OTR 120

[Jarkko Mattheiszen]

Ok, I popped the amp open and started tracing from the input to the end of the PI. Definitely some differences in there. I duplicated your spreadsheet and added my OTR in there:

https://docs.google.com/a/taintedstudio ... sp=sharing

I bolded the differing values and marked a ? in the spots I didn't check. There's also a V1B coupling cap (47nF) I didn't see in your list. Apart from those listed, the 1M grid leak resistor is nowhere to be found and there's the added V1A plate resistor bypass cap (500pf). The black wire from the input jack to the board is indeed a ground wire, and the input lead's shield has been disconnected from V1A ground. The major differences not listed are the PI and MV. I made a quick drawing of how they're wired in my amp (might not be 100% accurate, but I'm pretty sure I got it right):



I'll tidy up the lead dress a bit and take some better, hi-res pics of the amp then. It seems parts of it differ from the OTR schematic and the later production models (all the other OTR's I've seen), while others are in line with both the early and later 70's OR120 schems. Also, the board is loaded with both old and new components, like the rainbow coupling caps. From what I can tell, the only aftermarket modding is the PT, the V1A plate resistor bypass cap and the input ground wire - everything else looks like it's always been that way. Could this actually be a prototype amp?

[fiveightandten]

Yup, this was indeed confirmed when finding the old pics! I'm pretty sure the new PT is this one from TAD, or at least I haven't been able to find anything else suitable that looks identical: http://www.tubeampdoctor.com/en/shop_Ma ... k_MKII_133

That may very well be it. Are the color codes the same? I'm colorblind, so I'm pretty bad at hatching that stuff out in pictures.

Makes sense. I've now heard of three of these amps, all with the same serious oscillation problems from the factory. Comparing to pics from a friend's Superbass from the same era, the same rerouting has been done to that one.

It does happen with something of them. I wouldn't consider it common, but it's possible in any of these amps where the OT wiring isn't nice and neat. It needs to be tucked away in the corner, and away from the inputs as much as possible. Most of these amps have shields around the inputs, and that's likely why they were installed. If the shield is missing, I wouldn't be surprised if the amp has oscillation issues.

I might be completely wrong here, but as far as I know, the production of the earliest post-Gibson Oranges were handled by Matamp. I know it should be taken with a grain of salt and it's just additional trivia really, but there are some pretty interesting photos on Wheeler's site like this one showing an OTR faceplate under a Green one: http://www.planetoftheamps.com/orange-a ... atamp.html

That's pretty wild. I wonder why they bothered putting an Orange face panel behind it (maybe the Matamp one is just plastic and needs a backing). That must be a tight squeeze to get the shafts through 2 panels with enough threading to work with.

I actually sent Orange an e-mail about this amp along with the photos. I'm not sure if anyone has the time to check them out or if they even have any records on these, but it's worth a shot!

I once emailed Orange to get info on my GRO100. It took them a year and a half to respond, but when I got an email, Cliff Cooper himself responded. I think they're a lot better with emails nowadays. But there likely aren't a lot of people in the company that were around back when your amp was made.

Ok, I popped the amp open and started tracing from the input to the end of the PI. Definitely some differences in there. I duplicated your spreadsheet and added my OTR in there:

https://docs.google.com/a/taintedstudio ... sp=sharing

I bolded the differing values and marked a ? in the spots I didn't check. There's also a V1B coupling cap (47nF) I didn't see in your list. Apart from those listed, the 1M grid leak resistor is nowhere to be found and there's the added V1A plate resistor bypass cap (500pf). The black wire from the input jack to the board is indeed a ground wire, and the input lead's shield has been disconnected from V1A ground. The major differences not listed are the PI and MV. I made a quick drawing of how they're wired in my amp (might not be 100% accurate, but I'm pretty sure I got it right):



I'll tidy up the lead dress a bit and take some better, hi-res pics of the amp then. It seems parts of it differ from the OTR schematic and the later production models (all the other OTR's I've seen), while others are in line with both the early and later 70's OR120 schems. Also, the board is loaded with both old and new components, like the rainbow coupling caps. From what I can tell, the only aftermarket modding is the PT, the V1A plate resistor bypass cap and the input ground wire - everything else looks like it's always been that way. Could this actually be a prototype amp?

