Hello all,

That's it: let's discuss about this small but great amp.
In resume, structure very similar to a JCM800 preamp, a few changes. I've see basic structure of 4 stages there.



Just a few ideas, observations:
-V1B or V2A are good candidates to tweak cathode resistors to turn them to cold clipper, maybe tamming a bit and compressing that OD whrn gain know is pushing.
-R48 and R38 twaked, adding variable resistor+ cap for a Presence for NFB?
-Increase some coupling caps to get more bass thru the signal?
-Where/How could be tweaked Fx loop triodes to get a few more boost from there, withour distortion?
-For PPIMV dual shaft pot, some kind of attenuator, where Resistors would be replaced with variable resistor?

What is it that you think needs to change? How?

It looks over-the-top to me. If you put more bass in it it will just get flubby, or motorboat.

Quote from: PRR on November 04, 2023, 09:24:36 PMWhat is it that you think needs to change? How?

It looks over-the-top to me. If you put more bass in it it will just get flubby, or motorboat.

100%.   Usually one would be reducing coupling caps to lower bass, if anything...guitar really doesn't sound very good with too much bass, and it over-stresses the power supply, contributes to blocking distortion and so on.

Messing around with gain stage AC bypassing is an extension of the above.

Yes, you could experiment with stage biasing if you wanted. The audible 'improvements' in how it overdrives might be less dramatic than expected, though!

Maybe modeling the tone stack and seeing if any values there are worth playing with (or an entirely different tone stack) could be worthwhile?   

I think one has to ask - what's wrong with it now that it requires any mods? 

Quote from: Plexi on November 04, 2023, 03:31:20 PMIn resume, structure very similar to a JCM800 preamp, a few changes. I've see basic structure of 4 stages there.

IMHO, any "similarity" effectively ends to the tonestack and actually even that one is different.

- There's none of characteristic "partial" cathode bypass to limit lows flabbing out during distortion (Orange employs very low coupling capacitance instead)
- OR15 doesn't exactly favor "treble-peaking" circuits either, on a contrary, I counted at least two plate bypass lo-pass filters
- The tonestack is plate driven so no Marshally CC+CF overdrive stage there with its rather symmetric clipping characteristics; OR15 more prominently has an extra voltage gain stage, and the only thing preventing over-the-top distortion is that unorthodox "dual" gain control
- OR15 employs very conventional "symmetric" bias points and maximum voltage gains of the preliminary stages pre FX loop. We see none of skewed bias points of 2K7 or 10K (!!!) cathode bias of Marshall 2203, not to mention lesser gain ratios of such bias points.

So, no, there's particularly nothing JCM800-ish in there.

QuoteJust a few ideas, observations:
-V1B or V2A are good candidates to tweak cathode resistors to turn them to cold clipper, maybe tamming a bit and compressing that OD whrn gain know is pushing.
...
-Increase some coupling caps to get more bass thru the signal?

Want to convert this to Marshall, there are major differences I pointed out. Orange's method to reduce farting distortion caused by too much low end is to employ rather low coupling capacitances; Marshall employs partial cathode bypass and treble-peaking circuits instead (compare e.g. 2203 circuit diagram for reference). There are many ways to skin the cat but most importantly: at what set of frequencies those filters operate in OR15 or in a Marshall? Those plate bypass caps will concentrate the gain of the OR15 even more readily to mid-range frequencies reducing "fizz", yet a 2203 JCM800 will undoubtedly be much brighter and aggressive overall.
 

Quote-Where/How could be tweaked Fx loop triodes to get a few more boost from there, withour distortion?

Pre or post loop? The negative feedback around V3B controls voltage gain before loop. V3A could be made to have more gain with cathode bypass or alternatively just tweaking that attenuator after it to attenuate less.

1) When comparing the low-frequency cutting by bypass capacitors in Marshall and interstage capacitors in Orange, you are confusing yourself. These capacitors operate differently and have different effects on sound. The Orange method is basic; what the interstage capacitor cuts off cannot be compensated by the bypass capacitor.

The Orange definitely has large bypass capacitors and a thick (perhaps excessively) sound, you can replace the bypass capacitors with a smaller value of 680nf-1uf, complete with smaller capacitance interstage capacitors the sound will be more taut than the Marshall, in the style of modern Friedman amplifiers.

2) The absence of a cathode follower, in addition to asymmetric compression, the cathode follower affects the slope of the tonal stack, without it there will be less mid and high frequencies, usually this is compensated by a decrease in the master volume value.

3) Lack of cold clipper which is part of the jcm 800 sound which affects the waveform and overall sound.

4) Different values ��of interstage filters and cathode resistors and capacitors.

5) Different amplifier circuits

6) Orange has no presence Why bother changing a cool modern amp into a vintage amp? buy yourself a Marshall SC20h

A capacitor across R48, the 4.7K feedback resistor, would help tame the highs and also help improve stability.

Reducing R13 would give you more gain and less bass.

In general, reducing the value of coupling capacitors will cut the bass but you need to determine how much you want to cut and what the slope of the turnover should be.

Do not cut C22 and C23, the coupling caps to the output tube grids because they are in a feedback loop and the phase shift may cause some instability.  If there is any rise in the output at either the low or high end between the inverter and the speaker outputs, that is a sign of instability.

Thank you all for explain a bit more about it! I'm reading and learning about some functions and design.
I guess I've started witht the wrong consign for the thread; I'm not looking to improve anything about OR15 (which I haven't, yet: but played and really liked it). Just I've found a great platform to understand and learn a bit more about amps structure and how to tweak them could change gain and freq response.

Please, don't hesistate to keep going with observations and suggestions 

I'm looking for some difficult schematics to check around this.
Orange Rocker 15 terror, or 32 rocker.
Friedman Runt 50 or 20.

Where do you usually found them? Turning google results upside down.. but maybe there's some hide forum or blog around.

I'm looking them to compare and learn. Mostly interested in known how Friedman reach those almost high gain Marshally tones using 3 or 4 triodes in the preamp.

I don't know if the cold clipper trick used by Marshall on JCM800 is the only reason to "compress" a bit more without diodes clipping or similar.

A cool tweak to this amp, could be jump and insert input direct to second triode.
More clean headroom I guess¿

I've seen it over Dark Terror (90% same circuit).

EDIT: not jumpling first triode, but another good option,
https://forum.orangeamps.com/viewtopic.php?t=46197

Would worth to put a Choke?
Where? Replacing R50?

Just a reminder for those who want to change a tone stack, there is a calculator here:

https://www.duncanamps.com/tsc/

Quote from: amptramp on April 07, 2024, 08:49:39 AMJust a reminder for those who want to change a tone stack, there is a calculator here:

https://www.duncanamps.com/tsc/

Great, thanks for share..!
Amp is very quiet over freqs; doesn't cut or boost too much. In a future where I can crank almost all the way the volume, I'll try sugested mod to Rockerverb values.

About the amp, great overdrive and distortion variety. I'm waiting to test in jam and band context to see how is it going. I feel it doesn't need any booster in the front; just maybe boost mids or highs for solos.