OR15 presence

[jontheid]

It is probably bigger because it is a higher wattage resistor, but it has a really small value.
Putting it back in won't cause any harm.
You might even be able to get away with resoldering R48 as well and just disconnecting the NFB connecting wire (or it might get a bit noisy, hard to tell).

[Luisito_les]

Yes, I notice a little bit more noice after I disconnected the NFB. But the tone cut solved the problem. To have control on the amp's clarity makes it more versatile. Pinch harmonics, rakes, palm muted... everything is easier. BTW, I tested the amp with a 2x12: Greeback and Eminence The Governor. The result was excellent.

[Squirrels]

Sorry to dig up an old thread but I've been wondering if you can make a depth/resonance control out of r48 by replacing it with a 1 meg pot and .0047uF cap? I'm basing this solely on a depth/resonance mod I was reading about for a JCM800. I play metal and blues at mostly low volumes and could would love to be able to tighten this amp up a bit more. I know the #4 probably would have been a better choice for metal but the OR15 sounds fantastic regardless. It sounds great with my Green Rhino and Big Muff but there is always room for improvement.

Also since the OR15 and #4 share the same circuit board and supposedly most of the components is there a way to make the two circuits switchable? I have very limited knowledge about circuits and wiring diagrams and my experience is mostly from tweaking a Valve Jr. with Bitmo mods and various things I've read on the web so I'm probably way off with that line of thinking.

Anyway, would a depth/resonance control work like that or am I way off? Thanks.

[Squirrels]

I did a little more research on it and saw that the NFB resistor doesn't get removed so I got that figured out. Maybe I should just experiment with it and share my findings. Worse case scenario, I'll remove it.

[Squirrels]

I picked up the parts today for the resonance/depth control. After opening it up, my only problem is where to solder to the output. I know it goes to an 8 ohm jack, but I'm not familiar with the terminals on a Cliff jack.

[Randy Bass]

Hopefully this will be useful:

[Squirrels]

Thanks for the diagram, I appreciate it. I tried it out and it didn't work. So I put the .0047 uF cap in parallel with r48 instead to be a fixed depth mod to see if that had any result and it seemed to tighten it up a bit, not too much though. I guess since it's a pretty dark amp I might not get much more from it. Ehh, there was nothing really wrong with it in the first place.

[jontheid]

I think you should be a bit careful doing mods when you have no idea how they work.

The negative feedback signal is taken from the output via R48 to the phase inverter tube.
Putting a capacitor in parallel with this resistor with make it easier for higher-frequency negative feedback signals to get through, as impedance decreases with frequency.
More high frequency components to the NFB signal means that the amp will be less bright as HF will be attenuated more.

The only way you boost the HF via the NFB loop is by shunting away some of the NFB signal to earth.

[Squirrels]

No, you're right about not being very knowledgable about amps, so I'm very careful to make sure the caps are drained before doing anything. My experience is very limited: Bitmo mods, changing some components out (following directions) of a Valve Jr. and Champion 600, a few pedal mods and building a Valvecaster, which hardly makes me an expert. I'd love to learn more about the whole process about how an amplifier works, any suggestions on a book or anything?

But anyway, it seemed liked a pretty straight forward mod to the amp, so I figured why not give it a shot... wire a 1 meg pot with a .0047 uF cap in-between the negative feedback resistor and the hot output of the 8 ohm speaker out, or the 8 ohm tap (I wasn't 100% sure which one). Neither seemed to work, so I tried the fixed depth mod I read about by soldering the cap in parallel with the NFB resistor. Seemed to tighten up the low end for a heavier tone but maybe it was just a mental thing considering I just altered the amp a bit.

I found another OR15 schematic here, but feedback isn't listed under R48: http://music-electronics-forum.com/atta ... v-230v.pdf

So you're saying that I pretty much did the opposite of what I was hoping to do then? And seriously, any suggestions on where to start learning about amps from a technical standpoint? I'm starting to think I've just been lucky with what I've worked on.

Thanks.

[jontheid]

There are lots of sites where you can learn the basics about passive components - resistors, inductors, capacitors, transformers.
Get a basic working knowledge of simple DC and then AC circuits with these components.
For the next step:
I always recommend http://www.valvewizard.co.uk/" onclick="window.open(this.href);return false; if you are trying to learn about tube amps.
Start with 'the triode gain stage' - it really is the fundamental building block for all tube amps.
Then do 'single ended output stage'.

If you can understand both of these well enough, then you know how simple amps, like the Fender Champ, work.

