JCM800 depth mod

[Dmoney]

Just tying to figure something out for a friend. I could test but i thought I'd ask for an opinion.
adding a depth mod to a 2204 copy. do the gents here that go for that sort of thing, lower the 100K NFB resistor, or leave it as it is?

[darkbluemurder]

If the amp does not sound too bright with the presence turned way down I would leave the 100k, otherwise I would change to 47k. With the 47k the depth control may work more effective because there is more negative feedback to begin with (the depth decouples the negative feedback for low frequencies).

Cheers Stephan

[ericsabbath]

39k works better and makes the overall tone warmer and a bit middier and less gritty

bet a 60's style 27k would work well too

[Dmoney]

cheers.
I've added a depth mod to a 2205 before, but not a 2204 or 2203. The preamps and pretty different.
I might start with the 100K... or actually pop in a spare 100K trimpot and dial the NFB in.

I'm converting an amp I made into a JCM800. my mate wants an 80's vibe. Misfits, TSOL, Adolescents, Dead Kennedys.
He's not super into guitar and never had a valve amp before, but I thought the 2204 circuit was a good bet for him that he'd probably recognise the sound of, rather than trying to build some ampeg lee jackson head or something.

he wants it warm, so I thought the depth mod would be handy. I have some other ideas too.

[ericsabbath]

he wants it warm, so I thought the depth mod would be handy. I have some other ideas too.

adding a depth control won't make it any warmer
considerably lowering the nfb resistance will

[Dmoney]

i kiiiind of agree, but while decreasing NFB resistance should tighten bass it should also dampen the high end and kind of round out the sound a bit. I wouldn't want to decrease it so much that the amp starts to sound too "modern" or compressed. I think leaving it with a little grind i'd associate with less NFB will keep up the desired vibe.

I've played about with completely variable negative feedback before. a 250K pot just wired as a variable NFB resistor. It's a pretty cool control. it is a big difference going from lots of nfb to a little nfb. It's interesting to hear.

the person it's for hasn't been playing long and is used to a marshall AVT. I was told by a mutual friend that the descriptions for what he wants might not be that accurate because he won't be sure of ways to describe what he wants in a familiar way. Not like how people here describe things to each other... if that makes sense.

That's why I'm favoring the depth mod. I think the perceived added low-end will come out as warmth, I might also decrease the NFB but I might stick a 100K trim pot in as the NFB resistor and decrease it to suit. I might add a bright switch on the gain control and clip out some of the usual high F bypass caps mentioned in JCM800 mods too.

[ericsabbath]

i kiiiind of agree, but while decreasing NFB resistance should tighten bass it should also dampen the high end and kind of round out the sound a bit. I wouldn't want to decrease it so much that the amp starts to sound too "modern" or compressed. I think leaving it with a little grind i'd associate with less NFB will keep up the desired vibe.

low NFB Res won't make the bass tighter, just not as loud
it actually feels fatter, as the amp gets less dry and middier
you can have the high end grind back adjusting the presence control or you could just add some treble gain on the preamp circuit

lowering the NFB resistance  doesn't make the amp any more or less "modern"
mesa rectifiers modern mode completely cuts the NFB, which is the opposite of an uberschall, that, as far as I remember, doesn't even have a NFB resistor, and they both are in the same ballpark

had a negative feedback pot in an amp, but that was a pain in the ass to dial, since volume oscillates a lot with it
and I always ended up preferring the more organic sound of lower settings

[Dmoney]

The Uberschall uses a cutsom inductor in the NFB loop I can't remember if that replaces the NFB resistor but if it does it will have variable resistance across different frequencies like other inductors i imagine.

with variable nfb, increasing the total amount to my ears is ok to a point, until you get to a stage where it's like there is a blanket over your rig. At any individual point the presence and depth are going to vary in how effective they are. I'm not saying I wouldn't lower it, but there is a point where it just doesn't sound great. I don't feel like the presence can make up for whats lost in the same way. The presence seems to make more sense to me as a control for high end when using less NFB. I played around using variable nfb with a vox style high cut control and that works well, you can dial in the character of the amp and then tame some fizz with the high cut. obviously a traditional presence wouldn't be as effective with variable nfb. This is similar to the Mesa approach right? If i remember correctly the presence hangs off the tone stack, so you get some bass control and the way to dial out harsh high end that isn't dependent on NFB. I wouldn't say a dual rec was as tight sounding as an Uberschall.

I tried decreasing the amount of negative feedback present in an SLO style build once, and that actually didn't sound very good to my ears. I prefer it with the 39K NFB resistor.

With the amp i have here with an nfb pot it in, I think I like it where the bass sits nice and it riding the volume knob seems to give more of a result. too little nfb and the bass just gets bigger until it washes out (hard to describe). too much and the amp just sounds flat. It goes quieter with more nfb as I'm sure you know, but there is a point were one way it feels like a more... almost scooped kind of tone thats pretty smooth, and the other way gets you some more bass and rawkus vibe to it.

[darkbluemurder]

he wants it warm, so I thought the depth mod would be handy. I have some other ideas too.

adding a depth control won't make it any warmer
considerably lowering the nfb resistance will

I agree. Adding a depth control will add more bottom end punch. This is especially beneficial for low volume playing.

I would not consider the 2203/04 circuit as "warm". It is rather known as one of the brightest amps ever, especially at lower volumes. The good thing is it is an easy preamp to modify for more warmth and slightly more gain if needed. Check out the Mod5, UYI-Mod and Egnater Seminar.

Cheers Stephan 

[Dmoney]

I know what the depth will do. I have 2 amps with one stock, I've owned a 5150, and I've installed it on two amps I've built, and a friends laney GH100TI.

I also wouldn't consider the 2204 circuit as that warm, but there are a lot of details on the net about possible mods. I found some clips of the UYI and Mod 5 and wasn't that into them. I'll try and find schems a bit later. The Egnater schem has some stuff in I was thinking of trying... namely fiddling with cathode bypass and coupling caps. The Egnater has a bigger bypass cap on the V1 cathode and then its dropping some lowend with a 1nf coupling cap after that stage. That kind of thing is something I might give a go. I also have some other ideas for tone shaping that I've experimented with before.

I'll probably end up building a stock 2204 circuit to begin with and then taking it from there.

[darkbluemurder]

I'll probably end up building a stock 2204 circuit to begin with and then taking it from there.

Agreed. Gotta have a baseline IMHO.

Cheers Stephan

[Dmoney]

cheers for pointing me to those mods by the way. some good inspiration.