Updates to my Single Ended Marshall-esque build

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jarrodthebobo
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Updates to my Single Ended Marshall-esque build

Post by jarrodthebobo » Sun Apr 08, 2018 10:10 pm

Posted this amp up a little while ago (i Think it was in the #36 thread), but I've done quite a bit of work on it since. Thought ya'll would get a kick out of it.

[youtube]https://www.youtube.com/watch?v=vut2VbTbKfc[/youtube]

Just a heads up; I realize I kept screwing up resistance and caps in terms of "ohms" and "ks" and "microfarads" but it was 3 am, give me a break please :lol:

Anyways, when I get some free time I would like to do a recording either with a mic or direct in via the effects loop so you can hear how it really sounds; but this phone recording actually came through pretty good so I think you'll get the jist of what it's capable of.

Only question I have for ya'll is; how come when the 6v6/el34 and the el84 are both in (in parrallel) it seems as if a ton of treble tone is lost, UNTIL the master volume is up loud enough for the el84 to break up? It seems kind of odd. Been wanting to go ahead and throw another octal socket in the amp so I can have two 6v6 or el34s in parralel just to see how that sounds; but I'm not sure if the stock Epiphone valve Junior power tranny can handle two el34s at once.
"I hear you knocking but you can't come in-
I know you've been drinking Gin." -Louis Jordan

jarrodthebobo
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Posts: 22
Joined: Wed Jan 17, 2018 9:10 am
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Re: Updates to my Single Ended Marshall-esque build

Post by jarrodthebobo » Sat May 26, 2018 4:08 am

Made some more updates:

[youtube]https://www.youtube.com/watch?time_cont ... I-zBLrnf-s[/youtube]

The amp is basically schematically a JCM800 with a ton of mods, as well as a single ended power amp.

The biggest mod to this amp as of late was removing the el84 noval socket, which never got use as of late, and replaced it with a octal socket. There are now two octal sockets; one biased with a 250ohm resistor on the cathode, and the other with about 450ohms.

With this setup you can do a few things here: Run a big bottle tube in the hotter socket (such as an el34 as that socket has a bridged cathode to plate 1 connection, allowing the full use of it), or another 6v6. However, you can also run one big bottle tube, and one 6v6 in parallel for some really cool tonal variations. I feel it'd be a bad idea to run an el34 and 6v6 together as an el34 draws a lot of current, so when I parallel the tubes, I usually only do two 6v6s, or a 6v6 and a 5881.

Being able to mix and match wildly different power tubes gives a whole new dimension to this amp and really makes it come alive. My favorite combo is the 6v6 and 5881: for some loud clean bass, and for some gritty crunch (as well as the added bonus of some extra volume!). Two 6v6s are really cool too; especially with a JJ 6v6 in the 'hot' socket, and a vintage one in the 'cool' one. Very interesting. Fairly loud :headbang . Defiantly get some awesome cleans with the two tubes!

Aside from the power section, there are a couple of tweaks to the preamp circuit; mainly just resistor tweaks to get a little more gain out of the circuit. Also changed the 2nd triode's cathode bias switch back to 1.5k with a 10uf cap, 10k, and 10k with a .1uf cap. When using the clean channel, as like a JCM800 the first stage triode is bypassed. In order to get enough volume to drive the power amp well, the NEW 1st stage (so the second triode) needs to have a higher gain and thus a lower cathode bypass cap (as 10k has nearly no gain; just clipping).

The clean channel then becomes a 1.5k 10uf stage running into an 820ohm stage that can have either no bypass cap, or the choice between .47uf and 10uf for some extra gain-y goodness. This then runs through a cathode follower, some signal reduction/impedance matching, and then out to the effects pedals.

the return of the effects loop also functions as a "fake Phase inverter", or really just another weak gain stage in which negative feedback from the amps speaker out can be utilized. This stage is biased at 1.5k, which limits additional clipping.

This "fake PI" stage receives NFB from the speaker outs and runs through selectable resistors for different NFB responses: 22k, 75k, and around 140k. Each setting has its own advantages, but I prefer the 22k as it lets the most NFB through; tightening up the bass and the whole guitar sound as a whole. The other two settings give you more harmonic content though, and the 140k resistor sounds pretty cool with the JCM800 'mode' engaged: very raw.

There is also a new resonance control, which honestly doesn't do too much: I currently have an extra hole on the front of the amp and it needs to be filled with SOMETHING of use, and I had run all out of ideas!! So resonance knob it is... unfortunately, with the type of amp this is (Single ended) not a ton of low bass is really generated or passed through the Output Transformer, so any benefit I'd get from the Resonance control is near non-existent unless you REALLY crank the master volume; and then even so its only really noticeable on the clean channel.

The "modified Marshall" channel follows the same chain as above, however its an extra triode stage in front of the other triodes mentioned above, biased at 2.7k with a .68uf cap. It also has a 220k plate resistor which leads to more compression and distortion/gain being generated in the stage. The output of this stage leads to a coupling cap that can be switched from .022uf to .0047uf; stock 'fatter' tone, and modified 'thinner' tone. Really changes the feeling of the amp.

In this "modified Marshall" channel, the real magic comes from the second stage; 10k = JCM800, 10k + .1uf = S.I.R #34 (with 3rd stage cathode @ .47uf aswell), 1.5k = Over the top Modded cascaded marshall (Along the lines of Randy Rhoads perhaps?). Bright caps and everything else is tweak-able to taste, but as I have shown in my videos, I have a few generalized "settings/channels" that match schematics/ general values used in other famous circuits such as the #34 or the Mod 5.

It's hard to get much more flexable than this amp without going digital! It's also hard to turn this amp up much louder then 1/4 on the master because oo boy, for 5-12 watts she sure is one loud son of a gun. It's become almost a joke with my bandmates as every-time they hear it, it seems to have gotten louder (first time they heard it, biases were wrong and caps were dead; second time they heard it, it was just running a 6v6, third time a 6v6 and el84... and now, they've heard it with a 5881 and 6v6 paralleled).

I can't wait to get my hands on a greenback and toss her on in there!
"I hear you knocking but you can't come in-
I know you've been drinking Gin." -Louis Jordan

jarrodthebobo
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Posts: 22
Joined: Wed Jan 17, 2018 9:10 am
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Re: Updates to my Single Ended Marshall-esque build

Post by jarrodthebobo » Sat Jun 09, 2018 5:56 pm

https://youtu.be/E2sboNxvlFg

Got some clipping diode experimentation here; trying for some of those mythical VH1 Jose tones (that probably don't exist)
"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: Updates to my Single Ended Marshall-esque build

Post by danman » Sun Jun 10, 2018 6:40 pm

Good job on such a complicated scheme! It sounds pretty good from what I can tell in the videos. :hairband:

jarrodthebobo
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Posts: 22
Joined: Wed Jan 17, 2018 9:10 am
Just the numbers in order: 13492

Re: Updates to my Single Ended Marshall-esque build

Post by jarrodthebobo » Sun Jun 10, 2018 11:02 pm

Thanks a ton man!! Just uploaded another video this afternoon and posted a new thread (The Big1) in regards to the new circuit that I have available in this single ended amp. It sounds HUGE and I can only imagine how big it will sound in an actual JCM800.
"I hear you knocking but you can't come in-
I know you've been drinking Gin." -Louis Jordan

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