It doesn't look like that sheets document is public. It won't let me view it. Yeah, I didn't include the V2B coupling cap because it's the same value in all of the schematics (.068u...it sounds like yours is different). I really only included components with values that differed from circuit to circuit.

So, you have large value coupling caps in there, Vox style. Instead of the OTR PI, it looks like you have what I'm calling the "Type 2", as it's direct coupled and just has the simplified PI with plate and cathode resistors (no grid load). Interesting that there are 47K's bridging the pot wipers and outer lugs. I don't think I've seen that.

[Jarkko Mattheiszen]

That may very well be it. Are the color codes the same? I'm colorblind, so I'm pretty bad at hatching that stuff out in pictures.

Yeah, seems like they are!

I once emailed Orange to get info on my GRO100. It took them a year and a half to respond, but when I got an email, Cliff Cooper himself responded. I think they're a lot better with emails nowadays. But there likely aren't a lot of people in the company that were around back when your amp was made.

That's really cool! I'm hoping they have some sort of records on these, or at least some note on who made these

It doesn't look like that sheets document is public. It won't let me view it. Yeah, I didn't include the V2B coupling cap because it's the same value in all of the schematics (.068u...it sounds like yours is different). I really only included components with values that differed from circuit to circuit.

Ah, my bad! I made it public now. The different values in the tonestack are quite curious. Playing with TSC it looks like there's a considerable bump at 100Hz with the controls "flat" compared to the other models.

So, you have large value coupling caps in there, Vox style. Instead of the OTR PI, it looks like you have what I'm calling the "Type 2", as it's direct coupled and just has the simplified PI with plate and cathode resistors (no grid load). Interesting that there are 47K's bridging the pot wipers and outer lugs. I don't think I've seen that.

I haven't seen that kind of a MV anywhere either, and I had to go over it thrice to make sure I got it right. It would be an easy conversion to the regular one, might have to give it a go.

[Jarkko Mattheiszen]

I took some hi-res photos of the amp, here's a link so I won't kill anyone's connection:

https://www.dropbox.com/sh/bovtxquuqfli ... KgkFa?dl=0

And here's what I know so far:

- Built no earlier than 1997 and no later than 1999
- 1990’s chassis and sleeve
- Chassis has “Model OR” stamped and serial 54 hand written, no indication of OTR except on the front panel
- OT markings: 112-120 Q369 (last character not certain)
- Choke markings: 112-120 E346
- The PT has been replaced at some point to a TAD OR120 transformer
- Old style impedance and voltage selectors
- Standby switch is not labeled, it says “PRE AMP / SLAVE OUT” instead
- 70’s PCB stamped “Orange Graphic MKIII” (never heard of a MKIII, what's that?)
- Letters “BI” hand written inside the chassis
- “N/O/DRIVE” hand written on the PCB next to the rectifier diodes
- Mix and match of vintage and modern components on the PCB
- Tonestack has more bass and a larger boost at 100Hz than a regular OTR
- Mods done afterwards: PT replaced, wiring cleaned up, V1A plate resistor bypass cap added, input jack grounded to PCB, probably to remove oscillation issues

Differences to the OTR schematic drawn by Adrian Emsley at Orange:
- Tonestack bass top cap is 1000pF instead of 2200pF
- TS bass bottom cap is 0.01uF instead of 0.022uF (same as post 1974 OR120’s)
- TS treble bottom cap is 4700pF instead of 0.01uF
- V1A and V1B coupling caps are 0.047uF instead of 0.022uF and 0.068uF
- V2A plate resistor is 390k instead of 220k
- V2A is direct coupled to V2B (same as post 1974 OR120’s)
- V2B cathode resistor is 100k (same as all 1970’s OR120’s)
- No second V2B cathode resistor (same as post 1974 OR120’s)
- PI is wired the same as post 1974 OR 120’s
- Different master volume wiring

[Jarkko Mattheiszen]

Ok, I got a very helpful reply from Orange. It's not a Matamp build after all - seems like it's a custom amp built by Adrian himself at Denmark Street in 1997-1998 using 70's parts. Really cool, I'm taking the "for sale" ads down right now

[bclaire]

Ok, I got a very helpful reply from Orange. It's not a Matamp build after all - seems like it's a custom amp built by Adrian himself at Denmark Street in 1997-1998 using 70's parts. Really cool, I'm taking the "for sale" ads down right now

Ahh... that's the most logical explanation! It's a very cool amp!