy artifacts.

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Great tones come in small glass jars!

Important to note, though, that all 3 tube types have unique pin-outs so the wiring will need to be shuffled.

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** b0b0 **
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- This Space For Rent -

zaphod, b0b0, Thanks for the info. I will look into those tubes.

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Les

http://www.soundclick.com/unimind

unimind...did you ever get this resolved?

Nope. I never could get rid of the fizz with el84s. When I put in 6CM7s the fizz was gone. See this thread viewtopic.php?f=7&t=3294

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Les

http://www.soundclick.com/unimind

ezcomes if i remember you built the plexi 1 right ? If so yo might want to try to change the cathode resister to 820 r instead of the 470 r. That at least fixed it for me on the normal Chanel.

thats right...for the most part though, i only use the TMB channel...so i don't know how much that would help...i noticed it more after i removed R8...but at the same time i also did the cascade mod...so...now i'm struggling to remember if it did it before...

today i'm going to put R8 back in...and go from there...

the other thing i noticed...screwing around yesterday...with the VRM at noon, gain at about 3pm same with TMB volume i hit my tubescreamer to practice a solo (sounded awesome) and then when i turned it off...it almost seemed like all the gain was gone...and then slowly built back up after a few seconds...

Well hey it does help for me a little on the tmb site. There's a few threads about it here and on ampgarage and 18 watt you can look at if so desired.

I have have some more success by changing the master volume pot down to 250k

The following is at your own risk. Cause I don't know how far you can push the power supply in this amp. (need expert advice on this)
The clip I did in brown down was with and extra 2k2 dropping resistor and it own cap for v1 alone. Which was a little too hard sounding in the end. I have since removed the cap and resistor and change the last dropping resistor from 2k2 to 3k9. and ist almost there. (think I'm gonna have too glue two 2k2's together)




As for your other issue haven't run in to that

I am resurrecting this old thread for an update on my brown build.. I rebuilt my Trinity 18 watt to Unimind's original "brown" specs back when he first posted the specs and was pretty happy with it but suffered from the same fizzy decay on low notes that we were all chasing down in this thread. In my case I was able to get rid of them by using the Paul Ruby zener mod that worked great until I installed a vvr. Once I installed the vvr the Ruby clamp worked great at high voltages but as I dialed down the vvr the Ruby clamp would lose its effectiveness as the voltage dropped way under the zener values. I managed to largely solve this by using really low voltage zeners (3v) which allowed me to dial down the vvr quite a bit more before any fizzies would show up, but it was not perfect

However today I was tinkering with my amp trying to coax a little more bass out of it (it sounds a little thin vs some other plexi type builds I have) and the result was not only a little more bass as I had hoped for but I also managed to completely get rid of the fizzies by changing the .68uf cap on v2 to a 25uf as per the "Mod 5" and some other plexi layouts on the Metro board.

Now there is a drawback to this as this bigger cap does make the bass pretty loose vs the .68, but what coaxed me to this "brown" layout in the first place was the early EVH tones Unimind managed to coax out of his original build, and those old EVH tones are not exactly modern high gain tight bass. I get some pretty nice brown tones from this amp and now no longer seem to have any fizzies even when I dial the vvr right down - I was very surprised that this cap made such a difference on this, it was an unexpected bonus in my attempt to coax a little more bass out of my amp.

I am using an 820r on my amp in place of the 2.7k/2.7k with boost switch - I'm thinking that the .68uf over 820r on v2 must have introduced just the right amount of gain or perhaps an odd time constant to cause some sort of weird phasing that carried through the preamp and caused excessive fizzies on my amp - I do not know the math or theory well enough to really explain it other than when I switched the cap I had less fizzies. If anything I expected them to be worse with the extra low frequencies bypassed by the cap but that was not the case - Still has the Ruby mod on it as well

thought some of you that have brown builds may want to try it - YMMV

Thanks for the details Lindz and good work. One could put all these values on the switch and make both work with some combination. Might be worth a try, kind of Fat / Thin switch with gain.

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

For me it was just changing the 2k2 power resistor to a 4k7. Has the 500 k master volume. And I did go back and try to pi tail back to 470R (unsuccessful) put the 820r (really 785) there may be some value between 470 and 820 that would work

This is with a 2k7 in the first stage by passes .68 (this always sounded more to my liking ) and efectively 820 bypassed with .68 in the second stage.

Update: I put back a .68 just to be sure I was not losing my mind as it just seemed too good to be true to me that a cap in this position would affect things the way it did. Interestingly I have less fizzy swirl even with this .68 in it now - its not the same one I had in there as I had melted part of the covering on the old one due to sloppy soldering in the past and wanted a pretty one in there. I wonder if I had some sort of messed up cap in it before? I am tempted to go dumpster diving to see if that old cap was causing grief - not sure how it would necessarily cause the fizzies though. I guess this cap switch was not a magic fix after all

I kind of feel like a dope for even posting last night before confirming

Thanks for the update anyway. When we switch in the .68, it's in parallel wit two 2K7s.

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

Sounds like a bad tube or tubes somewhere in the amp.

Think I solved mine for the 10th time. Seem to have gone for now by putting the vrm on the first power supply node not before (my vrm is on the whole amp) don't have the ruby mode in.

So B+ , diode , 32uf ,vrm, plates+1k5, 32uf, screen (also 220k +.1uf to ground off first cap)