"The Ultimate Small-Watter" Amp Build (Single Ended modded Plexi tones!)

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jarrodthebobo
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"The Ultimate Small-Watter" Amp Build (Single Ended modded Plexi tones!)

Post by jarrodthebobo » Mon Feb 05, 2018 1:53 pm

Hey guys, I just posted another thread about an Issue I'm having with this amp (About a plate bypass cap), but the thread was getting long winded with the descriptions of the amp, so I decided to make a new thread for it in order to better 'introduce' it.

This amp started off as a project amp that's been sitting around for a bit; One of my old amp building buds had modded my 'eh' Epiphone Valve Special into a... kinda 'meh' Vox..ish, marshal-ish, fender-ish kinda thing. It was a single ended design utilizing 3 Preamp tubes, and either a single 6v6 or single el84. It had 2 channels, one clean channel utilizing two tubes for gain and tone, and another that only had 1 stage and a simple tone knob. I never really liked it too much, but decided to keep it since it was hand wired and might make a cool building platform one day.

About a month or 2 ago I decided to finally play with it; and play with it I did. Initially it started off with me going easy and rebuilding the preamp into an SEplex; which is a cool little design I'd seen floating around on SEwatt and the such (http://chasingtone.com/dvnator/the-seplex/). The idea of having a 'fake PI' in a single ended amp seemed pretty cool to me, as the PI distorting is a pretty big part of the whole marshall sound. The amp sounded pretty cool with this design, and I was happy for a bit; but the will to tweak took over, and I jumped off into the deep end.

First I added a switch that added an extra gain stage between the first two stages; biased cold with a knob that let you choose between 10k of cathode resistance up to 40k of cathode resistance. It got that really cool soldano tone, but it was pretty noisy; and I'm honestly not a fan of the 'liquid' distortion. So I decided to play some more...

I added switches to change every bypass cap (3 way switches), and got rid of the resistance knob; instead replacing it with a 3 way switch so along with changing the resistance, I could add caps in parallel as well. That was fun too; but I needed more.

Then I found a schematic for the SIR #39 and I fell even FURTHER down the rabbit hole! Tried building that circuit; but it was WAY too noisy... so I went with the #34. I REALLY liked this one.... but again; MORE IS MORE when it comes to tweaks!

From the #34 with all these switches, I've moved into the following circuit;

Switch 1: Changes the bypass cap for the 470k resistor to gain pot 1; Values of 2200pf, 500pf, and nothing.
Switch 2: Changes the bright cap between 5000pf(Mod 5 value), 1000pf (#34 value), and 220pf (I prefer this over the 500pf).
Switch 3: Changes bypass cap over v1a's cathode; .68uf, 10uf, and nothing.
Switch 4: Changes coupling cap between v1a and v1b from 22nf to 4.7nf (will be 2.2nf when I get the part).
Switch 5: Changes the cathode for v1b from 10k to 1k. 1k has a bypass cap of 10uf, 10k has the option for no cap, or .1uf (ala the #34).
Switch 6: Changes the bypass cap over the 470k that leads to v2a (former voltage divider) between 100pf, 500pf, and no cap.
Switch 7: Changes cathode bypass of v2a between .47uf, 10uf, and nothing. (didn't notice huge difference between .68uf and .47uf, so kept the .47uf for the #34 spec).

Pre gain 1 is a 1 meg pot.
Pre gain 2 (pot after v1b) is a 1 meg pot.
Slope resistor was replaced with a 100k pot (with a 33k resistor in series).
Treble cap is 430pf. Other tone caps are 22nf.
All other coupling caps are .22nf, except for the aforementioned switchable cap, and the 'PI' coupling cap, which is 44nf.
Plate bypass cap over first stage 100k is 500pf.
Plate to cathode bypass cap on v2a is 220pf.
Treble wiper goes to a 100k resistor, with a 50k resistor to ground. This leads into a 100k pot, which goes to a send jack.
Return jack has approximately 1 meg going to ground, and enters the 'fake pi' which is basically just another gain stage; so the 'recovery stage' when the effects loop is used.

The PI stage is biased at 100k on plate and 1.5k on the cathode; so it doesn't really add much more distortion; just volume. It can be pushed however. The cool part about this 'fake PI'/ recover stage, is that I can use it for some negative feedback loopage here. I have a 100k resistor between the 16ohm tap and the cathode of this stage; for some moderately tight NFB.
Presence knob runs off this stage as well; and uses a 5k pot with a 5.6k resistor (couldn't find a 4.7k in my parts drawer) and a .1uf cap. It was initially a .68uf, and I'm still trying to decide which cap I liked better.

Weird thing about the presence knob is that when its cranked towards it's 'full on' setting, the amp appears quite a bit louder.

Preamp section 1 complete... However, there is a second channel!

I had a left over triode; so I made a 1.5k biased channel with a 20uf over the cathode. This runs directly into the stage before the tone stack (so the 820ohm stage; v2a), where it just flows through the rest of the circuit. I typically turn the slope knob up to around 60-100k using this channel as its a bit more 'fendery' and clean. Can get some nice grind when cranked though; especially with the master pushed.

