18 Watt Plexi Build

[dbharris]

To provide an update on this...the VRM worked as installed but made lots of weird noises (crackle, buzz, light saber, etc.). I added a 16uf F&T cap at the vrm as the first filter node. Grounded to the power amp star. Cured all the weird noises. I also placed a 2W 220K metal oxide resistor in parallel with that cap to drain power when turning the amp off.

I changed the speaker jacks to Marshall style Cliff isolated jacks. Ran a ground wire back to the power amp star. Maybe lowered the background hum a bit.

Lastly, I added the 2 470K resistors as a voltage divider after the gain pot. No treble peaker cap. Since I tend to run that pot fairly high I am considering adding the same network after the volume pot too.

I forgot to grab pics before putting the chassis back in the head cab. I will do that the next time I crack her open.

I think I will still add global negative feedback and make it switchable. But going to enjoy playing the amp as is for a month or so.

I am liking the sound quite a bit. I am probably attenuating volume something like 8-9 decibels with the vrm and that also warms up the amp. So far have only plugged a guitar straight in, I will connect it to my pedal board this weekend. I run the pedal board with the 4 cable method on my mesa amp, so need to rework a couple things to put it all in front of this one.

Thanks again to everyone that helped me out!

-Dan

[jwhousley]

Question: I have seen some builds of this 18 watt Plexi with the tubes facing down instead of parallel with the floor. Where is this type of chassis available to purchase???

[dbharris]

I think what you mean is a head chassis versus a combo chassis. A chassis designed for a head cab will have the tubes pointing up or down, perpendicular to the floor with the chassis on your bench. A chassis designed for a combo cab will have the tubes parallel to the floor with it on your bench.

My amp is a kit from Trinity Amps. They sell kits for either orientation, but not just a chassis. You could look at Mojotone, Weber, and some other vendors for chassis options. Lots of choices out there for 18W variants.

Though, with 6V6 tubes like mine, you are pushing closer to 30W than 18W.

-Dan

[jwhousley]

I think what you mean is a head chassis versus a combo chassis. A chassis designed for a head cab will have the tubes pointing up or down, perpendicular to the floor with the chassis on your bench. A chassis designed for a combo cab will have the tubes parallel to the floor with it on your bench.

My amp is a kit from Trinity Amps. They sell kits for either orientation, but not just a chassis. You could look at Mojotone, Weber, and some other vendors for chassis options. Lots of choices out there for 18W variants.

Though, with 6V6 tubes like mine, you are pushing closer to 30W than 18W.

-Dan

Thanks Dan! Yes, but what I am seeing is the same chassis used for both. I guess it's possible that the pics I am seeing may be inaccurate.
These appear to not be pointed down...

https://www.mojotone.com/kits/BritishAmpKits_x#/
https://www.mojotone.com/amp-parts/Chas ... le-Chassis

The Weber chassis appears to be correct.

https://tedweber.com/6m18ch/

[sluckey]

Ceriatone also has the correct chassis.

http://www.ceriatone.com/british-style-18w/

Look under Add-On/Parts

[jwhousley]

Ceriatone also has the correct chassis.

http://www.ceriatone.com/british-style-18w/

Look under Add-On/Parts

Thank you!

[dbharris]

Somebody on the Trinity forum asked for a clip. I still feel like I have some tweaking to do for this amp. Going to add GNFB in January and then go from there.

Please excuse the sloppy playing...



The Ocean is Neck/Bridge (LP), Middle (Strat)

Sunshine of your Love is Neck (LP) with volume rolled down to 8 for main riffs and and 10 for solo with tone on 0 for solo only, Neck (Strat) and same vol/tone changes on guitar

Pawn Shop is Bridge (LP) Bridge (Strat)

Then I play some open chords and roll the volume back on the guitar to clean up the tone

Last, is playing each channel separately and then jumpered together.

-Dan

[M Fowler]

I built several 18w combos using GDS amps tubes down combo chassis with their transformers.

I have also used Weber tubes down combo chassis with Heyboer transformers.

Good products.

Mark

[pullshocks]

Great thread.

What is a VRM that you mentioned installing?

Thanks.

[dbharris]

Thanks! Trinity calls it VRM for Voltage Regulation Module. It's just a switched pot, a mosfet, and some resistors and caps. Same idea as Power Scaling or other marketing terms.

Essentially it allows you to drop power to the entire amp as if it were hooked up to a variac, but with the advantage of not being able to turn voltage up over your line voltage.

For this amp it is great to use sparingly. I am probably dropping 6-8db of output with it. The amp is still plenty loud but not wall rattling. You loose some power tube distortion the more you turn down the voltage. Also, to the extent you ride your volume knob a lot, that becomes less effective with lower headroom/voltage.

[dbharris]

Quick update, I spent some time working on the amp today and I am very satisfied with the results. I know the guts are a bit sloppy since I have chopsticked around so much and done many rounds of mods, but all of the connections are solid and sound is great.

I started out chopsticking. Was able to determine all of the instability in the amp was due to a micorphonic tube in V1. It seemed good back when I was rolling tubes, so I guess it must have been a little microphonic and got worse as I played the amp and it got rattled around. It was a new Mullard long plate. Now I have an EHX 7025 there. All good and not running off into feedback at the drop of a hat.

I alligator clipped in a couple small snubber caps across the plate resistor for the second gain stage. Settled on 470p and soldered it in. This really helped calm the ice pick harmonics of this amp.

Then, I made all of the several changes necessary to add Global Negative Feedback on a switch...PI plates, feedback resistor, changes to long tail pair part of PI, moving where the channels meet the PI and adding mixing resistors. I fired up the amp and it wasn't howling but was definitely out of phase and unstable. Switched the OT leads and all good.

This sounds like a big box Marshall to me! Nice and tight even with lots of gain. Now that I have the fat boost and GNFB switches there is a lot of variation that can be dialed in with those and different amounts of gain. Still cleans up incredibly well with the volume knob on a guitar. I am calling this one complete. If there is anything I might have done different, it would be going with a 220p or 330p snubber since the GNFB kills some high end too. But the difference is not worth opening her back up for that one cap.

-Dan