Hoffman Plexi 50 build - distortion sounds a bit gritty

[JD79]

Evening all (or morning, depending on timezones)

I'll quickly introduce myself, then get down to business. My name is Joel, and I'm a rather poor guitarist and a compulsive tinkerer in all things audio. Turntables, homemade guitars, guitar amps, hi-fi gear etc. No real electrical qualifications, but I understand bits and pieces of valve operation and can solder to some degree.

About 5 years ago I decided I couldn't justify buying a Plexi head, and thought that building one would be cheaper and infinitely more enjoyable. I was right on both counts :) I came across the schematics and layouts for a Hoffman Plexi 50 online, bought the bits and pieces from an eBay seller here in Oz, and went to it. After some minor debugging, I would up with an amp that sounded pretty decent to my ears at the time. Certainly the clean tone is spot on, and the overdrive wasn't too bad. However, the overdrive has never really sat too well with me. The best way i can describe it is brittle and rather harsh, without much in the way of that old Marshall creaminess. Almost like a fuzz distortion, which tells me that I may have muffed something up in the build (Ok, pun intended there :) ). There doesn't seem to be much of a gradual or dynamic transition between clean on lightly picked notes and the aforementioned fuzzy drive on heavier notes. For one reason or another I haven't got around to working out exactly what I've done wrong, but I'd like to have a go at improving the tone.

Has anyone here encountered anything like this with Hoffman builds? Any suggestions to get the ball rolling?

And finally, looking around the site here, I can't seem to find the schematic I used back then, and sadly I've lost the original document. Any idea where I can source it from?

Thanks in advance for any suggestions,

Joel.

[sluckey]

We need more info on your amp. The bright channel on a Plexi can be brittle sounding. Try the other channel. It's a little darker. Or, better yet, jumper the two channels together for a much greater variety of tones.

Doug didn't provide a schematic for his Plexi 50. Does the following layout look familiar? It's hiding in the tube amp Library of information.

[tubenit]

Is the schematic that you used this one?  We need to know what you have before we can offer suggestions for smoothing the tone?

What are your voltages like?

What type of non-electrolytic caps did you use?

I attached an editable SCH version, that you can edit if need be to have a schematic that matches your amp.

With respect, Tubenit

[Fresh_Start]

What tubes are in the amp? That could make all the difference by itself.

Respectfully,

Chip

[JD79]

Thanks for the replies :)

That looks like the one I was working off Sluckey. It was a while ago, but it does look familiar. I think there were a few more pages of hookup info that I had to work from as well.

I've opened the amp up for the first time since soon after I build it. I didn't remember it being such a rats nest. Not the cleanest build, but I hope I don't have to redo much of it. I'll take a photo in a bit to show you all what it looks like.

The non-electroytic caps I used were polypropylene film except for one silver mica I think. All purchased from a guy who specializes in valve amp parts. Not name brand caps though as far as I know.

I checked the voltages against those on the schematic provided by Tubenit, and they're fairly close, but not spot on.

I get:
V1) 1:201 3:1.6 6:202 8:1.5
V2) 1:175 3:0.9 6:288 8:176
V3) 1:222 3:35.4 6:222 8:35.4
V4) 3:466 4:458 5:-40 8:0.041
V5) 3:466 4:459 5:-41 8:0.041

I'm getting 42mv across each of the 1ohm bias resistors.

Are these values close enough to spec?

As for the tubes, 12ax7's in the pre, and EL34's in the power stage, all electroharmonix.

I'll spend a while working bit by bit over the schematic and comparing everything tomorrow when I've got a bit more free time.

[JD79]

Here's a pic of the innards of my build. Like I said, not the prettiest, and I'm not sure if layout issues may be an issue.

[tubenit]

Changing the cathode resistor and caps will remove some grit and add smoothness.

The smoothing caps (220p &100p) will smooth out the OD.  Be sure to have the 100p be after the .02 coupling caps and NOT before them.

Changing the CF cathode from 100k to 82k will add some compression.

I think these changes will smooth out the harshness of the amp.

With respect, Tubenit

[JD79]

Thanks Tubenit.

I'll get to ordering the required components tomorrow, and should be able to start trying the mods over the weekend if all goes well.
A few further (rookie) questions though.
What voltage ratings will I need for the 2.2uf and 5uf caps (assuming electrolytics given the plarity on the schematic)?
Should I use polypropylene film caps for the 220p and the 100p caps?
And which mods would you suggest I try first?

Thanks again :)
Joel

[tubenit]

Any cathode caps rating of 25v or more is fine.

Silver mica for the 220p & 100p

I'd first change the V1a cathode resistor and cap. And then add the smoothing caps.  You don't need a 220uf cathode cap.

With respect, Tubenit

[JD79]

I managed to source the required components today and started the mods.

First I made the V1a cathode changes as suggested and tried it out. I like the change. Much less sharp and wasp like when overdriven, but still a bit harsh and gritty. Not quite the creamy overdrive I'm hoping for. I seem to have lost maybe a bit of sparkle on clean tones, but fiddling with the presence and tone controls brings most of it back.

Then I skipped ahead and put the smoothing cap across the PI anode resister. This seemed to smooth the driven tone out quite a bit more, without sounding muddy or losing attack. I'm much, much happier with the sound now :)

I'm temped to undo the first mod and see how it sounds with just that smoothing cap, but I'll hold off on that for a while and enjoy it the way it is.

Many thanks for the advice Tubenit. You're a legend!

[tubenit]

OK, that sounds like progress. The 220uf cap is really for a bass guitar and anything above 25uf will not get you much for a guitar tone range. If you need more than 5uf, then try 10uf,  ....... then try 25uf or 22uf.

Next thing I'd try is the cap across V1-1 & V1-3 pins. You can use isolated/insulated alligator clipped wires to try this carefully.  Remember this is very high voltage.  Drain the voltage before removing the alligator clipped cap.

With respect, Tubenit