THE POWER AMP: I actually hadn't changed much in this circuit from what I started with (my friend's design), so I don't know too much about this section. I changed the grid stopper on the 6v6 however to 4k, and added a 5.6k grid stopper going to the el84. Initially the two tubes had shared one grid stopper, so paralleling the tubes was impossible. Now you can however, and with both tubes in the amp its actually VERY loud; I dare to say as loud as my Bogner Alchemist which is 40 watts. I'm sure this amp isn't pushing much more than 10-15 watts, but it definatly sounds louder.

I believe the 6v6 isn't biased properly; as its much quieter on its own than the el84 is on its own, as well as having a slight hum when in use. The el84 doesn't have this hum; and its not dependent on the tube as I've swapped different brands in, so something isn't 100% here. However, the sound with both paralleled is fantastic when they're pushed; the 6v6 seems to break up a bit earlier (which is odd since el84s usually break up early), and the fat 6v6 distortion combining with the brightness of the el84 is pure tonal ecstacy.

The output tranny has been replaced with a 15watt single ended tranny from classic transformers. It's so much more robust than the stock one. The power tranny is still the same as the valve special however; as it was pretty overspec stock.

Speaker is a WGS Veteran 30.

I think that covers anything! I'll be posting clips soon, and some video.
"I hear you knocking but you can't come in-
I know you've been drinking Gin." -Louis Jordan

jarrodthebobo
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Re: "The Ultimate Small-Watter" Amp Build (Single Ended modded Plexi tones!)

Post by jarrodthebobo » Mon Feb 05, 2018 1:54 pm

Here is an older video from before quite a few of these mods/tweaks. Forgot to mention I also built a new cabinet for the amp as the Valve Special just wasn't cutting it looks/size wise. I think this 18 watt plexi style cab suites it much better. The wine tolex is pretty yummy too :thumbsup:
https://youtu.be/0A8YbKjZx84
"I hear you knocking but you can't come in-
I know you've been drinking Gin." -Louis Jordan

danman
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Re: "The Ultimate Small-Watter" Amp Build (Single Ended modded Plexi tones!)

Post by danman » Tue Feb 06, 2018 8:43 pm

Sounds good...very aggressive! I may have to hunt down this 34 schematic and make some changes to my se el84 amp. Adding another gain stage or two sounds like a good idea... :rocker:

jarrodthebobo
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Joined: Wed Jan 17, 2018 9:10 am
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Re: "The Ultimate Small-Watter" Amp Build (Single Ended modded Plexi tones!)

Post by jarrodthebobo » Wed Feb 07, 2018 1:33 am

It's basically a JCM800 but with a .1uf cap bypassing the 10k cathode resistor, a 150k resistor from the gain pot's wiper to ground, and a .68uf cap for the presence control. All coupling caps are 22nf and the voltage dividers (470k) each have 500pf caps bypassing them.

Ive seen some schematics with the first 470k (before the volume pot) having either 500pf or 2200pf; and I actually prefer the 2200pf; gives a slight bit more mid push. The bright cap on the gain pot should also be 1000pf; which is pretty important to the mid-range cocked wah sound.

The final gain stage, the 820ohm stage after the 10k stage, also needs a .47uf cap across it. I found this really helps dial in the needed extra gain; especially in combination with the .1uf cap over the 10k resistor. Except a great deal of treble, and potentially a lot of oscillations if your wiring isn't great.

Besides that, the only major modification is just making sure the NFB is 100k off of the 4ohm tap. I however have 100k off of the 16ohm tap; mainly because I prefer a bit more NFB in the single ended amp design. The SE designs tend to have a bit more 'bite' to them I've found and can sound a slight bit unstable in the high end, so the extra NFB really helps.

Aside from the #34, I'll try to get a schematic of my amp here built up when I can! It'll probably just be the preamp though; as I'm not too knowledgable on the power amp side of the amp. Again, I'll try to get some clips out whenever I can; University is just taking all my time at the moment.

Also an observation; I removed the 500pf plate bypass capacitor and some of the oscillations actually decreased a slight bit; which doesn't make too much sense to me. Instead of a plate bypass cap, I now just have a 220pf cap across the cathode and plate. I feel like the amp has a bit more presence now.

Wish I could figure out why the PResence knob adds so much volume to the amp when its turned up however; it almost acts as a volume boost pedal honestly with how much extra sound is pushed out at max levels; it's very odd.

Also noticed that I have a great deal less definition and presence when I have the el84 and 6v6 in parallel at certain volume levels; mainly when I have the amp turned up to where it's 'loud' but before the power tubes begin to distort. Having the el84 on its own, or the 6v6 on its own, or even an EL34 on its own does not have this 'treble roll off' effect though. I'm sure this is due to some weird capacitance issue. I'll have to play with the power amp a bit and try to understand it a bit better.
"I hear you knocking but you can't come in-
I know you've been drinking Gin." -Louis Jordan

danman
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Joined: Tue Sep 10, 2013 9:09 pm
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Re: "The Ultimate Small-Watter" Amp Build (Single Ended modded Plexi tones!)

Post by danman » Wed Feb 07, 2018 8:06 pm

Thanks for the info. Mine is just a simple Fender Champ circuit which can run a single 6v6 or el84. It gets very hairy with the el84 and it is only two gain stages driving it with a simple tone control. Been wanting to try adding another gain stage or two but i'm sure it would take some tweaking to control all the added gain.

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