Next do 'cathodyne phase inverter' followed by 'push-pull output stage'.
Once that is covered you are well on the way to understanding most of what is going on with the OR15.

All in all there is a lot to learn and some of the concepts can be quite tricky at first.
I have a degree in electronics, but they taught us nothing about tubes as when I did my degree (93-97) they were thought no longer relevant! Ha!
So I have had to teach myself all the tube stuff from scratch, and even with an electronics background I still found some things hard to understand, so don't let that put you off!
Cheers
Jon

[jontheid]

I found another OR15 schematic here, but feedback isn't listed under R48

The NFB signal comes of the 8 ohm tap of the output transformer.
R48 and R38 form a potential divider which then applies a fraction of the NFB signal to one of the inputs of the phase inverter (via C21) and also a small amount to the bottom of the 'long tail' resistor of the phase inverter.

The usual place where you would put a presence control is across R38 - you could put a variable resistor in series with a capacitor across it.
This means you can 'throw away' a variable amount of the high frequency parts of the NFB signal.

The result of this is boosting of the higher frequency signals.
The problem with the OR15 is that the value of R38 is very small, so that the associated capacitor across it needs to be very large to give an effective treble boost at useful frequencies. There's discussion about this earlier on in the htread.

Cheers
Jon

[Squirrels]

Thanks for the information Jon, I'll have to get around to reading it.

What I'm trying to do is boost bass frequencies, like how Jet City JCA20 owners have been doing to tighten up their low end. Apparently JCM800 owners have been doing it as well, since I've read that the JCM and OR15 are similar, I thought it may be useful. I'm primarily a metal player and with all the gain the OR15 has it gets very close but with the resonance control I figured that I could tighten it up more to really get me there and have an overall more versatile amplifier.

The mod seemed simple enough to try out but I couldn't get it to work. Either I got oscillating noise, or the amp would shut off when I turned the pot down. I also read that wiring the cap in parallel with the feedback resistor would make it a fixed mod, giving the effect that the pot would be dialed in at 10, it sure sounds that way.

Here is the diagram I used: http://home.comcast.net/%7Ejbjdav26/Resonance1.jpg

[Man1ak]

Maybe offtopic, but the JCM is not similar to the OR15. I would say they are quite disimilar.

For the lower frequencies I believe you should try first to change the EQ cap values to the ones used in the Jim Root. Opinions go that the JR is more modern sounding than the OR. There was this thread http://forum.orangeamps.com/viewtopic.php?f=2&t=52248" onclick="window.open(this.href);return false; and one user (sizzlingbanger) has compared OR and JR. Not particularly useful for the bass, but maybe he replies in this thread with the info.

[Squirrels]

Thanks for the link to the other thread, I'll have to check it out later on.

Page three of this thread mentioned some similarities to a JCM800 a few times. I love the over all tone of the OR, I don't really want a drastic change, I just think the depth mod could be beneficial for more variation in EQ settings, like the presence or tone cut would.

[a.hun]

What I'm trying to do is boost bass frequencies, like how Jet City JCA20 owners have been doing to tighten up their low end.

Sorry if this seems like nit-picking but I see this a lot. While many people seem to think that 'tight bass' means the same as 'big full bass' it actually doesn't mean the same at all. Tight bass is well controlled and punchy, often / usually less full though!

What do you actually want here, tighter or fuller bass?

BTW don't forget that EQ-ing is all about the balance of different frequencies. If you want to move the balance towards the bass for example than cutting the treble is often the way forward, whether you are doing it with the amp's front panel controls or with internal tweaks. Sort of thing I was talking about here:

Good guitar amp EQs (especially on valve amps) are often very interactive, so changing the level of one control will also affect (and be affected by) the levels of the other controls. This means there are usually several ways to go about getting your sounds. The balance of frequencies are all relative to each other, so if you want more bass for example try cutting the treble a little instead for a slightly different result. Or if you want more treble try cutting the bass. And since you have control of both ends of the frequency range, you can also control them in comparison to the mid range...

You may have to also mess about with volume controls when changing the EQ, as they will usually affect gain too. But the interaction of the various controls of valve amps is a huge part of what makes them so rewarding to play compared to many solid state amps. With those the treble control often only affects the treble frequencies, mid the mids, bass the bass, with no real interaction between them. Really boring and - believe it or not - often pretty limiting too in comparison to a really good valve amp EQ...

Hope that helps clarify things and helps you get where you want to be.


Andy.